Starting at TDC, combustion is already under way and the exhaust opens (EO) at 110–120° ATDC to promote a rapid blow-down before the inlet opens (IO) about 20–30° later (130–150° ATDC). In this way the inertia of the exhaust gases, moving at about the speed of sound, is contrived to encourage the incoming air to flow quickly through the cylinder with a minimum of mixing, because any unexpelled exhaust gas detracts from the weight of air entrained for the next stroke. The exhaust should close before the inlet on the compression stroke to maximize the charge, but the geometry of the engine may prevent this if the two events are piston.
FIG.: 2 STROKE VALVE TIMING DIAGRAM
controlled. It can be done in an engine with exhaust valves, but otherwise the inlet and exhaust closure in a single-piston engine will mirror their opening. At all events the inlet ports will be closed as many degrees ABDC as they opened before it (i.e. again 130–150° BTDC) and the exhaust in the same region.
