Gas Exchange Organs

As noted, the fresh gas stream entering the cylinder in the case of inflow scavenging and the exhaust stream leaving the cylinder in the case of loop scavenging are controlled by ports in the cylinder wall and the ascending and descending piston. A feature of port control is that a large flow cross section can be opened and closed within comparatively small crank angle ranges in comparison to conventional valve actuation in the cylinder head. High nominal speeds can, therefore, be obtained with port controlled two-stroke engines. A characteristic quantity used in designing and determining the gas flow rate through a port is the (opening) time cross section.

This defines the time integral over the respective port cross-sectional area from the opening to the closing of the respective port. Without additional measures, symmetrical timing results for port-controlled two-stroke engines at the dead centers of the crankshaft. With the goal of improving the charging of the combustion chamber with an asymmetrical intake timing diagram, a series of two-stroke spark-ignition engines were equipped with tubular and roller rotary disk valves and then later with disk type rotary disk valves. With asymmetrical timing of the rotary-disk valves, the start of intake is substantially earlier than with port control. Since the vacuum in the crank chamber is comparatively low at this point in time, the air column in the intake tract is excited to form gas column oscillations comparatively less at low and average speeds.

This produces more continuous torque characteristics and favorable conditions for the formation of a fuel-air mixture with a very constant air-fuel ratio in the carburetor. Instead of rotary-disk valves, modem two-stroke spark-ignition engines have frequently used reed valves in recent years. These act as nonreturn valves and automatically open given a specific pressure gradient toward the crank chamber, and they independently close given an opposite pressure gradient. Figure 10-57 shows the construction of a reed valve for two-stroke engines.

Fig. 10-57. Illustration of the constructions of a reed valve for use in an intake system of a two-stroke engine

The basic body (made of die-cast aluminum or plastic) in the form of a gable to reduce flow resistance is generally sprayed with a thin elastomer coating at the area where the reeds are contacted to reduce mechanical load and improve the seal and acoustics. The reeds fixed to one side of the basic body (mechanical replacement model: cantilever with a surface load) are made of either 0.15-0.2 mm thick Cr-Ni sheet steel or more recently 0.4-0.6 mm thick fiberglass-reinforced epoxide resin plates. Given the same length and width, the natural frequencies of steel and epoxide resin reeds are approximately the same since the quotients of their elasticity modulus and density are about the same.

Since the reeds open more as the pressure differential increases, a linear relationship between the pressure differential and mass flow results in a first approximation. To prevent the reeds from moving in an undefined manner (opening too wide with subsequent premature closing of the reeds, vibration in the second eigenform, etc.), reed valves with arched stops of sheet steel are provided that the reeds contact as they execute a rolling off movement when they open. The natural frequency of the reeds should be at least 1.3 times that of the opening frequency (intake frequency of the engine). Reed valves are placed either directly on the crank chamber or as shown in Fig. 10-58 and are used together with the piston intake control.

Fig. 10-58. Intake system with combined piston edge/reed valve control

Fig. 10-59. Section of a loop-scavenged cylinder with an exhaust port pivot valve according

With the goal of compensating for the disadvantages of symmetrical timing of port-controlled loop scavenging, some modern high-performance spark-ignition engines use flat-seat valves, pivot valves, or rotary-disk valves. This can improve the fresh gas charging, the torque and performance curve, or, as is the case with the Honda AR combustion method (activated radical), the ignition of the air-fuel mixture. Figure 10-59 shows a section of such a cylinder.