An engine is a machine that converts chemical energy into mechanical energy; this conversion process is essentially a working cycle. Simply put, fuel is burned inside the cylinder to generate kinetic energy, driving the piston to move back and forth. This movement drives the connecting rod and the attached crank to rotate around the crankshaft's center, thereby outputting power.
The operation of a four-stroke gasoline engine is a complex process consisting of four strokes: intake, compression, combustion/expansion, and exhaust.
Intake Stroke
During this stroke, the piston is driven by the crankshaft to move from Top Dead Center (TDC) toward Bottom Dead Center (BDC); simultaneously, the intake valve opens while the exhaust valve remains closed. As the piston moves downward, the volume above it increases, causing the pressure inside the cylinder to drop and creating a partial vacuum. Because the intake valve is open, the cylinder connects to the intake manifold, allowing the air-fuel mixture to be drawn into the cylinder. By the time the piston reaches BDC, the cylinder is filled with a mixture of fresh air-fuel and residual exhaust gas left over from the previous cycle.
Compression Stroke
The piston moves from BDC to TDC with both intake and exhaust valves closed. Driven by the inertia of the flywheel, the crankshaft rotates and pushes the piston upward via the connecting rod. The volume of gas within the cylinder decreases, compressing the gas and causing both the pressure and temperature of the mixture to rise.
Power Stroke
At this stage, both valves remain closed, and the spark plug ignites the mixture. The mixture burns violently, causing a rapid rise in temperature and pressure within the cylinder. The high-temperature, high-pressure gases force the piston downward, driving the crankshaft to rotate via the connecting rod. Of the engine's four strokes, this is the only one where thermal energy is converted into mechanical energy; hence, it is known as the power stroke.
Exhaust Stroke
During this stroke, the exhaust valve opens, and the piston moves from BDC to TDC, expelling the exhaust gases from the cylinder as it rises. Due to resistance in the exhaust system and the inherent volume of the combustion chamber, it is impossible to completely expel all exhaust gases by the end of the exhaust stroke; the gas remaining in the cylinder is known as residual exhaust gas. This residual gas not only hinders the intake of a fresh charge but also adversely affects the combustion process.
At the end of the exhaust stroke, the piston returns to Top Dead Center (TDC), completing one operating cycle. Driven by the inertia of the flywheel, the crankshaft continues to rotate, initiating the next cycle. This continuous repetition keeps the engine running.
