Now that the power stroke has done its job, there are noxious byproducts in the form of burned gases that
have to be removed. The exhaust stroke takes care of this task. Once again, the wonder of exact mechanical
timing results in the removal of burned exhaust gas in the combustion chamber. This happens thousands of
times a minute in a typical internal-combustion engine.
Compression Stroke
From BDC the piston starts to travel upward, compressing the air-fuel mixture
within the sealed combustion chamber. This compression increases the volatility
of the mixture. When the piston reaches the top of its travel, which is called
TDC (top dead center), the air-fuel mixture is at maximum compression
(extremely volatile). At this point the fuel-air mixture is ready for combustion.
Power Stroke
At this point, a spark is generated from the ignition system. The spark jumps the
air gap of the plug and ignites the highly volatile air-fuel mixture. The controlled
burn of the air-fuel mixture forces the piston downward once again to BDC, thus
powering the engine.
Exhaust Stroke
After the air-fuel mixture has burned and provided the energy for the power
stroke, there are gaseous byproducts that must be removed from the system. As