Schematic diagram of four-stroke internal combustion engine
The working principle of a four-stroke internal combustion engine: Four-stroke engine (Four-stroke): An engine that completes a working cycle when the engine crankshaft rotates every two revolutions, that is, the piston reciprocates four strokes up and down.
The working cycle refers to the working process of the engine consisting of intake, compression, combustion and expansion (work) and exhaust stroke. The engine completes an intake, the process of compression, work, and exhaust is called a working cycle, also known as a cycle. â‘ . Explanation of several main terms
A. Dead point: The piston (piston) is connected to the crank connecting rod assembly. The piston has two upper and lower limit positions in the cylinder. The upper limit position is called the top dead center. It is the largest distance from the center line of the crankshaft. The lower limit position is called the bottom dead center, and it is the smallest distance from the centerline of the crankshaft.
B. Piston stroke: The distance that the piston moves from top dead center to bottom dead center is called stroke, also known as stroke.
C. Cylinder working volume: the space swept in the cylinder by the piston moving one stroke.
D. Working volume of combustion chamber: the space formed by the top of the piston and the top of the combustion chamber in the center of the cylinder head when the piston is at top dead center.
E. Total cylinder volume: the sum of the working volume of the cylinder and the working volume of the combustion chamber.
F. Compression ratio: an important structural parameter of the engine, which directly affects the engine power.
Compression ratio (ε) = total cylinder volume / combustion chamber working volume
= 1+ cylinder working volume / combustion chamber working volume
generally:
Diesel engine: ε = 14 ～ 20
Gasoline engine: ε = 6 ～ 10
First Stroke --- Intake Stroke: When the piston is at a specified crank angle before top dead center, the intake valve opens and the combustible mixture is drawn into the cylinder. When the piston moves from top dead center to bottom dead center, the exhaust valve closes at a certain crank angle of the top dead center, and the cylinder volume above the piston increases, making the cylinder form a vacuum. The combustible mixture continues to pass through the intake air Door suction. When the piston travels to a certain crank angle after the bottom dead center, the intake valve closes. At this time, the intake process ends.
The second stroke ----- compression stroke: the piston moves from the bottom dead center to the top dead center. When the intake process is completed, the intake valve and the exhaust valve are both closed. At this time, the combustible mixture in the cylinder The gas begins to be compressed.
The third stroke ----- Combustion expansion work stroke: In the compression stroke, when the piston travels up to a certain crank angle before top dead center, sparks are emitted between the electrodes of the spark plug to ignite the compressed combustible mixture. The burning combustible gas mixture makes the temperature and pressure in the cylinder rise sharply, and the piston moves from the top dead center to the bottom dead center under the action of this high temperature and high pressure, and the crankshaft is driven by the connecting rod to make useful work.
Fourth stroke ---- Exhaust stroke: During the combustion and expansion stroke, when the piston travels to a certain crank angle before the bottom dead center, the exhaust valve opens, and the exhaust gas begins to be discharged through the exhaust valve. The crankshaft continues to rotate and push the piston to move from bottom dead center to top dead center. Push the exhaust gas out of the cylinder. This exhaust process ends until the piston reaches a certain crank angle after the top dead center, and the exhaust valve is closed.
Advantages and disadvantages of four-stroke engines:
advantage:
* Intake, compression, expansion (burst), and exhaust process are carried out separately, so the work is reliable, efficient, and stable. The speed range from low speed to high speed is wide (more than 500 ～ 1000rpm).
* There is no blow-by gas return loss like a two-stroke engine, and the fuel consumption rate is low.
* Low-speed operation is stable, relying on lubrication system lubrication, not easy to overheat.
* The intake process and compression process take a long time, and the volumetric efficiency and average effective pressure are high.
* The thermal load is smaller than the two-stroke engine. Don't worry about deformation and ablation. Large displacement, can be designed as a high-power engine.
Disadvantages:
* The valve valve mechanism is complex, with many parts and difficult maintenance
* High mechanical noise
* Because the crankshaft explodes once every 2 revolutions, the rotation balance is unstable
Four-stroke engines are used in almost all automotive engines including gasoline and diesel engines,
The process of the internal combustion engine converting fuel heat energy into mechanical energy is carried out according to certain rules. The four continuous processes of inflation (ie intake), compression, work, and exhaust constitute the working cycle of a four-stroke engine, and each working process is carried out in turn.
Each working cycle of a four-stroke engine is completed by moving the piston up and down twice, that is, in four strokes, as shown in the figure:
1. Intake (inflation) stroke: The fresh mixture of fuel and air enters the cylinder, and the piston moves downward.
2. Compression stroke: As the intake valve and exhaust valve close, the piston moves upward and the combustible mixture is compressed.
3. Power stroke: The combustible gas mixture ignites and burns, pushing the piston down.
4. Exhaust stroke: The upwardly moving piston expels the burned exhaust gas from the exhaust valve.
The four strokes of a four-stroke engine are used as a working cycle and are repeated thousands of times in 1 minute. The four-stroke engine is also known as a four-cycle system, also known as the Otto cycle.
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