The high Swirl Scavenging Valves

There is a high pressure difference over the scavenging valves of the Z engine when the valves open. Therefore, the speed of intake air increases to the corresponding speed of sound in the valves. To achieve high swirl for Z combustion, it is necessary to lead the intake air tangentially into the cylinder. The shrouds can be used to prevent air flowing backwards the swirl (figure 1). The corresponding speed of sound is maximum speed achieved in common poppet valve. The valve seen in the picture is designed to enable supersonic speed after the valves. The valve forms a Laval type narrowing/widening nozzle while it is open. The maximum flow speed is over 500 m/s depending of the pressure difference. If needed, the swirl rate can be adjusted by rotating the shrouded intake valves. The swirl rate decreases, when the intake air is not lead tangentially into the cylinder. 31.10.2002

Figure 1: The scavenging valve

The schematic figure 1 presents a valve that is optimal for achieving high flow speeds. However, the edge piece outside the sealing surface and the shroud cause thermal stresses. This type of valve outer diameter is quite big in respect to the flow cross section area. Also, the shrouded valve is more complex to manufacture. An alternative design is presented in the figure 2. The outer diameter of the valve is narrowed by changing the angle of the edge piece outside the sealing surface. According to the simulations, the flow speed is reduces only about 10 % and the flow direction stays almost the same. To prevent the flow backwards the swirl, a certain sector of the intake duct above the intake valve is masked. In addition, the cylinder head is shaped to be enclosed to the valve edge piece with a small gab on the sector that the flow is not desired. In the prototype engine, small notches were milled into the cylinder head valve area to form fluid diodes / labyrinth sealing to intensify the restraining effect on the flow prevention sector.

Figure 2: Alternative design of the scavenging valve

Theoretical flow behavior in the valves can be seen the figure 3. This is an example with 20 bar pressure in the intake manifold. The theoretical flow speed is calculated assuming valve to be an optimized Laval nozzle. The pressure difference has been received from a simulation of the engine. Needed ratio between valves exit area and throat (at the inner seat) area has been calculated from the flow speed and specific volume of the flow. Calculated exit area / throat area –ratio has been calculated from the valve lift data and valve dimensions. The valve in this example is of the type in the figure 1.

Figure 3: Valve flow