# SI engine combustion. CO2 dissociation?

Hey guys,
Had this over in the mechanical engineering section but I think I might have more luck with getting an answer over here.

The question is, what is happening to make the extra power when using a rich mixture in an SI engine?

An air fuel ratio (AF:R) of 14.7:1 is stoichiometric for petrol (gas), but an AF:R of 12.6:1 makes more power.

One common theory I have heard is that not all the fuel and oxygen react in a stoichiometric mix, so by upping the fuel there is more chance of all the oxygen reacting with the excess of fuel.

For some reason I have got the notion that when the CO2 in the cylinder dissociates due to the heat of combustion, the liberated oxygen is free to react with the excess fuel.
But I can't find any sources that state this, they all talk about equilibrium reactions and NOx production.

The information in this link gives some insight:
http://www1.gantep.edu.tr/~ozcan/kit...n/7699X_13.pdf [Broken]

The author [above] states:
"If there is no dissociation then the peak pressure is always reached at the stoichiometric ratio. However, when dissociation occurs the [air fuel ratio] at which the peak pressure occurs is moved to the rich region. This is because dissociation tends to increase the amount of substance in the products..."
The author doesn't state what he means by "substance in the products".
Anybody have any idea what he might be talking about?

Gordon Blair (design and simulation of four stroke engines) states on the subject of CO2 dissociation that:
"at the height of combustion......the gas composition would now be 4.3% CO, 9.36% CO2, and 2.1% O2."
With 2.1% free oxygen in the cylinder, surely some would react with the excess fuel present in a rich mixture.

I'm not making 2 + 2 = 5 am I?

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