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How Gasoline Engines Work

Gasoline engines convert the chemical energy in gasoline to pressure and heat, which are in turn converted to mechanical energy. The process by which most modern automobile engines do this is reasonably simple. First, air is allowed into the cylinder, when the piston is moving down and the pressure is decreasing so that air will be pulled in in the greatest possible quantities.  Gasoline is injected into a cylinder (hence the term 'fuel injection') when the piston has closed again to compress the air, and the spark plug , an electrical device [Rotating engine piston] inserted into the cylinder, gives off a spark that ignites the gasoline. The gasoline then burns in the air, creating heat and a strong, expanding force. This causes a piston, a stopper at the bottom of the cylinder that can move up and down, to be forced to the bottom of the cylinder, as shown in the animation. The cylinder is connected through an ingenious mechanism to the engine shaft, so when it goes up and down, the shaft rotates. After the piston reaches the bottom of its path, a valve in the top of the cylinder opens up. The engine's momentum, or energy from the pressure in another piston, pushes the piston back up, which in turn pushes the remnants of the reaction up through the valve and into the exhaust system.

In standard cars, there are several cylinders attached to the engine shaft. Small cars often have four; large cars and small trucks commonly have six, and large trucks generally have eight. Almost all handheld gasoline-powered tools the size of a manually-propelled lawn mower or smaller only have one. A larger number of cylinders usually means more power, since more fuel is burned per revolution of the engine shaft, but it also means lower fuel economy for the same reason. The diameter of the cylinder is also a factor, since larger cylinders mean that more fuel can be burned in a cylinder.

[Descriptions of the Labeled Parts of the Cylinder]
Animation of a single cylinder, taken from (

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