The Mechanics
Combustion on modern automotive gasoline 
engines occurs within a cylinder (like a soda 
can) where the energy of combustion pushes 
down on a piston attached to a rotating shaft, 
similar to the way a bicycle pedal is pushed 
down by your feet, and turning in a circle.  
Each cylinder is like another pedal- the more 
pedals the faster a bike can go.  The same is 
true with cars.  This is why most 8 cylinder 
engines make more power than 4 cylinder 
engines.  Modern cars have between 4 and 12 
cylinders turning the engine.  
      
            The engine cylinder also 
behaves like an air pump, the 
one you might use to inflate a 
bicycle tire.  Air is sucked in 
one side, squeezed, and 
pushed out of the other side.  
This same airflow happens 
within an engine cylinder.  The 
intake valve opens to let air 
(and fuel) into the cylinder and 
the exhaust valve opens to let 
the burned air and fuel out to 
the exhaust.  This occurs in a 
specific sequence called the 4 
stroke cycle (invented in 1867 
for the history buffs).