Heat of combustión of a mixture of two substances?

[Saharka]

Hello chemists, need some help here.

I'm trying to calculate the power output of a certain otto cycle using different fuels, specifically gasoline, ethanol and different combinations of the two like 85% ethanol and 15% gasoline or 50% ethanol and 50% gasoline. However I have found it difficult to find a reliable source for the heat of combustion values of those mixed substances so I wondered if I just calculated the average heat of combustion given the proportions of the substances used or if there is some weird chemical stuff going on that I'm not aware of, this is out of my area of expertise so I figured I should ask somebody that knows.

The short version is: can I just assume that the heat of combustion value of a mixture of substances to be the average value of the heats of combustion of the substances that make the mixture given the proportions or not?

For example:

If gasoline has a heat of combustion of 47,000 Kj/Kg and ethanol 28,500 Kj/Kg would it be correct to assume that the heat of combustion of a mixture of 50% ethanol and 50% gasoline has a heat of combustion of: (47,000+28,500)/2 = 37,750 KJ/Kg is that correct?

 

[oz93666]

For example:

If gasoline has a heat of combustion of 47,000 Kj/Kg and ethanol 28,500 Kj/Kg would it be correct to assume that the heat of combustion of a mixture of 50% ethanol and 50% gasoline has a heat of combustion of: (47,000+28,500)/2 = 37,750 KJ/Kg is that correct?

Yes... I'm sure that's OK ... but monitor exhaust products to make sure the degree of complete combustion is not affected by mixing ...

But also this is heat of combustion don't assume that the % of work output from the engine is constant for all fuel mixes ... for example if a particular fuel makes the engine run hotter , it will be more efficient ie a greater % of the heat of combustion is turned into useful power and less wasted in heat.

 

[Ygggdrasil]

If gasoline has a heat of combustion of 47,000 Kj/Kg and ethanol 28,500 Kj/Kg would it be correct to assume that the heat of combustion of a mixture of 50% ethanol and 50% gasoline has a heat of combustion of: (47,000+28,500)/2 = 37,750 KJ/Kg is that correct?

Depends what the 50% means. Are you mixing equal amounts of ethanol and gasoline by volume (v/v %) or equal amounts by mass (w/w %)?

 

[Saharka]

Yes... I'm sure that's OK ... but monitor exhaust products to make sure the degree of complete combustion is not affected by mixing ...

But also this is heat of combustion don't assume that the % of work output from the engine is constant for all fuel mixes ... for example if a particular fuel makes the engine run hotter , it will be more efficient ie a greater % of the heat of combustion is turned into useful power and less wasted in heat.

I'm doing it all theoretical for now so I only have to adjust the air to fuel ratio when changing fuels so I think that's ok.
I'm also using the fuel

Depends what the 50% means. Are you mixing equal amounts of ethanol and gasoline by volume (v/v %) or equal amounts by mass (w/w %)?

Mass

 

[Saharka]

Yes... I'm sure that's OK ... but monitor exhaust products to make sure the degree of complete combustion is not affected by mixing ...

But also this is heat of combustion don't assume that the % of work output from the engine is constant for all fuel mixes ... for example if a particular fuel makes the engine run hotter , it will be more efficient ie a greater % of the heat of combustion is turned into useful power and less wasted in heat.

So I have to do a stoichiometric analysis for each mix?

 

[Ygggdrasil]

Mass

Ok, then your approach is correct. 1 kg of fuel would contain 0.5 kg of ethanol and 0.5 kg of gasoline, so the heat of combustion would just be the heat generated by the combustion of 0.5 kg of ethanol plus the heat generated by the combustion of 0.5 kg of gasoline.

 

[Saharka]

Great, thanks :D

 

[oz93666]

So I have to do a stoichiometric analysis for each mix?

Well ... if any C particles or CO emitted in exhaust then combustion is not complete , many garages have equipment for measuring emissions, I don't know if this is significant and worth bothering about ... temperature of engine could be measured by thermocouple for comparison .
This is a practical problem , not something you can do on a piece of paper ...