During atomization the fuel still remains in liquid form, it's just broken up into small droplets consisting of multiple fuel molecules.

Once this mixture of fuel droplets, and air is inside the cylinder, and a spark occurs do the air molecules gain kinetic energy, then collide into the atomised fuel, and the individual fuel molecules break apart thus turning fuel from a liquid to a gas (vaporisation), then those fuel molecules combines with the air molecule, then combustion occurs? If this turns out to be correct, once the fuel molecule combines with the air molecule, how does combustion occur from there?Does the fuel molecule just rapidly release energy, thus the nearby air molecule start moving rapidly, and push on the piston head to produce power?

As stated above, I'm explaining how I see the process occurring, but I'm not exactly sure I'm correct. I'm just confused on how molecules actually perform during this process. I'm trying grasp a better understanding overall on how scenarios like cold starting, or running too rich or lean can affect combustion on a molecular level.

Any help is appreciated, thank you.

  • Best asked on chemistry then....
    – Solar Mike
    Oct 7, 2017 at 15:44
  • 1
    This really is more appropriate in the chemistry community, but just a really short answer, the spark causes the gas molecules to react with oxygen thus releasing the chemical energy stored in the gas molecule. That energy is released as heat.
    – BillDOe
    Oct 7, 2017 at 18:29
  • ditto chemistry.stackexchange.com
    – agentp
    Oct 7, 2017 at 21:06

1 Answer 1


Under high pressure and temperatures O2 ionizes to a high energy state that seeks out the fuel to fill its electron ring. Under very high temperature and pressure it becomes a free radical that reacts violently, i.e. knock.

  • So will the oxygen ionize at the end of the compression stroke before ignition or only once the pressure starts to rise with the combustion flame front?
    – Solar Mike
    Oct 7, 2017 at 21:06
  • The study I read stated that the oxygen readily ionizes to an ignition level. At standard conditions the oxygen ionized in a match flame to ignite gasoline fumes. The spark kernel and flame front in an engine exhibits localized ionization that travels with the flame front.
    – TomO
    Oct 7, 2017 at 21:22

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