Combustion chamber heat

[Dirk Wright]

On Thu, 26 Sep 1996, talltom wrote:

> 
>   I've noticed a little discrepency here in relation to combustion chambwer
> heat.
> It seems that some say it's necessary to go over rich to cool and prevent
> preignition/detonation. Others say that thermal barrier coatings are the
> neat trick,
> which would make the combustion chamber hotter, and to me more knock prone.

I have no hard facts to back me up, but I would agree that using a 
thermal barrier coating on the pistons/combustion chamber/valves would 
create a situation where detonation/knock would be more likely. Since 
less heat would be abosrbed by the engine, as the flame front progressed 
through the cylinder onthe power stroke, the remaining unburned fuel 
would be heated more rapidly.

I think there would indeed be a penalty in that the spark would ahve to 
be retarded more, and the mixture enriched to prevent this. Another 
alternative of course, is water injection, which I am more and more 
convinced is a good thing for air cooled engines at least, which tend to 
run hotter.

I think, though that you could get away with coating the exhaust and 
intake ports without any penalties. The idea behind using coatings in the 
first place was to reduce heat in the engine and thus extend its life. 
Likewise with the exhaust header. For the intake port, the charge would 
stay a little cooler before entering the chamber. For the exhaust port, 
the exhaust gases would stay hotter, leading to better scavanging at the 
expense of running the exhaust valve hotter.

Everything's a tradeoff, it seems.

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Dirk Wright 					            wright at uspto.gov
"I speak for myself and not my employer."               1974 Porsche 914 2.0
"A real hifi glows in the dark and has horns."            1965 Goodman House
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