The Engine
At the heart of any car is its engine, affectionately refered to as the powerplant, motor, or prime mover depending on which circles one frequents. when someone asks what you have under the hood, they are refering to the engine.

The engine's primary purpose is to convert fuel and oxygen into motion. It does this through the combustion process which is simply a controlled explosion of the gasoline and air mixture. The combustion process is comprised of four primary functions or cycles. Here's how each of the four cycles occurs:

The Intake Stroke: We'll start with the piston at its highest point, referred to as top-dead-center. From this point, the piston is pulled down by the rotating crankshaft through the connection rod. It is during this cycle that the intake valve (see intake system) is open allowing the air-fuel mixture into the combustion chamber. When the piston reaches the bottom of its travel, the intake valve closes.
The compression stroke: As the crankshaft continues to rotate, it begins to push the piston up, again through the connecting rod. This upward movement of the piston compresses the air-fuel mixture, making it more dense. This compression continues until the piston again reaches the top of its travel.
The power stroke: At the point at which the piston reaches the top of its travel, a pulse of high voltage (see ignition system) is sent to the spark plug, causing a spark to jump across the gap between the electrodes. This spark ignites the air-fuel mixture forcing the piston downward, thus turning the crankshaft. The timing of the combustion is such that the explosion of the air-fuel mixture lasts for approximately the same amount of time that it takes for the piston to reach the bottom of its travel.
The Exhaust Stroke: Once the air-fuel mixture has burned, the byproducts of the combustion must be removed from the combustion chamber. This is handled during the exhaust stroke. As the rotating crankshaft again pushes the piston upward after the power stroke, the exhaust valve opens, allowing the piston to push the exhaust gasses out of the combustion chamber so that a fresh air-fuel mixture can be drawn in on the next intake stroke. The exhaust valve remains open until the piston again reaches top-dead-center at which point it closes. The engine is now ready for the next intake stroke.

This cycle is repeated over and over again as the engine runs. Since most cars have 4, 6, or 8 cylinders, each cylinder will go through these same 4 cycles. Normally, each cylinder is timed such that there is equal crankshaft rotation between the the power strokes of each cylinder, allowing a smoother operating engine. One could imagine how rough an engine would operate if all of its cylinders were on the power stroke at the same time.

The rotating crankshaft of the engine transfers the linear motion of all of an engine's pistons into a circular motion, but that motion must be transfered from the engine to the wheels of the car. This is dealt with in the drive train.

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