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This video contains a four-stroke cycle of an engine
Intake Stroke
When the intake stroke occurs, as you can see, this piston moves from top dead center to bottom dead center, opening the intake valve. By the way, this is both an intake valve and an exhaust valve. As the piston moves towards the BDC, negative pressure is created in the cylinder. And the ambient atmospheric pressure forces the air-fuel mixture, or just air in a diesel engine, into the cylinder through the open intake valve, filling the vacuum zone created by the movement of the piston. As the piston begins to change direction, the cylinder continues to expand through the BDC while the air-fuel mixture continues to flow according to its own inertia. Depending on the engine design, the intake valve may remain open after the crankshaft has rotated a few degrees from bottom dead center, after which the intake valve closes, sealing the air-fuel mixture in the cylinder. This is the compression stroke time
Compression Stroke
The compression stroke is when the air-fuel mixture trapped inside the cylinder is compressed. The combustion chamber is sealed to form the charge. In this scenario both valves are closed. Charge is the amount of compressed air-fuel mixture trapped in the combustion chamber ready for ignition. Compressing the air-fuel mixture releases more energy when the charge is ignited. Compression is the process of reducing or compressing a charge from a large volume to a smaller volume within the combustion chamber. A flywheel helps maintain the momentum needed to compress the charge.
Fuel vaporization increases as the heat generated causes the small droplets of fuel to vaporize more completely. The increased droplet surface area exposed to the pilot flame allows for more complete combustion of the charge within the combustion chamber. Only gasoline vapor will ignite. Increasing the surface area of the droplet allows the gasoline to release more vapor instead of remaining liquid.
The compression ratio of an engine is the comparison of the volume of the combustion chamber with the piston at bottom dead center to the volume of the combustion chamber with the piston at top dead center. This area, along with combustion chamber design and style, determines the compression ratio. Gasoline engines typically have compression ratios in the range of 6:1 to 10:1. The higher the compression ratio, the more economical the engine.
Ignition Event
The spark at the spark plug begins to fire at approximately 20° of crankshaft rotation before the top dead center (BTDC). Atmospheric oxygen and fuel vapors are consumed by the progressive flame front
Power Stroke
The power stroke is the engine operating cycle in which hot, expanding gases force the piston crown out of the cylinder head. The force and subsequent movement of the piston is transmitted through the connecting rod, applying torque to the crankshaft. The applied torque initiates rotation of the crankshaft. Both valves are closed during a power cycle
Exhaust Stroke
The exhaust stroke occurs when spent gases are expelled from the combustion chamber and released into the exhaust system. The exhaust stroke is the last stroke and occurs when the exhaust valve is open and the intake valve is closed. The movement of the piston expels the exhaust gases into the exhaust system.
When the piston reaches bottom dead center during the power stroke, combustion is complete and the cylinder fills with exhaust gases. The exhaust valve opens and the inertia of the flywheel and other moving parts pushes the piston back to top dead center, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at top dead center and completes one cycle of motion
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