How does combustion of fuel inside engine cylinder occurs?

The cylinder's intake valve opens to let in a pre-mixed dose of atomized fuel and air. The cylinder moves upward, squeezing this mixture from it's original size to about 1/8th of it's original size. At the top of the stroke, a spark plug is timed to fire, the spark of which ignites the fuel/air mixture causing it to burn rapidly (it does not explode, it burns), and produces a huge volume of gasses that forcibly push the piston back down - this energy is transmitted through gears to the wheels of the car.