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Today, let's talk about the working principle of automobile engines. As mentioned earlier, automobile engines are divided into several categories: four-stroke gasoline engines, four-stroke diesel engines, two-stroke gasoline engines, two-stroke diesel engines, and rotary engines. Among these engines, the most common ones are four-stroke gasoline engines and four-stroke diesel engines. Two-stroke gasoline engines, two-stroke diesel engines, and rotary engines are less common, and the working principle of four-stroke diesel engines is almost the same as that of four-stroke gasoline engines. So today, we will take the most common four-stroke gasoline engine as an example to talk about the working principle of automobile engines.
If you search online for the working principle of automobile engines, you will find countless answers. These answers almost all mention that the engine's operation is divided into four strokes: intake, compression, power, and exhaust, each stroke involving the temperature, pressure, and piston movement status inside the cylinder. For us ordinary car owners, this knowledge is not very significant. Today, I will explain the entire process of how an engine starts and operates normally from a different perspective, using as simple language as possible.
Before the engine starts, it is in a stationary state. To make it work, an external force must first drive it. This external force is usually provided by the starter. Of course, in early engines, there were no starters, and people usually used manual methods to drive the engine, such as cranks or pull ropes. When the battery is disconnected, the engine can also be started by pushing the car backward to rotate the engine. In some particularly large diesel engines, the starting torque is particularly large, and a regular electric starter cannot drive it. Auxiliary starting devices are generally used, such as pneumatic starters or additional engine starters.
Most modern cars now use electric starters. Before starting, first turn the ignition switch to the on position, the fuel pump starts to supply pressure to the fuel supply system, the primary winding of the ignition coil is energized, and the engine electronic control system is ready. Then turn the ignition switch to the start position, the starter is energized and runs, driving the crankshaft to rotate, and the engine starts to work.
Most modern car engines are multi-cylinder engines, and we only observe one of the cylinders.
The starter drives the crankshaft to rotate, and the piston starts to move downward from the top dead center. The sealed cylinder generates a vacuum suction force, while at the same time, the intake valve opens, and external air or combustible mixture is sucked into the cylinder under the action of the vacuum suction force, similar to how a syringe draws in liquid. This is called the intake stroke. When the piston reaches the bottom dead center, the intake stroke ends.
For naturally aspirated engines, the intake pressure can never be greater than atmospheric pressure, so when the intake ends, the pressure inside the cylinder is slightly lower than atmospheric pressure, generally between 0.075 and 0.09 MPa, and the temperature is the same as the atmospheric temperature; for turbocharged engines, the external air is first compressed by the turbocharger, so the intake pressure can be greater than atmospheric pressure, and the intake temperature will be higher, the specific pressure and temperature depend on the turbocharger's working condition and engine speed.
When the intake stroke ends, the starter continues to drive the crankshaft to rotate, and the piston moves upward from the bottom dead center. At the same time, both the intake valve and the exhaust valve are closed, trapping the gas sucked into the cylinder during the intake stroke in a sealed space. As the piston moves upward, this space becomes smaller and smaller, compressing the gas inside the cylinder, increasing both temperature and pressure. This is called the compression stroke. When the piston reaches the top dead center, the compression stroke ends.
At the end of the compression stroke, the pressure and temperature inside the cylinder are very high. Generally, it will exceed 1.0 MPa, and the temperature will exceed 400°C. The pressure and temperature of diesel engines will be higher, and the pressure and temperature of turbocharged engines will be higher than those of naturally aspirated engines.
When the compression stroke ends, the gas inside the cylinder is compressed into a very small space, which is the combustion chamber. At this point, the spark plug installed in the combustion chamber begins to ignite. The high-temperature and high-pressure combustible mixture encounters the electric spark and is instantly ignited, then begins to burn vigorously, rapidly increasing in temperature and expanding in volume, generating extremely high pressure. Since the cylinder head is fixed, these high-temperature and high-pressure gases can only push the piston downward. At this point, the piston changes from being passive to active, and begins to push the crankshaft to rotate, which is called the power stroke.
The power stroke is the only stroke in which the engine generates power. During the power stroke, the highest temperature inside the engine cylinder can reach up to 2000°C, and the pressure can reach 5 MPa. The noise and vibration of the engine are emitted from this stroke. As the piston moves downward, both temperature and pressure gradually decrease until the piston reaches the bottom dead center, ending the power stroke.
After the power stroke ends, the crankshaft has gained some kinetic energy and stores some of it in the flywheel, so it can rotate independently without the need for the starter's assistance.
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