Why does an engine overheat if it runs lean?

[mrmr96]

Per title. I've heard this is the case but I'm struggling to understand why this is. Can anyone post a decent explaination or a link to an article where I can read more?

Many thanks.
(PS - My car's fine, this is purely out of interest!)

[crolandc]

Remember when using a bunsen burner at school,the more air you gave it the hotter(more fierce)the flame.Same thing!!

[banzai_dan]

In simple terms, its because the fuel has a much higher specific heat capacity than the air.
When the fuel air mixture enters the cylinder it is at a lower temperature than the walls of the combustion chamber.
The laws of physics say that heat energy flows from a high temperature reservoir (the cylinder walls) to a low temperature reservoir (the fuel/air mixture).
As a rich fuel mixture has a higher specific heat capacity it takes more energy from the engine to raise it's temperature, hence the engine cools down more; and the opposite occurs when the mixture is lean.
I'm not sure if I'm 100% correct though so I'm sure someone else will come along and correct me if I've missunderstood something.

[ELAN+2]

in laymans terms, a lean mix burns hotter, the bunsen burner analogy is a fair one. Modern fuels burn hotter than the old leaded fuels any way, it helps to keep the emissions down, this is one of the reasons classics can be "unhappy" on unleaded BTW

[Herman Toothrot]

Also running lean can cause det which causes the ignition point to get even hotter. So say a machine mark on the head is under the lean conditions causeing ignition then the fact it has the explosion start from it then the explosion makes it even hotter and the effect is self propogattion and it gets even hotter and det even worse untilt he engine dies.

[mrmr96]

ELAN+2 said:

in laymans terms, a lean mix burns hotter.

Ok. But _why_ does a lean mix burn hotter?

[ELAN+2]

mrmr96 said:

ELAN+2 said:

in laymans terms, a lean mix burns hotter.

Ok. But _why_ does a lean mix burn hotter?

I do not understand the physics/chemistry of the process, however as with the bunsen burner example given earlier think along the lines of an oxy/acetylene cutting torch, you heat the steel until its red hot, then inject a large amount of oxygen and the flame temperature soars and cuts the steel.

If you have any form of combustion(log fire) and blow air into it, the fire becomes more intense and hotter.

[esselte]

mrmr96 said:

ELAN+2 said:

in laymans terms, a lean mix burns hotter.

Ok. But _why_ does a lean mix burn hotter?

Is it because in a less than Stoichiometric engine all the fuel is burnt with none left to absorb any of the heat? If you see what I mean?

[anonymous-user]

In extreme cases, fuel is used as a coolant. By that I mean you inject more fuel than you actually need for combustion. In racing terms this is common practice (for me at least).

So...the fuel you require for combustion, is burnt. The rest is excess, either exiting the cylinder 'raw' - i.e. unburnt, in the same state it was in when it was in the tank, or alternatively, burnt through cylinder heat (not through ignition).

A lean fuel mixture will burn hotter as the air content in the cylinder at that time, will be greater than that of the fuel, and as a gas, will be easier to superheat.

HTH a bit