A simple question on running lean...

A simple question on running lean...

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TheEnd

Original Poster:

15,370 posts

187 months

Saturday 1st August 2015
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Assuming a petrol engine here anyway, but I get the general ideas of stoich vs a little bit rich and lean.

Running rich means you use up all the oxygen available which is the limiting factor, and waste some fuel at the end, and running lean means you make the most of all of the fuel even if there is air left over that could have done something.
Stoichiometric is fine on paper, but inside a cylinder it isn't always as perfectly mixed as it could be.

The main question is why is running lean hotter?
I have seen some people say it burns slower, others saying it burns faster, some saying it is missing the cooling effect of evaporating fuel.

More heat would mean more power, but there is less power produced, presumably from the lower amounts of big volume-hungry exhaust gasses which are also pushing along with the thermal expansion.

So what is the mechanism of hotter combustion and exhaust gasses if there is less fuel about?

bearman68

4,642 posts

131 months

Saturday 1st August 2015
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Speed of combustion.
The fuel has a more ready supply of O2, and so burns faster. Because it burns faster, the peak temperature is higher, but the overall chemical energy available is less.


TheEnd

Original Poster:

15,370 posts

187 months

Sunday 2nd August 2015
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More searching on the matter came up with this quote-

"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."


The question arose after speaking to a guy who was running out of fuel in his Porsche (~1991 car). The engine pretty much cut out, but he could get it going by nailing the pedal which gave him about 1000rpm and that allowed him to limp into a nearby petrol station.
When he got there, he noticed smoke coming from the engine and he could see the cats and manifolds smoking which suggest it got pretty hot in the exhaust.

So if running lean makes the combustion slower, which allows more heat transfer into the engine, that doesn't automatically mean it is burning any hotter inside the cylinder, just it has more time to transfer the heat.

But we do know the exhaust got hotter too, so would that have been from slower burning gasses still combusting out through the exhaust, or a cumulative collection of heat in the engine over time meaning after each combustion it's going to be a little hotter for the next one and so on until it all stacks up.



AER

1,142 posts

269 months

Monday 3rd August 2015
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When people say "it burns hotter" when running lean, they are never specific about what is actually hotter. Exhaust gas? Combustion chamber? Valve seat? Spark plug?

Thing is, combustion engines are a multi-factorial problem and whether it's hotter depends on your baseline. Your baseline depends on the engine, speed, load, fuel grade, coolant temperature, air temperature and other more minor factors.




fatjon

2,142 posts

212 months

Monday 3rd August 2015
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My understanding;

A stoich or slightly rich mixture is ignited a few degrees before TDC and burns to provide optimal peak pressure on the piston where it has the best mechanical leverage on the crank, pressure peaking around 17 degrees ATDC. The combustion is pretty much over as the piston passes 60 degrees ATDC. Most of the energy is used in shoving the piston with minimal heat transfer to the combustion chamber, piston and walls. The heat is confined to the small area above the piston and by the time the piston has exposed more of the cylinder wall the combustion is over.

A lean mixture burns slower therefore imparts more heat to head and cylinder walls as the combustion does not all take place quickly and in the confined area when the piston is near and just after TDC. In extreme cases the mixture is till burning as it is shoved out past the exhaust valve causing valve and seat damage and huge heat transfer to the head via the exhaust ports and glowing manifolds.

It could be rubbish but it's what I have learned from various books on the subject and it sounds quite logical to me.

AER

1,142 posts

269 months

Tuesday 4th August 2015
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Yup, there's a bit of rubbish there.

Peak cylinder pressure and peak cylinder temperature are closely related, so most of the heat transfer occurs around this point where the temperature differential between the gas and the combustion chamber walls is literally enormous and the surface-volume ratio is also at its worst/highest. At this point gas temperatures are well into the thousands of degrees C.

By the time the exhaust valve is opening gas temperature is back down to "only" hundreds of degrees and surface-volume ratio has much improved, lowering heat transfer rates by an order of magnitude or more.

Exhaust valves and seats can overheat if they leak because they'll leak worst with the highest temperature gas at Pmax.

Most of the exhaust valve temperature comes from the cylinder head and high pressure gas at Pmax, rather than the flow of gas over it during the exhaust stroke.

fatjon

2,142 posts

212 months

Tuesday 4th August 2015
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Cheers, thanks for the correction. Sounds logical to me as I suppose temperatures in the high hundreds would be enough for glowing manifolds.


R8VXF

6,788 posts

114 months

Sunday 30th August 2015
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