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] 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.  The two-stroke process begins with the fuel entering the  engine and concludes as it exits as exhaust. 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] 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] High crankcase pressure closes reed valve Exhaust port Reed valve closed Transfer port Low crankcase pressure opens reed valve Reed valve open Transfer port Fuel/Air/Oil Mixture Figure 4-4.  Reed valve is open with low pressure and closes when  the pressure increases in a two-stroke engine. Figure 4-5.  Intake rotary valve for a two-stroke engine. Crank shaft operates/rotates rotary valve Open port shown for air/fuel intake Two-Stroke Process Crankcase Vacuum Intake Stroke—Piston Moving  Up Crankcase Compression Stroke—Piston Moving  Down 4-4