How does Lean-burn actually save fuel?

[t vago]

Originally Posted by mnmarcus

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?

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.

[mnmarcus]

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

Air-fuel ratio (AFR) is the mass ratio of air to fuel present during combustion. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture (often abbreviated to stoich). AFR is an important measure for anti-pollution and performance tuning reasons. Lambda (λ) is an alternative way to represent AFR.

[t vago]

Originally Posted by mnmarcus

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?

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.

[saand]

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?

[user removed]

Originally Posted by 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.

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

[Tygen1]

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.

[abently]

Originally Posted by Old Mechanic

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.

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.

[redyaris]

Originally Posted by 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.

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.

[redyaris]

Originally Posted by Old Mechanic

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

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.

[redyaris]

Originally Posted by abently

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.

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