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]](anim/piston.png)
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.
