OK, I am doing a report on the DIY WB O2 sensor. My question is that in an ideal world the proper air/fuel ratio is 14.7 to one. When max power is desired, the a/f ratio drops to around 12 to one or so. Could someone please enlighten me to the reasons for this?


My hypothesis is that the mixture at WOT is not perfectly homogenous and more fuel is needed to mix with the oxygen in the intake charge. Once the a/f ratio becomes too rich, the fuel displaces the oxygen and thus results in power loss. Too lean, and there is not enough fuel i enough time to mix with the oxygen. Thanks!!

 

Fuel helps cool the chambers so I imagine it may have something to do with the affect of temp on the mixture but also has something to do with not exploding pistons with runaway detonation that can be induced from the sudden rise in temp.

Dont know for sure.

 

Vader, Ed? Anyone?

 

14.7:1 is the best balance of power and efficiency.
15:1 , better milage. 14:1 , better power.
you have to sacrifice some fuel economy to achive the best a/f ratio for power and vice versa.

 

Thanks for responding but I know that. But why?

 

It's all about the chemistry...a given number of moles of oxygen will oxidize a given quantity of gasoline. Compromises are made in automotive design, even the ideal 12.8:1 WOT ratio (richer than stoich) is a compromise to prevent engine failure.

Whip out a chemistry book and go to town, you'll learn everything you need to know about stoichiometric air:fuel ratio.

 

Even my Internal COmbustion book is unclear why this compromise is made. How does it prevent engine failure? So a pro stock car or somehthing would run leaner ratios b/c of less concern with engine failure? I was under the impression that the richer than stoich ratio actually did make more power. Thanks

 

alot of people forget that stoichometric is "theoretical optimum" and also assuming "ideal conditions".
in theory if you mix air/fuel 14.7:1 and ignite it there will be no extra fuel or oxygen left over.
in a real engine it takes an air/fuel ratio of between 12.5-12.8:1 to burn all the oxygen, due to real world conditions.
runnning stoich in a real engine leaves unburnt oxygen in the exhaust.
hope this clears up a few things.

 

if you lean out the mix to something like 15:1 the temperature of the flame will increase. when you richen up the mix, it will cool down the flame.

i believe what happens, is when you make the mixture rich like 12:1 it takes longer for the flame to completely burn through, and i suspect that only lower rpms can benifit from this. the higher rpms will just spit out flames out of the exhaust port.

just my .02

if you want to find more information on this, you need to crack open a thermodynamics book. if you look in the back in the chemical reactions chapter you can find what you are looking for.

andrew

 

ok, last night i talked to one of the M.E. seniors here at my college and he gave me a reason for your question in a very simplified way.

if you run an engine slightly lean, you get good emissions and a better flame burn, but it is not good for the overall life and dexterity of the engine. now when you run lean, the flame gets colder, but heat transfer inside of the engine increases, making the cylinder, heads, and exhaust valve hotter.

if you run the engine slightly rich, you get more power, but loose on the emissions and fuel economy. the flame also burns hotter when the engine is run rich.

stoich is all around optimal, but there are many variables that define what stoich is for a particular engine. these include fuel type, inlet temp, humidity, and who knows what else.

hope this is correct, and that it helps with your paper

andrew