Hi guys quick noob question on a/f Ratio. A short technical answer will be fine.

Why do our cars run on such a low a/f ratio as compared to stoichiometric? Is it because of the higher temperatures/pressures present in the cumbustion chamber due to the turbo so we run a bit rich to prevent detonation?

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Cheers

stoichiometric is defined as the best burn rate for fuel/air, although it doesnt necessarilly relate to the safest ratio, or the best ratio for performance.

realistically you should monitor EGT (exhaust gas temp) and examine that in comparison with AFR.

an EGT over 620/650 celcius is lean (regardless of fuel) tempuratures below that are rich.

an EGT of 620 celcius under full load will produce the most optimal torque.

AFR's relate closely to EGT in that above and below the optimal EGT you will either be lean or rich depending on where you find AFR's that produce the best torque (this also will be the optimal EGT)

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Yeah stoichiometric is the right amount of fuel combined with the right amount of air (O2) so that when burnt, there is no mols of air (O2) or fuel left over. less fuel = burns hotter so egt = higher. more fuel = cooler so lower egt. So does the a/f ratio depend on safe temps for the engine internals? indicated by egt?

Wouldn't it be more efficient to run at stoichiometric. Since the cars run rich it just means we are dumping fuel out the exhaust because it ran out of air to react with. But because of the increased temp/pressure from the turbo the charge might spontaneously combust instead of creating a controlled flame front?? I understand that egt can give you an indication of whether you are running rich or lean (compared to 11.X:1) if you know what the temps are supposed to be at a safe mixture.

So i suppose regardless of efficiency you need a richer mixture to ensure low enough exhaust gas temperatures which indicate safe operating conditions inside the engine? Do safe EGTs differ between cars? surely as you increase boost (and hence temp) you would need to increase a/f ratio to compensate for the higher temperature?

And on top of that there would be a different safe temperature and pressure for different a/f ratios before the onset of detonation? or does it only depend on fuel type?

Sorry for all the questions, and cheers for your answer!! I'm just curious and my brain works itself overtime on things like this.

I just realised i asked for a short answer and i have just written a novel myself. Apologies.

as i mentioned, a stoichiometric mixture doesnt necessarilly equate to the safest mixture or best horsepower.

theres no direct ratio relationship between an AFR and EGT, but they do relate to determining (together) the best method of finding the sweet spot of the engine.

620 degrees C is the best burn rate for -any- fuel, be it unleaded/race/gas. so finding that EGT at WOT will determine the optimal AFR for that motor.

remembering that EGT is also influenced heavily by ignition angle, retarding ignition will increase an EGT, yet increasing the ignition will lower the EGT, also the further you increase the ignition angle, the more prone you are to inducing knock/ping/rattle. so the balance is to drive your ignition foward as much as possible and keep the burn as cool as possible to avoid knock (by richening the mixture) but without moving too rich that you cannot reach the target EGT (620).

its a fine balancing act and theres many other factors at play.

be considerate too that most EGT/AFR readings are taken at a merge collector point in the headers or behind the turbo, so these readings are going to be reflective of the average temp/ratio of all the cylinders, so individual cylinder temps/ratios may cause a knock/lean/rich/hot/cool scenario without being noticed by the average.

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Thanks mate!!

Looks like there is a variety of things to consider.

Is there any reason why 620 is the best egt?

Also, if you have higher initial temp of the a/f mix, does that mean that you try to alter the other factors to make sure the heat released in the reaction when it burns + the initial temp = approx 620? probably better/easier to lower the intake temp somehow (better IC) but am i on the right track?

Thanks for the post! Interesting stuff!!

unsure why 620 C is the best EGT, but its common EGT target for all aircraft, jet turbine, top-fuel drags, motor racing, diesel etc!

regarding lowering your EGT's, controlling the environment is one thing, but impractical for all-scenarios, thats why it comes down to tuning correctly for air temp and water temp correction maps on ignition/fuel.

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