Omega thermocouples

[Mike Turner]

Evert.Rosseel @ rug.ac.be> @ INTERNET WROTE:
>
>Because the combustion of the carbon in the fuel to CO gives less 
>than half the energy that combustion to CO2 gives (10.2 MJ/kg and 
>33.6 MJ/kg repectively). This means that - for a given mass of 
>oxygen (or air) in the cylinder - combustion to CO involves an energy 
>loss.
>Slightly rich doesn't reduce power because the combustion proceeds 
>faster.
>
>> Why does a lean mixture burn more slowly?  There's plenty of O2 around.
>> If it does burn more slowly why is a lean mixture more prone to 
>> detonation?  I would think that a slow burn would make it more resistant 
>> to detonation.
>
>It burns slower because the temperature in the reaction zone is lower 
>(extra air is heated up "for no real purpose"). 
>(Flame speed is always maximum close to stoichiometric).
>
>Because the combustion is slower, the end gas has more time to 
>autoignite (before it gets burned). (Although for lean mixtures the 
>ignition delay is longer, the effect of flame speed is more 
>important).
>
>> Why does retarded timing cool off the combustion temp.?  Does it have to 
>> do with when the peak presure occurs thus peak temp. (i.e. PV=nRT)
>
>Because the combustion occurs later, when the piston has already 
>descended somewhat. The equivalent expansion results in a lower 
>temperature DURING COMBUSTION (and therefore also less NOX and lower 
>knock sensitivity).
>
>AFTER COMBUSTION the temperatures will be lower. Because the 
>combustion phasing is "too" late, the efficiency of the engine goes 
>down. Because all energy that is not converted to work shows up in 
>the exhaust gas (and heat exchange with the engine 
>parts), higher exhaust gas temperatures result. Also, some of the 
>engine parts may get too hot : especially the exhaust valves.
> 
>> Why does an overheating engine require MORE advance?  I would think that 
>> cooling things off with a retarded timing would be the prudent thing to do.
>
>A retarded timing will always increase the temperature of the exhaust 
>gas, and therefore increases thermal load.
>Increasing the advance will - in my opinion - only give a real benefit if 
>the advance was considerably too small.
>
>One of the classic remedies to reduce thermal load is too run richer :
>because the extra mass also must be heated up, lower temperatures 
>result. (Of course efficiency suffers greatly).
> 
>Evert

I wish I had been smart enough to ask those questions. I've been wondering for 
a long time why the
things observed did what they did. Thanks.