The Four Strokes
In the first stroke, the intake stroke, the piston begins near the top of the cylinder. The exhaust valve is closed, and the intake valve is open. The piston moves downward and, like a pump, creates a vacuum that draws air through the intake valve into the combustion chamber. Once the piston reaches the bottom of the cylinder, the intake valve closes, trapping the air inside the cylinder. On the compression stroke, both valves remain closed as the piston moves upward, compressing air inside the cylinder to about 300 to 500 pounds per square inch (psi), depending on the design of the engine. According to the laws of physics, when air is compressed, its temperature increases. In the case of diesel engines, the air must get extremely hot because diesel fuel won't ignite at temperatures much below 750 F. As the piston gets close to the top of the compression stroke, it approaches what's known as "top dead center." At that point, the fuel injector sprays into the cylinder a carefully calibrated amount of fuel that begins to burn, causing the fuel/air mixture to expand rapidly. Both valves remain closed, so the expanding gases force the piston to the bottom of the cylinder, which causes the crankshaft to rotate. This is the power stroke. When the piston reaches the bottom of the power stroke, the rolling inertia of the flywheel carries the crankshaft another half turn back upward in the cylinder for the exhaust stroke. On the exhaust stroke, the exhaust valve opens, and the piston again acts like a pump, pushing gases out of the cylinder through the exhaust valve.