Four Stroke Gasoline Engine


Four Stroke Gasoline Engine

In a passenger car or truck, the engine provide the rotating power to drive the wheels through the transmission and driving axles. All automobile engines, both gasoline and diesel, are classified as internal combustion because the combustion or burning takes place inside the engine. These systems require an air/fuel mixture that arrives in the combustion chamber at the correct time and an engines constructed to withstand the temperatures and pressures created by the burning of thousands of fuel droplets.

Intake Stroke. The first stroke of the cycle is the intake stroke. As the piston moves away from top dead center (TDC), the intake valve opens. The downward movement of the piston increases the volume of the cylinder above it. This reduces the pressure in the cylinder. This reduced pressure, commonly referred to as engine vacuum, causes the atmospheric pressure to push a mixture of air and fuel through the open intake valve. As the piston reaches the bottom of its stroke, the reduction in pressure stops. This causes the intake of air/fuel mixture to slow down. It does not stop because of the weight and movement of the air/fuel mixture. It continues to enter the cylinder until the intake valve closes. The intake valve closes after the piston has reached bottom dead center (BDC). This delayed closing of the valve increases the volumetric efficiency of the cylinder by packing as much air and fuel into it as possible.

Compression Stroke.The compression stroke begins as the piston starts to move from BDC. The intake valve closes, trapping the air/fuel mixture in the cylinder. The upward movement of the piston compresses the air/fuel mixture, thus heating it up. At TDC, the piston and cylinder walls from a combustion chamber in which the fuel will be burned. The volume of the cylinder with the piston at BDC compared to the volume of the cylinder with the piston at TDC determines the compression ratio of the engine.

Power Stroke. The power stroke begins as the compressed fuel mixture is ignited. An electrical spark across the electrodes of a spark plug ignites the air/fuel mixture. The burning fuel rapidly expands, creating a very high pressure against the top of the piston. This drives the piston down toward BDC. The downward movement of the piston is transmitted through the connecting rod to the crankshaft.

Exhaust Stroke. The exhaust valve opens just before the piston reaches BDC on the power stroke. Pressure within the cylinder causes the exhaust gas to rush past the open valve and into the exhaust system. Movement of the piston from BDC pushes most of the remaining exhaust gas from the cylinder. As the piston nears TDC, the exhaust valve begins to close as the intake valve starts to open. The exhaust stroke completes the four-stroke cycle. The opening of the intake valve begins the cycle again. This cycle occurs in each cylinder and is repeated over and over, as long as the engine is running.

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