Page:EB1911 - Volume 18.djvu/959

HEAVY] steam is used, and where the whole of the parts are maintained in the best condition. The amount of petroleum spirit, or of paraffin, required to propel a steam vehicle 1 m. would, other conditions being equal, propel a vehicle fitted with an internal-combustion engine over a distance of 3 m.

The essential parts of any internal-combustion system are: the carburetter; the engine; the radiator; the clutch; the change-speed gears and the final transmission. The carburetter is a vessel in which the liquid fuel is converted into a combustible gas or Vapour, for, as there is no Connexion to any gas main, the ordinary petrol engine has to make its gas “on the premises.” The production of the gas is automatic, and calls for practically no attention from the driver, because, once the engine is started, the necessary aspiration to draw through the correct quantities of air and fuel is provided by the action of the valves and pistons.

A smart turn of the starting handle is required to set the pistons and crankshaft in motion, so that an initial supply of the combustible mixture may reach one of the cylinders. This first charge of gas is automatically ignited by an electric spark, the current for which is furnished and controlled without the necessity for any hand regulation, and there is then nothing further for the driver to do, as regards power, except to move a convenient lever which opens or closes a “throttle” valve between the cylinders and the carburetter.

An internal-combustion engine would get very hot if no precaution were taken to cool it, and it is usual to surround the cylinder with water spaces. These spaces are called jackets, and the water is forced through them, either by a pump or by thermo-siphon (natural circulation) action. It is expedient to keep down the weight of water, and for that reason pipes, tubes or small boxes are built up in such a manner that a large cooling surface is exposed to the air. A fan, which is driven from the crankshaft of the engine by gear or a belt, is employed to aid this cooling by reason of the increased volume of air that passes round the outside of the components of the radiator members. The general scheme is the same, both for heavy and light motor-cars.

It is very important that the driver should have a convenient means of separating the engine from the driving mechanism, and of putting the two in connexion again, whenever it becomes necessary, without jar or shock. The common practice is to use a leather-faced, circular member with a coned face, and to control the amount of “grip” between this member and a corresponding enclosing member attached to the engine flywheel by means of a pedal and springs. When the driver wishes to disengage the two members, he has merely to depress the foot lever. It will be clear that a clutch of this description can be made to engage without any difficulty, there being no fixed positions or steps such as one associates with the ordinary jaw-clutch, and this gradual application of the load can only be accomplished by the aid of two or more surfaces in frictional contact, and by the holding together of these surfaces by the pressure of one or more strong springs. The Hele-Shaw multiple-disk clutch gives very good results, and is easy for drivers to use in traffic.

An internal-combustion engine cannot develop power unless the crank-shaft can rotate at a relatively high number of revolutions, and the rate of doing work is lowest when the angular velocity is at its minimum. It is, therefore, necessary to introduce a system of levers between the engine and the road wheels, in order to permit the number of revolutions of the crankshaft to be maintained when hill climbing, or when the vehicle is carrying a heavy load, and the common practice is to introduce three or four sets of different sizes of toothed wheels, any pair of which can be put into engagement by the movement of a single lever, which lever is placed near the driver’s right hand as a rule. The lowest of these gear ratios, i.e. the one which allows the crankshaft to make the greatest number of revolutions to one revolution of the road wheels, is required for starting purposes, and the highest gear ratio, i.e. the one which allows the road wheels to make the greatest number of revolutions in relation to those of the crankshaft, is employed for high-speed travelling on the road. From the last change-speed shaft the power must be transmitted to the road wheels through a differential gear and through one or other of the types of final drive which are now employed by representative makers. The great distinction from the axle of a horse-drawn vehicle is that there must be both a mechanical connexion, yet a differential action, between the two back wheels. The wheels on horse vehicles revolve loosely on the axle, and one can overrun the other at curves, but the special device known as the “differential gear” has to be introduced into all motor vehicles between the change-speed gears and the driven road wheels. Such a device permits one of the two driving wheels to be driven round at a quicker angular speed than the other, the difference being determined by the radius of the curve around which the vehicle is turning.

The most common form of final drive is, perhaps, that in which two “roller” or “silent” chains transmit the power from