The Two-Stroke Cycle

Picture shows the sequence of events in a typical 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 (or air piston where the engine has two pistons per cylinder). The exhaust may be via ports adjacent to the air ports and controlled by the same piston (loop scavenge) or via piston-controlled exhaust ports or poppet exhaust valves at the other end of the cylinder (uniflow scavenge).

The principles of the cycle apply in all cases. 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 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. The inlet closure (IC) may be retarded relative to exhaust closure (EC) in an opposed piston engine to a degree which depends on the ability of the designer and users to accept greater out-of-balance forces. 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. Where there are two cranks and they are not in phase, the timing is usually related to that coupled to the piston controlling the air ports. The de-phasing is described as ‘exhaust lead’.Injection commences at about 10–20∞ BTDC depending on speed, and combustion occurs over 30–50∞, as with the four-stroke engine.