# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: scientizscht on 12/06/2018 17:29:45

Title: What forces are generated per volume of petrol or diesel combusted?
Post by: scientizscht on 12/06/2018 17:29:45
Hello!

I would like to know what forces are generated per ml of petrol/diesel combusted.

Any idea?

Thanks!
Title: Re: What forces are generated per volume of petrol or diesel combusted?
Post by: chiralSPO on 12/06/2018 18:47:23
The force generated will depend more on the conditions of the combustion than what is combusting.

However, it is fairly easy to predict the energy released from combustion (heat of combustion). See here: https://en.wikipedia.org/wiki/Heat_of_combustion

BTW: please format the title of each new thread as a QUESTION!
Title: Re: What forces are generated per volume of petrol or diesel combusted?
Post by: scientizscht on 12/06/2018 20:19:50
The force generated will depend more on the conditions of the combustion than what is combusting.

However, it is fairly easy to predict the energy released from combustion (heat of combustion). See here: https://en.wikipedia.org/wiki/Heat_of_combustion

BTW: please format the title of each new thread as a QUESTION!

Thanks, but this doesn't help much. I mean, should I take the energy released and divide it let's say by 50cm and it will give me the average force on the piston for that movement? Is that accurate?

I am mainly interested in average piston forces per gram of combusted fuel in stoichiometric conditions.
Title: Re: What forces are generated per volume of petrol or diesel combusted?
Post by: chiralSPO on 12/06/2018 22:01:21
As I mentioned, it is fairly easy to find the energy released, but as you noticed, that is not very useful.

The problem is that only a fraction of the energy can actually go towards useful work, and the rest goes to waste heat. Determining how much of the energy can do work depends on the temperature and pressure inside the engine, and the temperature and pressure outside, as well as the shape and size of the piston, how quickly the pressure changes, and several other parameters that are neither simple to model nor simple to measure.

https://en.wikipedia.org/wiki/Carnot_cycle
https://en.wikipedia.org/wiki/Carnot%27s_theorem_(thermodynamics)
https://en.wikipedia.org/wiki/Work_(thermodynamics)
https://en.wikipedia.org/wiki/Engine_efficiency
Title: Re: What forces are generated per volume of petrol or diesel combusted?
Post by: evan_au on 12/06/2018 22:44:27
A good place to start is with the chemical equation. Petrol can be approximated by octane (C8H18) and Diesel fuel approximated by decane (C10H22).

Then you write the equation:
2C8H18 + 25O2 = 16CO2 + 18H2O

Bear in mind that air is about 80% nitrogen, which doesn't contribute any energy, but must be heated up in the reaction (and does produce some pollutants):
100N2 + 2C8H18 + 25O2 = 100N2 + 16CO2 + 18H2O

For efficient burning, you must ensure the fuel is a well-mixed vapor. But you don't want to produce too much pollution, and you don't want an explosion that will wreck the engine.

There are a lot of tradeoffs!

Edit: It's been quite a while since I had to balance chemical equations! I'm still not sure I got it exactly right?
Title: Re: What forces are generated per volume of petrol or diesel combusted?
Post by: chris on 12/06/2018 22:54:02
This is quite an interesting interview with Mani Sarathy, from KAUST, about fuels, fuel development, and fuels of the future (https://www.thenakedscientists.com/articles/interviews/reinventing-diesel).
Title: Re: What forces are generated per volume of petrol or diesel combusted?
Post by: Bored chemist on 12/06/2018 23:02:23
2C8H18 + 25O2 = 16CO2 + 18H2
Cough!