Omega thermocouples

[Evert Rosseel]

>  Why does power decrease if you go too rich?  Too lean makes simple sense 
> but rich?

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
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*    Dr. ir. Evert Rosseel                                             *
*    Laboratory for Machines                                   *
*    Department of Mechanical and Thermal Engineering          *
*    University Gent                                           *
*    Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium            *
*    Tel : ++32 9 264.33.06                                    *
*    Fax : ++32 9 264.35.86                                    *
*    Email : Evert.Rosseel at rug.ac.be                           *
*    WWW : http://allserv.rug.ac.be/~erosseel/motoren.htm      *
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