The Work Cycle of the Z engine

The Z process combines the best parts from 4-stoke and 2-stroke processes. The Z engine cylinder produces work on every crankshaft rotation like 2-stroke-engines. However, the exhaust cycle is almost similar to 4-stroke process. The Z engine also has no scavenging ports in the cylinder liner.

The Z process is explained in the figure 1. Fuel is injected into cylinder when the piston is near top dead center. Fuel ignites spontaneously (normal diesel type combustion). The expansion stroke is like in common 4-stroke engines. Exhaust valves are opened before bottom dead center. After BTC, the piston rises pushing rest of the exhaust gases from the cylinder. Near TDC (approximately 40° BTDC), the exhaust valves are closed and intake valves are opened. A small overlap can be allowed to prevent the cylinder pressure rising before intake valve is opened. The intake air is compressed externally to high pressure (for example to 12...25 bars). The intake valves are open only approximately 20° on crankshaft. After the intake valves have closed, the final compression is done in the cylinder. The cylinder temperature rises to the self-ignition temperature during final compression. The amount of the internal EGR, intake air mass and pressure levels can be controlled by the valve timing.

Figure 1

The intake air compression is explained in the figure 2. Because of the high pressure, intake air has to be compressed in 2 stage. The pre-compressor could be for example Lysholm-type screw compressor (in the figure) or a turbo. If the Lysholm-compressor is used, the intake mass flow could be controlled by variable transmission from the crankshaft. The pre-compressor pressure varies from 1 bar (absolute pressure) to 2.6 bar depending on the engine load and speed. The main compressor is piston type. The main compressor can be controlled efficiently by adjusting the compression volume.

Figure 2

To ensure that the temperature is high enough for ignition and to optimize the efficiency, the intercoolers have bypasses. The electronic control unit (ECU) controls the bypass valves. The temperature after intercooler 2 is allowed to rise at small loads. The temperature after intercooler 1 is also adjusted while reducing the main compressor pressure at small loads.