How Four-Stroke Combustion Engines Work

The internal combustion engine has been the primary power source for automobiles for over a century. Specifically, the four-stroke engine remains the standard design for most passenger vehicles on the road today. It works by converting the chemical energy stored in gasoline into the mechanical energy required to turn the vehicle's wheels. This conversion is achieved through a precise, cyclical process consisting of four distinct stages: intake, compression, power, and exhaust.

The cycle begins with the intake stroke. The engine's piston, housed within a precision-machined metal cylinder, moves in a downward direction. At the same time, an intake valve at the top of the cylinder opens. The downward movement of the piston creates a vacuum, pulling a carefully metered mixture of atomized fuel and air into the combustion chamber. Once the piston reaches the bottom of its travel, the intake valve closes, perfectly sealing the cylinder.

Next comes the compression stroke. With both the intake and exhaust valves firmly closed, the piston travels back up the cylinder. This upward movement compresses the trapped air-fuel mixture into a very small volume at the top of the combustion chamber. Compressing the mixture significantly raises its temperature and pressure, highly increasing its volatile potential energy and preparing it for an explosive chemical reaction.

The third stage is the power stroke, which generates the actual force that drives the vehicle forward. Just before the piston reaches the top of its compression stroke, an electrical spark plug ignites the highly pressurized air-fuel mixture. The resulting controlled explosion generates rapidly expanding, superheated gases. These expanding gases force the piston back down the cylinder with tremendous force. This linear motion is transferred via a connecting rod to the crankshaft, which translates it into rotational motion.

Finally, the exhaust stroke clears the cylinder for the next cycle. The exhaust valve opens, and the piston moves upward once more. This upward motion forces the spent, useless combustion gases out of the cylinder and into the vehicle's exhaust pipe. Once the piston reaches the top, the exhaust valve closes, the intake valve opens, and the entire four-stroke cycle begins again.