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.



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.





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! )

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! )

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]

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.

[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.

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