Fig. 29.—General Arrangement of a Poppet Valve (A) with Tappet (K) and Cam (H).
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.
Fig. 30.—Sectional Drawing of the Cylinder of a Sleeve Valve Engine, with Inlet Ports uncovered.
Fig. 31.—Sectional Drawing of the Cylinder of a Sleeve Valve Engine, with Exhaust Ports uncovered.
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.