A four-cylinder engine has four spark plugs. An important part of the power-generating process, each spark plug is responsible for igniting the fuel and air mixture inside of its cylinder.
At the beginning of a stroke, a piston moves downward in its cylinder, drawing in a precise mixture of gas and air. After reaching the bottom, the piston moves back up, heavily compressing the mixture. Once it reaches the top, the spark plug ignites the mixture, and the explosion forces the piston down into the cylinder. This mechanical movement is harnessed and used to power the vehicle. Some large engines may have two spark plugs per cylinder to ensure that complete combustion is achieved and maximum power is generated.