Why do high performance engines run rich?

[Caito]

I do know that engines running lean overheat. A lean running engine produces more power? When I cut the fuel of my RC Model plane engine, it revs up. That makes me think it produces more power when running lean.

But, why do race cars run so rich, to the extent of having flames throught the exhaust?


Thank you very much.

Bye bye

Caito.-

[strad]

It's not that they are so rich so much as fuel mixture passed by the exhaust valve during overlap..

[mx_tifoso]

...

But, why do race cars run so rich, to the extent of having flames throught the exhaust?

...

Some good reading on that:
Exhaust flames on overrun

[machin]

Petrol engines produce highest power when they are slightly over stoichiometric, i.e. slightly rich, any more or any less and they produce less power. They also run cooler the richer they are, and hotter the leaner they are.

As track engines run at higher compression ratios, higher time at full throttle etc, it is common to run them slightly richer than the optimum level for power (i.e. slightly rich) to keep them cool.

Cheers!

Machin.

[madtown77]

Air is the limiting factor in gasoline SI combustion. So you put a little more fuel to ensure that you use all of the air in the cylinder. Extra air does no good since it is mostly nitrodgen and therefore inert. Having excess nitrodgen just gives you more stuff to heat up during combustion decreasing peak temperatures and pressure.

Technically you can run exactally at stoic and get the same power, but statistically its unlikely. Its hard to measure air flow that accurately in an engine so you ere on the side of caution and greater power.

[marcush.]

and the old word of :
if you wanna go mean go lean...is not correct?

the exhaust flames on overrun are a matter of fuel cut off engaged or not on overrun conditions thats a matter of activating programme features.If you cut off the fuel supply completely you get into an overrun condition there is no flames.you can as well produce spectacular fireworks in braking areas off throttle...

[747heavy]

yeap - something like this for example:





in some cases you don´t cut fuel in overrun condition, to improve throttle reponse
when back on power.
Until ~2008 V8Supercars in Australia are very prone to flame in overrun, you will see it if you watch some video clips from that time. Today with the E85 fuel, they don´t do it that much.

[strad]

BS...It makes the most power the leanest you can run it without burning it up..Rich equals cooler and as was pointed out earlier...cooler means less power.
Why do you think they spend so much time studying the color on the plugs and the tops of the pistons?
I don't want to be so lean I'm pulling material off the plugs or pistons but I damn sure don't want a bunch of over dark carboning up.

[Edis]

I do know that engines running lean overheat. A lean running engine produces more power? When I cut the fuel of my RC Model plane engine, it revs up. That makes me think it produces more power when running lean.

But, why do race cars run so rich, to the extent of having flames throught the exhaust?


Thank you very much.

Bye bye

Caito.-

A spark ignition engine produce peak power with a slightly rich mixture. This depends on the engine but can typically be found in the region lambda 0.75-0.95. Going leaner or richer than that will reduce power output. Going leaner will however affect fuel consumption in a positive way.

Engine temperatures tend to peak slightly above lambda 1, upward lambda 1.1. Going richer or leaner than that will reduce temperatures. A homogeneous charge spark ignition engine will however be uncapable to use very lean mixtures. When a spark ignition engine go very lean, combustion speed will slow down which increase exhaust temperature, HC emissions and reduce efficiency. The throttle response also suffers and the engine will start to misfire, then stop running.

The flames are a result of not cutting the fuel injection when the engine is off throttle. This introduce fuel and air into the exhaust, which will then self ignite due to the hot exhaust. This is done to improve throttle response and engine cooling. Running rich under full or partial throttle do not produce flames as there is no oxygen availible in the exhaust.

[conni]

as a keen rc flier myself and a maker of F1 cars I shall explain

when you set your plane up for flight you SHOULD set it to max rpm the back it off a 1/4 of a turn on the mixture screw to take into account the thinner air in flight and the high G,s youll pull in turns and flips so your plane will always be running overly rich but when it runs out of fuel it will reach its max rpm just before it gets to lean to run

conni

[autogyro]

I spent many years setting up carbs for racing, mainly Weber side and downdraft.
Choosing jets was done mainly on the dynos we had, looking for maximum power.
However, final choice was to fit jets one setting richer than best power.
This was to keep the top end cool and max power jets would only be fitted if the race was crucial for results.
Even so, jets were changed during circuit testing sometimes by fairly large amounts due to intake design and atmospheric conditions.
This was achieved by turning off the engine on entering the pits and pulling the plugs for a visual check on colour.
We also used a mobile hub dyno in the pits towards the later era of carbs to track tune power output.
I demonstrated such a mobile dyno for Gordan Murray at McLarens.
We suggested these dynos for the FIA to use, to check F1 engines and also to use at one make events to establish a level playing field on power.
Of course ECUs and injection/telemetry prevented this initiative.
These days the engine is more or less just a part bolted to the car, to be changed if a problem, much like in most road car garages.

[G37Sam]

BS...It makes the most power the leanest you can run it without burning it up..Rich equals cooler and as was pointed out earlier...cooler means less power.
Why do you think they spend so much time studying the color on the plugs and the tops of the pistons?
I don't want to be so lean I'm pulling material off the plugs or pistons but I damn sure don't want a bunch of over dark carboning up.

"BS"?? That's not very nice to say now is it..

The cooler the mix, the less prone it is to detonation, meaning you can run higher compressions and bring ignition timing closer to ideal. Why do you think people run intercoolers/meth spray etc.. all to cool the mix

[machin]

"BS"?? That's not very nice to say now is it..

Its not nice.. but its funny because he's wrong!!!!

[xxChrisxx]

"BS"?? That's not very nice to say now is it..

Its not nice.. but its funny because he's wrong!!!!

I was thinking that, a classic "NO U!!" post by someone who has learnt something (the guy obviously knows a little bit about engines) but has come to a horribly wrong conclusion.

Strad, may I suggest you think through what the goal of an engine in, and where it gets the power from. It's fairly logical to work out why what you said is wrong.

Hint: Less is not more, more is more.

[strad]

The engine is nothing more than an air pump...the more you can push thru the more power you're gonna make.
This guy says the same just in a way you like better..

I spent many years setting up carbs for racing, mainly Weber side and downdraft.
Choosing jets was done mainly on the dynos we had, looking for maximum power.
However, final choice was to fit jets one setting richer than best power.
This was to keep the top end cool and max power jets would only be fitted if the race was crucial for results.
Even so, jets were changed during circuit testing sometimes by fairly large amounts due to intake design and atmospheric conditions.

The best power he mentions is,,how shall I say this so ya get it..On the lean side of the range of mixture you could run,,but you run just a little richer so you don't melt the exhaust valve and literally pull metal off the spark plug and piston.,,break out your little Champion Spark Plug viewer and you can see the bits of aluminum.
And he is right about the intake and exhaust in the real world altering the jetting from the dyno settings.
And the method of testing is correct,,clutch in, ignition off at speed,,ya want a nice cocoa color and you ain't gonna get that from bein rich.
A little to cool things is one thing but you want to be able to burn it ALL..
Well except for that pulled thru on overlap,,,where you get your real cooling effect from the mixture..as mentioned earlier.

I am not here to toot my own horn as some seem to be..but I will tell you that I have raced on and off for 40 years in various series and in 1980 My C/ED took the
Division 7 NHRA Winston Championship Series championship..I stay in touch with mostly drag racers but I do stay in touch with things like engine development.

I can only think that I must mis explain or not make my point right cause I can assure you..I am not wrong ,,You make the most power when it's as lean as you can get away with....