How does Lean-burn actually save fuel?

[IamIan]

Just wanted to add one more bit I didn't notice mentioned previously.

Sense Leaner Air to Fuel Ratios burn slower ... some engines that make use of significant Lean Burn modes ( like the Gen-1 Insight ) ... also tend to lower the ICEs operating RPMs during high Lean Burn operations ... lower RPMs = less friction.

Attached is part of one EPA test describing both the BSFC already discussed and the friction benefits as well.

[t vago]

Originally Posted by IamIan

Just wanted to add one more bit I didn't notice mentioned previously.

Sense Leaner Air to Fuel Ratios burn slower ... some engines that make use of significant Lean Burn modes ( like the Gen-1 Insight ) ... also tend to lower the ICEs operating RPMs during high Lean Burn operations ... lower RPMs = less friction.

Attached is part of one EPA test describing both the BSFC already discussed and the friction benefits as well.

Book alert: The reason why lean burn is not done at higher RPMs is because combustion of the air fuel mixture is more complete than at richer AFRs.

At stoich or richer, during a combustion event, there's a small layer of air-fuel mix, hugging the combustion chamber and piston surfaces, that remains unburnt. This serves as an thermally insulating layer against the high temperatures associated with combustion, and is considered a good thing for this reason. Now, disrupt this layer, and the surfaces will now be directly exposed to those high temperatures (which are normally much higher than the melting point of aluminum alloys used for pistons and cylinder heads).

If the surfaces are not allowed sufficient time to cool off (say, when the heat energy moves from the metal surface into the meat of the metal engine, and then into the coolant and oil), the surface temperatures rise to the point where air-fuel mix will spontaneously ignite once it reaches those hot surfaces. This is called pre-ignition. If this pre-ignition occurs while the cylinder is in its compression stage, the air-fuel mix will combust and raise the temperature of the now-ignited charge just sucked in, and the temperature will then be further raised by the compression itself (basic thermodynamics). This will make the surfaces that much hotter, and if this is left unchecked, will quickly destroy an engine.

Unfortunately, lean burn tends to strip away this small insulating layer of unburnt air-fuel mixture simply because of the fact that the combustion process tends to be more complete. Usually, though, this in itself is not enough to cause problem, because at lower RPMs, the high surface temperatures can usually propagate through the metal surfaces into the coolant. However, sharp edges and carbon deposits may tend to become much hotter anyway, and this will lead to pre-ignition.

Now, there's no real way to sense for pre-ignition, but we can sense for a related bad thing called detonation. This is where air-fuel mixture actually explodes instead of burning normally. The explosion causes the flame front to move faster than the speed of sound, and the shock waves created by this flame front impact on the combustion chamber surfaces, disrupting that small insulating layer previously mentioned. Detonation can be heard as a pinging sound, or the sound as though rocks were being thrown against the engine. The engine computer can detect detonation, and enrich fueling and retard spark advance to compensate.

For lean burn engines, detonation would typically occur as a result of pre-ignition, and at lower RPMs, the engine computer can react in time to prevent engine damage. Combustion chamber surface temperatures as a whole won't become high enough to melt.

However, at higher RPMs, even the engine computer can't sense detonation in time to prevent engine damage. Once a lean-burn system detonates at high RPMs, it's too late. You just melted something.

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

[abently]

Originally Posted by redyaris

What changes with rpm is the volume per unit time. The volume per rev is constant. The changes are in MASS and PRESSURE.

Half right, the pressure changes thereby affecting the mass of air induced.

When I stated 'actual volume flow' that was in reference to the mass of air at atmospheric pressure, but from now on I will talk about it as mass only.

So can someone confirm or disprove that as vacuum trends to 0.Hg, air mass increases? That equation on the Eng Tool Box suggests otherwise....

[woodydel]

Let's make this REALLY simple. You're a little guy inside the cylinder head standing next to the intake valve. The valve opens the the piston draws down. You've got a bucket of fuel in one hand and an eyedropper in the other. If you dump the bucket when the valve opens, the fuel zooms into the air streaming by and disappears into the cylinder. The next time the valve opens you squirt the fuel in your eyedropper into the air streaming by and disappears into the cylinder. The eyedropper is the LEAN BURN. You will NEVER use as much fuel as the bucket. I don't care about how efficient the design of the engine is either. Don't care about variable valve timing, turbocharging, high lift cams or any other trick you might find being used on engines. Any particular engine is still just an air pump. The ATMOSPHERE is the driving force pushing the air into the engine. If you mix less fuel with the air supplied to the engine you save. Your driving habits and unreasonable expectations of engine performance is the real problem. Less fuel being pumped into the engine yields less BTU's of energy available per revolution of the engine. It is going to take you longer to climb that hill. You are going to have a harder time fighting against a head wind. There is only so much available energy in each gram of fuel. You cannot get something for nothing. If you insist upon no compromise, your lean burn efforts will be negated. NOTHING in any formula or theory is going to change this. Smokey Yunick produced the leanest burning engines ever. The engines ran so hot it was like they were inside a furnace while running. Lean burn runs hot. Some engine parts can literally melt. Lean burn will produce extremely high engine efficiencies. Smokey Yunick was the KING of lean burn super efficient engine design but even he could not extract more BTU's from each gram of fuel than what was available.

[abently]

Less fuel being pumped into the engine yields less BTU's of energy available per revolution of the engine.

Correct, refer back to the first post. If you need say 8kw of power to cruise at 60km/hr and that originally occurred at 1500rpm, you are now going to need to cruise at say 1700rpm during lean-burn to output the same amount of power (energy) and therefore cruise at 60km/hr.

Now tell me something I don't know.

[t vago]

Less fuel being injected may mean less available energy, but in the case of lean burn, it also means a larger percentage of that energy being used to do useful work.

You also have to figure out what your volumetric efficiency (VE) is at 1500 and 1700 RPMs. This is important, because lower RPMs doe not automagically mean better VE.

F'r instance, the 4.7L V8 and transmission package used in my truck originally had cams and intake that provided decent performance for a truck, though not anything special. The transmission computer that controls my transmission originally gave 4 forward gears (one overdrive).

The High Output version of this engine/transmission combo was developed for the Jeep Grand Cherokee, and this involved (among other more subtle things) changing out the cams and intake manifold for items that had better VE at lower RPMs, thus allowing the engine to gain more torque at the low end. This dovetailed rather nicely with the transmission computer reprogramming that allowed the Jeep Grand Cherokee to have 5 forward gears (2 overdrive gears) using the same exact transmission hardware.

Now, I retrofitted my engine to have these High Output cams and intake manifold, and retrofitted the newer transmission computer to my truck about 2 months after I swapped out the other parts. As a result, I could also have that second overdrive gear which allowed me to go down the highway about 300 RPMs lower than before, and I saw a gain of about 1 MPH with the new transmission computer, as opposed to the old one.

Other Dakota owners with the 4.7L/transmission combo and the programming for 4 forward gears, on another website I frequent, only changed the transmission computer by itself, while leaving their intake manifolds and cams alone. They reported different results, in that they saw their RPMs drop by about the same amount as what I saw, but also saw losses of fuel efficiency as a result of the revised transmission computer. Their engines did not have the modifications that improved VE at lower RPMs, thus their engines had to work harder than they did before, just to suck in air-fuel mixture at those lower RPMs. As a result, their FE dropped even though they added a second overdrive gear.

[abently]

Originally Posted by t vago

Less fuel being injected may mean less available energy, but in the case of lean burn, it also means a larger percentage of that energy being used to do useful work.

You also have to figure out what your volumetric efficiency (VE) is at 1500 and 1700 RPMs. This is important, because lower RPMs doe not automagically mean better VE.

The first part, correct, slower flame front.

Generally VE goes up with RPM, I think the only engines that would high VE under 2000 rpm would be some diesels and tractors etc.

Its actually surprising that so far, no one has been able to answer conclusively the question of how 'lean burn actually saves fuel?'........

I can very easily hook up an accurate RPM meter to the vehicle and prove whether or not RPM goes Up, Down or same at cruise with 'Lean burn' compared to stoich but at a cost of $50 (same cost as a tank of petrol) I think I'll pass till I am tempted to do otherwise.

[woodydel]

Originally Posted by abently

Correct, refer back to the first post. If you need say 8kw of power to cruise at 60km/hr and that originally occurred at 1500rpm, you are now going to need to cruise at say 1700rpm during lean-burn to output the same amount of power (energy) and therefore cruise at 60km/hr.

Now tell me something I don't know.

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.

By the way, your increase in RPM's from 1500 to 1700 is a 13% increase. Did you know that?
Do you think that maybe it's impacting your results? So if you're pumping 13% more air, do you think you might be pumping 13% more fuel with it?

[t vago]

Originally Posted by abently

Its actually surprising that so far, no one has been able to answer conclusively the question of how 'lean burn actually saves fuel?'........

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.

[SVOboy]

Something I'm not sure I saw addressed:

Are you speaking specifically of lean-burn systems that have been used by a number of manufacturers, most notably Honda, GM, etc, or are you just talking about leaning out the AFR on any vehicle without other concomitant changes?