It spreads out more fuel molecules to combine with oxygen, so the oxygen doesn't have to 'search' for fuel, there is no 'waiting' for it to find partners because there are so many extra fuel molecules hanging around. What we really want is to nurse the piston through TDC, then wait until the rod approaches useful angles for torque, and THEN react as many air and fuel molecules as possible through that window.
This is why there is such a large timing window for modern engines (where it seems like no additional power is gained with +/- large increments as long as it falls within some range). So if your mixture contains the exact amount of fuel necessary from one end to the other, you will lose a lot of torque because there was no extra fuel component to react during the critical rod angles where torque is produced through as maximum mixture reaction is desired.Īs the piston descends the volume of the chamber is going up so the design of the chamber and piston surface is often controlled to facilitate the reacting of fuel and air through the critical rod angles more-so than the beginning. Reaction at the beginning and end is all wasted fuel. In fact as the reaction builds and reaches TDC there is 0 net torque(forcexlength = 0 at TDC) even though a very high pressure may be produced in the chamber, which is using up fuel molecules and oxygen. When the reaction starts you are pre-TDC and some of the fuel is being burnt before it has a chance to produce torque Now imagine if you put the exact amount of air and fuel necessary to complete the reaction. Look at the graph of pressure inside the cylinder
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