As the core power source of modern automobile, four-stroke gasoline engine converts the chemical energy of gasoline into the mechanical energy that propels the automobile forward through precise mechanical movement and thermodynamic transformation. It works on four consecutive strokes: intake, compression, power and exhaust. Each stroke corresponds to a specific piston movement trajectory and valve opening andclosing state to complete a complete working cycle. The following is an in-depth analysis of the physical process, energy conversion mechanisms and key technical parameters of the four-stroke.
I. Inlet stroke: preparation of inlet and airfuel mixture
Inlet stroke is the starting point of engine operation. When the piston moves from the top to the bottom of the cylinder, the intake valve opens and the exhaust valve closes. As the piston descends, the volume of the cylinder increases, causing the internal pressure to drop below atmospheric pressure, forming a negative pressure zone. At this time, the ideal gas to oil ratio of 14.7: 1 is pushed into the cylinder through the intake manifold and valve through atmospheric pressure. In the example of the a 1.5L naturally aspirated engine, the piston has a downforce of 8-10m/s and the cylinder has an instantaneous vacuum of -80 kPa, ensuring sufficient petrol-electric hybrid power.
Key technical parameters of this stroke include the opening time of intake valve (usually 10-30° crankshaft angle earlier than top dead point) and closing time (40-60° crankshaft angle later than bottom dead point), as well as the design of the intake manifold length and diameter. Modern engine adopts variable valve timing technology to adjust the opening and closing time of inlet valve dynamically in order to optimize the efficiency of inlet valve at different engine speeds. Honda's i-VTEC system, for example, can improve charging efficiency by extending the opening time of the intake valve at engine speed.
ii. Compression Stroke: When Increased Energy Density and creating Combustion Conditions to compress stroke, both intake and exhaust valves are closed, the piston moves from bottom to top dead spot, the cylinder volume decreases, and the gasoline mixture is compressed. During this process, mechanical energy is converted into the internal energy of the air fuel mixture, causing a significant increase in its pressure and temperature. For engines with a compression ratio of 10.5: 1, the air-fuel mixture in the cylinder has a pressure of 1.2-1.8 MPa and a temperature increase of 300 -400°C at the end of the compression stroke.
The compression ratio is the core parameter of the stroke and is defined as the ratio of total volume of the cylinder to the chamber volume of the cylinder. A higher compression ratio can improve heat efficiency, but the risk of knocking must be balanced. Modern engines use high-precision fuel injection systems (such as direct injection) and detonation sensors to monitor combustion conditions in real time and dynamically adjust ignition advance angle. For example, the Volkswagen EA211 1.4T engine uses direct injection technology, which injects fuel directly into the cylinder and uses a a stratified combustion with a 10: 1 compression ratio, reducing the tendency to explode.
III. Power stroke: The core stage of engine energy output is to dynamically convert the energy inside the engine into mechanical stroke. As the piston approaches top dead spot, the spark plug produces a high-voltage electrical spark (20-30kV) that ignites the compressed air fuel mixture. The combustion reaction is completed in 0.001 seconds, releasing a large amount of heat energy that causes the gas pressure inside the cylinder to surge to 6-8 MPa and reach a temperature of 2000-2500°C. High temperature and high pressure gas push the piston from top to bottom to the dead point, turning linear motion into crankshaft rotation through connecting rod, producing mechanical work.
The efficiency of this process depends on combustion rate and energy release control. Modern engines optimize fuel atomization through porous injectors such as six-hole injectors injectors and combine them with turbocharger technology to increase intake pressure and achieve more comprehensive combustion. BMW B48 2.0T, for example, uses a twin-axle turbo engine that converts exhaust energy into inlet pressure, increasing cylinder pressure by 20% and power output by 15% during a power trip.
IV. INTRODUCTION INTRODUCTION Introduction: Exhaust stroke: Exhaust Trail, the exhaust valve opens, intake valve closes, piston from bottom to top dead point to remove the combusted exhaust gas from the cylinder. Exhaust gas temperatures can reach 800-1000 degrees Celsius with a pressure of approximately 0.3-0.5 MPa. To improve exhaust efficiency, Hyundai has adopted a dual overhead camshaft (DOHC) design that reduces exhaust residue by independently controlling the opening and closing timing of intake and exhaust valves. the Toyota Dynamic Force 2.5L engine, for example, optimizes the exhaust valve lift curve, reducing exhaust residue to less than 5% of the exhaust exhaust.
In addition, exhaust gases must be treated by a three-phase catalytic converter to convert carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) into harmless carbon dioxide (CO2), water (H2O) and nitrogen (N2). Modern engines use closed-loop control and oxygen sensors to monitor exhaust composition real time and dynamically adjust the air-to-flame ratio to ensure emissions meet China VI emission standards.
Conclusion: Synergy between four-stroke engine and engine evolution
Through precise timing control and energy conversion, the four-stroke gasoline engine achieves an efficient transition from chemical energy to mechanical energy. From the preparation of the gasoline-oil mixture during intake, to the increase in energy density during compression, to the release of explosive energy during power, to the completion of cycle preparation during exhaust stroke, each stage requires a rigorous match to ensure stable engine performance. With the popularization of turbocharging, direct injection and frequency conversion speed regulation, the thermal efficiency of modern engines has exceeded 40%, which provides core support for energy conservation emission reduction and performance improvement in the automotive industry.
