A simple question on running lean...
Discussion
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?
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?
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.
"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.
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