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I asked this because if I have a small portion of fossil fuel and light it, the fuel will only burn. But inside a combustion chamber of a vehicle this same combustion will propel the piston downward keeping the vehicle moving; why?

  • LostPecti: please edit if my changes do not capture your intent. – Fred Wilson Apr 20 '16 at 3:01
  • No you captured my question perfectly. Thanks for your help. – LostPecti Apr 20 '16 at 3:11
  • The main thing is that you're turning the fuel into a fine mist. If you light a puddle, it will burn slowly, if you atomize it and light the cloud, it will explode. Pretty much anything is dangerous when its small enough & in high enough quantities mixed with the air. Just google about flour mill explosions, even flour is explosive when mixed well with air making flour mills super dangerous. We compress it so there is even more oxygen around the substance we want to burn. Add these together and you get quite the explosive force. – Trotski94 Apr 20 '16 at 11:57
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During combustion, the pressure in the combustion chamber is increased, and this pressure pushes down the piston. There are two reasons for this:

Increase of the amount of gas molecules

Let's say we use hexane as fuel. To burn one hexane molecule consisting of 6 carbon and 14 hydrogen atoms, we need 13 oxygen atoms (6.5 oxygen molecules) and get 7 water and 6 carbondioxide molecules:

 1* Hexan           + 6.5* oxygen -> 7* water  + 6* carbondioxide

     H H H H H H
     | | | | | |
1* H-C-C-C-C-C-C-H  + 6.5* O-O    ->  7* H-O-H + 6* O-C-O
     | | | | | |
     H H H H H H

Because air consists of just 20% oxygen and 80% nitrogen, there are four nitrogen molecules for each oxygen molecule in the chamber. They do should not react during combustion, so you simply add 26 molecules nitrogen on both sides.

So, before combustion, there are 1+6.5+26=33.5 molecules and after, there are 7+6+26=39 molecules.

One interesting fact about (ideal) gases is that a certain volume at certain temperature and pressure always contains the same amount of molecules, regardless which kind or mixture of molecules.

Let's say we still have the same volume in the combustion chamber and neglect the increase in temperature, the increase of number of molecules by a factor of 39/33.5=1.16 results in an increase of pressure by a factor of 1.16, too.

Heat expansion

If you increase the temperature of a gas, it would expand. If it can't because it's enclosed in the combustion chamber, the pressure rises instead. For example any (constant) volume of ideal gas at room temperature (20°C) increases its pressure by a factor fo 4.3 when heated to 1000°C.

All together

During combustion, the pressure increases by a factor of 1.16 as the number of molecules rises, and another factor of 4.3 due to the temperature, leading to a total factor of 5 in pressure increase. Let's say the combustion chamber has a piston of 8cm diameter (typical bore), which corresponds to a surface of 50cm². A pressure of 5000hPa (difference to environmental pressure of 1013 hPa) will apply a force of 2500N (or 560lbf) onto the piston and push it down.

What I didn't say here is that a real motor first compresses the air/fuel mixture by a factor of about 14, which increases temperature and pressure in the cylinder. (It invests energy here, but it gets it back after combustion) Also, I don't know which temperature is reached during combustion.

Also, this is a very basic calculation neglecting some effects, but I think it clearly shows how the force on the piston is created.

Oh, and if you ignite a little fuel in an enclosed bin, you will also notice an increase in pressure. But since the process is quite slow, most of the heat leaves the bin, it doesn't become that hot, and the pressure isn't that high. (But be careful: Fuel vapors can explode, and then you have the high pressure...)

  • Great answer. I can add a few comments here. Exhaust manifold temps can reach 1200-1600 F. The ratio of pressures before and after combustion is usually modeled by automotive engineers as "explosion ratio". In this way they don't have to worry too much about the chemistry behind the scenes – Zaid Apr 20 '16 at 9:58
  • First off thanks you for your detailed answer. Let me see if I understand. The idea of a fuel is to make enough pressure to push the piston down. It does this using two methods. The first is using molecules, the more molecules added the more pressure in the cylinder. The other method is using heat, since we know gas have atoms and can expand better then solid atoms or liquid atoms. When the atoms in the gas are heated they expand causing even more pressure. So in all combustion chambers rely on high amount of pressure. I have a question why do air/few have to be compressed first? – LostPecti Apr 20 '16 at 23:40
  • @LostPecti I think your question is great. Your other question in your comment, "Why does air/fuel need to be compressed?" is another question. You should ask it. :-) Cheers! – DucatiKiller Apr 21 '16 at 5:03
  • @LostPecti: Well, atoms/molecules don't expand, they are more like small boxers keeping their neighbors at distance by punching them. Heat is their dope (->more strength, more distance, more space needed by all boxers) and of course more boxers want more space. The piston also takes lots of punches, which force it down. (Hmm, should have put that into the answer...). But you are right, the idea of a piston engine using fuel is to generate pressure inside the cylinder to push the piston down. Steam engines generate pressure outside and lead it into the cylinder. – sweber Apr 21 '16 at 6:37
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AFAIK the way this works when you burn the fuel, you have it in liquid state at atmospheric pressure. In actuality you're not burning the liquid, you're burning the vapors that form over the liquid.

When in the engine, fuel is disbursed into droplets which creates a lot of surface area for the fuel to expel vapor. What the ECU does is create a very specific mixture of this fuel, vapor, and air that is needed to create the high energy burn that makes the engine turn.

Without that very specific ratio you don't get the more energetic burn. The chemistry behind why that is I don't know.

I hope that helps.

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