Demonstration of Function in Single-Cylinder Engines

Increasing Air Expenditure by Dynamic Supercharging. In the following, air expenditure λ_L is used as a reference for dynamic supercharging. The air expenditure describes the ratio between the measured air mass and theoretical air mass flow rate that is formed from the stroke volume V_H and speed n. Since in the first approximation the torque rises in a linear relationship to the air expenditure in a faster-running engine given a stochiometric air ratio λ = 1, λ_L represents a suitable comparative quantity for evaluating charge cycle processes.

Figure 10-89 shows λ_L in a slow-operating test engine as a function of the rpm. Because of the short induction pipe length, λ_L is quite low at low rpm. If the air stroke valve is opened twice in the intake phase given the same rpm, the air expenditure is substantially greater given optimal control times, and the increase rises strongly as the engine speed decreases. At n = 1000 min^-1, the gain in air expenditure is approximately 13% in contrast to the pure induction without valve control. The rise in air expenditure predicted by charge cycle calculations can be almost completely achieved with this prototype.

Fig. 10-89. Increase in air expenditure from an air stroke valve with two cycles in the intake phase in a slow-running engine

To illustrate the charge cycle processes occurring in the intake system, Fig. 10-90 plots the pressure in the induction pipe measured with induced low pressure against the crank angle in a slow-running engine. One pressure measuring site was upstream from the air stroke valve and, hence, characterizes the pressure level of the induction pipes in the direction of the manifold. The second measuring site was in the volume between the air stroke valve and intake valve. To provide orientation in the engine cycle, the stroke curves of the intake and exhaust valves as well as the movement of the air stroke valves are also plotted. The pressure characteristics shown at a speed of n = 1000 min^-1 clearly illustrate the physical processes when an engine is dynamically supercharged.

Fig. 10-90. Pressure characteristics before and after the air stroke valve with dual cycles in the intake phase in a slow-running engine

Because of the large valve overlap between the intake and exhaust valves, it is necessary to keep the air stroke valve closed when the intake valve starts opening to prevent the inflow of exhaust. Only when the exhaust valve is largely closed is the intake cross section of the air stroke valve released and the delayed inflow process begun. During the intake valve stroke, the valve is closed. The pressure in the chamber between the air stroke valve and intake valve quickly drops to 150 mbar due to piston movement. Shortly before BDC in the charge cycle, the valve is briefly opened a second time.

Because of the pressure difference, another air flow shoots into the combustion chamber, which causes a rise in pressure of nearly 100 mbar because of the reflection from the piston head in contrast to the induction pipe pressure. This pressure peak is retained in the cylinder chamber for dynamic supercharging, which significantly increases the density caused by the additional mass introduced into the combustion chamber. In Fig. 10-90, we see the pressure drop in the chamber between the air stroke valve and intake valve after the intake valve closes that arises from leakage across the air stroke valve.