<P> The reed valve is a simple but highly effective form of check valve commonly fitted in the intake tract of the piston - controlled port . They allow asymmetric intake of the fuel charge, improving power and economy, while widening the power band . They are widely used in motorcycle, ATV and marine outboard engines . </P> <P> The intake pathway is opened and closed by a rotating member . A familiar type sometimes seen on small motorcycles is a slotted disk attached to the crankshaft which covers and uncovers an opening in the end of the crankcase, allowing charge to enter during one portion of the cycle (aka disc valve). </P> <P> Another form of rotary inlet valve used on two - stroke engines employs two cylindrical members with suitable cutouts arranged to rotate one within the other - the inlet pipe having passage to the crankcase only when the two cutouts coincide . The crankshaft itself may form one of the members, as in most glow plug model engines . In another embodiment, the crank disc is arranged to be a close - clearance fit in the crankcase, and is provided with a cutout which lines up with an inlet passage in the crankcase wall at the appropriate time, as in Vespa motor scooters . </P> <P> The advantage of a rotary valve is that it enables the two - stroke engine's intake timing to be asymmetrical, which is not possible with piston - port type engines . The piston - port type engine's intake timing opens and closes before and after top dead center at the same crank angle, making it symmetrical, whereas the rotary valve allows the opening to begin and close earlier . </P>

State the two compression out on petrol engines