30 replies to this topic

[charliedurrant]

Hi,

I was just thinking...not something I do too often...

My understanding is that running lean causes the engine to run hotter as there is additional liquid when rich to allow additional energy transfer to the exhaust gasses.

So if the mixture is perfect i.e. o2 is proportionally correct for the fuel then is it simply the additional water that cools the engine?

Charlie

[Sam Walters]

In simple terms in comparison to stoichiometric conditions lean conditions produce a slower burn rate of the fuel. This in turn means the fuel takes longer to burn. Thus more heat is transferred into the cylinder walls and thus into the cooling system. Its worth nothing that the reality is the power comes from the heat produced in the engine. But its a simple case of F=MA. Thus more power is delivered from an instantaneous bang as opposed to a slower burn. Running lean can also effect the distribution of fuel within the mixture. Meaning that it doesn't all combust at the same time. meaning that fuel can burn when the piston is at a different position within the engine. This in combination with the excess heat generated in the engine`s cylinder walls can start to effect flame fronts within the combustion chamber to the point where multiple flame fronts are crated that crash into each other. AKA detonation. Ouch!

[Ethel]

It's part chemistry 'n part that Boyle's Law from your school physics.

Lean means there's excess oxygen for the fuel so it can burn quicker & hotter (and run the risk of burning other stuff like valves & pistons). Because it's burning quicker and hotter it can lead to much higher pressure, which in turn means more temperature (Boyle's Law). If you didn't alter the ignition timing you could end up with a situation where the cylinder pressure get high enough before TDC to try pushing the engine in reverse which really would send the pressure & temperature soaring.

[charliedurrant]

Thanks for the replies - conflicting opinions.....umm

In simple terms a slower burn during the compression cycle, I would think, would allow more time for conduction of energy into the block and head.

But, if we have a lean mixture then there is more oxygen to be next to fuel across the statem at any one time so burn rate should be faster.

I'd liken it to a wood burning stove. To allow more conduction into the cast iron casing you slow the burn rate by reducing o2 in by shutting off vents.

I still like the idea of liquids allowing better conduction of energy out of the system ....

I've probably misread both answers so if, and I realize this is purely an academic question, there is any reference material or further explination that would be great.

Charlie

[R1minimagic]

The more fuel you put in the cylinder, the cooler the cylinder charge on the intake stroke. This is because as the piston moves downwards the fuel vaporises and absorbs energy in doing so. If you run lean, you dont get the same cooling effect, i.e. start of combustion is hotter.

[charliedurrant]

The more fuel you put in the cylinder, the cooler the cylinder charge on the intake stroke. This is because as the piston moves downwards the fuel vaporises and absorbs energy in doing so. If you run lean, you dont get the same cooling effect, i.e. start of combustion is hotter.

R1,

Thanks, to me the reasoning above makes complete sense.

Charlie

[Sprocket]

yep, lean mixture burns faster. The fact that the lean mix burns faster also means that its peak temperature will happen sooner, and be in the cylinder longer, thus the cooling system will pick up more heat.

[Sam Walters]

Sorry i must have stated incorrectly. as a general rule a leaner mixture will burn faster and hotter. think blowing on a fire to get it going. However it generally has a less well distributed mixture. thus burn at different points.

[charliedurrant]

yep, lean mixture burns faster. The fact that the lean mix burns faster also means that its peak temperature will happen sooner, and be in the cylinder longer, thus the cooling system will pick up more heat.

Sprocket,

The problem I see with this is that a perfect mixture will burn just as quickly assuming that the air and fuel will mix homogeneously before the spark ignites the mixture which I think would occur...

Charlie

[mini-luke]

yep, lean mixture burns faster. The fact that the lean mix burns faster also means that its peak temperature will happen sooner, and be in the cylinder longer, thus the cooling system will pick up more heat.

Sprocket,

The problem I see with this is that a perfect mixture will burn just as quickly assuming that the air and fuel will mix homogeneously before the spark ignites the mixture which I think would occur...

Charlie

Say, for the sake of simplicity that in each cycle of the engine running at stoich 100 molecules of oxygen and 100 of fuel are burned, so there is one molecule of fuel per one of oxygen. If you reduce the number of fuel molecules to 80 they will be more likely to ''meet'' a molecule of oxygen and combust as there are more available per fuel molecule. This means that it takes less time for them all to combust.

All engines (LC and AC) and liquid cooled in some way, the liquid being fuel. Converting atomised liquid fuel to a gas requires energy, this energy is removed from the engine as heat, cooling the engine. The more fuel, the more heat required to vaporise it.

[R1minimagic]

yep, lean mixture burns faster. The fact that the lean mix burns faster also means that its peak temperature will happen sooner, and be in the cylinder longer, thus the cooling system will pick up more heat.

Sprocket,

The problem I see with this is that a perfect mixture will burn just as quickly assuming that the air and fuel will mix homogeneously before the spark ignites the mixture which I think would occur...

Charlie

Say, for the sake of simplicity that in each cycle of the engine running at stoich 100 molecules of oxygen and 100 of fuel are burned, so there is one molecule of fuel per one of oxygen. If you reduce the number of fuel molecules to 80 they will be more likely to ''meet'' a molecule of oxygen and combust as there are more available per fuel molecule. This means that it takes less time for them all to combust.

This is not true, they are less likely to meet as you have reduced the concentration of fuel molecules!

[mini-luke]

yep, lean mixture burns faster. The fact that the lean mix burns faster also means that its peak temperature will happen sooner, and be in the cylinder longer, thus the cooling system will pick up more heat.

Sprocket,

The problem I see with this is that a perfect mixture will burn just as quickly assuming that the air and fuel will mix homogeneously before the spark ignites the mixture which I think would occur...

Charlie

Say, for the sake of simplicity that in each cycle of the engine running at stoich 100 molecules of oxygen and 100 of fuel are burned, so there is one molecule of fuel per one of oxygen. If you reduce the number of fuel molecules to 80 they will be more likely to ''meet'' a molecule of oxygen and combust as there are more available per fuel molecule. This means that it takes less time for them all to combust.

This is not true, they are less likely to meet as you have reduced the concentration of fuel molecules!

The 80 fuel molecules are going to react faster as there are more oxygen molecules per fuel molecule. It's basic stoichiometry. Yes, overall there will be less heat energy, but it will be released over a shorter period of time.

[R1minimagic]

It doesn't matter what the ratio is, you reduced the concentration so the rate of reaction will be slower, basic reaction kinetics

Rate = k x {fuel} x {O2}

[charliedurrant]

The 80 fuel molecules are going to react faster as there are more oxygen molecules per fuel molecule. It's basic stoichiometry. Yes, overall there will be less heat energy, but it will be released over a shorter period of time.

It doesn't matter what the ratio is, you reduced the concentration so the rate of reaction will be slower, basic reaction kinetics

Rate = k x {fuel} x {O2}

Isn't all the talk about reaction kinetics reasonably irrelevant. In the highly charged (literally) and high temperature reaction chamber the reaction will occur much quicker than the rate of conductivity of the cast iron. The only plausible explination (in my mind) is the energy transfer to the incoming fuel, the outgoing water molecules and fuel when in excess of the oxygen.

Charlie

[charliedurrant]

Doesn't retarding the spark cause overheating so essentially reaction kinetics really can be ignored? Head hurts....

Edited by charliedurrant, 06 December 2011 - 10:04 PM.