The Book of the Villiers Engine - A Complete and Fully Illustrated Instruction Manual on the Construction, Running, and Repair of Villiers Engines - Pitman's Motor Cyclists Library
By Cyril Grange
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The Book of the Villiers Engine - A Complete and Fully Illustrated Instruction Manual on the Construction, Running, and Repair of Villiers Engines - Pitman's Motor Cyclists Library - Cyril Grange
ENGINE
CHAPTER I
HOW THE TWO-STROKE ENGINE WORKS
IN the opening and introductory chapter of this book we will, for the benefit of those readers of small technical knowledge and at the risk of boring the more advanced student of motor-cycling matters, outline briefly the principle on which the two-stroke engine works, and in succeeding chapters we will deal with the care, maintenance, and running of the Villiers motor-cycle engines, all of which are of two-stroke design. Before considering the actual cycle of operations, it is necessary first to reflect upon the construction of the two-stroke engine.
The Working Parts. The two-stroke engine is vastly simpler than the four-stroke type ; there is no complicated valve mechanism needing constant adjustment and overhauling. Therein lies the beauty of the two-stroke. Simplicity is the key-note throughout. For utility purposes, and for the rider who has no time for constantly working on his engine, the two-stroke engine has no equal. Moreover, the two-stroke is well known to possess a degree of reliability as yet unattained by other types of power units.
Fig. 1 illustrates the moving parts of a two-stroke engine, and it will be noticed that they are three in number ; the PISTON, the CONNECTING ROD, and the CRANKSHAFT ASSEMBLY. Let us deal with them in this order.
The Piston. This is a cylindrical body of cast iron or aluminium alloy fitted with spring rings which enable a gas-tight fit constantly to be maintained between the piston and the walls of the cylinder in which it reciprocates. A pin known as the gudgeon pin passes through the diametrical centre, and forms the axis for the small-end bearing of the connecting rod. The top of the piston is shaped to form a deflector, the duty of which will be understood upon referring to the cycle of operations explained later.
The Connecting Rod. The connecting rod is a stiff H
section rod connecting the piston to the crankshaft, the duty of which is to transmit the downward thrust of the piston to the crankshaft, thereby converting reciprocating, into rotary movement. Its lower end reciprocates on the crank pin, and this is known as the big-end bearing.
The Crankshaft Assembly. This comprises the crank pin with nuts, the cranks with their projecting shafts, and the balance weights. This crankshaft is carried by means of bearings in each side of the crankcase, in which the shaft rotates. Mounted on the crankshaft outside the crankcase is a heavy-rimmed fly-wheel, which stores up energy and helps the engine to run evenly, at the same time enabling it to maintain its revolutions when the power impulses have ceased.
FIG. 1. THE MOVING PARTS OF A TWO-STROKE ENGINE
All other parts of the engine are stationary, such as the carburettor, which supplies an explosive mixture, the magneto which generates a spark at the plug points and causes the explosive charge to be fired. These two components are described in separate chapters later.
Bolted on to the crankcase is the cylinder in which are ports for the entry and exit of the gases.
An internal combustion engine derives its power from gases which are forced into the cylinder, where they are exploded, and the consequent pressure developed forces down the piston which, in turn, rotates the crankshaft. This cycle of operations is achieved in the simplest manner in a two-stroke engine. There are types of internal combustion engines requiring valves with complicated operating mechanisms, or in some instances, special sleeves which have to describe involved rotating and reciprocating movements, but in the two-stroke engine the piston itself is arranged to cover all these duties, thus enabling the minimum of parts to be employed to give the same results. Apart from its simplicity, the two-stroke engine has a big advantage in giving a power impulse for every revolution, whereas other types of engines only give a power impulse for two revolutions. It will be understood, therefore, that the torque, which means the power thrust, is much more even and, in the two-stroke, has no harshness.
FIG. 2. CYCLE OF OPERATIONS OF A TWO-STROKE ENGINE
Above are shown Four Consecutive Phases
Let us now study the actual cycle of operations which is made clear by the following description and reference to Fig. 2.
The Two-stroke Cycle. It is important that the crankcase of a two-stroke engine should be perfectly gas-tight, because this compartment acts as a compression chamber. The piston itself compresses the gases at the top and the underside of its head. The diagrams, Fig. 2, show clearly the various ports in the cylinder. The carburettor which supplies the mixture to the engine is attached to the inlet port, whilst the transfer port is a connecting passage between the crankcase and the cylinder barrel. The exhaust port is, of course, the final exit for the burned gases.
Assume, first of all, that the piston is at the bottom of its stroke and is about to ascend, thus creating suction in the crankcase ; on ascending the piston will uncover the inlet port from the carburettor, as shown in Fig. 2 (B), allowing fresh gases to enter the crankcase. Meanwhile, in the combustion chamber the burned gases from a previous explosion have been driven out through the exhaust port and the piston is compressing a fresh charge (Fig. 2 (C)). When the gases are compressed in the cylinder head, a spark occurs at the sparking plug igniting these gases, causing an explosion which forces the piston on its next downward stroke, and in descending it compresses the gases which have been collected in the crankcase, and the top of the piston first uncovers the exhaust port, allowing the exploded gases to pass from the combustion chamber, and then a little farther down its stroke uncovers the transfer port. The compressed gases in the crankcase then force their way through the transfer port, and are directed by means of the deflector of the piston head upwards into the combustion chamber (Fig. 2 (A)). The object of the deflector on the piston is now apparent. If the piston had a flat top the gases from the transfer port would shoot straight across, and a big proportion would be lost through the exhaust port. The deflector therefore projects the gases upwards, and they actually help to push out burned gases. Thus the cycle of operations is continued, the gases being drawn from the carburettor, compressed in the crankcase, transferred to the cylinder head, then exploded and expelled:
It will be seen that the functioning of a two-stroke engine, therefore, is exceedingly simple, and, probably most important of all, there are no delicate parts to get out of order. The absence of gear wheels and tappets for valve operating mechanism alsó makes the two-stroke engine mechanically silent.
CHAPTER II
TYPES OF VILLIERS TWO-STROKE MOTOR-CYCLE ENGINES
THE Villiers Engineering Co., Ltd., market various different sizes and types of engines ranging from 98 c.c. (1 h.p.) to 346 c.c. (3 1/2 h.p.). Here it might be convenient to point out that c.c. means cubic centimetres,
and represents the actual volume of the cylinder swept by the piston. As a convenient formula, 100 c.c. is taken to represent 1 h.p., so that it will be seen that the small 147 c.c. engine is really 1.47 h.p., or 1 1/2 h.p.
Actually, the power developed by each of the engines is considerably more than the nominal figure obtained by this formula, which, however, is a convenient method of describing them. The range of Villiers engines covers all needs, and includes models for low priced utility motor-cycles ; for sports machines, and for lightweight sidecar