How does Lean-burn actually save fuel?
 

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?

:)

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.

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.

:)

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?

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

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.

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

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.

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.

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.:)

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?

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?

Why not? Lean burn gasoline systems do not do well with catalytic converter systems that consumers are willing to accept. Your modern catalytic converter system requires an exhaust input that has some little bit of unburned hydrocarbons, some little bit of oxides of nitrogen, and some little bit of carbon monoxide (just not all at once). This is why gasoline cars generally burn near stoich at idle and crusing loads. Lean burn causes carbon monoxide emissions to generally go down, at the expense of higher oxides of nitrogen and unburned hydrocarbon emissions.

You can redesign a catalytic converter system to work with lean burn, but it'd add a lot of cost to the price of a car.

Thanks t vago,
Generally, could this additional be off set by the increased milage? In your opinion are the overall emissions worse for an aftermarket lean burn car than a regular?

From Air-fuel ratio - Wikipedia, the free encyclopedia

Yer welcome.

If you're looking at a point where you recoup the expense involved in converting a cat system to efficiently convert oxides of nitrogen, I'd say that's simply not possible.

From a technical standpoint, you'd need NOx sensors, some sort of NOx-storing catalytic converter, and some way to automatically enrich the fuel-air mix to transform all of that stored NOx back into harmless emissions.

You're going to have to balance the increase in fuel economy you're seeing, with the fact that you're also emitting higher NOx emissions. You can offset that by using EGR, but be prepared to suffer a performance hit in the process.

tygen1, can you give any details about how you have set up your lean burn, where the voltage dividers go, what values ...
also how have you gotten around closed loop operation, as far as i know when changing the MAF sensor output, it works when in open loop so until the o2 sensor heats up but as soon as its warm and sending back usable voltages the car goes into closed loop mode so stops looking at the MAF sensor.

I ask as i am trying to do lean burn without buying a wide band o2 sensor and controller.

Also when i started going lean using my standard narrow band o2 sensor i had to increase the idle control screw so that the car wouldn't stall, have you done this as well?

Compression ratios are calculated by considering the bore and stroke as well as the volume of the combustion chamber area that is still present at TDC.

That represents the theoretical maximum compression ratio. In reality any restriction present that reduces the amount of air that is actually in the cylinder at BDC reduces the usable (I call it effective) compression of the engine. In some cases cam timing can allow the atmospheric pressure to actually push more air in the cylinder than the measured theoretical compression.

When you have a manifold reading of 14 inches, you are only allowing the remaining inches of measured air into the cylinders, so to say that the same amount of air always passes through and engine is incorrect. You can confirm this by just considering the exhaust exiting the tailpipe. At idle it is a very small amount, Rev the engine up and the amount vastly increases.

Lean burn, when properly designed and utilized compensates for engine operation states where manifold vacuum is fairly high, probably above 40% of atmospheric pressure. Effective compression is much lower than the potential maximum.

Since higher effective compression creates more pressure during combustion, lean burn allows for the compression to be slightly higher, while preventing the power from being more than is necessary.

regards
Mech

I would suggest a wideband sensor with a display, most likely it won't pay for itself very quickly, but you would have some peace of mind.
The trick is to get your car to stay in open lope, maybe by disconecting an oxygen sensor. This may or may not work depending on the vehicle. Then, if your MAF reads in volts, you can just bleed voltage off the signal volt with a pot so the pcm believes it's getting less air than it actually is. Pretty simple, however my pcm doesn't seem to like being tricked :)
I have mine on a switch so I can go back to stoich at any time, so no need to up the idle.

There is a significant flaw in this statement, RPM increases exhaust volume significantly.

But I have also been looking at this >

Vacuum and Flow Measurement

And I have more questions than answers.... I was under the impression that real air consumption 'actual volume flow' would go up as vacuum was decreased, towards positive pressure... but now I'm just confusing myself. :confused:

For the perpose of better understanding I would add that; what the throatle changes in a cylinder is the density [m/v] the volume is alwas the same. So the change is the mass of air entering the cylinder. the problem is in the way we use the word amount because it can refer to both mass and volume, and most of us do not diferentiat between the two. I hope this helps.:)

The compresion ratio is = volume at BDC/volume at TDC [ie 10:1]
What the throatle plate does is control the MASS of air entering the cylinder.
Because the mass of air is reduced, at part throatle, when the intake valve closes the pressure in the cylinder is reduced, therefor what changes at Top Dead Center is the Pressure. The compresion ratio is fixed as is the vollume, and does not change. What changes is the compression pressure.
I make these comments for better clarity I hope it helps.:)

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

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.

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....

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.

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. ;)

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.

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.

:)

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?

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.

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?

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.

:)

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.

:)

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

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.

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...

:)

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

Good post ... :thumbup:

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.

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.

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?

Interesting article:

Fuel Economy Tuning - Tech Review - Honda Tuning Magazine

http://image.hondatuningmagazine.com...onomy_02_z.jpg