How does Lean-burn actually save fuel?

[abently]

So from everything I have seen, the only way lean-burn seems to help anyone save fuel is by allowing the engine to operate in a more efficient BSFC range. I.e. more RPM's but same output power to cruise at 60km/hr etc.

Which would explain why I'm only seeing a 12% gain, when I have leaned out the Closed Loop mixtures by 20%.

Correct, wrong or not exactly right?

[saand]

The RPMs will be the same at a given speed assuming you have a manual or have your torque converter locked.
As i understand it the main purpose of lean burn is to allow the engine to pass more air when under lower load conditions. At low load conditions the throttle is almost closed which creates a high vacuum, energy is required to create this vacuum. If more air can pass through the throttle when burning lean it means less energy is used as there is less vacuum.

I am currently looking into burning lean myself, how have you leaned out your closed loop mixture by 20%?

also 12% is a good improvement as far as modifications go.

[abently]

The RPM's can't be the same, because say you use 8Kw of power to cruise at 60km/hr, that 8Kw of power at 14.7:1 occurs at say 1500rpm, and at 17.7:1 occurs potentially at 1700rpm. Therefore shifting your cruise rpm into a lower bsfc zone.

Remember max power for most engines occurs around 12.5:1 AFR's and anything beyond that reduces power.

I had a wideband system from way back (LM-1) that outputs a narrow-band signal so I've used that to set my Closed Loop mixtures.

[Ryland]

Originally Posted by abently

The RPM's can't be the same, because say you use 8Kw of power to cruise at 60km/hr, that 8Kw of power at 14.7:1 occurs at say 1500rpm, and at 17.7:1 occurs potentially at 1700rpm. Therefore shifting your cruise rpm into a lower bsfc zone.

Most engines are putting out more power then needed at cruising speed tho, so say your car requires that 8kw/10hp to maintain speed, the engine is most likely putting out closer to 12kw/15hp, so with a lean burn engine you keep the same engine speed but cut out some of the fuel so it run leaner, you get a drop in power but it's a low load condition so you don't need that power.

[mnmarcus]

Most engines are putting out more power then needed at cruising speed tho, so say your car requires that 8kw/10hp to maintain speed, the engine is most likely putting out closer to 12kw/15hp, so with a lean burn engine you keep the same engine speed but cut out some of the fuel so it run leaner, you get a drop in power but it's a low load condition so you don't need that power.

This can't be right... If your engine is creating more hp than the car needs for the speed it has to put the difference into acceleration. If not where does the extra work go?

[user removed]

Lean burn is effective because it uses a higher mixture ratio instead of a more restricted throttle position. Lean burn allows higher BSFC due to leaner mixture, when loads are only a fraction of best BSFC points in an otherwise identical engine without lean burn.

Combustion characteristics when lean burn is incorporated only exist when effective in cylinder compression is much lower than possible with high loads applied to the engine.

One way to look at it is to just consider lean burn as displacement reduction. While you are not actually reducing the calculated displacement of the engine, partial throttle positions that maintain a manifold vacuum of less than 50% of atmospheric pressure reduce the "effective" displacement of the engine as well as the actual compression pressure at the instant before combustion occurs.

That same lower effective compression is the essential component of lean burn, but for lean burn to be most effective better atomization of the fuel-air mixture is another component of the strategy.

regards
Mech

[woodydel]

The engine is a pump. For every revolution the same amount of air is pumped. Some engines pump air better at differing RPM's. That doesn't matter. As the air is pumped through the engine, if you squirt less fuel per revolution you HAVE TO USE LESS FUEL. So you call it lean burn because there is an excess of air available for any particular volume of fuel squirted into the airstream in a given amount of time.

All the talk about whether you get more or less power means NOTHING. It doesn't matter if you're able to climb a hill or not. Working your throttle to hold back the amount of air the engine can pump DOES NOT change anything I stated in the previous paragraph.

Squirting less fuel in any given time period when the engine is running has to use less fuel. The consequences of the "lean burn" don't mean anything. A "lean burn" is just harder to maintain. It does not matter how you squirt the fuel into the engine. Carburetor, fuel injection, a BUCKET will not make any difference. Obviously a BUCKET will not be a "lean burn". All you're doing is measuring how much fuel goes through the engine with the air.

I think people are making this out to be complicated for no reason. Even Einstein knew enough to look at things simply.

[t vago]

Originally Posted by mnmarcus

This can't be right... If your engine is creating more hp than the car needs for the speed it has to put the difference into acceleration. If not where does the extra work go?

He should have said "the engine is capable of putting out more hp than the car needs." Throttling takes care of that extra hp.

[mnmarcus]

I think this is a good question but has not been answered yet. I think Old Mechanics response is the best so far. I am sure I do not know the answers to this question, but would like to be enlightened if possible. Quotes are from original poster.

So from everything I have seen, the only way lean-burn seems to help anyone save fuel is by allowing the engine to operate in a more efficient BSFC range. I.e. more RPMs but same output power to cruise at 60km/hr etc.

I believe this is essentially true, ex. when "low" power requirements exist the lean burn is more efficient (BSFC) because instead of throttling back to get to that low power requirement, which would consequently increase pumping losses, less fuel is injected for the same(?!) HP at the same rpm. I believe the actually answer would be much more complex, for instance during maximum acceleration would lean burn be more efficient? I would guess not because the throttle is wide open which means minimum pumping losses. Your mpg may still go up because you are putting less fuel through the motor, but your bsfc would be worse because you are not getting as much power per unit of fuel.

Which would explain why I'm only seeing a 12% gain, when I have leaned out the Closed Loop mixtures by 20%.

Again I'd say this is essentially true. Leaning the fuel to air mixture would only coincidentally equal an MPG gain. If there was a direct one to one relationship we could just inject zero fuel to achieve unlimited mpg.

Correct, wrong or not exactly right?

I think not exactly right but close, and I am sure I am not exactly right either but perhaps someone can explain it better or more accurately?

Some additional questions I have that seem to relate;

If there are 2 identical cars traveling at the same steady speed which require 25 HP and the only difference is one car runs (say whatever%) leaner how would you describe the differences between the 2 cars engines?

The engine rpms would have to be the same for the same gear and same speed.

The compression would be identical because it never changes in an engine, does it?

The lean car is injecting less fuel per stroke so it has to use less fuel per minute.

The only difference would be the throttle is opened wider in the leaner car which means the less fuel makes less HP but less pumping losses as well so same net HP to the transmission. Right?

My next question would be why aren't all cars lean burn? I assume lean burn turns on and off or is adjusted by the engine control unit? I'd also assume there are times lean burn is not more efficient mpg wise, right?

[Tygen1]

Originally Posted by abently

The RPM's can't be the same, because say you use 8Kw of power to cruise at 60km/hr, that 8Kw of power at 14.7:1 occurs at say 1500rpm, and at 17.7:1 occurs potentially at 1700rpm. Therefore shifting your cruise rpm into a lower bsfc zone.

Remember max power for most engines occurs around 12.5:1 AFR's and anything beyond that reduces power.

I had a wideband system from way back (LM-1) that outputs a narrow-band signal so I've used that to set my Closed Loop mixtures.

12% is a very nice gain! Good Job

I use a voltage divider on my MAF to achieve lean AFR's. I usually target 17:1. Typically, on flat ground, don't need to increase rpms, however the LOD on my SGII goes up. Climbing hills while lean requires more rpm though.

Can you detail how you have your lean burn system wired up?