During Power Range, Engine Requires Richer Mixture, for -
1. Providing the best/more power
2. Preventing the overheating of engine components, as ENRICHING THE MIXTURE REDUCES THE FLAME TEMPERATURE AND CYLINDER TEMPERATURE.
So, my question is how this flame temperature gets reduced by using a rich mixture? Isn't the fact true that we are having more fuel particles in the richer mixture, means more heat will be released and so the flame temperature will be more for the richer mixture?
Basically, the optimal anything, not just combustion, is a maximum or minimum. The whatever (flame temperature) falls off on either side of the optimum. In the case here, some of the heat of combustion must also warm up the molecules not participating in the reaction. Just like the combustion temperature in pure oxygen is much higher than air because some of the heat of combustion is spent warming up the nitrogen, argon, and water vapor.
A rich mixture has excess fuel. The quantity of fuel which undergoes combustion is limited by the amount of air available. Assuming that complete combustion occurs at stoichiometric conditions. by adding more fuel, two effects occur - i) Some fuel can only be partly oxidised - this forms products like CO and the corresponding energy release is lower and ii) some fuel remains unburned, and absorbs the energy released during combustion. The net effect is that the product temperature is lowered.
Saleel is right. The energy of complete combustion products (CO2, SO2) is always lower than incomplete ones (CO, SO) so that less heat is released during the reaction.
Dear Suraj R. Karanjikar ,
The maximum heat release for any fuel oxidizer combination takes near the stoichiometry. Other then that on both the sides (lean or rich) heat release reduces. On rich side the temperature due incomplete combustion of fuel and on lean side it reduces due to surplus oxidizer absorbing the available heat due to combustion.
Dear Suraj K.,
A rich mixture have a higher combustion rate. So, it leads to lower heat loss to the combustion chamber and cylinder walls. And so, they have lower temperature than with lean mixture combustion.
The flame temperature can be reduced though rich mixture zone because of the dissociation of combustion products.
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