Why does a lean engine run hot?

[infamous]

Originally Posted by 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)

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.

[DFINITLYDISTRUBD]

Originally Posted by dr249

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.)

[chilehed]

Originally Posted by dr249

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.

[alexander]

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 :P

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...?

[DFINITLYDISTRUBD]

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)

[alexander]

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?

[marcringuette]

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

[DFINITLYDISTRUBD]

The bit that causes melted pistons is that lean mixtures extend the combustion duration due to the slow flame speed,

I don't buy that... it is my experience that
lean mixes should burn faster and hotter...run too lean you detonate which hammers the hell out of your engines internals....run to rich your changing plugs but the engine's fine....it's really no different from cranking the oxygen too high on an acetylene torch eventually you reach the point where you get machinegun like reports or one big bang that's the leanest of lean super hot super quick.....course at the other end is the sooty pure acetylene flame which burns fairly cool but very sluggishly...even the sound of this flame lighting is very sluggish and sounds a lot like a bubble in mud.

[marcringuette]

I understand your reaction, but the textbooks seem to be saying that lean burning is slower, not faster.

From the book "Lean Combustion" by Derek Dunn-Rankin, p.104 : "A disadvantage of lean operation is that the burning rate is reduced compared to combustion under stoichiometric conditions. ... The reduction in burning rate results in an increase in the overall combustion duration, which in turn leads to increased heat transfer losses to the cylinder walls and a decrease in the overall engine thermal efficiency."

Of course, once the cylinder walls are heated up by this effect, preignition can start happening and the usual process of "flame front from spark outward" is messed up. Before that point, though, lean combustion will start with the spark, as intended, but will be slower and less consistent, with some remaining regions of fuel-air mixture not burning until later in the piston stroke.

Anyhow, because I was only seeing the "no evaporation of excess fuel" explanation for why lean engines run hot, I wanted to introduce this seemingly important alternate explanation. Late burning of fuel leaves heat around in the cylinder for longer and causes a thermodynamically less efficient cycle. Cylinder walls heat up, and badness begins.

Cheers
Marc

Ref: search inside "Lean Combustion" book for term "combustion duration"
at URL amazon.com/gp/reader/012370619X

[dingotj]

I was wondering if someone could shed some light on the effects of lean fuel mixtures in a diesel engine. I've had people tell me that it works the same as gas enginges and it burns hotter, and I've had people tell me that they work differently and it burns cooler. Thanks for the help