pressures to which the piston and cylinders are exposed while the engine is running at the lower loads. This is important both for smooth running and good wearing qualities. Theoretically, better results could be obtained from the point of view of economy by retaining a constant compression pressure, constant charge of air, and producing ignition, somewhat in the manner of the Diesel engine. Such a method, however, would have the disadvantage of producing practically the same maximum pressure for all loads, and this would tend to give an engine which would not run smoothly at slow speeds.
As has been said, tube ignition was speedily abandoned for electric ignition by accumulator, induction coil distributor and sparking plug. This in its turn was largely displaced by the low-tension magneto system, in which the spark was formed between contacts which were mechanically separated within the cylinders. The separable contacts gave rise to complications, and at present the most popular system of ignition is undoubtedly that of the high-tension magneto. In this system the ordinary high-tension sparking plugs are used, and the high-tension current is generated in a secondary winding on the armature of the magneto, and reaches the sparking plugs by way of a rotary distributor. In many cases the high-tension magneto system is used for the ordinary running of the engine, combined with an accumulator or battery and induction coil for starting the engine from rest. Such systems are called dual ignition systems. Sometimes the same ignition plugs are adapted to spark from either source, and in other cases separate plugs are used. The magneto systems have the great advantage of generating current without battery, and by their use noise is reduced to a minimum. All electrical systems are now arranged to allow of advancing and retarding the spark from the steering wheel. In modern magneto methods, however, the spark is automatically retarded when the engine slows and advanced when the speed rises, so that less change is required from the wheel than is necessary with battery and coil.
Sir Oliver Lodge has invented a most interesting system of electric ignition, depending upon the production of an extra oscillatory current of enormous tension produced by the combined use of spark gap and condenser. This extra spark passes freely even under water, and it is impossible to stop it by any ordinary sooting or fouling of the ignition plug.
The most popular engines are now of the four and six cylinder types.
Fig. 5 shows a modern four-cylinder engine in longitudinal and transverse sections as made by the Wolseley Company. A, A are the cylinders; B, B, water jackets; G', oil scoops on the large ends of the connecting-rods. These scoops take up oil from the crank chamber. Forced lubrication is used. The oil pump M is of the toothed wheel type, and it is driven by skew gearing. An oil sump is arranged at L, and the oil is pumped from this sump by the pump described. The overflow from the main bearings supplies the channels in the crank case from which the oil scoops take their charge. It will be seen that the two inside pistons are attached to cranks of coincident centres, and this is true of the two outside pistons also. This is the usual arrangement in four-cylinder engines. By this device the primary forces are balanced; but a small secondary unbalanced force remains, due to the difference in motion of the pistons at the up and down portions of their stroke. A six-cylinder engine has the advantage of getting rid of this secondary unbalanced force; but it requires a longer and more rigid crank chamber. In this engine the inlet and exhaust valves of each cylinder are placed in the same pocket and are driven from one cam-shaft. This is a very favourite arrangement; but many engines are constructed in which the inlet and exhaust valves operate on opposite sides of the cylinder in separate ports and are driven from separate camshafts. Dual ignition is applied to this engine; that is, an ignition composed of high-tension magneto and also battery and coil for starting. U is the high-tension magneto. Under the figure there is shown a list of parts which sufficiently indicate the nature of the engine.
An interesting and novel form of engine is shown at fig. 6. This is a well-known engine designed by Mr Knight, an American inventor, and now made by the Daimler and other companies. It will be observed in the figure that the ordinary lift valves are entirely dispensed with, and slide valves are used of the cylindrical shell type. The engine operates on the ordinary Otto cycle, and all the valve actions necessary to admit charge and discharge exhaust gases are accomplished by means of two sleeves sliding one within the other.
The outer sleeve slides in the main cylinder and the inner sleeve slides within the outer sleeve. The piston fits within the inner sleeve. The sleeves receive separate motions from short connecting links C and E, driven by eccentrics carried on a shaft W. This shaft is driven from the main crank-shaft by a strong chain so as to make half the revolutions of the crank-shaft in the usual manner of the Otto cycle. The inlet port is formed on one side of the cylinder and is marked I. The exhaust port is arranged on the other side and marked J. These ports are segmental. A water-jacketed cylinder head carries stationary rings L, K, which press outwards. These are clearly shown in the drawing. The inner sleeve ports run past the lower broad ring L when compression is to be accomplished, and the contents of the cylinder are retained within the cylinder and compression space by the piston rings and the fixed rings referred to.
Fig. 6.
The outer sleeve does not require rings at all. Its function is simply to distribute the gases so that the exhaust port is closed by the outer sleeve when the inlet port is open. The outer sleeve acts really as a distributor; the inner sleeve supplies the pressure tightness required to resist compression and explosion. The idea of working exhaust and inlet by two sleeves within which the main piston operates is very daring and ingenious; and for these small engines the sleeve valve system works admirably. There are many advantages; the shape of the compression space is a most favourable one for reducing loss by cooling. All the valve ports required in ordinary lift valve engines are entirely dispensed with; that is, the surface exposed to the explosion causing loss of heat is reduced to a minimum. The engines are found in use to be very flexible and economical.
The petrol engines hitherto described, although light compared to the old stationary gas engines, are heavy when compared with recent motors developed for the purpose of aeroplanes. Many of these motors have been produced, but two only will be noticed here—the Anzani, because Bleriot's great flight
