Two views of a crankchamber of modern design are shown in Figs. [36] and [37]. In these figures A is the top half of the crankchamber which rests upon the chassis or framework of the car, being bolted to an underframe at B and C. The cylinders are attached to the chamber at the flange H by means of studs and nuts. This portion, the top half of the crankchamber, requires to be very strong and stiff, because the upward pressure of the explosions acts on the crown of the cylinder and tends to tear the cylinder off the flange H, while at the same time it exerts a great force on the piston, pushing it downwards and tending to force the crankshaft down out of its bearings. In the best practice the whole weight of the crankshaft is supported from the top half of the crankchamber and is carried on the bearing bolts as shown at S, so that they also receive the downward thrust of the piston and in their turn transmit it to the main casting.
The bottom half of the crankchamber then becomes merely an oil container, or reservoir, and dust cover; it should be so arranged and situated that it may be readily removed for inspection of shaft and bearings from underneath. Sometimes the crankchamber has long arms, which can be attached directly to the side members of the chassis, or it may be supported in the chassis by a tubular cross member.
In Fig. [37] the camshaft is shown at T; the magneto would be carried on the bracket E and driven by gearing from the crankshaft. The facing at G is for the water pump, which, in this case, is intended to be mounted on an extension of the camshaft T. The oil pump would be fixed at F, preferably towards the rear of the engine, so as to secure an adequate supply of oil for the pump when the car is climbing a steep hill. The oil could be drawn off and the reservoir emptied by unscrewing the large plug shown in the centre of D in Fig. [37]. The timing wheel housing or casing is shown at Q; the oil ducts and connexions for supplying the main bearings with oil are not shown in these drawings, nor are the inspection openings and covers. The upper half of the crankchamber frequently becomes very hot, due to conduction of heat from the metal of the cylinders, and for this reason it has from time to time been proposed to draw the air supply of the carburettor through the crankchamber to serve the dual purpose of cooling the bearings and heating the air supply to the carburettor; but the idea has not found favour, as there is considerable risk of dust and grit finding its way into the bearings and causing trouble due to abrasion.
[CHAPTER V]
THE CARBURETTOR AND CARBURATION
A carburettor is a contrivance for supplying an explosive mixture of air and petrol vapour to a petrol engine. Petrol, although a liquid fuel, is a combination of carbon and hydrogen which, when supplied with the necessary air, can be burnt and thus evolve heat, which heat is turned into work inside the engine cylinder. What we have to supply to the engine is really a mixture of air and petrol vapour in certain proportions, such a mixture being often spoken of as carburetted air on account of the carbon contained in it. About two parts of petrol vapour (by volume) are required to every one hundred parts of mixture, or fifteen pounds of air to every pound of petrol vapour (by weight). This carburetted air must be of the required strength and form a homogeneous mixture in the form of a vapour. The problem of carburation consists in forming a mixture of the correct strength and character. Air may be carburetted by passing it over the surface of liquid petrol in a surface carburettor, or by drawing it over or among wicks saturated with liquid petrol as in the wick type of carburettor, but both these methods have been largely superseded by the use of what is now known as a jet or spray type of carburettor, in which the petrol is sprayed from a fine jet and mixes with air which is passing up rapidly round the outside of the jet. In all cases, however, the liquid petrol must be vaporized before entering the engine, and to do this heat must be supplied to the mixture, just as water has to be heated before it can be vaporized and turned into steam. Under ordinary circumstances sufficient heat can be obtained from the incoming air to effect vaporization of the liquid petrol if it issues in the form of a very finely divided spray, but when the demand for mixture, from the engine, is great the air cannot supply the requisite heat without its temperature falling below the vaporization point; hence most carburettors of up-to-date pattern are fitted with a mixing chamber surrounded by a hot-water jacket. The essential features of the carburetting plant are shown diagrammatically in Fig. [38], in which A is the petrol tank fitted with the petrol tap G, to which is coupled the petrol pipe F. Some form of petrol filter as indicated at B should be placed between the tank and the carburettor C. The throttle valve of the carburettor is shown at H, the extra-air valve at E, and the engine induction pipe at D.
Fig. 38.—General arrangement of the Carburetting Plant, showing Petrol Tank (A), Petrol Filter (B), Carburettor (C), and Extra-air Valve (E).
The carburettor proper may be constructed in a variety of forms, but the elements of which it is composed are: (1) the float chamber A, (2) the petrol jet B, (3) the choke tube C, (4) the mixing chamber D, and (5) the throttle valve E, as shown in Fig. [39].