Why does a lean engine run hot?

[dr249]

This is more of a practical question than the typical discussions I've seen here but interesting nontheless. Any mechanic knows an engine which is running lean (less fuel than stoiciometric) runs hotter than one which is properly tuned. 2 stroke motorcycles can actually seize from overheating if run too lean. Normal logic would suggest that with less fuel there would be less heat generated, but it is not the case. My Internal Combustion Engines course professor did not even know about the phenomena (he was the type who knows theory but has someone else change his car's oil).

Any ideas why this happens?

One idea I had is a properly tuned engine could actually be tuned slightly richer than stoiciometric and the excess fuel provides cooling as it evaporates. This doesn't sound like it would maximize efficiency though. My nitro fuel powered RC car uses this technique for cooling but efficiency isn't important in that application.

[infamous]

Originally Posted by dr249

One idea I had is a properly tuned engine could actually be tuned slightly richer than stoiciometric and the excess fuel provides cooling as it evaporates. .

This is a good observation dr249, it is quite true that the excess fuel will result in better cooling. This does however reach a point of diminishing returns, keeping in mind that it is heat that produces horse power. Like everthing else in physics, there is a balance between forces and an ideal compromise is always seemingly just one adjustment away. Because high compression engines produce more heat, they will also make more horsepower. However, if this heat reaches beyond a critical level, preignition along with greater frictional loses will start leaching away any extra power that was gained in the first place. A slightly lean mixture will always make more power because total combustion is achieved, the question is however, for how long will the engine tolerate the abuse. I drag race with a small block Mopar, and I personally like to keep the fuel mixture slightly rich just to be on the safe side. I learned this the hard way by burning the top ring land off of a piston on more than one occasion.

[Erasmus00]

Originally Posted by dr249

This is more of a practical question than the typical discussions I've seen here but interesting nontheless. Any mechanic knows an engine which is running lean (less fuel than stoiciometric) runs hotter than one which is properly tuned. 2 stroke motorcycles can actually seize from overheating if run too lean. Normal logic would suggest that with less fuel there would be less heat generated, but it is not the case. My Internal Combustion Engines course professor did not even know about the phenomena (he was the type who knows theory but has someone else change his car's oil).

Any ideas why this happens?

I think the higher air/fuel ratio allows the fuel to burn hotter.
-Will

[dr249]

Originally Posted by infamous

A slightly lean mixture will always make more power because total combustion is achieved, the question is however, for how long will the engine tolerate the abuse.

So you think normal engines are running richer than stoich in order to protect the parts from over-temperature? If that is the case it would be interesting to substitute something cheap like water instead of the extra fuel (water also has an extremely high latent heat of vaporization so less water would be needed than the extra fuel). Either that or somehow provide the necessary cooling without hurting efficiency. I've heard of water injection before, maybe that's what they were doing but I thought it was knock prevention so they could run higher compression ratios and get more efficiency and power that way.

[C1ay]

Changing the mixture on a carburetor does not change the displacement of the engine, it will still pull in a fixed quantity of air/fuel mixture. Removing fuel from a particular air/fuel quantity is the same as increasing the air in the mixture. This is like blowing on a smaller air/fuel quantity or blowing on a fire with a bellows to make it burn hotter.

[Qfwfq]

Originally Posted by dr249

If that is the case it would be interesting to substitute something cheap like water instead of the extra fuel (water also has an extremely high latent heat of vaporization so less water would be needed than the extra fuel).

It's an old idea that does increase efficiency because the water vapour has a much greater volume than the liquid water. It has been tried but usually deemed not worth the trouble, imv this is because it has never been applied in the best possible manner.

The next time I'm rich, I'll build a completely re-designed engine and revolutionize the motor industry.

[infamous]

Originally Posted by C1ay

Changing the mixture on a carburetor does not change the displacement of the engine, it will still pull in a fixed quantity of air/fuel mixture. .

This is certainly true C1ay, but only if both examples are feed with the same cylinder head. I own a flow bench which I have used to flow test the volumetric efficency of the cylinder heads on my race car. Over the last year or so I have spent many hours porting these heads in an effort to increase the power output of this engine. As a result, I have seen an increase from 560 to about 650 horsepower. This increase is attained, not by raising the overall cubic inch displacement of the engine, but, by allowing the air fuel mixture to more freely enter the combustion chamber of the motor. By increasing the velocity and volume of air fuel mixture entering the chamber, through cam timing and intake port efficiency, a much greater volume of air fuel mixture is placed within the cylinder for ignition. As any good physicist knows, atmospheric pressure is responsible for the flow of air from ambient pressure to an evacuated chamber. The ease and speed with which this can occur will result in a greater volume filling the cylinder. Upon ignition, greater cylinder pressure is obtained raising the power output of the engine.

[UncleAl]

Good point! Consider a four-cycle four-cylinder engine doing a modest 3000 rpm. One intake occurs in 0.5 revolution. There is no more than 10 milliseconds to fill the cylinder, including valve opening and and closing. Nothing is running at equilibrium. With only 14.7 psi pushing the intake it is amazing anything gets in there at all.

[infamous]

Originally Posted by UncleAl

Good point! Consider a four-cycle four-cylinder engine doing a modest 3000 rpm. One intake occurs in 0.5 revolution. There is no more than 10 milliseconds to fill the cylinder, including valve opening and and closing. Nothing is running at equilibrium. With only 14.7 psi pushing the intake it is amazing anything gets in there at all.

Absolutely UncleAl; Things get even more critical at engine speeds of 8600 rpm. Cam timing and spring pressure along with reciprocating weight of valve train components become severely important. I use titanium intake valves to reduce valve weight along with special light weight solid lifters. Cam timing at 8600 rpm usually works better with at least 90 degrees of overlap. I have found that lobe separation of 108 degrees, and duration of around 300 works the best also. All these factors must work togeather to allow that 14.7 psi of atmospheric pressure to do it's job.

[brianthepoet]

the oldies will remember the days of charcoal gas producers, but my late father-in-law used to take the fan belt off and run on power kerosene. he said it made the engine run extremely hot, but did little or no damage other than creating a need for de-coking the head more often. Funny the things you remember isn't it - he told me that about 1975!













Organic gardening is a lot of rot (brianthepoet)