Heat of combustión of a mixture of two substances?

Click For Summary

Discussion Overview

The discussion revolves around calculating the heat of combustion for mixtures of gasoline and ethanol in the context of an Otto cycle engine. Participants explore whether the heat of combustion for a mixture can be determined by averaging the values of the individual components based on their proportions, while also considering factors that may affect combustion efficiency.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions if the heat of combustion of a mixture can be calculated as the average of the individual heats of combustion based on their proportions.
  • Another participant suggests that while averaging may be acceptable, one should monitor exhaust products to ensure complete combustion is not affected by the mixture.
  • There is a clarification about whether the 50% mixture refers to volume or mass, with a later response confirming that the mass approach is being used.
  • Concerns are raised about the efficiency of the engine and how different fuel mixtures may affect the percentage of heat converted to useful work.
  • A participant mentions the need for stoichiometric analysis for each fuel mix to ensure complete combustion, while also noting practical considerations such as measuring emissions and engine temperature.

Areas of Agreement / Disagreement

Participants express varying views on the appropriateness of averaging heat of combustion values, with some agreeing that it may be acceptable under certain conditions, while others emphasize the importance of monitoring combustion efficiency and emissions. The discussion remains unresolved regarding the best approach to calculating the heat of combustion for fuel mixtures.

Contextual Notes

Participants highlight the dependence on definitions of mixing proportions (mass vs. volume) and the implications of combustion efficiency on the calculations. There is also mention of practical challenges in measuring combustion outcomes, indicating that theoretical calculations may not fully capture real-world performance.

Saharka
Messages
6
Reaction score
0
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?
 
Chemistry news on Phys.org
Saharka said:
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.
 
Saharka said:
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 %)?
 
oz93666 said:
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
Ygggdrasil said:
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
 
oz93666 said:
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?
 
Saharka said:
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.
 
Great, thanks :D
 
Saharka said:
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 ...
 

Similar threads

Can the heat of combustion only be found experimentally?
  • · Replies 12 ·
Replies
12
Views
4K
Calculating time to reduce alcohol in wine using heating method
  • · Replies 131 ·
5
Replies
131
Views
10K
Otto cycle combustion heat question
  • · Replies 4 ·
Replies
4
Views
3K
Change in Free Energy when Heating a Substance
  • · Replies 9 ·
Replies
9
Views
3K
Calculate Heat of Combustion of C2H2
  • · Replies 1 ·
Replies
1
Views
4K
Specific heat capacity of a mixture
  • · Replies 1 ·
Replies
1
Views
5K
Thermodynamics.... Constant volume pressure of combustion
  • · Replies 33 ·
2
Replies
33
Views
9K
Heat transfer and combustion problem
  • · Replies 7 ·
Replies
7
Views
4K
Enthelpy Change of Combustion.
  • · Replies 1 ·
Replies
1
Views
5K
Determining Heat of Combustion for a Fuel and Oxygen Mixture
  • · Replies 5 ·
Replies
5
Views
5K