Combustion chamber heat

[Shaun Brady]

At 09:40 AM 9/26/96 -0400, you wrote:
>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.
>
Unforutunately, current practice in everything from Formula 1 to stationary
diesels refutes this reasoning.

My seat of the pants rationalization is that the coatings reduce the
likelyhood of hotspots and thus reduce the likelyhood of detonation, or
don't affect the ignition event at all and enhance the expansion.

I use thermally reflective coatings in my motorcycle racing engine, but
didn't have a good baseline prior to applying them and don't have hard
numbers on the improvement.  Vizard feels that they are worth a 5-10% power
improvement.  The coatings are relatively cheap(about $15 per surface) when
compared to what must be spent for similar improvements.  They are more of a
messy pain in the ass & scheduling hassle than an expense really.  I can't
think of another modification (other than electronic engine management) that
improves both power and economy in equal measure.

I'm also from the school of thought that the amount of ignition advance
required for max power is a direct reflection on the efficiency of the
combustion chamber.  34-36 degrees is a pretty typical number for an
amazingly wide range of applications.  Max power at 32 degrees would
indicate to me that trying to improve the combustion efficiency would be a
waste of time.  40 degrees would suggest the chamber is pretty pathetic.
Mismatches between dynamic compression ratios and fuel octane will skew
this, of course.  If the combustion chamber is working well, and the fuel is
matched to the application, power will fall off several degrees before
detonation.  These numbers are based on working with naturally aspirated
motors with power peaks from 5000 to 12000 rpm and specific outputs from
1-2.5 hp/cid.

These are just my generalizations based on a narrow range of experiences
from a huge range of possibilities, so don't take them too seriously. Be
carefull, detonation will kill a motor far faster than anything you can do
with the fuel curve.

SBrady