How does Lean-burn actually save fuel?

[t vago]

Originally Posted by abently

That's correct.

Re-read what the Old Mechanic said;



Same power with less fuel.... lol

No, it didn't help that he phrased his statement the way he did.

He mentioned stuffing more air into the cylinder with less percentage of fuel. Holding the amount of fuel steady while stuffing more air into the cylinder would result in the same amount of fuel being burnt as before, thus getting the same energy out of a power cycle as before.

Along with that, if you get more air into the cylinder, then that means a lower engine vacuum, which implies a more open throttle. Thus, both throttling and pumping losses are reduced. As a result, a larger percentage than before of that work being developed, as described in the paragraph above, can be used to actually do something useful, and it will cause the vehicle in question to accelerate unless the fuel flow rate was reduced. It then becomes necessary to reduce the fuel flow rate a little bit to maintain the same vehicle speed as before.

Therefore, with lean burn you get the same power with less fuel.

[IamIan]

Originally Posted by abently

Speed maintained, does not mean RPMs are maintained.

With a fixed transmission ratio ... yes it does... the wheel RPMs are directly connected to the engine RPMs and restricted via the fixed transmission ratio.

Originally Posted by abently

I'll explain it with a ridiculous example. Say during lean-burn (less fuel is injected), power at cruise RPM (say 10Kw) is halved, to maintain speed, you must correct RPM such that power is bumped back up to 10Kw. Now that RPM increase may not be so pronounced due to the improvement in efficiency with more throttle, but it will be there.

RPM changes ... are not required for changes in engine power output.

There are a wide variety of ways to change engine power output that do not directly require a change in engine RPMs to get that change in power output.

Originally Posted by abently

RPM may even decrease under IamIan example, but I doubt that would be the case unless designed to do so.

I do not know any ICEs designed to operate in Lean Burn that are not also designed to lower the operating RPMs when operating in lean burn ... as touched on previously there are several good engineering reasons to do this.

If you know of any ICEs that do not please post ... I'd like to read / research such an engine design myself, would be interesting.

[gone-ot]

...the old Chrysler "Lean-Burn" engines used ignition-timing control, not throttle control, to speed-up/retard engine idle, as well as driving response, as I vaguely recall.

[abently]

Originally Posted by t vago

Therefore, with lean burn you get the same power with less fuel.

Correct, assuming the efficiency gains are large enough, to offset the reduction of fuel.

[abently]

Originally Posted by IamIan

With a fixed transmission ratio ... yes it does... the wheel RPMs are directly connected to the engine RPMs and restricted via the fixed transmission ratio.

Bingo, thanks!

Air mass intake must be increased to maintain power (torque) output, via change in throttle, to thereby stop the RPM's from dropping when going into 'Lean-burn.' The fact that efficiency increases with this change in throttle angle is the sole source of improved fuel consumption.

I think this thread can finally be closed, or sticky'd!

[IamIan]

Originally Posted by abently

The fact that efficiency increases with this change in throttle angle is the sole source of improved fuel consumption.

"Sole source" ... is incorrect... It is one source... there are others that also contribute.

Originally Posted by abently

Air mass intake must be increased to maintain power (torque) output,

No.
Power can be increased or maintained in other ways ... Air mass is not the only way.

Originally Posted by abently

to thereby stop the RPM's from dropping when going into 'Lean-burn.'

As posted previously ... there are very good engineering reasons to want the RPMs to be lower when in 'Lean-Burn'... The Lower RPMs during Lean Burn is desired and intentionally designed in.

[abently]

As far as I am aware, if RPM is maintained from stoich to lean burn, then the throttle/pumping losses are the sole (most significant) contributor to improved fuel efficiency. We can get into semantics about maybe thermo efficiency, combustion pressures, harmonics and tuned intakes/exhausts but many of these changes have little effect in say a 200 RPM band in the low region anyway.

'Power can be increased or maintained in other ways' again..... of course power can be increased in other ways such as with more fuel, lol. More Ignition timing in a modern engine will not improve power or fuel consumption measurably.

I'm not rehashing the last point either, I understand why lower RPM's are desired during lean burn, so save your breath.

[IamIan]

Originally Posted by abently

the sole (most significant) contributor to improved fuel efficiency. We can get into semantics about maybe thermo efficiency, combustion pressures, harmonics and tuned intakes/exhausts but many of these changes have little effect in say a 200 RPM band in the low region anyway.

200 RPMs is pretty low... most ICEs have a hard time even producing enough power to overcome pumping losses at 200 RPMs.

My bad ... I made the assumption you were using the term Sole in the same way as in a 'Sole Proprietorship'.

Thanks for clarifying ... based on the above it seems when you use the term ... you have your own personal deviation from the standard means ... you seem to mean only ( most significant ) ... significant being subjective as you choose to ignore the other contributors that are effecting things as 'semantics'... thanks for clarifying.

Originally Posted by abently

of course power can be increased in other ways such as with more fuel, lol.

Glad you see the error of your previous statement.

Originally Posted by abently

More Ignition timing in a modern engine will not improve power or fuel consumption measurably.

Incorrect.
We know the effect these things have on combustion because it can be measured... if it were not a measurable difference OEMs would not spend the engineering time and production costs to include it.

Originally Posted by abently

I'm not rehashing the last point either, I understand why lower RPM's are desired during lean burn, so save your breath.

Glad you see the error of your previous statement.