A work cycle consists of four piston strokes: intake stroke, compression stroke, expansion stroke (power stroke), and exhaust stroke.
Intake Stroke
During this process, the intake valve of the engine opens, while the exhaust valve closes. As the piston moves from top dead center to bottom dead center, the cylinder volume above the piston increases, lowering the pressure inside the cylinder below atmospheric pressure, creating a vacuum. This allows air to be drawn into the cylinder through the intake manifold and valve, while the fuel injectors spray atomized gasoline, which thoroughly mixes with the air. At the end of the intake stroke, the gas pressure inside the cylinder is approximately 0.075-0.09 MPa. At this point, the temperature of the combustible mixture inside the cylinder has risen to 370-400K.
Compressed Itinerary
To ensure a rapid combustion of the combustible mixture within the intake cylinder, which generates higher pressure for increased engine power output, it is necessary to compress the mixture before combustion. This compression reduces its volume, increases its density, and elevates its temperature, necessitating a compression process. During this phase, both the intake and exhaust valves are closed, and the crankshaft drives the piston from the bottom dead center to the top dead center, completing a stroke known as the compression stroke. At this point, the mixture pressure increases to 0.6-1.2 MPa, and the temperature can reach 600-700 K.
During this journey, an important concept is the compression ratio. The compression ratio refers to the ratio of the large volume of gas in the cylinder before compression to the small volume after compression. Generally, the higher the compression ratio, the higher the pressure and temperature of the mixed gas at the end of compression, and the faster the combustion rate, leading to greater engine power and better economy. Typically, the compression ratio in passenger cars ranges between 8-10, but the Polo that has been launched recently boasts a high compression ratio of 10.5, thus delivering a relatively good torque performance. However, when the compression ratio is too high, it not only fails to further improve combustion but can also result in abnormal combustion phenomena such as detonation and surface ignition.
An explosion is an abnormal combustion caused by excessively high gas pressure and temperature, leading to spontaneous combustion of the combustible mixture at the end of the combustion chamber, which is far from the ignition center. During an explosion, flames spread outward at high speed, even as the gas does not have time to expand, causing a rapid increase in temperature and pressure, forming pressure waves that advance forward at the speed of sound. When these pressure waves strike the walls of the combustion chamber, they produce a sharp knocking sound. Additionally, they can lead to a series of adverse effects, such as engine overheating, power loss, and increased fuel consumption. Severe explosions can even result in damage to components such as burned valves, cracked shaft bearings, and pierced spark plug insulators.
In addition to detonation, engines with excessively high compression ratios may also face another issue: surface ignition. This is another form of abnormal combustion caused by the ignition of a mixture of air and fuel on the hot surfaces within the cylinder (such as the exhaust valve head, spark plug electrodes, and carbon deposits). When surface ignition occurs, it is accompanied by a strong knocking sound (more muted), which generates high pressure, increasing engine load and reducing lifespan.
Expansion Journey
During this process, the intake and exhaust valves remain closed. As the piston approaches the top dead center, the spark plug emits an electrical spark, igniting the compressed combustible mixture. After combustion, a significant amount of heat is released, causing the pressure and temperature of the gas to rapidly increase. The pressure can reach up to 3-5 MPa, with temperatures soaring to 2200-2800 K. The high-pressure, high-temperature gas pushes the piston from top dead center to bottom dead center, moving the connecting rod and rotating the crankshaft to output mechanical energy. This energy not only sustains the engine's continuous operation but also performs external work. As the piston moves, the cylinder volume increases, and both gas pressure and temperature rapidly decrease. By the end of this stroke, the pressure drops to 0.3-0.5 MPa, and the temperature is 1300-1600 K.
Exhaust Stroke
As the expansion stroke (power stroke) nears completion, the exhaust valve opens, allowing free exhaust through the pressure of the exhaust gases. When the piston moves from the bottom dead center to the top dead center, the exhaust gases are forced out into the atmosphere. This is the exhaust stroke. During this stroke, the pressure within the cylinder is slightly higher than atmospheric pressure, approximately 0.105-0.115 MPa. The exhaust stroke ends as the piston approaches the top dead center, with the exhaust gas temperature around 900-1200K.
We have now completed the introduction of one working cycle of the engine, during which the piston has moved back and forth between the top and bottom dead centers four times, correspondingly rotating the crankshaft two full turns.
A gasoline engine equipped with an alternator forms a gasoline generator set.