This is a very good post by zth member, Sifu F8. :
"i would urge you guys to be a little bit more critical when you think. first off, i hope no one would take any of the opinions expressed here anymore than that, opinions, and not facts. so take what you read with a pinch of salt in zth.
before i go further, let me say that we're talking about petrol spark ignition engines. therefore, stoichiometry is about 14.7:1 air:fuel ratio by mass. by lean, i refer to air:fuel ratio > 14.7:1. by rich, i mean air:fuel ratio <14.7:1.
when you say the leaner you run, the more power you get until detonation occurs, this is simply untrue. for emission purposes, ie optimal operation of the catalytic converter, the air fuel ratio should be about 14.7:1. but for economical purposes, you can run it even leaner at about 15.4:1.
however, if it is ultimate power, for any given ignition advance timing, the figure is between 12.3:1 to 13.2:1, depending on the engine with turbo cars generally preferring the richer mixture as it aids in cylinder cooling(*more later). this is not to be confused with the ability to run more ignition advance, which is true as well. i'm just saying, even if you retained your ignition value at a point which wasn't knocking for a really lean and knock prone fueling, if you richen it up to about 12.5:1, you will get more torque.
however, people often run it even richer to aid even more in cylinder cooling so that the knock threshold is pushed further back. in the end, the resultant effect is a battle between the gain with the even higher ignition advance(higher ie closer to optimum value, but not past that) and the loss with running it more rich than is optimum from an air:fuel ratio point of view.
as for you getting more power with leaner mixtures, i think it has to do with the fact that you haven't advanced your ignition to the optimum yet for your current burn rate. and by running leaner, you increase your in cylinder temperature and speeding up the burn rate. the effect is similar to advancing your ignition except you don't get as much as you would have done if you had advanced it and richened your mixture. not to mention the thermal stress you place on your engine and its effect on durability. but before that, what did you use to measure your air fuel ratio? and what do you mean by lean and rich in terms of numbers? lets be a bit more specific and quantitative here.
finally, the leaner you run, the higher the exhaust gas temp is also incorrect. all else being equal, ie ignition advance stays fixed, exhaust gas temp is highest at stoichiometry of 14.7:1. if you start going leaner, the temperature actually starts going down. similarly, if you start going richer than stoich, the temp also goes down. so when you see that your egt is lower, you need to know which side of the lower you are in.
but more importantly, egt is a result of not just the air fuel ratio. it also depends on your ignition timing, compression ratio etc. the more retarded your ignition, the higher your egt even if your air fuel ratio was constant.
therefore, please don't use egt as the sole indication of your state of tune. definitely use a wide band lambda sensor for fueling, and a knock sensor wired to an amplifier and a pair of headphones to listen out for knocking. however, the most optimum is still tuning it on a dyno. so what if air fuel ratio and ignition was this and that if the torque output is now lower? the fuel and ignition settings that give the higest output is that. and i can assure you there won't be knocking at that point because the output would've come down if there was.
*-this is basically a more expensive means of water injection because fuel has lower heat of latency than water and thus absorb less heat, and is more expensive than water although it vaporises more easily.
so, it is true that stoich makes the highest EGT and power. and forget about ignition timing and other things...
no. stoich makes the highest egt but not the highest power. and no, you cannot forget about ignition timing and other things.
GT500 wrote:
1)Does carbon deposite that build in the cylinder (due to richer fuel) effect the EGT?As i know carbon deposite can raise the compression ratio(the surpace of the cylinder getting thicker)
2)To f8,can i say EGT is secondary in tuning process?I mean AFR is that important?
i'm not so sure the carbon deposits give an appreciable rise in egt due to the higher compression ratio. there may well be depending on how badly deposited your piston crowns are but i think the more apparent effect is pre-ignition as these carbon deposits act as hot spots.
Think of AFR and then EGT as secondary tuning aids. firstly, you tune it on the dyno. once its optimum in terms of power for any given rpm and load site, you observe the air fuel ratio and egt. next time if you need to retune and for some reasons can't do it on a dyno, then do it on the road with a wideband afr meter and egt gauge until the air fuel ratio corresponds to what it was on the dyno via fueling, and then whilst keeping the fueling constant, tweak your ignition until you get the same egt.
But seriously, yes, i would still think the egt will go down as you go from stoich to leaner. the reason is, at stoich the combustion is perfect so maximum heat is extracted. past stoich into lean, since you put in less fuel, there should be less heat until ultimately, the engine dies of fuel starvation"
