Otto cycle combustion heat question

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1. Dec 16, 2016

Saharka

I have been trying to determine the efficiency of an ideal Otto cycle based on the compression ratio of my car and the heat of combustion of gasoline and I think I'm not entierly wrong but there is something quite off. When trying to calculate the temperature of the combustion stage my result gives me the amazingly high temperature of 51,621kelvin which is obviously silly.
Here is the data I used:

I used the heat of combustion of gasoline, in this case 42,500 Kj as Qin.
662.385kelvin as the temperature of the previous stage of the cycle.
0.834 Kj/KgK as the sepecific heat of air.

So according to me the temperature should be given by the following equation:

Qin=CvΔT

So I need one of the temperatures so it becomes:

T3=(Qin/Cv)+T2

Which translates to:

T3=(42,500/0.834)+662.385

And finally:

T3= 51,161kelvin

I suspect that I'm using the wrong number for the specific heat of air but I'm not entierly sure, any ideas?

2. Dec 17, 2016

mfig

You have ok numbers for air. The problem is that you only took into account part of the energy balance. You need to consider the enthalpy of formation for fuel and products, not just the fuel. Remember, you don't get those product bonds without an energy exchange.

3. Dec 17, 2016

Saharka

Thanks for the reply, but I'm still confused, I thought the heat of combustion already considered the enthalpy of formation of the products or am I missing the point?

4. Dec 17, 2016

256bits

Right.
But you are using 1 kg of gasoline, by combustion, to raise the temperature of 1 kg of air.
Would that be correct?
https://en.wikipedia.org/wiki/Air–fuel_ratio

5. Dec 17, 2016

Saharka

Ohhh right, should have been a duh, thanks.

So no I think the air fuel ratio is 14.7 I'll adjust for this thanks again.