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stroke engine performs the same functions as a four-stroke engine in half the number of strokes. 

A wide range of valve systems are found on two-stroke engines for the purpose of opening and closing ports in the cylinder to let fuel in and exhaust out at the proper time. This is similar to the intake and exhaust valves on a four-stroke engine. One-way pressure valves, called spring, reed, or poppet valves, open when the pressure drops within the crankcase, pulling the fuel from the carburetor into the crankcase. [Figure 4-4] 

Mechanical rotary valves are driven off the engine, rotate to provide an opening at the precise time, and can be on the intake and exhaust ports. [Figure 4-5] 

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Piston porting does not use any valves. The fuel inlet port is opened and closed by the piston position as it moves up and down in the cylinder. This is called a “piston ported inlet” and is used in the two-stroke process description that follows. 

## Two-Stroke Process 

The two-stroke process begins with the fuel entering the engine and concludes as it exits as exhaust. 

## Crankcase Vacuum Intake Stroke—Piston Moving Up 

The upward stroke of the piston [Figure 4-6A] creates a vacuum in the crankcase and pulls the fuel/air/oil mixture into the crankcase through the intake valve system from the carburetor. [Figure 4-6B]  This can be a pressure-actuated reed valve, a rotary valve, or a ported inlet system where the lower piston skirt provides an opening for the fuel/air/oil mixture to flow in when the piston is reaching its highest point of top dead center (TDC). At this point, the greatest portion of the fuel/oil/air mixture has filled the crankcase. [Figure 4-6B] 

## Crankcase Compression Stroke—Piston Moving Down 

During the downward stroke, the pressure valve is forcibly closed by the increased crankcase pressure, the mechanical rotary valve closes, or the piston closes off the fuel/air oil mixture intake port as shown. The fuel/oil/air mixture is then compressed in the crankcase during the downward stroke of the piston. [Figures 4-6B to  4-6D] 

