Lesson 2/Learning Event 1
After the first amount of fuel is ignited, the piston is being forced down on the power stroke. The sweeping
action of the swirling air keeps lifting some of the fuel up out of the combustion chamber pocket. As the fuel
is lifted up into the burning fuel in the center of the combustion chamber, it also starts to burn. The pressure
from the burning expanding gases pushes down on the piston. The original 5 percent of fuel that was first
ignited is now acting as a spark plug igniting the remainder of the fuel charge. The slow, steady sweeping of
fuel from the pocket in the piston head during the power stroke produces a long, effective power stroke. This
steady burning of fuel, especially when using gasoline, eliminates a detonation knock.
Just before the bottom of the power stroke the exhaust valve opens. The piston then passes BDC and starts up
on the exhaust stroke, pushing the exhaust gases out the open exhaust valve. Just before the piston reaches
TDC on the exhaust stroke, the intake valve opens and both valves are open. If the engine uses a
turbosupercharger, the turbosupercharger forces clean fresh air past the intake valve into the cylinder. The
clean air clears the cylinder of any remaining exhaust gases by pushing it out the open exhaust valve. This is
called scavenging the cylinder.
When the piston passes TDC and starts down on the intake stroke, the exhaust valve closes. This starts the
four-stroke-cycle over again. You can now see that the difference between the multifuel engine and most
straight diesel engines is the combustion process of controlled slow burning of the fuel. This allows the engine
to operate on many fuels with no adjustment needed.