TWO STROKE INTERNAL COMBUSTION ENGINE

An engine requiring one revolution to complete one working cycle.

The Two-Stroke Cycle

The two-stroke cycle, which, as the name implies, is accomplished in one complete revolution of the crank. Two-stroke engines invariably have ports to admit air when uncovered by the descending piston. The exhaust may be via ports adjacent to the air ports and controlled by the same piston (loop scavenge) or via poppet exhaust valves at the other end of the cylinder (uniflow scavenge). Starting at TDC combustion is already under the way and the exhaust opens (EO) at 110-120 deg ATDC to promote a rapid blow-down before the inlet opens (IO) about 20-30 deg later (130-150 deg 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 un-expelled 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 controlled. It can be done in an engine with exhaust valves. At all events the inlet ports will be closed as many degrees ABDC as opened before it (i.e. again 130-150 deg BTDC) and the exhaust in the same region. Injection commences at about 10-20 deg BTDC depending on speed and combustion lasts 30-50 deg, as with the four-stroke.