Isn't the stoichiometric ratio of gasoline 14.7:1? That is where there is precisely enough air in the mix to burn all gasoline, right?

So, what factors keep folks tuning on the rich side? Is it because of unsafe temperatures? Insufficient fuel atomization? I know this is a straight-up newbie question to some of y'all, but after learning how the cats more or less burn off unburned fuel in the exhaust, I have to ask if it is possible to burn ALL of the gas in the combustion chamber, effectively negating the need to even use cats.

Am I onto sumpin, or has this been tried already?

 

Well since there will always be factors out of your control in the combustion chamber the reaction has the potential to vary. Temperature for example will not always be the same, this can affect the mixture between the air and the gasoline. But running rich you can get away with...but if you start to run too rich then your car may become damaged. Hoped that helped.

 

its because the object is not to burn all of the oxygen correctly.
its to either make max power (slightly rich)
or to make max MPG (slightly lean)

the only reason it bounces around 14.67 stock, is that the OEMS want that perfect o2 burn half the time for the emissions. tuners generally dont worry about that as much.

 

I think his question is why does a rich mixture make more power than stoichiometric. To ensure that as much air as possible gets burned there must be more gas than needed for ideal combustion because of imperfect mixing/vaporization. But you reach a point of diminishing returns because eventually the extra fuel will just cool the combustion. It's just a matter of finding the peak of the power vs a/f curve.

 

I'll add one thing. You DO run 14.7:1 at idle and partial throttle. When your car goes into closed loop, the o2's keep it pretty much dead on 14.7. Unless you have some bad split blm's (might only be a problem with lt1's, not sure) you should never run 'rich' at idle or partial throttle.

 

to add, 16:1 would give best effeciency but would burn pistons, 14.7:1 considered best safe lean by OEM

 

I kinda get the picture. Figured the engine parts might not be up to the higher temps at WOT, as well as imperfect atomization. Thanks for the enlightenment, y'all.

 

Excess air can cool an engine's combustion just as well as excess fuel. The problem with a gasoline engine is the distribution of the fuel on a molecular level. Natural gas engines can be run much leaner than stoich. On the molecular level liquid fuels tend to clump, whereas there is better fuel distribution with a gaseous fuel. Combustion temperatures (and production of NOx) peak near stoichiometric combustion. An engine could have it's compression ratio and timing optimised to survive full throttle with a stoiciometric air fuel ratio, but you would make more power with a similar engine that's designed to run a rich mixture. Vehicles with three way catalyst require excess HC and O2 to make the catalyst work. For the catalyst in your car to operate, the engine must be slightly rich. The excess oxygen comes from incomplete combustion and probably valve overlap in these engines. The catalyst completes the conversion of HC to H20 and C02, NOX to N2 and 02, CO to CO2. The catalyst will only operate in a range slightly richer than stoich, which is where our vehicles try to operate. Once lean of stoich combustion chamber temperatures will drop, however the gasoline engine can be only taken so lean, and not so lean as to meet emmission requirements. I was going to attempt a "lean-cruise" mode tuning in my car for better economy under light loads, as my catalysts are removed, and I do not have any sort of emission testing here. I will post my results with the board, but it might be a long time waiting...

 

Holden's lean cruise mode for sustained driving (hours and hours) at low load on the motorway runs often in excess of 16:1 AFR with no ill effect. I suspect the low RPM that it operates keeps combustion chamber temps low enough that this is safe.

With this in mind I have been running 15.4:1 under 2400 RPM and 50kpa map for a few months with no issues for ECT temp, knock or otherwise so far.

 

In response to the original question, 14.7 is a theory. In reality, imperfections in the combustion process would leave unburned oxygen. We can inject as much fuel as we need, but we can only ingest as much oxygen as the engine design allows. This means that to ensure that all available oxygen is burned, a theoretically rich mixture must be maintained for best power. Also, you can only run a certain amount of spark advance before causing detonation from excess pressure or loss of power simply because of combustion occurring too early in the piston's cycle. Oddly enough, gasoline/air mix burns the quickest at around 11.5/1 mixture. This does not give best power, though, because 11.5/1 has so much fuel that it burns cooler. All of these various factors influence power production. Also, on a side note, it is true that under heavy load conditions a lean mixture can hurt the motor. However, in a low-load lean-cruise situation, the total amount of fuel being burned is not enough to cause overheating or other problems, even at 16/1 AF.

 

Please explain how 16:1 will burn pistons during "max efficiency" mode

 

Simple answer is, you throw at it what you have in
surplus. For power, you can only get so much air in,
so maximize its use with excess fuel. For economy,
the opposite, give it air until you lose efficiency or
just your emissions certification.

 

i doubt a factory aluminum piston could handle the combustion temps associated with that ratio

 

This is deeper than I realized.

So Jimmy, by your graph the stoichiometric ratio is pretty much where you get the most possible of both power and efficiency, right?

I need to read more, I can tell.

 

they do from the factory.... all over the world, except the USA... where the EPA has decided that its "cheating" to do that... along with some emissions BS that doesnt apply to real life, is based on theorys, and they made laws based on them anyway.

 

Ric, Stoich is the amount of fuel and air that chemically for gasoline at least results in a perfect combination with CO2, H2O output (eg minimum emissions)

You need 14.7 parts O2 for each part of petrol in ideal world.

So CxHx + O2 => CO2 + H2O + Heat (Not sure what petrol is maybe C6H12)

Orange at low map there is no problem with 16:1 as mentioned Holdens have run it for years as lean cruise. Would you want to run it at WOT, no. But thats not what anyone is suggesting.

 

i didnt know we were talking about the world car industry


we dont have lean cruise here, so there are no safeguards in the PCM to run that lean AFR....

 

C8H15 is gasoline

So it goes (assuming atmosphere is 21% oxygen and 79%Nitrogen.. all others are considered negligible for calcuation purposes)

C8H15 + (x)O2 + (x)3.76N2 --> (x)CO2 + (x) H20 + (x)NOx + (x)CO + Engergy!! + etc..

I dont feel like balancing equaitons so that is just a basic equation..

 

There is not any safeguards in the Holden PCM I can see either other than trigger points to enable and disable it.

These are ECT, RPM, g/cyl and VSS.

Basically RPM and g/cyl is a map with AFR in it. Stock is lean 1200 RPM to 3200 RPM and 0.12g/cyl to 0.40g/cyl.

If you run open loop then you can make your own lean cruise by leaning up the bottom half of the commanded fuel table. I just did it under decel and at city speeds. After that its pretty much stoich till 80kpa. As mentioned ECT is fine.

 

GM used to run lean cruise on a number of models in the late eighties/early nineties until the EPA objected. It wouldn't surprise me if a number of later but related PCMs retained the capability if only we knew where to look...

BTW, re Stoich and emissions, the converter needs excess oxygen to burn/oxidize the engine's output of carbon monoxide and assorted hydrocarbons, but not to reduce its oxides of nitrogen, so calibrations do not run at a steady stoichiometric ratio, but rather 'dither' back and forth a tenth or two either side at ~10 cycles per second. This way, the converter can store oxygen during the lean excursions to use during the rich swings, and thus catalyze both oxidation and reduction reactions.

BTW2, 'gasoline' hasn't been a specific molecule for almost a century. Pump fuel formulas are myriad and ever-evolving and often consist of hundreds of compounds, which affects the actual stoichiometric ratio, although except in the case of significant alcohol content usually only by a tenth or two.