What is a stroke? It’s the movement of the piston from TDC (top dead center) to BDC (bottom dead center), OR BDC to TDC. A 4-stroke engine has the following "strokes", intake, compression, power, and exhaust.
A 4-stroke gasoline engine uses "internal" combustion, meaning that the heat that causes the air in the cylinder to expand is generated "internally". (A steam engine is actually an "external combustion engine" as its heat source is outside the cylinder!)Those 4 strokes must include the 5 key events common to all internal combustion engines - Intake, Compression, IGNITION, Power & Exhaust. Let’s look at a simplified model. Note that the valves are ONLY open during their respective strokes, IE: intake valve open ONLY during the intake stroke, exhaust valve only during the exhaust stroke. Both are CLOSED during compression and power!
The intake stroke starts with the exhaust valve closed, the inlet valve opening, and the piston at its highest point, top dead center.
It starts to move down, increasing the volume above the top of the piston. This makes pressure inside the cylinder lower than the pressure outside. This higher outside air pressure forces the air-fuel mixture into the cylinder. The piston reaches bottom dead center, the inlet valve closes, and the intake stroke ends.
Both intake and exhaust valves stay closed as the piston leaves bottom dead center. The piston moves up, squeezing the air-fuel mixture into a smaller and smaller volume, which compresses it. That causes the air/fuel charges temperature to rise, and that makes ignition easier and combustion (burning of fuel) more complete.
Just before the piston reaches top dead center, the next key event occurs - Ignition. The air expanding in the cylinder pushes the piston down the cylinder. This is the Power stroke that drives the engine.
The piston now moves from bottom dead center to top dead center. The exhaust valve opens, and the piston pushes out the leftover gases.
Let’s look at a complete 4-stroke cycle.
Source: CDX Global & Wikipedia
