The internal combustion engine comes down to,
at its most basic, a cylinder and a piston. To keep these two portions of the engine working, or to support their working, is the whole purpose of every other part of the internal combustion engine: fans cool the parts to keep them moving; the camshafts tell the valves when to open during the cycling of the pistons in the cylinders; the spark plugs provide a little needed oomph’ at just the right point in the piston’s cycle in the cylinder.
The four-stroke cycle engine was first patented in 1862 by a Frenchman, Alphonse Beau de Rochas. It wasn’t until 1867 that a practical working model was built by the German, Nikolaus August Otto. There have been many incremental changes to the internal combustion engine since then, and the similar diesel engine has also been developed, but these are all simply small improvements on the four-stroke engine that Otto first built and manufactured.
The term four-stroke cycle refers to four strokes of the piston in the cylinder. After completing the four strokes of the cycle, the piston returns to the initial condition and repeats the process indefinitely. Important to the cycle is also the operation of three pieces of equipment that are part of the cylinder head: The intake valve, the exhaust valve, and the spark plug.
At the beginning of the cycle, the head of the piston is at the top of its motion in the cylinder. As the piston begins drawing down the length of the cylinder the intake valve opens, allowing the motion of the cylinder to draw a fuel-air mixture into the cylinder. This stroke is called the intake stroke, for obvious reasons. At the bottom of the piston’s movement, the intake valve shuts, sealing the cylinder. End of the first stroke.
The second stroke is the compression stroke. Now the piston travels back up the cylinder compressing the fuel-air mixture in the cylinder several times over. At the top of the piston’s stroke, when the fuel-air mixture is at its most compressed, and just before the piston begins moving out of the cylinder again, the spark plug sparks, igniting the fuel-air mixture.
The third stroke begins with the explosion in the head of the cylinder, pushing down on the piston, delivering the power of the explosion of the mixed fuel and air to the piston, which transfers that motion to the drive shaft of the motor, where the power is harnessed to be delivered to the wheels of the car through the transmission. The third stroke is called, for obvious reasons, the power stroke.
The fourth stroke begins with the cylinder full of combustion products, and the piston at its maximum extension down the cylinder. As the piston begins moving back within the cylinder the exhaust valve opens, allowing the piston to push the spent combustion products out of the cylinder. At the end of the exhaust stroke, the exhaust valve shuts, and the piston is back at the top of the cylinder, just where it began this cycle, ready to begin again.
This full cycle is still sometimes called the Otto cycle in honor of Nikolaus August Otto, and it still runs most automobiles today.