Lean burning engine for better MPG?

[Trav]

I understand the theory.I also understand that in theory a lean or rich mixture will burn cooler,in reality the lean mixture because of incomplete burn due to excess oxygen in the mixture causes detonation which causes the rise in chamber temp.So while the actual flame temp may well be hottest at stoichiometric it is also the ideal mixture for as complete a combustion as possible.

 

[XS650]

Originally Posted By: edhackett
Simple chemistry. A lean mixture has more oxygen left after the fuel is combusted. This extra oxygen at combustion temperatures oxidizes(burns) the metal of the valves.

Ed


Bingo, like how a cutting torch works, except slower.

 

[Trav]

Yes there is no doubt about that.Thats the point when things get too lean the excess of oxygen on hot parts causes temps to climb in the chamber and things to burn.The cutting torch example is perfect,I don't know but i would guess the flame temp at the torch head is the same when the extra oxygen is introduced but the effect on the parts the flame is aimed at will be much hotter as will the area around where the cut was made.

 

[tom slick]

the torch's flame brings the steel near ignition temp. The oxygen reacts with the near molten iron to rapidly oxidize into iron oxide.

 

[XS650]

The real heat in a cutting operation is from the steel or iron being burned. You could cut off the acetylene once a cut starts and it would still continue cutting.

 

[mechtech2]

Anybody remember Chrysler's 'Lean Burn' engines?
Or Honda's lean burning engines with a third 'jet valve' to initiate combustion of the very lean mixture?
No melting problems.

A leaky valve can burn, and detonation or pre ignition can harm them as well.

 

[Lazy JW]

Originally Posted By: edhackett
Simple chemistry. A lean mixture has more oxygen left after the fuel is combusted. This extra oxygen at combustion temperatures oxidizes(burns) the metal of the valves.

Ed


Also, the lean mixture may burn slower (assuming no timing adjsutments), so there is still combustion going on late in the cycle and the exhaust valve "sees" hotter gases than it would if the combustion were completed in a timely manner. There is still less total heat but a bigger percentage is wasted late in the cycle: less power and reduced efficiency results.
Joe

 

[Shannow]

The Exhaust gas temperature trends are similar in shape to combustion chamber trends, highest at an equivalence ratio of one, and lower richer or leaner.

 

[ikeepmychevytoo]

Lambda 1 is not stoichiometric per the proper terminology in senses chemistry might evoke. However industry like to use it in place of the lambda 1 (may be they feel like scientists ?). Whatever going on in the engine in such a little moment is very primitive measure for even mentioning the stoichiometry.

While represents an experimental balance point in a mechanical environment Lambda 1 is overly rich for stoichiometry that rather deals with calculation and obsevation.

correct me if I'm wrong

 

[Shannow]

Not sure, but the temp peaks are a little rich of the theoretical point.

Equilibrium states of CO, HC, CO2, N2, O2, and NOx throw the peak away from theoretical (unless you add them into the model)

 

[BuickGN]

FWIW, I tried this on the GN when I had to use it for a couple months as my commuter 210 miles round trip daily. That was fun...

I ran it on 91 with a splash of av gas. Kept factory timing which is fairly aggressive at cruise plus my compression is already half a point higher than stock. Then I started leaning it out under cruise until it started surging and backed it off a hair. This would not be acceptable to most people because if there was any acceleration involved it would buck and not go anywhere until I gave it enough pdeal to kick it out of cruise mode. I didn't have the wideband at the time and the factory 02 read 0mv at cruise.

With double the factory hp and half a liter bigger motor I managed 28mpg which is 6mpg more than the factory rating. With Inconel (sp) exhaust valves I wasn't too worried about high EGTs.

 

[oilyriser]

Lean gives slow flame propagation, and the mixture will still be burning after TDC. This means less energy is captured by the expansion stroke, and more is left in the exhaust. Advancing timing, or running lean only at low engine speeds, should be ok.

 

[BuickGN]

Maybe, but I've always made more power the leaner I could run it before detonation. I guess it's all relative because leaner to me is getting close to stoich at WOT so in absolute terms it's not lean, just lean for a FI motor at 29psi.