Guide To Four Stroke Motorcycle Engines
The deep, throaty sound of a large displacement four stroke motorcycle differs from the high-pitched buzz of a two stroke for several reasons. The intake noise of most two strokes nearly matches the exhaust in loudness. However, the four stroke noise is almost all exhaust note as the intake is baffled down to a whisper.
A two stroke engine has a firing pulse on each revolution, resulting in a higher-pitched tone than a four stroke, which fires only once every two revolutions. The riding characteristics of the two types of engines also differ considerably. A two stroke engine favors high rpm and fairly aggressive riding tactics. A four stroke is content to run at low to moderate rpm, giving good torque and hardly ever fouling spark plugs.
The reason why two strokes have disappeared from the street and are only used in a minority of off road situations and then again primarily in restricted racing applications, is due to the two strokes' propensity for high emissions. Unlike a four stroke which can have a variety of emission controlling adjuncts, the two strokes run very dirty and there is no economically feasible way of cleaning up their act. That's why street legal two strokes have gone the way of the dodo bird and likely never to return.
It's generally conceded that two strokes generate much more horsepower per cc, yet are lighter in weight than a similar sized four stroke. The weight difference is caused by the many extra parts required to make up the cam and valve train, which will be examined in detail after a short review of how a four stroke engine works.
Basic Four Stroke Configuration
Two- and four stroke motorcycle engines have similar crankshafts, pistons, rods, and cylinders which make up some of the basic components of both engines. Both have a power stroke, that is, the explosive burning of a compressed air-fuel mixture in a cylinder.
The objective of all other phases of an engine's activity lead to or clean up after the power stroke. Let's take a look at the phases required in a four stroke.
To fill the cylinder with a fresh charge of gas-air mixture, a valve must open as the piston travels down its bore, drawing the fresh mixture into the complete volume of the cylinder. This intake valve opens as the piston starts down and closes as the piston starts back up so none of the fresh mixture is lost.
With the cylinder volume filled with the fresh air-fuel mixture, the piston now comes up to squeeze the fresh mixture into a "bomb"-like charge in the combustion chamber.
The sparking of this compressed charge is not a "stroke" but, it is a critical factor in how well the four stroke engine runs. The spark plug is fired just before the piston reaching its top position (top dead center or TDC). By the time the piston reaches TDC the fuel charge is burning well enough to insure complete and effective combustion on the power stroke.