The fuel is pumped into the engine as an atomized mist. The perfect ratio is 1 part of fuel to 14.7 parts air.

So that air fuel mixture (the fuel is atomized, so it’s even distributed) then gets compressed by the engine to roundabout 1/10th of it’s original volume. For example, if each cylinder in the engine is 0.5 Liters, then that 0.5 L gets compressed 10x to fit into 0.05 L.

That’s when the spark plug creates an electric arc, igniting the compressed air/fuel mixture. Now, this is where all the chemistry comes to bear. The oxygen in the 14.7 parts of air is what actually burns. But the 1 part of fuel mixed evenly throughout the air is what spreads the flame quickly and evenly throughout the air. It works like an accelerant, kind of like lighter fluid on a barbecue.

The spark lights the fuel on fire, which then burns all the air… this creates a violent explosion and rapid expansion of hot gases that force the engine to spin and create usable power.