How does Lean-burn actually save fuel?

[abently]

Originally Posted by t vago

That's easy. You're forcing the throttle to be more open in order to get the engine to do the same amount of work as before. That reduces throttling losses at the throttle plate, and reduces pumping losses in that your engine vacuum is lower than before.

Shrink those two losses, and you shrink the total amount of load placed on the engine. Shrinking the load means using less fuel.

The first part, I agree with, which corresponds well with any BSFC map.

The second part, I'm not sure load is the right word, losses may be a better term to stick with.

[abently]

Originally Posted by woodydel

R P M

Just what does the M stand for? By increasing your RPM's you are PUMPING MORE AIR THROUGH THE ENGINE EVERY MINUTE. Pump more air by increasing your RPM's and you defeat your lean burn experiment. That increased volume of air must be mixed with fuel. Run your engine at the same number of RPM's as before. Lean burn is not your problem. Your engine can't properly handle the changes you have made and produce the same amount of power. Your insistence upon maintaining the same performance level is unrealistic.

I find it difficult to believe that an engine at 1500rpm and 17.7:1 (AFR) will produce more power than the same engine at 1500rpm and 14.7:1 (AFR) given that the latter will have more energy (i.e. fuel) in the mixture to combust and produce work. There is a reason why max power tends to occur around 12.5:1.

[SVOboy]

I'm not sure that anyone'se actually bothered with these two links:

Lean burn - Wikipedia, the free encyclopedia

InsightCentral.net - Encyclopedia - Honda Insight Lean-Burn Oxygen Sensor

[t vago]

Originally Posted by abently

The first part, I agree with, which corresponds well with any BSFC map.

The second part, I'm not sure load is the right word, losses may be a better term to stick with.

Total load is equal to the work needed to propel the car forward, plus friction losses (won't expand this one for the sake of brevity, but it includes drivetrain losses, aerodynamic losses, rolling losses, friction losses inside the transmission, friction losses inside the engine, etc...), plus parasitic losses, plus pumping losses, plus throttle losses.

For the sake of fuel consumption, total load determines at what rate fuel is to be consumed. This is an open system, so rates apply.

[abently]

Originally Posted by SVOboy

I'm not sure that anyone'se actually bothered with these two links:

Lean burn - Wikipedia, the free encyclopedia

InsightCentral.net - Encyclopedia - Honda Insight Lean-Burn Oxygen Sensor

Even both articles there, say little about the differing air mass, rpm with lean-burn and change in throttle position etc.

4 pages later and the first post still stands largely uncorrected...

[Tygen1]

Well then, my car requires approximatly 5% more throttle opening when in lean burn

[IamIan]

Originally Posted by t vago

That's why lean burn is not recommended at high RPMs.

Good post ...

I agree that is another motivation for running at lower RPMs during lean burn.

I only listed one motivation , the change in flame speed... I did not intend to suggest it was the only motivation for the lower RPMs ... sorry if I gave that impression.

Whatever the combination of reasons for it happens to be ... the results are the same ... running the engine at lower RPMs still reduce engine friction ... which is an additional benefit ... and the point I was trying to add.

[arcosine]

Did you advance the ignition timing? Lean mixtures burn slower. We use to run so lean they would knock when richened up with the manual choke or accelerator pump. Of course we had the spark gap set really wide too. I think now days theres more turbulence in the combustion chamber.

[t vago]

Originally Posted by IamIan

Good post ...

I agree that is another motivation for running at lower RPMs during lean burn.

I only listed one motivation , the change in flame speed... I did not intend to suggest it was the only motivation for the lower RPMs ... sorry if I gave that impression.

No worries. I just wanted to clarify why lower RPMs are desirable, from an engine longevity standpoint. I have read already on this board where one ecomodder tried to do lean burn (in the form of a throttle-less gasoline engine), and ended up destroying their engine.

That reminds me: Y'all realize that a diesel engine is by definition a lean burn engine, right?

[arcosine]

Interesting article:

Fuel Economy Tuning - Tech Review - Honda Tuning Magazine