How Does an Engine Work?
2 Answers
The working principle of an engine is to convert thermal energy into kinetic energy. Most engines operate on a four-stroke cycle, which includes the intake stroke, compression stroke, power stroke, and exhaust stroke. The vast majority of automotive engines are four-stroke. 1. Intake Stroke: The working medium entering the cylinder is pure air. Due to the lower resistance in the diesel engine's intake system, the intake end pressure pa=(0.85～0.95)p0 is higher than that of a gasoline engine. The intake end temperature Ta=300～340K is lower than that of a gasoline engine. 2. Compression Stroke: Since the compressed working medium is pure air, the compression ratio of a diesel engine is higher than that of a gasoline engine (typically ε=16～22). The compression end pressure is 3000～5000 kPa, and the compression end temperature is 750～1000K, significantly exceeding the auto-ignition temperature of diesel (approximately 520K). 3. Power Stroke: As the compression stroke nears completion, diesel fuel is injected into the cylinder combustion chamber at a high pressure of about 10MPa through the fuel injector under the action of the high-pressure fuel pump. It mixes with air and spontaneously ignites within a very short time. The pressure inside the cylinder rises rapidly, reaching a maximum of 5000～9000 kPa, with a peak temperature of 1800～2000K. Since diesel engines rely on compression for ignition, they are called compression-ignition engines. 4. Exhaust Stroke: The exhaust process of a diesel engine is similar to that of a gasoline engine, except that the exhaust temperature is lower. Typically, Tr=700～900K. For a single-cylinder engine, the speed is uneven, the engine operation is unstable, and vibrations are significant. This is because only one of the four strokes is the power stroke, while the other three strokes consume energy to prepare for the power stroke. To address this issue, the flywheel must have sufficient rotational inertia, which in turn increases the overall mass and size of the engine. Multi-cylinder engines can compensate for these shortcomings. Modern vehicles commonly use four-cylinder, six-cylinder, and eight-cylinder engines.
I've been studying engines for years, and they function much like a heart, primarily generating power through a four-stroke cycle. First, the intake stroke: the piston moves downward, drawing in a mixture of air and gasoline. Next, the compression stroke: the piston moves upward, compressing the mixture. Then comes the combustion stroke: the spark plug ignites the mixture, and the resulting explosion forces the piston downward, generating power. Finally, the exhaust stroke: the piston moves up again, pushing the exhaust gases out through the exhaust pipe. This entire process repeats continuously within the cylinder, with the crankshaft converting the piston's motion into wheel power. Modern gasoline engines improve efficiency through precise control of ignition timing and fuel injection—for example, turbocharged models deliver even stronger performance. However, maintenance is key. Regularly inspecting spark plugs, oil filters, and air filters, and ensuring proper sealing can prevent issues like air leaks or knocking, thereby extending engine life. When driving on the highway, maintaining steady throttle input keeps the engine running smoothly, saving fuel and protecting components.
