The basics of an internal combustion engine motorcycles and trikes is easy to understand.  Trikes are just like motorcycles on steroids so their engines work the same.  Fuel, usually gasoline, is mixed with air in the carburetor.  This air/fuel mixture is then drawn into the cylinder when the piston is on the down stroke.  The piston moving down creates a vacuum in the cylinder area which causes the air/fuel mixture to fill this cavity.  This mixture is allowed in through the intake valve.

At the bottom of the intake stroke of the piston the intake valve closes and the compression stroke starts.  The piston moves up and the air/fuel mixture is pressurized which means it is compacted into a smaller and smaller space.  This makes it more explosive.  At the top of the compression stroke of the piston the mixture is ignited by the spark plug.  The resulting explosion forces the piston back down.  Since the piston is connected to the crank shaft the shaft turns.  This is in turn connected to the transmission which turns.  The transmission is hooked to the wheels which then causes them to turn. 

Most internal combustion engines have a carburetor which will mix the air and gas together in the correct amount for maximum explosive power.  But what about engines with 0 carbs?

If an engine does not have a carb then it will use fuel injection.  The fuel, instead of being metered in the carburetor throat will be injected directly into the air stream at the correct time.  This allows for a very exacting fuel air mixture which does two things.  First, it cuts down on pollution because there is no extra gasoline left over after the combustion.  The air and fuel mixture is perfectly combined because there are no left over petroleum molecules after the burning of the fuel.  The engine is said to burn clean.

This clean burn also is much more efficient.  This makes sense because there is no wasted fuel going out the tail pipe.