Poppet Valves.—Valves are provided for the purpose of controlling the admission of the mixture to the cylinder and also for controlling the exhaust or ejection of the burnt gases at the end of the firing stroke. The most common form of valve is the mushroom or poppet type of valve shown in Fig. 29, in which A is the valve head, B is the valve stem, C is the valve seating, and D is the cotter hole for the cotter E. It will be seen that the general appearance of the valve is a disc of steel with a fine stem to it similar to a mushroom in general outline—hence its name. The valve has a coned face which is kept pressed down on a coned seating by means of the pressure of a powerful spring F acting on the washer G, which bears against the cotter E and thus presses down the valve stem. To ensure that the valve shall always come down correctly on its seating and make a gas-tight joint, the valve stem guide M is provided.

The cam H raises the valve off its seat at the required instant when the motion of the camshaft brings the cam under the roller R. The cam lifts the roller vertically and with it the tappet or push rod K, which slides vertically upwards in the guide P and lifts the valve. The tappet is provided with an adjustable head S kept in position by the locknut T. To adjust the clearance between the head of the tappet and the underside of the valve stem the locknut T must first be slackened back and then the head S can be screwed up or down as desired, the best clearance being about 1/64 of an inch; the locknut is then tightened down again. During this operation the valve must be down on its seat. Sometimes to reduce the noise arising from the tappet striking the valve stem, the head of the tappet is padded with some material such as hard vulcanite fibre, but this wears down more quickly than steel and requires frequent adjustment. The latest device for reducing the noise arising from the valve mechanism consists in totally enclosing the valve gear and springs either by metal plates bolted to the cylinder casting or by extending the crankchamber to cover it all in, and then it is certain to be well lubricated. The exhaust valve is always liable to give trouble either from leakage or seizure or other causes due to the great heat of the exhaust gases, so that the valves are often made now of tungsten steel alloy which is not much affected by heat. If a mushroom type valve leaks it can be ground in and made a tight fit on its seating, provision usually being made for this in the form of a slot cut in the valve head, as shown in Fig. 32, for the insertion of a screwdriver or special tool. To grind in a valve, remove the cap Q by unscrewing it, raise the spring F by pushing up the washer G and then withdraw the cotter E. Lift out the valve and smear the coned face with fine emery powder and oil (or water). Put the valve back and turn it to and fro on its seating by means of the screwdriver, keeping a firm pressure down on it; continue the operation until by examining the valve you ascertain that it touches on the seating all the way round, then couple up the spring again, after carefully removing all traces of the emery powder.

Sleeve Valves.—Another form of valve which has come very much into favour is the sleeve valve, two views of which are shown in Figs. 30 and 31. In this case the gases enter the cylinder through ports or slots P cut in the cylindrical cast iron sleeves S₁, S₂, which are placed between the piston K and the walls of the water-jacketed cylinder C. These sleeves are moved up and down inside the cylinder, while the piston travels up and down inside the inner sleeve S₂ just as though it constituted the cylinder C. Some engines have two sleeves, as shown in the figure, but others have only one sleeve, and there is very little to choose between the two types on the score of efficiency. The great claim made for the sleeve valve is that it is almost noiseless in action and gives very much fuller openings for inlet and outlet of the gases. The piston has the usual number of packing rings to keep it gas-tight, and there is also a deep packing ring provided in the head of the cylinder H to keep the sleeve S₂ gas-tight and prevent loss of compression pressure. The head of the cylinder is usually detachable, and has often separate water connexions in the form of pipes leading from the cylinder jackets. The sleeves receive their reciprocating motion from eccentrics and rods attached to pins shown at the bottom right-hand corner of each sleeve. It might be expected that the sleeves would get very hot or very dry and seize up, or the piston might seize, but in actual practice this has not occurred to any great extent, and on the whole they have been very successful. It is, however, necessary to keep the engine well lubricated, especially when the sleeves get worn, as the oil prevents loss of gas by leakage past the sleeves and piston. In Fig. 31 the two sleeves have come together in such a position that the ports coincide with the exhaust ports cut in the cylinder walls and therefore the exhaust is full open, and as the sleeves travel at times in opposite directions quick opening and closing of the ports is secured. The cylinder head is held down to the cylinder casting by screws or bolts and can be readily detached for cleaning or inspecting the interior of the cylinder. The great objection raised against the sleeve valves is their excessive weight and the unmechanical manner in which they are operated.

The Camshafts and Eccentric Shafts.—These are usually made from the same material as the crankshaft and machined from the solid bar, the projecting cams or eccentrics being afterwards cut to the correct shape. In the case of a camshaft it is very important that the shape of the cams should be such that they lift the valves quickly off their seats to the full extent of their opening (or lift), keep them open for as long a period as desirable, and then allow them to close quickly but without shock. Cams which have straight sides are more in favour than those with curved sides, but if the action of the cams is to be theoretically correct the side of the cam should be curved in such a manner that the valve is lifted at first with a uniformly increasing speed and afterwards with a uniformly decreasing speed, so that it will be at rest in its top position. If this is not done the valve tappet may jump a little above the cam each time the valve is lifted. In Fig. 33 the cam A is intended for the inlet valve and the cam B for the exhaust valve, the essential difference being that the exhaust valve must be kept open longer than the inlet valve, and therefore the exhaust valve cam is the wider of the two. The timing of the inlet and exhaust valves of an up-to-date engine may be explained by considering the crankpin circle as divided into 360 parts or degrees. If there were no lag or lead in the opening of the valves, then they would open when the crank was on its dead-centre and close when the crank was on its dead-centre. The inlet valve would open when the crank was on its top dead-centre and close when it had reached its bottom dead-centre, this representing the suction stroke of the engine. Then would follow compression and explosion, giving two strokes or one revolution before the exhaust valve commenced to open. The exhaust valve would then open when the crank was on its bottom dead-centre and close when the crank reached its top dead-centre corresponding to the completion of the exhaust stroke. It is very important that the pressure of the gases above the piston when it commences to move upwards on the exhaust stroke should be as low as possible, and this can only be secured by opening the exhaust valve towards the end of the explosion or power stroke, thus allowing the bulk of the gases to escape and leaving the piston with little resistance to encounter on its upward exhaust stroke. Therefore we give the exhaust valve a lead of about 30 degrees, which means that it begins to open when the engine crank is 30 degrees from the bottom dead-centre on the downward explosion stroke, and we give it a lag of about 5 degrees in closing. This means that we keep the exhaust valve open until the crank has moved 5 degrees over the top centre, so that we may fully utilize the momentum of the gases to clear out the cylinder or scavenge it. As the piston moves rapidly up the cylinder on the exhaust stroke it pushes the gases in front of it out through the exhaust opening, but when it gets to the top of its stroke the piston stops and then comes down again for the suction stroke, whereas the gases will tend to keep on moving if they are not unduly restricted in their passage through the exhaust system, so that we can generally obtain some slight advantage by giving the exhaust valve a small amount of lag in closing.

The pressure of the gases in the cylinder after the exhaust valve closes will nearly always be a little above atmospheric pressure, and therefore nothing is gained by opening the inlet valve immediately the exhaust closes—we generally allow an interval of 5 degrees, which means that the total lag of the inlet valve is 10 degrees in opening, or the inlet valve does not begin to open until the crank has moved 10 degrees off its top dead-centre on the downward suction stroke. At the end of the suction stroke the piston will again come to rest before moving up on the compression stroke, but the gases will continue to rush into the cylinder from the carburettor owing to their momentum if we leave the inlet valve open a little longer, hence we generally give it a lag of 20 degrees in closing, which means that the inlet valve does not close until the crank has moved 20 degrees up from the bottom dead-centre on the compression stroke.

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.

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.

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.

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.