Intake Stroke: Preparing the Air-Fuel Mixture
During the intake stroke, the piston moves downward, creating a vacuum in the cylinder, and air is drawn into the cylinder through the intake valve. Diesel engines typically use either a "direct injection" or "swirl" combustion chamber design, with the former injecting diesel directly into the cylinder and the latter promoting mixing through intake swirl. Modern engines are also equipped with turbochargers, which use exhaust gases to drive a turbine that compresses the intake air, increasing air density and thereby enhancing fuel combustion efficiency.
Compression Stroke: Key Preparation for Energy Conversion
After the intake valve closes, the piston moves upward, compressing the air in the cylinder. Diesel engines usually have a compression ratio as high as 16:1 to 22:1 (compared to about 8:1 to 12:1 for gasoline engines), and the high pressure raises the air temperature above 500°C, far exceeding the diesel autoignition point (about 220°C). This stage places high demands on engine materials and manufacturing processes, requiring high-strength engine blocks, high-temperature-resistant piston rings, and precise cooling systems.
Power Stroke: Energy Release and Power Output
As the piston approaches the top dead center, the injector sprays diesel into the combustion chamber in a mist, which auto-ignites when mixed with the high-temperature air. The high-temperature, high-pressure gases generated by combustion push the piston downward, driving the crankshaft via the connecting rod, converting chemical energy into mechanical energy. Precise control of injection timing and quantity is required at this stage to avoid knocking or incomplete combustion. Modern engines use an electronic control unit (ECU) to monitor engine speed, load, and temperature in real time, dynamically adjusting injection parameters. For example, China 6 engines must meet particulate matter (PM) and nitrogen oxide (NOx) emission standards and are equipped with selective catalytic reduction (SCR) and diesel particulate filter (DPF) after-treatment systems to further optimize combustion.
Exhaust Stroke: Emission Discharge and Preparation for the Next Cycle
After the power stroke, the exhaust valve opens, and the piston moves upward to expel exhaust gases from the cylinder. The exhaust contains unburned hydrocarbons, carbon monoxide, and nitrogen oxides, which need to be purified through a three-way catalytic converter or SCR system. Modern engines also use exhaust gas recirculation (EGR) technology to redirect part of the exhaust back into the intake system, lowering combustion temperature and reducing NOx formation.
