I do not know if this is the correct place to post this subject, I apologize if it is not. This is something I am really confused about: What is really the gas that pushes the piston down in an car fuel engine ? Is that the flame front gas, or that still unburnt gas which is ahead that flame front which is compressed by the expanding burnt gas?. I understand, within the combustion chamber, right after the ignition, we have gas which spreads out as it gets burnt. This gas travels with a speed what we can call "low speed", as I heard it is around 40 cms/second?. But, on the other hand, we have gas pending of being burnt which is compressed, and it seems that "pressure wave" moves at sound speed, more or less. Ok, I rather shut up now, and leave any expert or experts to let me know more about this. Thanks a lot in advance.