As its designation implies, the two-stroke petrol engine (Figure 1) completes its working cycle in only two strokes of the piston, so that a combustible charge is ignited at every revolution of the crankshaft. Although in its easy construction the two-stroke petrol engine needs no valves, the induction and exhaust process should be facilitated by a system of scavenging or forcible clearing of the cylinder gases. This may either take the form of a separate engine-driven pump, or use the motion of the engine piston itself in a sealed crankcase. The flow of gases entering and leaving the cylinder is controlled by the reciprocating motion of the engine piston, which thus acts as a slide valve in conjunction with ports cut in the cylinder wall. Although the two-stroke petrol engine was once favored in Europe for some little inexpensive passenger cars, it usually became obsolescent because of the difficulty in reducing its harmful exhaust emissions.


Fig.1 Cross-section of a two-stroke petrol engine (Honda).

At the 11th Sir Henry Royce Memorial Lecture in 1966, Cyril Lovesey recalled a remark by an additional distinguished Rolls-Royce aero engine specialist, Dr Stanley Hooker, who humorously described a four-stroke engine as one with ‘one stroke to produce power and three strokes to wear it out’. It is therefore maybe not surprising that from time to time attempts are made to restore interest in the two-stroke petrol engine for automotive use, albeit in much more sophisticated forms of which an illustration will be later described.

It may be of interest to recall that the two-stroke and the four-stroke engine both originated in the late 1870s, so it might sensibly be assumed that both types of engine started out in life with an equal chance of success. The fact that the four-stroke engine became by far the more extensively adopted type can most likely be explained by its having a greater potential for further development. This is a criterion that can often be applied to opponent ideas in all branches of engineering.

The first successful application of the two-stroke cycle of operation to an early gas engine is usually attributed to a Scottish mechanical engineer, Sir Dugald Clerk (1854–1932).

It is for this reason, of course, that the two-stroke cycle is occasionally referred to as the Clerk cycle. Dugald Clerk, like several other pioneer researchers of the internal combustion engine, was later to attain high academic distinction, culminating in his election as Fellow of the Royal Society in 1908.

A separate pumping cylinder scavenged the Clerk engine. A few early motor vehicle two-stroke petrol engines followed the same principle, but it later became established practice to use the underside of the piston in conjunction with a sealed crank chamber to shape the scavenge pump. This idea was patented in 1889 by Joseph Day & Son of Bath, England and represented the simplest type of two-cycle engine.

In the two-stroke or Clerk cycle, as applied by Day, the following sequence of events is incessantly repeated all the time the engine is running (Figure.2):


Fig.2 The two-stroke petrol engine cycle.

1. The induction-compression stroke.

A fresh charge of air and fuel is taken into the crank chamber as a result of the depression created under the piston as it advances towards the cylinder head. At the same time, last compression of the charge transferred earlier in the stroke beginning the crank chamber to the cylinder takes place above the advancing piston.

2. The power-exhaust stroke.

The combustible charge in the cylinder is ignited instantly preceding the power stroke, during which the gases enlarge and perform useful work on the retreating piston. At the same time, the previously induced charge trapped under the retreating piston is partially compressed. Towards the end of the stroke, the exhaust gases are evacuated from the cylinder, a process that is facilitated by the scavenging stroke of the new charge transferred from the crankcase.

The uncovering and covering of the cylinder ports of the piston, or port timing, is determined by considerations like to those affecting the valve timing of the four-stroke engine .

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