Four-Stroke Engine
To understand how a automobile moves lets use the comparison of a person riding a bicycle to an automotive engine moving a car forward. Things need a form of energy to move. A bicycle moves forward because a person pushes with his foot downwards on the peddle transferring his energy to a peddle, arm, and sprocket assembly onto the wheel.
In an engine your foot pushing down would be the explosion caused by igniting the fuel mixture in an engines cylinder. This is the energy. The peddle would be the piston contained in a cylinder pushing down on the connecting rod. The connecting rod connects to the engines crankshaft which transfers this energy on down the line ending up at the vehicles wheels.
Note that as one peddle is being pushed down the other peddle is being forced upwards by the energy from the first. This is true with an automotive engine piston because it is attached to the engines crankshaft by a connecting rod along with other pistons/rods. As one piston is being forced downwards the other piston (or really pistons) is also being affected by the downward motion powered by the first. These upwards and downwards piston motions are called strokes.
Each one-way piston movement from one end of the cylinder to the other is a single engine stroke. A bicycle is the example of a two stroke engine. For each power stroke the peddle move down once the other is forced up once. An automobile is an example of a four stroke engine: intake, compression, power, and exhaust. For each power stroke a piston travels up and down the cylinder bore four times.
An automotive engine needs an "air tight" cylinder to achieve compression. The engine actually squeezes or compresses the air/fuel mixture into a tight pocket before igniting it. To do this it uses valves like little doors to quickly open and close to allow air/fuel mixture in and to let the exhaust out while still achieving this compression.
When both of these valves are closed the piston will compress the air. If say the intake valve or "door" where open when the piston is moving downwards then the air/fuel mixture will in turn be drawn into the cylinder. If the exhaust valve were to be open when the piston moves upwards then the exhaust would be expelled from the cylinder. These piston strokes are completed in the following order:
#1 Intake: This is where the four-stroke cycle begins. The piston is at top dead center (the very top of the cylinder bore). As the piston draws downwards the intake valve opens and the vacuum from the piston fitting tightly in the cylinder pulls the air/fuel mixture into the cylinder.
#2 Compression: After the intake valve allows the mixture into the cylinder it quickly closes leaving that "air tight" cylinder mentioned earlier. Now on a bicycle while your foot is pushing down on one peddle the other peddle is being forced up. In an automobile engine while one piston is being forced down by a contained explosion the other is being forced up compressing the air fuel mixture in that cylinder. This is called the compression stroke.
#3 Power: Now after the piston reaches the top of the cylinder the fuel is completely compressed the spark plug ignites the air/fuel mixture (14:1 fourteen parts of air to one part fuel) and an explosion occurs forcing the piston downwards. This is referred to as the power stroke.
#4 Exhaust: After the explosion contained in the engines cylinder the burned fuel needs to be expelled. Now to achieve this the engines exhaust valve opens while the piston is traveling back upwards again. This allows the exhaust to travel out of the engine and into the exhaust system. This is the final stroke of a four stroke automotive engine. After this the cycle begins all over again. A series of these pistons in a block connected to the crankshaft by rods will eventually transfer the energy from the explosions though the transmission etc. to the wheels and off you go.
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