Fig. 10 shows an engine of the sleeve-valve type; in this case having two sleeves for oper ating the intake and the exhaust by the succes sive covering and uncovering of valve ports in the upper portions of the cylinder walls. First proposed in 1906 as an adjunct of an automobile exhibition in Chicago, this engine was subse quently perfected in England and France and is now widely used. The silence of its opera tion stimulated designers of poppet valve en gines to the improvements by which the latter have also been made silent, mainly by better gears for camshaft, pump, magneto and fan and by the boxing of valve mechanism.
Another type of sleeve-valve engine, with a single sleeve, is also in successful use, in Eng land, where it is made. Many efforts have been made to produce automobile engines with ro tary valves, usually arranged in the nature of a valve tube placed horizontally through the heads of the engine cylinders and arranged to open and close the ports in the cylinder heads by rotation and periodic registry with ports in the tube; but only one rotary valve engine is in practical use and has a vertical rotating sleeve at the top . of each cylinder. It is of Italian manufacture.
By looking over Figs. 6 to 10 one notices in all of them a number of features adapted for removing the shortcomings of the stationary engine. The castings are thin, reducing weight and permitting the heat to escape to the cooling water with the required rapidity. Normally a centrifugal pump is employed to drive the heated water from around the combustion chamber and the valves into the top portion of the radiator and further into the finely sub divided ducts in the latter, where it is exposed to the cooling effect of the atmosphere, the air being drawn rapidly through the interstices, partly by movement of the vehicle and partly by the suction from a fan placed immediately behind the radiator structure. This is either tubular or cellular, the former being more ro bust and the latter more finely subdivided. Often no centrifugal circulation pump is used and the water is circulated by reason of its own variations of temperature, this ‘thermo syphon* system to be effective requiring a con siderable difference in height between the top of the engine and the top of the radiator, on the same principle that makes a high chimney create a stronger draft than a low one.
The ignition has kept pace with the increas ing demands made upon it. So long as engine
speeds were low, a primary or small storage battery produced a current whose voltage was raised by means of an induction coil with trembler. The magneto gradually took the place of this equipment, being adapted for pro ducing a spark more promptly and always ready to work when mechanically actuated from the engine. Recently the employment of engine starters requiring the use of a storage battery has rendered it possible to dispense with the magneto for ignition, and improved induction coil devices are often used instead of it. See Ictsmots; MAGNET() Fig. 11 shows an example of the aluminum crankcase in which the shaft and the connect ing-rod knuckles turn and whose lower portion, known as the sump, is usually removable and serves as a reservoir for lubricating oil. The usual system is to lubricate the pistons and cylinder walls by splash; that is, by the oil splashed from a shallow tray placed over the sump by the rapid rotation of the connecting rod ends, which are often provided with small scoops for this purpose. The bearings of the shaft are oiled from a gear pump operated in the lowest part of the sump, the oil being either forced through a continuous duct in the shaft and crankarms and pins or through a pipe above the shaft bearings, whence it is fed to the latter by gravitation.
Pistons and connecting-rods are made as light as possible, the latter either of alloy steel or of aluminum. A steel piston lightened by per forations in its skirt is shown in Fig. 12. Pis tons and cylinders of aluminum are the excep tion but are not uncommon, although this metal expands and contracts about three times as much as iron or steel by changes of tempera ture. Aluminum cylinders are so far normally lined with thin steel.
Mufflers or silencers for reducing the noise of the exhaust gases formerly consumed an appreciable amount of power by hindering the escape of the gas—sometimes one to two horse-power— and by the back pressure and re tarding of the flow contributed greatly to over heating of the engine. The improved construc tions, while regulating, cooling and contracting the gases, create less resistance than would be encountered if the gas were released at its first temperature and sped directly against the atmosphere. Yet in most racing machines it is so released, perhaps with an eye to the spec tacular effect.