How does Lean-burn actually save fuel?

[brucepick]

OK. I know a bit about LB from driving my HX, with ScanGauge and MPGuino. SG tells me when it's in and out of LB, among other good stuff. Guino of course tells me the mpg.

There are two factors that enable lean burn to actually get you more mpg.

First and foremost, the air/fuel ratio that's actually maintained in a conventional modern gasoline engine ensures that a small amount of unburned fuel passes THROUGH the engine. This is intentional, so that it's available to burn inside the catalytic converter, keeping it at the desired very hot operating temperature. Whatever that amount is, it's "wasted" as far as getting motion from that fuel.

So - by leaning out the mixture - you don't waste that small percentage of fuel. It burns in the cylinder where it can help move the car forward.

The HX keeps the cat HOT by putting it inside the exhaust manifold. To get it any hotter/sooner, you'd have to put a mini-cat into each exhaust port on the head. Now there's an idea...

Now - when my Civic enters lean burn - I can feel the power drop just a bit, if my foot is dead steady on the accelerator and the road grade stays the same, etc. Of course there's a power drop at that instant because the air volume to the cylinder is unchanged, and the fuel qty dropped. Some of that air isn't "getting fuel". Less fuel is being burned in the cylinder - where it counts. But that's OK. I step into the gas a bit and keep the car moving at the speed I want. Net result is that mpg goes up, and I can see it on the MPGuino. I've seen 100-120 mpg on the highway, on a gentle downgrade where some power is needed to maintain speed. On a moderate upgrade, if I can maintain LB it will get 30-35 mpg. But if I take it out of LB, it gets about 25-30 mpg for the same climb. Dont' ask me about level roads, we don't have them here!! Up and down all the way, usually not too steep.

The second benefit - which was already mentioned somewhere in this thread - is the reduced vacuum. The throttle is more open in lean burn, for any given power/rpm combination, so there's less mechanical effort expended to create + maintain that vacuum.

[abently]

Originally Posted by brucepick

Of course there's a power drop at that instant because the air volume to the cylinder is unchanged, and the fuel qty dropped. Some of that air isn't "getting fuel". Less fuel is being burned in the cylinder - where it counts. But that's OK. I step into the gas a bit and keep the car moving at the speed I want.

And here, we have further confirmation of an increase in RPM, with Lean-burn.

Arcosine, thanks. Great article. Somehow I'm not convinced that it was manifold pressure impacting negatively on fuel consumption beyond 15.5:1. I really needed to see a BSFC map to believe it. Otherwise how/why do all other Lean-burn engines operate in the 20-25:1 AFR's?

Or maybe it does, which is another reason for manufacturers to set low-rpm limits under which Lean-burn happens?

[user removed]

If you get more air mass in the cylinder with a lower percentage of fuel then you can get the same power with less fuel.

Opening the throttle provides more air, which raises the actual compression pressure in the combustion chamber prior to ignition of the fuel air mix. Since IC engines use the "leverage" of the difference between compression pressure and combustion pressure to accomplish useful work you can get the same amount of power at the same RPM with lean burn while using less fuel and more air.

It will only work under light loads, when normal manifold vacuum would be fairly high, compared to higher loads that would require more power.

regards
Mech

[t vago]

Originally Posted by abently

Otherwise how/why do all other Lean-burn engines operate in the 20-25:1 AFR's?

The other engines you mentioned have direct gasoline injection. This enables them to do neat tricks, like inject just enough gas to make a super-lean mixture during induction, and inject a little more gasoline later on to create a stoichimetric pocket of air-fuel mix within the super-lean mixture, during compression. This gives a so-called stratified charge. The spark plug easily ignites the stoich pocket, and the burning pocket then goes on to reliably burn the rest of the super-lean mixture.

That's how you can get 20:1 to 25:1 AFRs with modern lean-burn engines.

[abently]

Originally Posted by Old Mechanic

If you get more air mass in the cylinder with a lower percentage of fuel then you can get the same power with less fuel.

I have to disagree with this statement. Oxygen alone does not provide useful energy. Remember anything beyond (leaner than) stoich means there is an excess of air within the cylinder just taking up space and therefore reducing power output.

[IamIan]

Originally Posted by abently

I have to disagree with this statement. Oxygen alone does not provide useful energy. Remember anything beyond (leaner than) stoich means there is an excess of air within the cylinder just taking up space and therefore reducing power output.

Don't forget about the effects of changes in efficiency ... If fuel consumption per engine cycle goes down 5% ... but the Efficiency goes up 6% ... there is a net 1% improvement in power , even with lower fuel consumption.

Separately ... I would also recommend keeping in mind the difference between power and energy ... MPG is about chemical energy consumed for a given distance... MPG is not directly about power ... just like MPH , or acceleration are about power but are not directly about energy.

Lastly ... Oxygen does contribute chemical energy to a chemical reaction like burning gasoline ... the total chemical energy is not exclusively from the fuel ( gasoline for example )... to clarify a bit ... if you react gasoline with something other than O2 you will get a different amount of chemical energy released ... it is not the gasoline alone that contains all the chemical energy ... All the reactants in a chemical reaction contribute to the result.

[brucepick]

Originally Posted by abently

And here, we have further confirmation of an increase in RPM, with Lean-burn.

Where is the RPM increase? I open the throttle to maintain vehicle speed. Speed maintained means RPMs are maintained. Transmission is a conventional 5-speed, not variable ratio. We have confirmation of a more open throttle, which does not necessarily mean more fuel if lean burn is in use. It just means more air.

No matter. I certainly encourage folks building systems with custom mapping of engine parameters. There's plenty opportunity for innovation there.

Meanwhile, I'll continue getting about 40 mpg in winter and even better in the summer, in a car that cost about a tenth the price of a Prius.

[t vago]

Originally Posted by abently

I have to disagree with this statement. Oxygen alone does not provide useful energy. Remember anything beyond (leaner than) stoich means there is an excess of air within the cylinder just taking up space and therefore reducing power output.

Disagree away all you want, but Old Mechanic was essentially correct.

One can model the power stroke of a 4-cycle engine as a thermodynamic process called adiabatic expansion. You can look it up on Wikipedia if you want, or search for "adaibatic process," but in any case, it can be shown that it does not matter how much air is inside the piston-cylinder arrangement, as long as the amount of energy input is the same.

In other words, you can burn a set amount of gasoline at stoich with a set amount of air, and you'll get a set amount of work extracted from the piston. Keep the gasoline constant, while increasing the amount of air inside the cylinder, and you'll still get the same exact amount of work out of the piston.

(I'm not going to go into heat losses into the metal, nor atmospheric drag losses acting on the piston, nor friction losses, or any intricacies about how gasoline ignites with air, because that would be missing the forest for the trees)

[abently]

Originally Posted by brucepick

Where is the RPM increase? I open the throttle to maintain vehicle speed. Speed maintained means RPMs are maintained. Transmission is a conventional 5-speed, not variable ratio. We have confirmation of a more open throttle, which does not necessarily mean more fuel if lean burn is in use. It just means more air.

Speed maintained, does not mean RPMs are maintained. 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 may even decrease under IamIan example, but I doubt that would be the case unless designed to do so.

[abently]

Originally Posted by t vago

In other words, you can burn a set amount of gasoline at stoich with a set amount of air, and you'll get a set amount of work extracted from the piston. Keep the gasoline constant, while increasing the amount of air inside the cylinder, and you'll still get the same exact amount of work out of the piston.)

That's correct.

Re-read what the Old Mechanic said;

If you get more air mass in the cylinder with a lower percentage of fuel then you can get the same power with less fuel.

Same power with less fuel.... lol

I think he is getting into the dynamics of air mass and consumption per revolution in trying to explain same power with less fuel, but it would good if he could elaborate on it much more, otherwise it makes little sense. An example always helps..