The effects of a weak Air-fuel ratio

[Scotracer]

This is more of a debate between my father and I than anything. We are both engineers, educated to university degree level (although my degree is Honours whereas his is not) so we certainly know our background...but we've come to logger-heads.

The problem is this: I'm very keen on Thermodynamics and today I was reviewing some papers about Air-fuel ratios for my FSAE project and came across a lot of information (which is also taught in my classes) that says that combustion temperature falls when you lower the equivalence ratio (i.e. make the charge lean). Fine, right? Less combusted fuel + more unreacted air to heat = lower combustion temperature. Fairly logical.

However, my father contradicts this showing that when you make an engine run lean (he has ~40 years of motocross experience) it will either cause the engine to seize or will burn a hole in the piston - he says this is due to higher combustion temperature. I countered this by saying there must be another phenomenon occuring in this case - but I'm unsure as to what that may be.

Any insight? Also, some relevant papers would be nice

[Belatti]

Jeje... is there any chance that the 40 years experience your father has in motocross is with 2 strokes aircooled engines?

If this is the case he is right: 2 strokes aircooled engines uses very rich mixtures because the extra unburnt fuel helps to cool down the engine. A leaner mixture would work pretty well in terms of combustion but doesnt help to lubricate (those engines doesnt have oil pump and are lubricated by adding 5% of oil to the fuel) and definitively doesnt help for cooling. This is one of the reasons why 2 strokes engines are not good from an enviromental point of view and also why is starting to be banned from various motocross/kart series all arround the globe.

Leaner fuel mixtures burns slower and for power to be produced, combustion must be complete by the time the exhaust valve opens. If you have less fuel burning you have less power, you have less temp and so your line of thinking is right. But in 2 stroke engines a "weak air-fuel ratio" is still above stechiometric.

[Scotracer]

This also occurs in 2-stroke engines that are not pre-mixed with oil

I asked a friend of mine and this is the answer I got:

"The adiabatic flame temperature is a property of the fuel. It peaks slightly rich of stoichiometric at around lambda 1.1 for most hydrocarbon fuels and falls off on either side of this peak. The laminar flame velocity also falls off on either side of a peak near this region of AFR. 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. "

[rjsa]

When I was at university (Naval Architect) we where lectured by a teacher that leaner fuel mixtures leaded to cooler engines. That was disputed by one of my coleagues that was having flight classes. He staded that flight manuals for the Piper he flew suggested enriching the mixture to cool down the engine, what he had done himself.

This was the first time I've seen this question. And every time the result is the richier the cooler.

[Jersey Tom]

Fuel... Heat of vaporization....

[Belatti]

Scotracer, you are right about the "lean mixtures extend the combustion duration due to the slow flame speed" and if you check I stated that in my previous post.

Still, this is not a lineal process with only 2 variables:
1)If you are standing a little before the peak your friend states, the extension in duration is not quite enough to rise the mean temp that high, but the lack of "extra cooling" mixture is enough to melt aluminum.
2) Then, if you are standing a little after the peak, in 2 strokes aircooled it maybe not still enough because those little monsters require quite rich mixtures.
3) At last, if you are standing way before the peak it may occur that the engine does not even provide enough power to make it hot.

In resume, I believe there is a superposition of causes that ends in a superposition of effects, besides air-fuel ratio: a bad lubrication (2 strokes) rises temp, the flame speed and its peak temp as you called, the valve timing and fuel burning in the exhaust, compression ratio, etc.

[CMSMJ1]

My (limited) knowledge is that a leaner engine runs hotter..or to flip it that a riucher motor runs cooler.

I run larger jets in my rear cylinders in my V4 motorbike to keep the cylinder temps cooler.

without anything to back it up would it run cooler when rich due to the unburned fuel stealing the heat when vapourised?

[riff_raff]

A slightly lean mixture in a homogeneous SI engine can produce mild detonation (autoignition)in the combustion end gases. This mild detonation may not be audible, but the higher cycle pressures and temperatures it creates can thermally overstress the thermally isolated parts of an aluminum piston crown. The damage can occur in a very short period of time, with cycle rates exceeding 30 Hz in a typical production auto engine.

As others noted, best power in an SI engine usually occurs at equivalence ratios slightly higher than stoichiometric (1.0). Lean mixtures or stratified mixtures are good for low fuel consumption, but can only be used at low engine loads. At higher engine loads, stratified mixtures or lean mixtures will tend to cause detonation.

With premix 2-strokes, the oil content of the fuel tends to reduce its octane rating, making the engine more susceptible to detonation. If the engine is also carburetted, the difficult acoustic environment created by the expansion chamber, reed valves, etc. makes accurate and consistent fuel metering very difficult.