An alternative to the piston-operated crankcase inlet port is to use a disc-valve attached to and driven by the crankshaft (Fig. (a)). This disc-valve is timed to open and close so that the fresh charge is induced to enter the crankcase as early as possible, and only at the point when the charge is about to be transferred into the cylinder is it closed. This method of controlling crankcase induction does not depend upon the piston displacement to uncover the port – it can therefore be so phased as to extend the filling period (Fig.). A further method of improving crankcase filling is the use of reed-valves (Fig. (b)). These valves are not timed to open and close, but operate automatically when the pressure difference between the crankcase and the air intake is sufficient to deflect the reed-spring. In other words, these valves sense the requirements of the crankcase and so adjust their opening and closing frequencies to match the demands of the engine.
