Engine Tuning Theory With : Fuel Injection

[cygnus x-1]

My whole thesis on this is basically extrapolated from clues certain tuners on h-t have been dropping for a while. When a tuner says that you can get better mpg from running leaner than stoich with more timing, that got me thinking. I didn't see how taking out fuel past 14.7 for a given map point would give you more power. As far as I know, it doesn't. So then I thought, wait a minute, maybe I'm not seeing the big picture. The point of tuning for part throttle is to get the most energy out of every pulsewidth, not out of a certain amount of air. So if it's more efficient, why not put that pulsewidth on a higher manifold press part of the map? If you have a vac/boost guage in your car, you know that at a certain cruise speed, your guage will have a certain reading. But what I am saying is totally counterintuitive because with what I'm describing, the guage will actually read a slightly higher manifold press, for the same cruise speed, yet the motor is actually using less fuel, since at that higher map press, the pulsewidth is able to generate more power, so you end up with a slightly shorter pulsewidth to achieve the same cruise speed (power). Most people would see the higher manifold press and assume that the motor is under a higher load/must be putting out more power, but it isn't. It's just running leaner with a very slightly shorter pulsewidth.
Thing is, how do you tune this. With tuning part throttle for a given manifold press, you just add/subtract fuel and timing until you're at the most power. Oem maps just put the fuel at 14.7 and put the timing either where it's safe or at mbt, if that's safe with the available fuel. But when tuning leaner than 14.7, you essentially have 3 variables at once coming into play, so it's not so simple. I was trying to eliminate one of the variables by describing the scenario of moving the certain pulsewidth up and down on the map scale.

Here's some real "food for thought". What is the advantage of drive by wire? (computer controlled throttle)....

If any of this is incorrect, if you do actually get more power for a certain map press by running leaner than stoich, I don't understand how that would be, if you know, please explain. I believe that you would get a lower bsfc (mpg) by running with more air because it increases the motor's dynamic compression. That is why diesels are more efficient than gasoline motors, they have a much higher mechanical compression ratio and don't have a throttle, so their dynamic compression is much higher. The btu content difference in fuels is minor in comparison.

[coope]

dame thats alot to read

[Ichiban]

Well, I seem to remember reading in my High Performance Honda Handbook (which is 8 hours away so I can't readily reference it), most ECU maps DO run considerably leaner than stoich under part throttle low load conditions. Heavier fueling is not required until MAP decreases. In fact, I do believe that under no load/no throttle/high MAP conditions the computer may augment the a/f ratio all the way to 22:1 (imagine engine braking while driving down a hill) This is simply to reduce afterfiring and emissions.

Anyways, I believe that part throttle "cruising" mixtures can be leaner than stoich, because you can achieve a rapid clean burn, and with little load you don't have the cylinder pressures present to force detonation. It's simply more efficient. I'm still not sure how this would relate to your wanting to tune to a given injector pulse with, and vary the intake pressure value.

Diesel has approximately 15% more energy per volume unit than gasoline.

Oh, and don't quote me on any numbers other than the diesel one and how far away the book is with the answers in it.

[cygnus x-1]

My whole thesis on this is basically extrapolated from clues certain tuners on h-t have been dropping for a while. When a tuner says that you can get better mpg from running leaner than stoich with more timing, that got me thinking. I didn't see how taking out fuel past 14.7 for a given map point would give you more power. As far as I know, it doesn't. So then I thought, wait a minute, maybe I'm not seeing the big picture. The point of tuning for part throttle is to get the most energy out of every pulsewidth, not out of a certain amount of air. So if it's more efficient, why not put that pulsewidth on a higher manifold press part of the map? If you have a vac/boost guage in your car, you know that at a certain cruise speed, your guage will have a certain reading. But what I am saying is totally counterintuitive because with what I'm describing, the guage will actually read a slightly higher manifold press, for the same cruise speed, yet the motor is actually using less fuel, since at that higher map press, the pulsewidth is able to generate more power, so you end up with a slightly shorter pulsewidth to achieve the same cruise speed (power). Most people would see the higher manifold press and assume that the motor is under a higher load/must be putting out more power, but it isn't. It's just running leaner with a very slightly shorter pulsewidth.

Ok. That's what I thought you were getting at. Effectively just running a leaner mixture for a given MAP-RPM point. The timing has to be advanced because the leaner mixture burns slower, and to get peak cylinder pressure at the right crank angle it has to be ignited sooner. This only works to the point where you can't ignite the mixture anymore, which is a misfire.



Thing is, how do you tune this. With tuning part throttle for a given manifold press, you just add/subtract fuel and timing until you're at the most power.

Yes, that's it. This is also by definition the point of maximum fuel efficiency, i.e. minimum fuel usage for maximum power.



Oem maps just put the fuel at 14.7 and put the timing either where it's safe or at mbt, if that's safe with the available fuel. But when tuning leaner than 14.7, you essentially have 3 variables at once coming into play, so it's not so simple. I was trying to eliminate one of the variables by describing the scenario of moving the certain pulsewidth up and down on the map scale.

Ahh. So you were assuming an OEM type ECU with a narrow band O2 sensor. With a narrow band sensor you are pretty much limited to tuning close to 14.7:1 AFR because narrow band sensors can only tell you if the AFR is richer or leaner than stoich. So the ECU has to waffle back and forth between too rich and too lean to get an average stoichiometric AFR.

With a wide band sensor you can adjust to any AFR you want so now there is the extra variable. When I was tuning my Megasquirt I read all kinds of tuning references just to see what I could figure out. One thing that I found was that AFR isn't so critical to power output as you might think. More fuel doesn't necessarily mean more power output, it just means more fuel usage. You aren't likely to see any difference in power output by changing from 13.2 to 13.5 AFR. All you need is *enough* fuel to produce the desired power. Supposedly you can run stoich even at WOT but I can't remember how exactly that works; or even where I read that.

I tried running 17:1 AFR in the light throttle areas, but I had problems with misfiring so I backed it down. Here are my current AFR and timing maps. I could probably go a little leaner but it hasn't been a priority yet.

http://bluegreenlabs.com/Prelude/Mods/FIConversion/AFR-200710112304.jpg

NOTE: FOR ANYONE READING THIS OLD POST, THE ORIGINAL TIMING MAP THAT WAS LINKED HERE SEEMS TO HAVE BEEN LOST, SO I'VE REPLACED IT WITH A MORE UP TO DATE VERSION. THIS ONE IS BETTER THAN THE OLD MAP ANYWAY.

http://bluegreenlabs.com/Prelude/Mods/FIConversion/SparkTable-10-10-2008.jpg



The timing table is only street tuned so I don't know if it's optimal or not. I also don't know how it compares to a stock map so any comments would be interesting.



Here's some real "food for thought". What is the advantage of drive by wire? (computer controlled throttle)....

Drive by wire allows the computer to read the rate of change of throttle position in order to better control engine operation under transient conditions. The hardest part of engine operation to tune is during changing throttle position, which means changing MAP. Giving the ECU direct control over the throttle allows it to manage changes in MAP in a way that it can handle, instead of just having to react instantaneously to changes in MAP. (Ahh! I didn't understand why you mentioned this until just now. Clever! :D)



If any of this is incorrect, if you do actually get more power for a certain map press by running leaner than stoich, I don't understand how that would be, if you know, please explain. I believe that you would get a lower bsfc (mpg) by running with more air because it increases the motor's dynamic compression. That is why diesels are more efficient than gasoline motors, they have a much higher mechanical compression ratio and don't have a throttle, so their dynamic compression is much higher. The btu content difference in fuels is minor in comparison.

I'm not an expert here but I believe higher compression just makes more efficient use of fuel overall. Higher cylinder pressure creates more torque at the crank. It's more efficient because the change in volume from fully compressed to least compressed is greater; which means more of the pressure energy is transfered to mechanical motion. Or something like that. Not a very good explanation.

C|

[cygnus x-1]

Now this is interesting...

http://en.wikipedia.org/wiki/Lean_burn

C|

[Accordtheory]

Ahh. So you were assuming an OEM type ECU with a narrow band O2 sensor.

Drive by wire allows the computer to read the rate of change of throttle position in order to better control engine operation under transient conditions. The hardest part of engine operation to tune is during changing throttle position, which means changing MAP. Giving the ECU direct control over the throttle allows it to manage changes in MAP in a way that it can handle, instead of just having to react instantaneously to changes in MAP. (Ahh! I didn't understand why you mentioned this until just now. Clever! :D)


I wasn't asuming a narrow band sensor at all. I'm guessing you got that from my mention of part throttle oem maps being tuned to 14.7? But that's not why oems are tuned to 14.7, (for the most part) either. They're tuned that way to optimize the function of the catalytic converters.

Basically, the right tune for a certain amount of power on the part throttle map is the right mixture of air, fuel, and timing. Each certain amount of power requires a different mix of those three floating variables. I think that would be extremely cool if there was a program that would allow you to isolate the fuel variable, and tune from it.
As an oem, you also have a 4th variable to get that certain amount of power most efficiently: the right rpm.
Rpm is a factor in the efficiency of power production, although it's not a tuning factor, it is a gearing factor. Ideally, you would match a brake specific fuel consumption chart graphed across load and rpm to set your gearing for mpg. There is always one point of map/rpm that will use the least fuel for a certain output. In other words, say you need x amount of hp to cruise at 80 mph, what rpm/manifold press will yield the lowest fuel consumption for that required hp output? It isn't always the lowest rpm. It was a trip when I learned that..

As far as drive by wire, I'll have to get into that more. After I do more research, that is. I do know that you can do acceleration enrichment via a map sensor, without even using a tps, and I suspect you can do a decel fuel cut too. I suspect that with drive by wire, the computer is able to simultaneously take into account all of the variables I have mentioned, and figure out the most efficient way to accelerate the vehicle based on your desired rate of acceleration. I also suspect that the computer can be (or are)tuned to create a more linear rate of acceleration over a range of speeds based on pedal position. Have you ever driven a vehicle that accelerated hard at first and then would just crawl at 45mph and up with the same pedal input? Really annoying, most fords come to mind when I think of that. My parents' old taurus and explorer both do that. The drive by wire could accomplish the opposite of that. But one of the most cool things it can do is cut/reduce engine output directly (without doing stupid shit like retarding the timing, which you wouldn't want to do with a turbo) while shifting, to preserve the tranny/driveline or to stop wheelspin on the shift for hard acceleration. GM calls that "torque management". I don't know what the real difference is in driveability/mpg actually is with drive by wire though, but I'd say it's pretty small, because your brain learns to control the right foot pretty well, and as long as the tranny shift points are ideal, you'll be at the same load/rpm points anyway.

[itzdave]

:huh: ya lost at hello...

[Accordtheory]

Yeah, if you think it takes effort to understand this, then try to write about it..shit. I had a headache earlier after I posted, as I was hoping everything I wrote was true and made sense.. It's always a hell of a lot harder to explain something than it is to just nod your head and say, "yeah, okay, it makes sense, I guess"..

As far as the lean burn/stratified charge combustion, I know a little about that. I started reading about that after Larry Widmer's old civic hatch was featured in hot rod magazine. 11.04 on a supercharged d16, I was like how the F did he do that?? Little did I know, he's on of the most educated, contributing people to the whole sport and to developement of engines overall that I've ever read about. I went to his site, endynracingengines.com, (used to be theoldone.com) and started reading the old articles. Turns out he used to build pro stock engines in the 80s, and was using compression ratios in the area of 23:1, And running a lot leaner than 14.7, under full throttle. The way he explained it was the combustion was controlled by the shape of the piston and chamber in a way that allowed a layered charge of varying a/f ratios, and the entire intake side of the piston interacted with the head as a quench zone, pushing all of the mixture to the exhaust side. If I'm going to write any more about that, I'll have to go back to the site and read for a while, which I don't feel like currently doing.
I should be putting my gsr head on my motor right now, but I don't know, I think I might upgrade my valve springs first. I plan on running the S2 Pro1 cams, and I don't want to have to take the damn head off again to get that set up.

[2ndGenGuy]

I was gonna say, I don't think you wanna try doing any of that lean burn stuff in a traditional engine unless you really know what you're doing. The shape of the combustion chamber, location of injectors, etc is what allows the cooling of the lean mixture so that you don't melt everything down. The old CVCC engines were able to burn lean, because the super-rich mixture in the pre-combustion chamber cooled down the primary mixture in the cylinder.

I've always wondered what the ratios of lean to rich are as far as those old CVCC motors. I thought it would be bitchen to one day get an actual functional CVCC turbo motor built that puts out some decent power, and see what kind of emissions it puts out. The Honda City Turbo manages to do it, but 110hp is not as much as I would be looking for.

I don't know a whole lot about this stuff. But it's sure interesting as hell to read about.

[cygnus x-1]

I wasn't asuming a narrow band sensor at all. I'm guessing you got that from my mention of part throttle oem maps being tuned to 14.7? But that's not why oems are tuned to 14.7, (for the most part) either. They're tuned that way to optimize the function of the catalytic converters.

Oh, right. Duh. And the O2 sensor was designed for 14.7 to match the converters.



As far as drive by wire, I'll have to get into that more. After I do more research, that is. I do know that you can do acceleration enrichment via a map sensor, without even using a tps, and I suspect you can do a decel fuel cut too.

Yes! Megasquirt has two methods for acceleration enrichment. The simple way just adds to the pulse width based on throttle position rate of change or MAP rate of change. For a greater rate of change you add more pulse width. With this method you can also set how much of it is based on MAP or throttle position; continuously variable from 100% TPS to 100% MAP. This method works but it doesn't reflect what's actually going on.

The other method is based on something called X-tau. X-tau is a way to model the effect of intake manifold wall wetting. This is a phenomenon where some of the fuel that's injected from the injector sticks to the wall of the intake for a brief period of time. It changes based on air flow (RPM). "X" is an amount of fuel that sticks to the walls and tau is the amount of time it takes to evaporate back into the mixture. So by setting X and tau for different RPMs you can get very good acceleration enrichment for most conditions. Very sharp throttle changes will still trip it up but it overall it works quite well. It also works for decel enleanment too but I haven't had much success with decel tuning. Right now I'm running X-tau for accel enrichment only, and it feels like an OEM tune.



I suspect that with drive by wire, the computer is able to simultaneously take into account all of the variables I have mentioned, and figure out the most efficient way to accelerate the vehicle based on your desired rate of acceleration. I also suspect that the computer can be (or are)tuned to create a more linear rate of acceleration over a range of speeds based on pedal position.

Exactly.



I don't know what the real difference is in driveability/mpg actually is with drive by wire though, but I'd say it's pretty small, because your brain learns to control the right foot pretty well, and as long as the tranny shift points are ideal, you'll be at the same load/rpm points anyway.

I believe the primary motivation for drive by wire at the moment is emissions. Acceleration and deceleration are the worst areas as far as emissions go. So if you can control the engines acceleration rate you can keep unwanted emissions under control.

My parents have a Toyota Avalon with drive by wire. I think it's annoying because it has a slight time lag in throttle response. It's small but if you're used to a very responsive throttle you can definitely feel it.

C|

[forrest89sei]

WOW, GREAT THREAD!

SO Much to learn about Fuel Injection!

[cygnus x-1]

Yeah, if you think it takes effort to understand this, then try to write about it..shit. I had a headache earlier after I posted, as I was hoping everything I wrote was true and made sense.. It's always a hell of a lot harder to explain something than it is to just nod your head and say, "yeah, okay, it makes sense, I guess"..

Damn, ain't that the truth! I've spent an hour or two on a single post. Just to make sure I'm saying what I really mean to say. Even then I don't always get it right.



As far as the lean burn/stratified charge combustion, I know a little about that. I started reading about that after Larry Widmer's old civic hatch was featured in hot rod magazine. 11.04 on a supercharged d16, I was like how the F did he do that?? Little did I know, he's on of the most educated, contributing people to the whole sport and to developement of engines overall that I've ever read about. I went to his site, endynracingengines.com, (used to be theoldone.com) and started reading the old articles. Turns out he used to build pro stock engines in the 80s, and was using compression ratios in the area of 23:1, And running a lot leaner than 14.7, under full throttle. The way he explained it was the combustion was controlled by the shape of the piston and chamber in a way that allowed a layered charge of varying a/f ratios, and the entire intake side of the piston interacted with the head as a quench zone, pushing all of the mixture to the exhaust side. If I'm going to write any more about that, I'll have to go back to the site and read for a while, which I don't feel like currently doing.

I read that article on the web just the other day. Very cool stuff. Lately though I've been looking into head and intake tuning. I still think there is more to be had out of the A20 head than we might think. Part of this includes a properly designed ITB setup. I've not yet seen an ITB setup on an A20 that was specifically designed for the lower RPM strength of these engines. The other idea is high velocity intake ports. I have a theory that the ports on A20 heads are too big and don't provide optimal cylinder filling at the lower RPMs where the A20s really shine. Hopefully I can get a chance to try out some new ideas when it warms up again.

C|

[Demon1024]

WOW, GREAT THREAD!

SO Much to learn about Fuel Injection!
x2!
I love this site!:)

[Accordtheory]

I was gonna say, I don't think you wanna try doing any of that lean burn stuff in a traditional engine unless you really know what you're doing. The shape of the combustion chamber, location of injectors, etc is what allows the cooling of the lean mixture so that you don't melt everything down. The old CVCC engines were able to burn lean, because the super-rich mixture in the pre-combustion chamber cooled down the primary mixture in the cylinder.

I've always wondered what the ratios of lean to rich are as far as those old CVCC motors. I thought it would be bitchen to one day get an actual functional CVCC turbo motor built that puts out some decent power, and see what kind of emissions it puts out. The Honda City Turbo manages to do it, but 110hp is not as much as I would be looking for.

I don't know a whole lot about this stuff. But it's sure interesting as hell to read about.

Well, for the most part, you can't really run like that on a normal motor.
I've been wondering for a while if it possible to have an injector set up so it could inject fuel with a short enough pulsewidth that it could be timed to only mix with part of the air in the cylinder. In other words, inject right at the bottom of the intake stroke so that when the piston rises, the layer of air+ fuel is just at the top, by the plug. At higher throttle positions, the injector would start spraying earlier. Another thing I've been wondering about is along the same lines, is possible to start the injection after the exhaust valve is already closed, so that you could run large cams without fuel being scavenged out of the exhaust.

More on the running lean thing, how exactly does running lean melt things anyway? I don't really get that. I never really did. It's not like you're getting more energy out by injecting less fuel. I remember when I took a class in aircraft engines, the motors would always rev way up when we cut the fuel. I was like, wtf, if they make more power lean, why don't they run like that all the time? Now when I think back on it, I suspect that since those motors were air cooled, they ran really rich at idle, and as the carbs ran dry, the mixture leaned out, crossing the stoich point before they died. I've seen the same thing on 2 cycle jet skis too. Anyway, the only thing I can come up with as far as melting parts is a lean mixture burns much more slowly, effectively transferring more heat into everything. But that still doesn't fully click in my head though, because if the stoich mixture gets hot and cools off more quickly, wouldn't it still transfer just as much heat, because despite it cooling off more quickly, it got to that high temp initially more quickly too??
..more thinking required.

When you use an acetylene torch, isn't the hottest flame produced by a stoichiometric mixture?

I know diesels and other certain motors, like cng/propane run on varying a/f ratios, without any of the problems encountered with gasoline motors. I remember reading about a motor that could run on a variety of fuels, including gasoline/ethanol propane, and hydrogen. With hydrogen, I believe it ran like a spark injected diesel, with the throttle open and the power controlled by the amount of hydrogen injected.. I guess with a diesel, the egts and temperature of combustion actually drop as you add more air. That's how a diesel can run 70lbs of boost. Add fuel, egt gets too high, turn the boost up, watch the egt drop, then add more fuel until your compressor can't flow any more.

Is that leanness thing why you "can't" run gasoline in a compression ignition engine? (or why it hasn't been done)? If an injection system was developed that could pressurize the non lubricating gasoline to the same extreme pressure, and inject it in a manner to achieve stratified charge combustion, that would be awesome. You'd have a motor that was almost as efficient as a diesel, (btu content difference) yet could rev and produce the power of a honda, ie 120 hp/liter normally aspirated. Someone needs to get on this.

[2ndGenGuy]

Well, for the most part, you can't really run like that on a normal motor.
I've been wondering for a while if it possible to have an injector set up so it could inject fuel with a short enough pulsewidth that it could be timed to only mix with part of the air in the cylinder. In other words, inject right at the bottom of the intake stroke so that when the piston rises, the layer of air+ fuel is just at the top, by the plug. At higher throttle positions, the injector would start spraying earlier.

You might be onto something with that. I do believe that is partially the theory behind manufacturers doing the direct cylinder injection. Being able to inject directly into the cylinders allows them to target where they want the fuel, instead of being more evenly dispersed like it would be if it was pulled in by the rushing air through the intake manifold.


Another thing I've been wondering about is along the same lines, is possible to start the injection after the exhaust valve is already closed, so that you could run large cams without fuel being scavenged out of the exhaust.

I don't think that would do you any good necessarily. The problem is that air will still get scavenged out of the cylinder before the fuel is injected. Though I suppose the same amount of air would be in there regardless. And if you could somehow manage to get enough fuel in to keep your ratios the same, you might achieve better fuel efficiency. Having only air escaping out the exhaust valve, and not and air fuel mixture.



More on the running lean thing, how exactly does running lean melt things anyway? I don't really get that. I never really did. It's not like you're getting more energy out by injecting less fuel. I remember when I took a class in aircraft engines, the motors would always rev way up when we cut the fuel. I was like, wtf, if they make more power lean, why don't they run like that all the time? Now when I think back on it, I suspect that since those motors were air cooled, they ran really rich at idle, and as the carbs ran dry, the mixture leaned out, crossing the stoich point before they died. I've seen the same thing on 2 cycle jet skis too. Anyway, the only thing I can come up with as far as melting parts is a lean mixture burns much more slowly, effectively transferring more heat into everything. But that still doesn't fully click in my head though, because if the stoich mixture gets hot and cools off more quickly, wouldn't it still transfer just as much heat, because despite it cooling off more quickly, it got to that high temp initially more quickly too??
..more thinking required.

I'm pretty sure that the air burning is what causes the higher temperatures. Fuel is supposed to burn at a much cooler temperature hence why it is so much more flammable. And I thought that the fuel is what gets the air burning, and a complete burn is what gives you the most expansion and hence the most power. Not necessarily the most heat. I'm sure that longevity plays a role in it too, as you can see the results of lean mixtures on burnt up valves and spark plugs, among other things.


When you use an acetylene torch, isn't the hottest flame produced by a stoichiometric mixture?

I know diesels and other certain motors, like cng/propane run on varying a/f ratios, without any of the problems encountered with gasoline motors. I remember reading about a motor that could run on a variety of fuels, including gasoline/ethanol propane, and hydrogen. With hydrogen, I believe it ran like a spark injected diesel, with the throttle open and the power controlled by the amount of hydrogen injected.. I guess with a diesel, the egts and temperature of combustion actually drop as you add more air. That's how a diesel can run 70lbs of boost. Add fuel, egt gets too high, turn the boost up, watch the egt drop, then add more fuel until your compressor can't flow any more.

Is that leanness thing why you "can't" run gasoline in a compression ignition engine? (or why it hasn't been done)? If an injection system was developed that could pressurize the non lubricating gasoline to the same extreme pressure, and inject it in a manner to achieve stratified charge combustion, that would be awesome. You'd have a motor that was almost as efficient as a diesel, (btu content difference) yet could rev and produce the power of a honda, ie 120 hp/liter normally aspirated. Someone needs to get on this.

The reason a diesel can't rev like a gasoline engine is that you have no control over spark. Ignition occurs when the level of compression gets so high, which is at the same point of the compression stroke every time. Thats the advantage of the gasoline, is that you can ignite it whenever you want by initiating the spark. Because, as you know, you need to advance the spark timing as RPMS climb. This is why diesel engines can't rev, you can't control ignition timing. Diesel fuel is not volatile enough really to ignite with simple spark.

Correct me if I'm wrong on any of this. I'm still an engine noob. :D

[Accordtheory]

I'm pretty sure that the air burning is what causes the higher temperatures. Fuel is supposed to burn at a much cooler temperature hence why it is so much more flammable. And I thought that the fuel is what gets the air burning, and a complete burn is what gives you the most expansion and hence the most power. Not necessarily the most heat. I'm sure that longevity plays a role in it too, as you can see the results of lean mixtures on burnt up valves and spark plugs, among other things.

The air burning? You mean the nitrogen being oxidized? I don't think that reaction really contributes any energy..it just causes smog. The energy comes only from the fuel being oxidized. And expansion/pressure is directly proportional to temperature. I think there's a little more to it, but I still think the damage of a lean mixture is mostly due to the slow burn.

As far as diesels, you can absolutely control the ignition timing, with a map of rpm vs manifold press just like a gas ignition table. The only difference is it is actually injection timing. Make sense? The reason why diesels are rpm limited is diesel has a much slower burn rate, at least according to what I've read. There are some people out there who claim to be able to rev their diesels to almost 5k though..

[2ndGenGuy]

The air burning? You mean the nitrogen being oxidized? I don't think that reaction really contributes any energy..it just causes smog. The energy comes only from the fuel being oxidized. And expansion/pressure is directly proportional to temperature. I think there's a little more to it, but I still think the damage of a lean mixture is mostly due to the slow burn.

I guess the way I was thinking about it, was that the air and fuel both burn and produce the energy. I was under the impression that the leaner mixture burned hotter and more powerful because of the oxygen it was burning. But what your saying makes sense.


As far as diesels, you can absolutely control the ignition timing, with a map of rpm vs manifold press just like a gas ignition table. The only difference is it is actually injection timing. Make sense? The reason why diesels are rpm limited is diesel has a much slower burn rate, at least according to what I've read. There are some people out there who claim to be able to rev their diesels to almost 5k though..

Yeah that makes sense. I read a bit more about how diesels run, and I guess that the fuel is injected after the air is compressed (thanks to Wikipedia). That just seems odd as hell to me, and makes me wonder how you get an even mixture. Seems like it would ignite right at the injector. I was always under the impression that diesel was injected and mixed with air like a normal gas engine. I guess was thinking it ran like my R/C airplane engine.

Man theres a ton of stuff to learn still. Thanks for clarifying a few things, and discussing it with me. I'll try not to jack your thread and leave the conversation to the guys who know what's going on.

[Accordtheory]

I guess the way I was thinking about it, was that the air and fuel both burn and produce the energy. I was under the impression that the leaner mixture burned hotter and more powerful because of the oxygen it was burning. But what your saying makes sense.

Well, "burning" means "combining with oxygen". Air is only about 20% oxygen, the rest is mostly nitrogen. Some of the nitrogen does end up combining with oxygen, but this undesireable. Oems use exhaust gas recirculation to specifically reduce this. By mixing in a small amount of inert gas (exhaust) into the intake, the peak combustion temperature is reduced, reducing the formation of oxides of nitrogen. Some diesels use this too.

Oh no..I'm coming up with another idea..
What if you designed a motor that would use egr to run with considerably higher manifold pressures for a given output, mixing in the right amount of fuel, exhaust, and air for the desired output? I wonder how much egr the motor would actually tolerate. The higher the dynamic compression, the lower the bsfc..not to mentioned reduced pumping losses, too.

As far as the injection timing for very large cams, I brought that up solely for the idea stopping fuel from going out of the exhaust, for economy/emissions. It doesn't matter if fresh air gets pulled out. The fresh air would of course fool any 02 sensor, but so would fresh air+fuel.

Anyone know why large cams don't idle for shit normally?

[Accordtheory]

Yeah that makes sense. I read a bit more about how diesels run, and I guess that the fuel is injected after the air is compressed (thanks to Wikipedia). That just seems odd as hell to me, and makes me wonder how you get an even mixture. Seems like it would ignite right at the injector. I was always under the impression that diesel was injected and mixed with air like a normal gas engine. I guess was thinking it ran like my R/C airplane engine.


How does the rc engine work? Is it a diesel?

[Ichiban]

This thread is fascinating, watching you guys learning about combustion and feeling your way through it. I obviously don't know it all, but I've studied the basics on and off throughout school and out of my own interest. Let's see if I can help.

Lean=hotter burn- Fire up a properly tuned chainsaw and hold it at WOT. Then begin to close off the high idle mixture screw, and watch the RPM climb. Do this long enough and you'll get a burnt up piston, or a ring stuck in a cylinder wall. Obviously, by leaning it out, you're reducing the lubricating oil that's mixed with the fuel, but why does it rev higher? Simple. There's more air available per unit of fuel, and like any fire, it burns hotter and faster when A/F ratio is shifted towards lean. Richen up the mixture, and we all know what happens. But why do lean mixtures wreck engines? A few reasons. Lean mixtures are more prone to detonation, because of the abundance of oxygen, and also because they tend to burn hotter. Remember that detonation is caused by the cumulative energy inputs of heat and pressure. Secondly, there is a thin layer of fuel/air mixture, the boundary layer, that adheres to the relatively cooler cylinder walls, chamber and piston top that does not burn with the rest of the A/F charge. This boundary layer usually protects the insides from heat damage somewhat, but leaning out the mixture reduces it's mass, and allows for it to burn. Then you have direct contact with fire to engine parts. I'm sure that other factors, like latent heat of vapourization of the fuel plays a part, which is also reduced in leaner mixtures, but I don't know enough about them to really get in to it.

I would assume that the majority of engine damage cause by lean mixtures is caused by detonation and reduction of the boundary layer.

I would also assume that richening the a/f ratio causes a reduction in power due to reduction in combustion temperature.

Diesels- Diesels function on compression ignition, and fuel induction is timed direct injection. Timing can be highly variable, just like the spark ignition in a gasoline engine. The combustion/injection cycle begins before TDC on compression when the fuel spray is injected into the hot, high pressure air inside the cylinder. Since the mist of pure diesel spray cannot ignite as it contains no air, ignition begins at the verges of the spray where it is able to mix with the air. A myriad of small explosions occur on the eddy currents of the fuel spray as turbulence inside the cylinder eventually mixes the air and fuel. The fuel is actually detonating, but doesn't cause damage like gasoline detonation as it's progressively detonating, not all at once like a homogenous charge would causing a massive pressure spike. (Ie gas engine) In fact, diesel ignition delay occurs when there isn't immediately enough energy (heat or pressure) to light up the diesel spray right away. More of the fuel mixes with the air than normal, and when it does go, you get a loud CLACK! as a partially homogenous diesel/air charge goes off with a pressure spike. As the engine heats up, the fuel begins its progressive detonation sooner, pressures rise more smoothly per power stroke, and the engine runs quieter. This is the cause behind the familiar diesel "cold knock."


Large cams and crappy idle- Look up a term called "intake reversion" I don't really feel like explaining it here. It's basically a fancy way of saying that a high RPM cam and intake are out of tune at lower RPM and idle, due to reasons of valve overlap and simple fluid dynamics. This doesn't take a genious to figure out.

Well, I hope all this shit is right, cause I'm kinda tired. Post up if you actually made it through all this.

[Accordtheory]

There's more air available per unit of fuel, and like any fire, it burns hotter and faster when A/F ratio is shifted towards lean.

Okay, first, when I say lean, I mean numerically higher than the 14.7:1 for gasoline. Second, it does Not burn more quickly. Proof, the lean backfire.
As far as burning hotter, I don't understand how that could occur. That is what I am seeking information on.

[Accordtheory]

You can actually calculate exactly how much energy is released during a chemical reaction, if you know a little about chemistry. I'm not going to go find my chemistry book and get into it, it was about 7 years ago when I took the class, and I failed it..asshole teacher, and I don't feel like refreshing my memory right now.. But obviously I still see the concept, basically that for different combinations of molecules/atoms, certain amounts of energy are released. The amount of energy depends on what the molecules/atoms are. In a motor, the highest energy is released from the oxygen in the air combining with the hydrocarbon fuel. So when you have the chemically correct (balanced chemical equation) ratio of air/fuel, 14.7:1, almost all of the fuel and oxygen combine with each other, leaving only tiny amounts of free 02 and fuel to combine with other gasses/byproducts, this releases the most possible energy, and since the mass of unreacted gas in the chamber is minimized by this correct ratio, this should produce the highest combustion temperature.

So how would burning less fuel than chemically balanced produce hotter combustion? I'm either not buying it at all or need a damn good explanation.

When I first learned about this, I learned that running rich burns cooler, because since there is too much fuel for the available oxygen, it can't burn, and just acts as an inert substance, absorbing heat from the combustion. Its properties of heat of vaporization and specific heat both cool the combustion. Same thing as injecting water through the chamber, just nowhere near as effective.

But I never understood how running lean didn't have the same exact effect. The extra air should also be inert and absorb energy, also lowering the overall cylinder's temp. With a diesel, it does, and it does with many other fuels too. Just not with gasoline. I'm beginning to think gasoline is a lousy fuel.

Now as far as idle and large cams, I think it is possible with efi to tune the motor to idle smoothly.

[A20A1]

Anyone know why large cams don't idle for shit normally?

Valve timing: In order to get enough exhaust out and air/fuel into the chambers at higher rpm the valves need to stay open longer which increases their overlap, which then can cause reversion at low rpms. You manifold pressure changes as well as the following mixture introduced to the cylinders... The low vacuum would be worse for carbed but luckily you can up the richness of the idle jets and fiddle with the timing perhaps to get enough fuel from the carb with a weaker vacuum signal.



I've read unmatched head exhaust ports to header ports will help reduce reversion because the larger exhaust port on the header doesn't transition to the head smoothly and will resist the back-flow, however I haven't looked into exhaust valves to control reversion and increase low rpm exhaust velocity, it may do more harm then good if it creates more back-pressure then velocity on the exhaust flow.



I found a few brands
Doug's Headers Electric Exhaust Cut-Out
Quick Time Electric Exhaust Cutout
Street Pro Cutout
http://www.streetpro.us/
Race Ready Performance Electric Exhaust Cutouts
http://www.racereadyperformance.com/

probably not worth the time and money as I don't see a one size fits all valve being right for every motor without some serious r&d.

[Ichiban]

I understand that a stoichiometric mixture should produce the most amount of energy, and I have a basic understanding of the chemistry and physics involved. I'm not saying that a lean mixture produces more power, I'm saying that it burns differently. A stoich mixture will burn smoothly at a given rate, and produce a smooth release of heat and a corresponding smooth pressure curve. A mixture lean on fuel will react more quickly, giving a sharper rise in both heat and pressure, but less net energy released. Go back to the lean tuned chainsaw. Under no load, it rev's higher, but load it and it will bog and die. I'm starting to think that lean mixtures will increase no load RPM under the assumption that they burn faster. And your lean backfire I believe to be caused by lean misfires that decide to combust in the superheated exhaust system, which was heated up by all the other lean running cylinders, and by it's own lean running.

[2ndGenGuy]

I thought that lean backfire was when the fuel is so lean that it detonates before the intake valve closes, hence backfiring through the intake manifold... I don't see how a slower burn could cause this, you'd think if it burned more slowly it wouldn't have time to ignite fully before the intake valve closes. :confused:

Also, doing a bit of reading last night, found this kinda interesting. I don't know how legit it is, but they claim to be able to run up to a 30:1 AFR by using a spring loaded piston that varys the compression ratio depending on load, and a really powerful spark. Some other interesting stuff to read there too, but doesn't answer all the questions I had about how it works:

http://www.leanburnignition.com/index.htm

[2drSE-i]

Ok heres what i dont understand about anything you guys have said so far. In a book im reading (engine building for dummies, basically haha) it states that 14.7 (stoich) is the most effecient AFR because stoich is the point at which the fuel and air mixture burns most effeciently. Would this not mean that the most power should be found at stoich? With anything over stoich you get too "slow" (if you will) of a burn, and anything leaner than stoick will burn too "quickly" (again, if you will. i should have said colder and hotter...) I don't believe that engines were tuned to stoich (respectively tuned) because of cats or o2 sensors, but because it is the most effecient AFR.

[MessyHonda]

when i got my car tuned i got it tuned with a wideband. and they did a good job that it killed my clutch.

[Accordtheory]

A mixture lean on fuel will react more quickly, giving a sharper rise in both heat and pressure

This is incorrect.. The damn chainsaw thing is a flawed analogy.

[Accordtheory]

I thought that lean backfire was when the fuel is so lean that it detonates before the intake valve closes, hence backfiring through the intake manifold... I don't see how a slower burn could cause this, you'd think if it burned more slowly it wouldn't have time to ignite fully before the intake valve closes. :confused:

What..? The lean backfire is caused by the air/fuel still burning on the overlap cycle. That's Proof of the slower burn of a lean mixture. The lean mixture is still burning on the exhaust stroke, popping out the exhaust, if it's bad enough, it's still burning when the intake valve opens on the top of the exhaust stroke, igniting the fresh mixture. That causes an intake backfire.

[Ichiban]

Sorry all, I thought you were refering to "afterfiring" caused by a lean mixture.


What..? The lean backfire is caused by the air/fuel still burning on the overlap cycle. That's Proof of the slower burn of a lean mixture. The lean mixture is still burning on the exhaust stroke, popping out the exhaust, if it's bad enough, it's still burning when the intake valve opens on the top of the exhaust stroke, igniting the fresh mixture. That causes an intake backfire.

Uh, if it goes off in the exhaust during overlap, it will be conducted back through the cylinder and into the intake, would it not?

Mistake I made earlier was that I thought you were talking about the afterfiring that happens as an engine leans out. To be sure, you're talking about the giant fireball out of the carb, right? I always thought that was indicitave of an ignition timing problem, more than a lean mixture.

[Ichiban]

This is incorrect.. The damn chainsaw thing is a flawed analogy.

So then why does a lean mixture burn hot? This is a fact you can't argue with. The analogy is not flawed. It's still an engine.

[Accordtheory]

Sorry all, I thought you were refering to "afterfiring" caused by a lean mixture.



Uh, if it goes off in the exhaust during overlap, it will be conducted back through the cylinder and into the intake, would it not?

Mistake I made earlier was that I thought you were talking about the afterfiring that happens as an engine leans out. To be sure, you're talking about the giant fireball out of the carb, right? I always thought that was indicitave of an ignition timing problem, more than a lean mixture.

Well, I've never hear the term "afterfiring", but I guess in this instance we mean the same thing. To me, a backfire means either flames out the exhaust or the intake. If your ign timing is far enough off to fire the spark plug on the intake stroke, obviously the motor won't run like that, and that's different from what I'm talking about. I'm saying the motor's already running, and if you lean it out and if you don't blow it up first, it will backfire. Esp with forced induction.

[Accordtheory]

So then why does a lean mixture burn hot? This is a fact you can't argue with. The analogy is not flawed. It's still an engine.

First, I Can and Have been arguing against that, all along! I've been saying it's not as hot as stoich, but burns for a longer time. The answer to the chainsaw thing lies in the fact that it bogs when a load is applied to it with that mixture at that rpm. Explain that.

[Accordtheory]

I don't believe that engines were tuned to stoich (respectively tuned) because of cats or o2 sensors, but because it is the most effecient AFR.
You must not have read anything I wrote about dynamic comression, extracting the most energy out of a given pulsewidth, etc. In other words, most of what I wrote.

[Ichiban]

Well, I've never hear the term "afterfiring", but I guess in this instance we mean the same thing. To me, a backfire means either flames out the exhaust or the intake. If your ign timing is far enough off to fire the spark plug on the intake stroke, obviously the motor won't run like that, and that's different from what I'm talking about. I'm saying the motor's already running, and if you lean it out and if you don't blow it up first, it will backfire. Esp with forced induction.

I guess there's confusion around the terms backfiring and afterfiring. Afterfiring happens out of the exhaust (everyone calls incorrectly calls it a backfire) while true backfires happen in the intake only.

I also just did some quick reading (no reputable sources though) that state lean mixtures may either:
-Ignite rapidly and detonate, raising temperatures and causing damage.
-Ignite and burn very unevenly/partially/slowly (apparently if fuel distribution is uneven it can burn in pockets and stay burning for awhile)
-Not ignite at all

So, what do you think, Accordtheory?

[Ichiban]

First, I Can and Have been arguing against that, all along! I've been saying it's not as hot as stoich, but burns for a longer time. The answer to the chainsaw thing lies in the fact that it bogs when a load is applied to it with that mixture at that rpm. Explain that.

Because lean mixtures burn holes in pistons, cook valves, and weld spark plug electrodes together. Google "lean mixture" and see what you get. Talk to anyone with a sled or bike, or any engine tuner. Lean engines run hot. EGT's by cylinder are used on multi carbed engines to tune them. Hot=lean, cold=rich.

Maybe the faster burning or detonating mixture in the chainsaw cylinder can push the non-loaded piston out of the way faster in the free rev situation, but doesn't have the force (torque) to compensate for the load applied. Diesel engines of comparable displacement will produce more torque than a gas engine, but less overall kW, so that idea is totally possible.

[89T]

hmm! you guy's are killing me.
has any one thought about flash points yet?
leaner mix = lower flash point??
detonation?
too much fuel = higher egt's???
hell i wish i could answer all but i cant plain and simple. I am not a tuner, i am a builder.

[Ichiban]

Don't vacuum leaks result in a higher, erratic idle? Guess more air makes engine go faster, and shittier.

[2ndGenGuy]

Would this explain why a rich mixture burns cooler (or a lean mixture burns hotter in relation)?



With a liquid fuel engine, the addition of more fuel also lowers the combustion temperature by the condensing effect. Here the fuel is evaporating and absorbing combustion heat.

[89T]

i think there is alot more to this than what you guy's are focusing on.

2g guy yep that's one reason the target afr for the most efficiant power is found at 12.5:1afr.

[2drSE-i]

You must not have read anything I wrote about dynamic comression, extracting the most energy out of a given pulsewidth, etc. In other words, most of what I wrote.


i did, i just didnt read it three times until after i posted...i wish i had then i wouldn't look so dumb haha

[cygnus x-1]

I know I've read in several places now that the hottest combustion temperature occurs at the stoich ratio, and that richer or leaner mixtures actually result in cooler combustion temps. The only reference I can find at the moment though is here:

http://www.foxvalleykart.com/egt.html

Now with burnt exhaust valves, I'm not sure it's as simple as: hotter combustion=hotter valves. I read an article about this somewhere on the web and now I can't find it. It might have been on team-integra.net. Still searching...

C|

[Accordtheory]

What sucks is that almost everything I've written here has no direct formal research published anywhere that I can find. I posted some of this on honda-tech, they didn't even touch it. I knew they wouldn't, tuners don't like to talk, but that's okay, because most of them don't know shit anyway. Seriously, there have been so many posts on h-t with tuners who can't even make a motor not blow up, not to mention tune for better fuel economy than stock. If they do have some info, they don't understand what is really going on inside the cylinders, how it works, and can't explain it. Even one of the best tuners on h-t, mase, couldn't explain the best rpm vs bsfc for a given output thing. He didn't even really try, he was just like, f it, I've got school, maybe later.

So far, not one of you guys has even come close to refuting what I've said about leaner isn't hotter than stoich, (except for the chainsaw thing, which I think is flawed and will get into later) but causes engine damage by burning more slowly, still at a high temp. Yet you guys keep arguing with that. Evidence of thermal damage doesn't necessarily correspond to a higher temperature. It's temp vs Time of exposure.

guyhatesmycar, that was an interesting article you PMd me with, I don't know why you didn't put the link on this thread. However, there was a key piece of info lacking from it. Those damn airplanes don't have widebands! The thing I found most interesting was the WOT/LOP egt operation, but I already knew there was something else to it..and there was, altitude. The manifold press was only 75% of atmospheric, and they said engine damage was a definite possibility doing this below 6000ft. I don't know what a/f produces the hottest egt. I'm guessing somewhere around 17, 18:1? After all, it still has to light off in the conventional motor..

As far as specifically how running lean causes engine damage, I haven't gotten into that yet, now I will. Primarily I would say it leads to detonation. There are many misconceptions in this post about lean a/f mixtures and detonation, a lean mix does not have a lower auto ignition point, it is Not more susceptible to detonation in itself. It doesn Not burn more quickly. It burns slower, and it burns hot, but not hotter than stoich, to the best of my knowledge. It is belief that this increased exposure time at high temp is what heats up everything in the cylinder more, esp the spark plug and anything susceptible, like sharp edges in the chamber, and gets that glowing exhaust valve even hotter, since it's hotter passing through it, since it's either still burning or has just stopped, unlike a stoich misture, which has stopped by the time that valve opens. Then I would say this increased heat left behind in the chamber causes the fuel to ignite before the plug fires. This causes the entire a/f charge to autoignite at once (detonation) just like if you advanced your ign timing. What I see happening is that the a/f is trying to axpand while the piston is still rising, and the reduction in chamber volume vs the burning mixture compresses the mixture to where it reaches it's autoignition point, like a diesel, and it explodes at once. If the ign point is later, you don't get the 2 forces fighting each other as much, and the a/f is able to burn like normal.

Just to reiterate, I have never read any of what i just wrote anywhere.

As far as the chainsaw, I am theorizing that the carb is unable to maintain the stoich ratio across its entire operating range. At peak rpm, the venturi becomes a restriction, limiting the rpm, (designed that way) and going rich. So now you close the screw, and it revs higher. But now it doesn't have any power once you put a load on it and drop the rpm, because at that rpm, now it's lean.

[Accordtheory]

I know I've read in several places now that the hottest combustion temperature occurs at the stoich ratio, and that richer or leaner mixtures actually result in cooler combustion temps. The only reference I can find at the moment though is here:

http://www.foxvalleykart.com/egt.html

Now with burnt exhaust valves, I'm not sure it's as simple as: hotter combustion=hotter valves. I read an article about this somewhere on the web and now I can't find it. It might have been on team-integra.net. Still searching...

C|

That was a decent article. The think about detonation and cylinder head temp is what I learned in an aircraft engine class I took, I guess they haven't heard of knock sensors. Air cooled aircraft engines are like 40 years behind cars.
About egt guages, first, I hate those things. They respond so slowly, not like a wideband, that they're almost meaningless under transient conditions. Second, what they described in that article, where the egt drops under acceleration as being an indicator of detonation, is not what true when it comes to a turbo car, at all. Your egt should drop a little, because you run richer under boost. My car was 1400 cruise, 1300-1350 under boost. But then, you also run less timing, which counters that by increasing the egts, since the combustion is timed to closer to the valve opening. See how worthless the egt guage is? Spend the $ on a wideband..

The exhaust valve could be damaged by anything that increases the egt. Either running lean or with less timing. I would say most valves are damaged by incorrect lash though. If you're bleeding combustion pressure through the valve, that will quickly destroy it.

[Accordtheory]

i think there is alot more to this than what you guy's are focusing on.

2g guy yep that's one reason the target afr for the most efficiant power is found at 12.5:1afr.

Where are you coming up with this? Almost everything I have written is about trying to get the most out of a certain pulsewidth, ie, efficiency, and you're talking about trying to get the most out of a given mass of air, ie, max power. None of us have even really gotten into this yet.

And 12.5:1 isn't even the most efficient for producing power, it's just a usually accepted compromise between chamber cooling and power production. For max power production, the goal is to make sure as much of the oxygen is combined with fuel as possible. This results in running just slightly richer than stoich. However, this usually is not tolerated by the motor, because of the heat production, and the motor starts to knock. Since more power can be preserved by adding fuel than subtracting timing, fuel is added.

I suppose we could take this discussion in another direction, and start talking about tuning for power..

I need to get my damn car running so I can start experimenting firsthand and blow it up..

[cygnus x-1]

More about lean mixtures and EGT...

http://www.sdsefi.com/techegt.htm
http://www.hydrogen-boost.com/june2004.html

C|

[Ichiban]

It makes perfect sense that EGT's, and correspondingly cylinder temperatures, would follow a curve up to stoich, top out there and then fall as the mixture is leaned. Obviously, less fuel=less energy=lower temps.
So lean mixtures are responsible for parts damage because they remain burning in the cylinder and cause preignition on the next cycle, effectively raising temperature and pressure to beyond the autoignition point of the a/f mixture? That's a pretty complicated way to end up with detonation.

In regards to exposure time to the flame front, which I assume you mean, isn't that dictated by how fast the engine is running?

I also want to know why inducing a vacuum leak will raise engine idle Rpm.

[cygnus x-1]

So far, not one of you guys has even come close to refuting what I've said about leaner isn't hotter than stoich, (except for the chainsaw thing, which I think is flawed and will get into later) but causes engine damage by burning more slowly, still at a high temp. Yet you guys keep arguing with that. Evidence of thermal damage doesn't necessarily correspond to a higher temperature. It's temp vs Time of exposure.

I'm not refuting it because I believe that very lean mixtures do not burn as hot as a stoich mixture.



As far as specifically how running lean causes engine damage, I haven't gotten into that yet, now I will. Primarily I would say it leads to detonation. There are many misconceptions in this post about lean a/f mixtures and detonation, a lean mix does not have a lower auto ignition point, it is Not more susceptible to detonation in itself. It doesn Not burn more quickly. It burns slower, and it burns hot, but not hotter than stoich, to the best of my knowledge.

Lean mixtures burn slower and are harder to ignite. And if you go too lean it becomes so difficult to ignite that it won't and you get misfires. I experienced lean misfires when trying to run 17:1 AFR with my megasquirt.



It is belief that this increased exposure time at high temp is what heats up everything in the cylinder more, esp the spark plug and anything susceptible, like sharp edges in the chamber, and gets that glowing exhaust valve even hotter, since it's hotter passing through it, since it's either still burning or has just stopped, unlike a stoich misture, which has stopped by the time that valve opens. Then I would say this increased heat left behind in the chamber causes the fuel to ignite before the plug fires. This causes the entire a/f charge to autoignite at once (detonation) just like if you advanced your ign timing. What I see happening is that the a/f is trying to axpand while the piston is still rising, and the reduction in chamber volume vs the burning mixture compresses the mixture to where it reaches it's autoignition point, like a diesel, and it explodes at once. If the ign point is later, you don't get the 2 forces fighting each other as much, and the a/f is able to burn like normal.

This is almost exactly what I've been thinking but not finding any references to support. Here is some more interesting reading:

http://ballisticmotorsports.org/forums/YaBB.pl?action=print;num=1186877915
http://en.wikipedia.org/wiki/Detonation
http://en.wikipedia.org/wiki/Deflagration


I think I'm going to buy John Heywood's book on combustion engine fundamentals. It doesn't go into more advanced stuff like lean burn engines but it does have all the fundamental theory.

C|

[cygnus x-1]

In regards to exposure time to the flame front, which I assume you mean, isn't that dictated by how fast the engine is running?

I think yes. This would explain why heavy throttle at low RPMs is hard on engines. With light throttle and low RPM there isn't much air/fuel charge in the cylinder so temperatures won't be so high. But if you go full throttle at low RPM you end up with a full cylinder of air/fuel charge and lots of exposure time. Prime conditions for detonation and engine damage.

C|

[cygnus x-1]

Here we go!!

http://books.google.com/books?id=Yn9F0RhJ0EkC&pg=PA323&lpg=PA323&dq=lean+mixture+burn+valves&source=web&ots=S7K6yPNCA8&sig=K3npX2FSer7Sdode70h2cv0QHLc

C|

[2drSE-i]

sweet that book basically just explained to me what the hell you guys are talking about. Is it sad that this shit really excites me?

[Ichiban]

Page 322 uses the chainsaw analogy!

[2drSE-i]

Page 322 uses the chainsaw analogy!

whats funny about that is anyone who understands anything about fuel injection can see the chainsaw analogy and realize that it doesnt quite work that way.

[Accordtheory]

I also want to know why inducing a vacuum leak will raise engine idle Rpm.

MAF, speed density, carbed? Obviously, with a honda, speed density, the idle is controlled by vacuum leak, aka the idle air control valve. The map sensor reads this, and picks the idle up even more by maintaining 14.7 at the new higher manifold press. MAF, you're now getting more air for the the same pulsewidth, which the concept that I started this entire thread on. Carbed, basically the same.

[Accordtheory]

I think yes. This would explain why heavy throttle at low RPMs is hard on engines. With light throttle and low RPM there isn't much air/fuel charge in the cylinder so temperatures won't be so high. But if you go full throttle at low RPM you end up with a full cylinder of air/fuel charge and lots of exposure time. Prime conditions for detonation and engine damage.

C|

I don't agree with the explanation above, but I do agree that high load low rpm operation can be damaging to a motor. The a/f might burn more quicky as the piston is rising more quickly, but the whole reason for ign timing being different at different engine speeds is trying to optimize the cyl press vs crankshaft angle by compensating for a the non changing or minimally changing burn speed.

So how is it going to be more harmful to a motor to have fewer firing impulses of the same power (torque output) for a given amount of time? Shit, if anything, I'd argue the opposite, within a certain rpm range. For a given torque output, the motor uses less ign timing at lower rpm. Since torque is a function of cyl press vs crankshaft angle, and the motor isn't fighting itself with higher overall press to compensate for the increased press before tdc at higher rpm, the motor actually has lower avg/peak cylinder press for the same torque at a lower rpm.

However, I would say that this is no longer the case at really low rpm. I don't have a number here really, but maybe somewhere below 2000rpm? There, I suspect the piston can't get out of the way quickly enough, and the motor knocks. Ever heard of "pulse detonation"? Light an a/f mixture in a given space that can't expand, restrict the flow out of the space, and light the mixture. The pressure increases to the point where it reaches it's autoignition point, and it detonates. If you design the setup properly, the detonation chamber can actually scavenge itself, by the velocity through the exhaust tube pulling in the new mixture through a valve. Weird form of aircraft propulsion, only problem is it's unimagineably loud. Supposedly it's actually more efficient, because the extremely fast flame propagation of detonation reduces the amount of energy conducted into the chamber by reducing exposure time, transferring more energy into increasing the temp/expansion, and therefore propulsion.

[2oodoor]

Yeah, if you think it takes effort to understand this, then try to write about it..shit. I had a headache earlier after I posted, as I was hoping everything I wrote was true and made sense.. It's always a hell of a lot harder to explain something than it is to just nod your head and say, "yeah, okay, it makes sense, I guess"..

.

You bring a lot of interesting information for us to sort out and make work in our head. That part I can appreciate. But as smart as you may be, trying to make out everyone else is an idiot seems kind of going in the wrong direction of educating us and leans (love that play on words? ) towards insulting. Not so lean man, I am enjoying it otherwise. I do tend to agree with what you said about H-T tuners, lol, but not so much about mase. Just like you say in the quote, it is hard to write this stuff and not have someone tear it apart for sentence structure, less than perfect terminology, or a misunderstood point.
I recently aquired a drive by wire vehicle, so I have made it a point to fully understand the theory of operation. I have already written some on another forum, but when I get time and lose this headache I will write some up on this thread I suppose.

[Ichiban]

I don't agree with the explanation above, but I do agree that high load low rpm operation can be damaging to a motor. The a/f might burn more quicky as the piston is rising more quickly, but the whole reason for ign timing being different at different engine speeds is trying to optimize the cyl press vs crankshaft angle by compensating for a the non changing or minimally changing burn speed.

So how is it going to be more harmful to a motor to have fewer firing impulses of the same power (torque output) for a given amount of time? Shit, if anything, I'd argue the opposite, within a certain rpm range. For a given torque output, the motor uses less ign timing at lower rpm. Since torque is a function of cyl press vs crankshaft angle, and the motor isn't fighting itself with higher overall press to compensate for the increased press before tdc at higher rpm, the motor actually has lower avg/peak cylinder press for the same torque at a lower rpm.

However, I would say that this is no longer the case at really low rpm. I don't have a number here really, but maybe somewhere below 2000rpm? There, I suspect the piston can't get out of the way quickly enough, and the motor knocks. Ever heard of "pulse detonation"? Light an a/f mixture in a given space that can't expand, restrict the flow out of the space, and light the mixture. The pressure increases to the point where it reaches it's autoignition point, and it detonates. If you design the setup properly, the detonation chamber can actually scavenge itself, by the velocity through the exhaust tube pulling in the new mixture through a valve. Weird form of aircraft propulsion, only problem is it's unimagineably loud. Supposedly it's actually more efficient, because the extremely fast flame propagation of detonation reduces the amount of energy conducted into the chamber by reducing exposure time, transferring more energy into increasing the temp/expansion, and therefore propulsion.


I think you just answered your own question there. At low RPM/high load/high throttle conditions the piston can't move out of the way quickly enough to compensate for the VOC of the mixture. Thus, cylinder pressure rises to above autoignition point and detonation of the remaining unburnt mixture occurs. A characteristic of early Toyota 22R engines is that they rattle LOTS when under high load and low RPM. Listen next time you pull up to a beater Toy truck, chances it'll rattle when it pulls away from the stop light. If your window is open, you'll hear it.

Also, in regards to "pulse detonation", ever light gunpowder with a match? It sits there and burns. Confine it inside a rifle cartridge with a big heavy bullet in the way and we all know what happens.

[Accordtheory]

Roodoo2, I'm not trying to make anyone out to look like an idiot, at all, and i don't think anyone on here is an idiot. I brought up the thing about mase on h-t because he is one of the most respected tuners on there, has actually started threads specifically about tuning, and when He doesn't want to get into something, that means it is Really hard to figure out/find info on it. I'll admit it does frustrate me when people either don't read what I've written, try to oversimplify it, (oversimplifying is not giving the subject the respect it deserves, and usually gives the wrong explanation) and then try to contest something about it that's already basically universally accepted and taught, like that a lean of stoich mixture burns slower. Yet I still attempted to illustrate that and explain that for how many posts? You want to criticize me, well, where is my credit for doing that? I could have just said fuck off, but I didn't, I kept going at it, trying my best to figure shit out and put it down in words. I don't have any problem with people contesting shit that isn't acepted when they actually Can contest it, esp when I straight up say, as I've said numerous times in my threads, that I am theorizing to my best efforts. I constantly see people coming up with explanations for things that aren't accurate/complete, not necessarily in this thread, but in the whole world, and when I see that, I'll try my best to figure out if that's what's really going on. The low speed operation thing is a perfect example of this, I knew the exposure time thing was wrong, but I knew that low speed engine operation under load can cause knock, even with the lower VE at that speed, but didn't know why. I didn't even think of the detonation vs piston speed thing until I had already written the 2 previous paragraphs, and had actually never even thought of it before or read about it anywhere in my life. But it makes sense though, doesn't it? It seems like that's the kind of shit you're criticizing me for though. Me criticizing someone for sentence structure, no, misunderstood point, maybe if they're trying to contest without understanding, pefect terminology, fuck no. I straight up said I had never heard of the term afterfiring and explained what I meant. The only time I gave anyone shit on this board about something sentence/grammar/whatever related was when I said I was going to clown 89turbo'ed for his apostrpohe use, and I was totally joking around, and he didn't get upset.
I know damn well I don't know everything, and I'm not trying to make myself look smart, or anyone else look dumb. I'm just busting my fucking ass trying to figure shit out for this thread, if you can't see that, then what more can I say? If anyone else thinks I'm trying to be insulting, speak up. Shit, if I were to take a guess on who'd be pissed at me on this thread, I'd say it'd be guyhatesmycar, for how I totally dismissed his chainsaw analogy.

[Accordtheory]

As far as the chainsaw, I am theorizing that the carb is unable to maintain the stoich ratio across its entire operating range. At peak rpm, the venturi becomes a restriction, limiting the rpm, (designed that way) and going rich. So now you close the screw, and it revs higher. But now it doesn't have any power once you put a load on it and drop the rpm, because at that rpm, now it's lean.

Come on, guyhatesmycar, I know you have something to say about this...

[2oodoor]

...decaf?
Im lurking from this point, good thread guys.

[Ichiban]

Come on, guyhatesmycar, I know you have something to say about this...

It is clear that the RPM of the chainsaw is limited normally by the richness of the mixture. Reducing the richness of the mixture makes it rev higher. For all I know, the saws are tuned rich and blow up if you run at stoich. After all, that's where the highest energy release is, mixture wise.

Wait, I finally got what you're saying. The over-reving saw is creating enough vacuum to pull enough fuel to run. Once the RPM drops due to load, vacuum drops and so does fuel delivery, so saw dies. Reasonable so far.

But why does the saw (or my car) increace speed at idle momentairly as it runs out of fuel? Less fuel=more energy? I doubt it. Any increase in RPM means an increase in energy output, you can't get something for nothing. Also, inducing a vacuum leak on a carbed vehicle will not only add air, but raise manifold pressure and further reduce fuel delivery. Yet idle still goes up. Why?

[Accordtheory]

Well, I said before that I didn't think the chainsaw runs at 14.7 at idle, just like the aircraft engines I learned about in a class I took, so that's why it would pick up, as it crosses 14.7. The aircraft motors I learned about idled really rich, around 10 something to 1. Basically just spitting black smoke to stay cool, with the very limited airflow available for cooling. Are you saying that with a carbureted car idling at 14.7, the idle will momentarily increase before it stalls if you cut the fuel? How would creating a vacuum leak on a carb setup reduce the amount of fuel going into the motor? The manifold press would go up for that rpm, reducing the vacuum and therefore airflow across the carb momentarily, but then the rpm would pick up, provided it didn't stall, and then you'd end up with around the same fuel but more air, no?
I'm going to have to buy an eighth before posting again, you guys might have to wait for anything else out of me..