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Why does a lean engine run hot?


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#1 dr249

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Posted 07 July 2005 - 04:37 PM

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.

#2 infamous

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Posted 07 July 2005 - 05:11 PM

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.

#3 Erasmus00

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Posted 07 July 2005 - 05:16 PM

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

#4 dr249

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Posted 07 July 2005 - 05:50 PM

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.

#5 C1ay

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Posted 07 July 2005 - 07:17 PM

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.

#6 Qfwfq

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Posted 08 July 2005 - 05:01 AM

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. :)

#7 infamous

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Posted 08 July 2005 - 09:26 AM

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.

#8 UncleAl

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Posted 09 July 2005 - 02:33 PM

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.

#9 infamous

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Posted 11 July 2005 - 08:05 PM

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.

#10 brianthepoet

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Posted 15 July 2005 - 07:03 PM

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)

#11 infamous

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Posted 15 July 2005 - 09:04 PM

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)

Thats a new one on me brianthepoet, most of the automobiles from the 1940's until the early 1960's had only 8 or 9:1 compression. Kerosene which is very similar to diesel fuel would require at least a 15 or 16:1 compression ratio. Maybe he overcame this by blending his kerosene with gasolene. This would, as you state, require regular de-carboning of the cylinder heads and valves, not to mention the possibility of burning a hole in one of the pistons because of the extra heat. In any case, thats an interesting story.

#12 DFINITLYDISTRUBD

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Posted 04 February 2008 - 05:18 PM

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.


Mercedes is supposedly working on a six cycle diesel which uses water alone as fuel on the fifth cycle.

I'd just like to figure out how to rig an exhaust heated water injection system which would fire a metered amount into the jugs with the fuel air mix...betcha I could gain a 20-30% increase in fuel economy easy. (The only real question is can that lil six handle it.)

#13 chilehed

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Posted 04 February 2008 - 06:33 PM

So you think normal engines are running richer than stoich in order to protect the parts from over-temperature?...

Yep, we do that, but only under high load because of the fuel economy hit. There are a number of engineering and manufacturing issues with water that make it unattractive on a mass-production car. Fuel doesn't freeze, the injection systems are already in place, most folks wouldn't like having another tank to fill every time they stop for gas, there'd be a significant cost hit...

One thing to remember is that the mixture isn't homogeneous, so even at stoich there are lean and rich zones in the burning charge - which means that in order to get every BTU out of the fuel you have to run leaner than stoich. But if you do that then you get more NOx, and have no way to get rid of it in the exhaust stream because the converters need to have alternating rich/lean excursions in order to function.

Of course, if you manage to get the aluminum to burn it gets REALLY hot. :)

#14 alexander

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Posted 07 February 2008 - 01:40 PM

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?


Actually much for the same reason that most drifters run their engines extra rich, especially if they are running a big turbo. Their reason for doing it is so that gas purposely does not all burn up and is ejected out of the engine, for the obvious reason that liquid fuel carries with it a lot of heat, it then burns up in the exhaust pipe, but bottom line is that it is out of the engine, by that cooling the engine down; heat is a big problem on cars that run 600+hp and run almost fully open all the time...

Just like chilehed said, most modern cars are actually ran richer at high loads to cool it down, hence if you run the engine lean, it runs hotter :lol:

Mercedes is supposedly working on a six cycle diesel which uses water alone as fuel on the fifth cycle.

they were supposed to present the engine at the Paris auto show last year, and i thought they did...?

#15 DFINITLYDISTRUBD

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Posted 07 February 2008 - 05:12 PM

they were supposed to present the engine at the Paris auto show last year, and i thought they did...?

They might have...The last I heard (which was quite a while ago I must admit)
they were still ironing out bugs. (excessive detonation issues mostly)

#16 alexander

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Posted 08 February 2008 - 09:45 AM

absolutely, last i heard they were still working out the bugs, but the engine was presented nonetheless... perhaps as a non-running concept at the time...

It's going to be powering their new super car too, the one with auto cambering wheels and stuff.... it's going to blow slr mclaren out of the water, lighter, faster, waay better handling, cant wait to see it ;)

On the note of cars, drifting, heat and stuff, anyone here play Live for Speed?

#17 marcringuette

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Posted 17 January 2009 - 05:40 PM

This thread has focused on the ability of excess fuel to carry away heat. However, I found an alternative explanation for why lean engines run hot that sounds very convincing to me:

"The bit that causes melted pistons is that lean mixtures extend the combustion duration due to the slow flame speed, and therefore the mean cycle temperature rises. Peak flame temperatures don't cause any damage to pistons, but it takes far lower mean cycle temperatures to damage them, usually first from lubrication failure before outright melting." -- TDIMeister, eng-tips.com/viewthread.cfm?qid=212876

However, I admit that I don't fully understand why slow combustion would raise the mean cycle temperature. It's not like the combustion products stop being hot once combustion is finished! My best guess is that if combustion is rapid (before the piston has moved far) then the expansion ratio is maximized and the gases can most quickly expand and cool down.


Marc