The Petrol Engine / A Text-book dealing with the Principles of Design and Construction, with a Special Chapter on the Two-stroke Engine

by Francis John Kean

Fig. 34.—Eccentric Sheave (A) and
Eccentric Rod (B) for operating
a Sleeve Valve.

The camshaft requires to be well supported in bearings to prevent it from sagging or bending under its load. If the shaft and the cams are not made from nickel steel or high-grade steel alloy, they require to be case-hardened (hardened on the surface) to prevent wear on the surfaces due to the pressure of the valve springs, which is considerable and may reach 100 lb. per valve easily; the same applies to the rollers of the tappets. When sleeve valves are fitted to the engine, eccentric sheaves must be used instead of cams, as no springs are employed. An eccentric sheave with its strap and rod are shown in Fig. [34]. The valve shaft or lay shaft is shown at C, and the sheave with the hole bored eccentrically is shown at A, and B is the combined eccentric strap and rod. The pin D operates the sleeve valve, giving it a reciprocating motion in a vertical direction, the angular movement being taken up by the oscillation of the rod about the pin D, which would be fixed into the sleeve. Sometimes a groove is formed round the periphery of the eccentric disc or sheave to keep the strap in position and prevent end movement. As the weight of the sleeves is very considerable, the pin D and the eccentric rod must be well proportioned to prevent breakage or undue wear.

Fig. 35.—A Pair of Timing Wheels.

The Timing Wheels.—As there is only one suction stroke and one exhaust stroke in every two revolutions of the engine crankshaft, it will be clear that the camshaft or eccentric shaft must be driven at half the speed of the engine crankshaft. This may be done by the use of two gear wheels or wheels having teeth cut on their periphery, such wheels when used for this purpose being called timing wheels, because the positions of the cams on the camshaft (or the eccentrics on the eccentric shaft) relative to the engine crankshaft when the teeth of the timing wheels are put into mesh determines the timing of the inlet and exhaust valves, i.e., the instant at which they will open or close. A pair of timing wheels is shown in Fig. [35]. The pinion A has twelve teeth and is keyed to the engine crankshaft, but the wheel B, which is keyed to the valve shaft, has twenty-four teeth, and hence the valve shaft runs at half the speed of the crankshaft. The wheels shown are spur gears, and the teeth run straight across the rim of the wheel; it is, however, quite common to find wheels with curved or helical teeth, as these run quieter. Sometimes when spur gearing is used, one of the wheels is made of fibre and the other of steel, but when helical gears are used the wheels are generally made from nickel steel of high tensile strength. The finer the pitch of the teeth (i.e. the distance between the centres of consecutive teeth) the quieter the gears will run, but the question of strength and the cost of production must also be considered. The latest practice is to use a silent chain drive; this originated with the introduction of the sleeve valve and eccentric shaft. When chains are used for the timing wheels provision must be made for taking up slack in the chain owing to stretching of the links, and as this cannot be done in the usual manner (by sliding the sprocket wheels further apart) owing to the centres of the crankshaft and the valve shaft being rigidly fixed by the bearings, a small jockey pulley (with teeth on it similar to those on the chain sprocket wheels) is provided attached to a short shaft or spindle, which can be raised or lowered at will, and thus keep the correct tension on the chain. The chain drive must be more expensive and require more attention; moreover, it cannot be so very much quieter in action than good well-cut helical gearing.

Fig. 36. and Fig. 37. Two Views of a
Crankchamber.

The Crankchamber.—The crankchamber, as its name implies, is the receptacle which contains and supports the crankshaft and also the camshaft. It is generally an aluminium casting, but frequently for commercial vehicle engines the top portion is made of cast iron and the bottom portion of sheet steel. In either case brass or gunmetal bearings, often lined with white metal, are fitted for the shafts to revolve in, and the engine cylinders are mounted on the top of the chamber. Provision should be made on the sides and ends of the crankchamber for fitting the magneto and oil pump and also the water pump, if required. There must also be some form of housing or extension of the chamber to enclose the timing wheels, and sometimes the whole of the valve gear is contained within the crankchamber to ensure proper lubrication for it and stop any noise from it reaching the outside world. It is also important that there should be large inspection openings fitted with proper oil-tight covers and some provision for easily pouring large quantities of oil down into the lower portion of the chamber. The design of a crankchamber necessitates careful forethought to ensure ample provision for all the necessary attachments and fittings and to secure the maximum accessibility of all parts. One or two vent pipes, consisting of upwardly projecting pipes having their outer end covered with wire gauze and screened from dust should be provided to allow hot air and gas to escape from the chamber.