Why does a lean engine run hot?

[alexander]

back home i was too young, now, i wish i had access to it, there are some REALLY nice vehicles in it :)

 

actually my dad used to work for some people that had a MAZ-547, it runs a 700bhp turbo diesel with direct injection in a really weird drive shaft configuration... it runs 6 axles front 3 pivot to turn, 3 front axles are linked separately to the transmission from the 3 rears, and they can be driven in different directions (3 axles going forward, 3 reverse) to dig the truck in, at 700bhp the truck can only go 40km/h, but has a fuel capacity range of about 500km. (the pressure in each tire can be adjusted separately too, and it comes with hydraulic stabilizing legs that, once the truck digs in, lift it completely off the ground and level it to an accuracy within less then a millimeter... oh the truck is designed to carry up to 80 tonnes, you know, two of those really big ballistic missiles with a 10000km range, though as a launch platform they can carry only one..

 

????:SS20 irbm.jpg ? ?????????

 

my dad got to ride one when they had to pull a tractor out of a marsh

 

they do have a new one MZKT - 79221 (there is a civilian modification) that's 8 axles (all pivot) and an 800bhp diesel that finished development in 97 and has started being delivered to the army in 2000, i think there are a total of 6 now, they look sweet too.

 

since i mostly deal with civilian engines, they are always quirky and different, the only reason i say i know that my dad's tdi has a butterfly valve is because i had just finished putting it back together after getting a glow plug machined out (so we had to take the head off, and that meant taking everything apart. It's no wrx, you cant take that engine out by yourself in 2 hours, or an s13... my friend, when not rushing, lifts his engine out in 45min, and with the amount of head gaskets he blows (something about his highly turboed, intercooled and auto nitris injected engine), i tell you, its a really neat trick to have...

[cbrennan]

...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...

 

Any ideas why this happens?

 

I realize that this thread is old, but as far as I can tell, no one has really answered the original poster's question.

 

The reason that a lean engine runs hotter is that a lean mixture has a greater concentration of oxygen than a rich mixture. As anyone who has spent time in a high school chemistry class will tell you, combustion of a hydrocarbon produces four results; CO, CO2, H2O and heat. With a higher oxygen atom concentration in a lean intake charge, the process is "encouraged" to form more CO2 and less CO. The formation of CO2 releases significantly more heat than the formation of CO (carbon monoxide).

 

Lean mixture = ^ Oxygen = ^ CO2 = ^ Heat

 

I hope this actually answers your question.

[tungs10]

Perhaps a good question would be, "What do you mean by lean?" Equivalence ratio is the ratio between a given fuel/air ratio and the stoichiometric mixture. So 1 would be stoich, less than 1 would be lean, greater than 1 would be rich. In 1977, some people at NASA were fooling around with a 1969 Cadillac engine in a dynamometer test lab. They measured thermal efficiency of the engine as a function of equivalence ratio. The results:

 

ratio efficiency

1.05 27.99

1.00 28.85

0.95 29.5

0.90 29.87

0.85 29.99

0.80 29.83

0.75 28.97

0.70 27.21

0.65 25

 

 

So from 1.0 all the way down to around 75% lean, the engine was actually more efficient. You may have a lower power output at a given RPM, but you are still using more of the energy that is latent in the fuel. Past 75% however, the engine began to get very inefficient, and if energy is not being converted to torque output, then it is getting converted to heat, which will cause the engine to be hotter.

 

I would guess that the reason for the lower efficiency is that when you go past a certain point, you have to move more air to get the same amount of fuel. The engine starts to become more of an air compressor, and air compressors can get really hot and they can use a lot of energy.

 

More efficient combustion would produce more heat but leaner ratio would mean less fuel which would mean less heat. The figures above show that the proportion of fuel goes down a lot faster than the efficiency goes up. So perhaps less energy is being released as the mixture gets leaner.

 

A lean engine might run hotter if it is too lean, but generally speaking this is not the case. A diesel engine does not really have a constant fuel air ratio. Air is pulled into the cylinder, fuel is injected a little past top dead center (TDC) and it burns pushing the piston down. Mass of oxygen versus mass of fuel is variable. With older engines, a throttle was used to control the amount of air admitted to the cylinder. More modern engines use exhaust gas recirculation (EGR). Some of the exhaust gas is diverted, cooled down, and recycled back to the engine. This displaces the oxygen and allows control of the output. The ratio of mass of oxygen to fuel is always oxygen in excess of the stoichiometric proportion so it is equivalent to lean for a spark ignited engine. This results in more efficient combustion and higher efficiency. That means less waste heat energy which means the engines runs cooler for a given load.