Why do lean cars run hotter?

[rattlsnak]

OK, since you're not satisfied with all of the above correct answers, do a search on AOPA.org, or Landings.com, or Flightinfo.com or such avaition forums, this has been a hot topic for YEARS !!!!!!!! This is why airplanes (piston powered) have EGT's and CHT gauges. It is really quite obvious and self explanatory to see and watch how the gauges react when you manually richen or lean the mixture.

[BK911]

Ok, now I am a little confused. TimT stated you get more power lean from stoi. That would imply an AFR above 14.7? I thought max power was somewhere around 13.5, which would be rich. ????

BK

[911pcars]

As to the question of what is rich, it's all relative. 14.7:1 is richer than 15:1; 13.5:1 is leaner than 13.0:1 and so on.

IMHO, the ideal air-fuel is whatever air-fuel mixture provides either max. power or max. fuel economy. Anything richer than this will not produce more power but might provide some side benefits/effects (higher/lower combustion temperature, higher/lower EGT, higher/lower heat transfer to the cooling system).

I was taught that leaner mixtures burn slower (molecules further apart). That's why cars have a vacuum advance mechanism (or equivalent electronic control) to provide additonal ign. timing during part throttle operating conditions where leaner mixtures are SOP). Slower burning mixtures also expose more of the cylinder to combustion heat as the piston travels toward BDC on the power stroke.

My $.02
Sherwood Lee
http://members.rennlist.org/911pcars

[Superman]

I have to go, and I did not read all posts here but I want to make a couple of comments. first, Warren is correct (always a safe bet) in that it is lean cars that have more hp, not rich ones. Race cars are not running rich, they are running on the edge of lean.

And I have heard lots of explanations of pinging that were intriguing, including the 'supersonic' thing (sorry Island, this is my month to pick on you I guess) until I heard the REAL explanation. From BA I think, but not sure. Anyway, when the fuel starts to burn it expands and increases pressure in the combustion chamber. Also, the piston it still on its way up, also increasing pressure. And there is heat in there already. When detonation happens, it is the spontaneous combustion of the remaining unburned fuel, when the temperature and pressure in the cylinder reach the combination that will spontaneously combust that fuel. This is why higher octane gas stops detonation, and also why retarding ignition does too. It starts with a flame front, and ends with a POP of the last few CCs.

[TimT]

OK one last time....

Most manufacturers of engine management systems, recommend to always start tuning and programing the EFI from a rich position. Electromotive and Haltech have "start up maps" in the software supplied, this gives a way to get the engine running and a baseline for your fine tuning.

While doing dyno pulls you continue to lean the mix until you achieve the most power and torque.

As you lean out the mix ( by adjusting injector dwell time). The power the engine makes increases, to a point. Then the too lean mix increases EGT and CHT temps..... and the power falls off as the "Ideal" combustion parameters have been missed.

I dont know the exact ratio that makes the most power. I am willing to bet it is different from engine to engine.

All said lean=hot

[island911]

Originally posted by Superman
I have to go, and I did not read all posts here but I want to make a couple of comments. first, Warren is correct (always a safe bet) in that it is lean cars that have more hp, not rich ones. . . . .

Super, is that what Warren said?
It is far too simplistic to say "lean cars that have more hp."
Without looking at the posts I can tell you Warren would not say such a thing.
I could believe he said something like; Race engines require a faster mean flame speed and pressure rise (than do road cars with slower piston speeds) and can take advantage of a leaner mix.

Also, Super, do you think your correcting me (or enlightening me) on the detonation thing? I cant figure how your "picking on me" (other than that Starbucks crack)
I used detonation as an example (an end point) of flame front speeds . . .used an extreme velocity most are familar with.

For the non-binary/non-linear thinkers out there here is a graph showing the best performance is achived with a relatively rich mix. It also shows you can slow the pressure rise by going leaner -or- richer. Also note the Combustion Time curve.


. . .it's not just leaner/richer, more/less power.

edit for refnc; Complete Combustion:
AF = Air Fuel Ratio = Mass Air / Mass Fuel
Perfect 100% octane
(C8H18 + O2 + N2 = CO2 + H2O + N2) AF = 15.1:1
Real Gas (SAE standard) AF = 14.7:1
oxygenated w/ 89% gasoline and 11% MTBE AF=14.281:1
( a good argument for an O2 sensor )

[island911]

Originally posted by 911pcars
As to the question of what is rich, it's all relative. 14.7:1 is richer than 15:1; 13.5:1 is leaner than 13.0:1 and so on.

IMHO, the ideal air-fuel is whatever air-fuel mixture provides either max. power or max. fuel economy. Anything richer than this will not produce more power but might provide some side benefits/effects (higher/lower combustion temperature, higher/lower EGT, higher/lower heat transfer to the cooling system).

I was taught that leaner mixtures burn slower (molecules further apart). That's why cars have a vacuum advance mechanism (or equivalent electronic control) to provide additonal ign. timing during part throttle operating conditions where leaner mixtures are SOP). Slower burning mixtures also expose more of the cylinder to combustion heat as the piston travels toward BDC on the power stroke.

My $.02
Sherwood Lee
http://members.rennlist.org/911pcars

Sherwood, I think your right on; except, just up to the part about "molecules further apart."
In general, the reaction is between the O2 and the octane molecules. The spacing between all molecules is determined by pressure.
The octane molecules just need to be close to the O2's . . .they don't need to be close to another octane to react with the O2's.

Re. Slower burning mixtures: these burn for a longertime, but much less hot.
I can see the logic of heat applied longer will "heat soak" better; BUT, the temperature difference of the fast burn, can easly out-pace the long application of the slow burn. This will put more more heat to the head, pistons, and such. In contrast, the lower temp, slower burn will but more work in to the crank.