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Understanding Lean Fuel Mixtures

Understanding AFR

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I watched a video on running lean which basically explained that engines don't run hot when they are lean, they actually run hottest at the stoichiometric AFR. What he explains is that it's not that the engines run hot when they have a lean AFR, but that they run hot when a rich AFR is not rich enough. As an example, although an AFR of 14.0:1 on pump gas is technically rich, it wouldn't be as rich as we would want under WOT conditions.

Is this accurate?

I included the referenced video below:


this video is very misleading in my opinion.

Regards Ross

Thank you for the reply. Is there a chance you could elaborate? Is the information in the video completely incorrect? I obviously don't have any plans on tuning a car to be lean at WOT, I'm just trying to better understand the concept.

Okay, I think I understand now but if somebody could confirm I'd appreciate it:

Generally, the highest combustion temperatures occur around 1.1 lambda and starts to drop off on either side whether you go lean or rich, but peak torque of the engine occurs on the rich side of the curve so you're getting the benefit of lower temperatures along with more power when running rich.

I would disagree with Ross - a possible point of discussion if he would be so kind as to say where he thought it misleading - I may be wrong.

Anyway, discussion points...

Part of the confusion, I suspect, is that there are actually a few different, but related, things to consider, such as

1/ the actual combustion process and the temperatures reached

2/ the environment within the cylinder where the combustion is occuring.

3/~80% of the gas in the engine isn't part of the combustion process, but is being acted on by heating - and the hot combusted gases are both being heated and part of the PVT equation but also have to pass that heat into the ~80% that is along for the ride.

4/ the engine works on gas pressure and expansion - PVT - with the combustion giving the change in T.

One may expect the peak energy release to be at stoich' as a purely chemical process, as all the fuel and oxidant (oxygen) is used up and there is no wastage. This assumes a homogenous (completely mixed) charge and this is rarely achieved, even with engines designed for high turbulence and mixing.

However, an engine doesn't actually run on the fuel - it uses the fuel to heat the gases in the cylinder, increasing the pressure on the piston(s), and there are several factors to take into account, besides the mixing mentioned above. The fuel is introduced as a liquid and both cools the charge by evaporation, and adds thermal mass that reduces the temperature in the cylinder as it's compressed, and the best use of the fuel for 'power' is when it gives the most heating - pressure gain - at the correct time - best PVT if you will, for the amount of fuel used, even if some is wasted as unburned (HC) or incomplete combustion (CO).

This is different from tuning for economy as there the focus is on the minimising amount of fuel and maximising the heating for the fuel used. Going back to the basic concept of the fuel being used to heat the gases in the cylinder, one may think that the leaner the mixture the better for economy. However, it isn't that simple, as there may be problems getting all the fuel hot enough to be ignited by the burning fuel in the cylinder as it may not be homogenous, and there is a loss of heat energy to the piston and the surfaces of the bore and combustion chamber in the head - this last part is a big part of why modern vehicles run much hotter than older vehicles, it's to reduce heat transfer from the combustion to the cooling system.

Modern vehicles try to maximise the mixing of the fuel and air using high swirl, high turbulence and similar engine designs, and even more modern vehicles use direct injection under very high pressures to improve atomisation, smaller fuel droplets which evaporate and mix better, and which direct it into the area around the spark plug to improve initial combustion.

That helps a lot, Gord. Thanks for the detailed explanation on the process.

I love these debates, If you run a engine in a dyno the hottest you will see a pyro is with a lean mixture this I turn heats the cooling system, in real life and not text book tests I have spent years on dyno tuning engines for top race teams where every Hp counts and finding the fine line between lean and melting has to be found to extract maximum power and engine life span. I have never seen a rich engine run hot (water temperature) but have seen a rich engine run hot pyros which is from fuel burning out side of the internal combustion process (fuel burning in the exhaust ).

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