Quite a few of the commercial vehicles use the unit power plant in which the gear box or transmission is bolted directly to a housing cast integral with the case and surrounding the flywheel, or by separate arms which are bolted to the rear motor arms. However this method is very rarely resorted to in the heavier types owing to the large size of the transmission, its weight and the necessity of providing a separate jack-shaft for chain driven vehicles.
Like all other units of the commercial car, there are various designs which have been worked out and giving excellent results, and the writer presents illustrations of the types conforming to the general outline of the motors referred to.
The writer will not endeavor to describe the various methods of mounting the engine in the vehicle frame, as this can be done more clearly by devoting a chapter to this feature which will cover all of the present methods pertaining to all units mounted in the frame.
Fig. 16 shows the valve operating mechanism used on a prominent L-head commercial car motor, illustrating how the valves are located side by side. The European type of valve, with conical seat and the most popular method of enclosing the valves to protect them from dust and foreign matter, are shown. The pushrod guides are pressed into the cylinder and can readily be removed from the bottom of the same. When the cylinder is removed from the crank case the entire valve mechanism, excepting the cams and their shaft, is removed with the cylinder, a noteworthy feature when the overhauling of an engine is considered. The pushrod is of the mushroom type and the outline of the cam is formed of curves, to further assist in_ obtaining a quieter valve action. The conventional type of pushrod adjustment incorporating a hardened steel screw and lock nut is used. This view also shows the large bearing provided for the cam shaft so that it may be removed through the forward end of the case. Helical teeth are used on the timing gears and the end thrust is taken by a hardened steel pin supported by a spring which transmits this thrust to hardened steel washer on the timing gear housing cover.
Fig. 17 illustrates the method of operating the valves when they are located .side by side in the cylinder head. The pushrods are similar to those previously described above, however, the adjustment is omitted here and incorporated with the rocker arm mounted on top of the cylinder. These rocker arms pivot on a shaft and their opposite ends bear against the valve stems. The operating rod placed between the push-rod and the adjustment in the rocker arm has a ball-shaped end to reduce wear at this point.
Fig. 18 illustrates the overhead type of valve mechanism for valve in the head cylinders with valves set at an angle on opposite sides and operated through one camshaft located on the cylinder heads. It will be noticed that a very light spring is used for the intake valve, while a very stiff spring is used for the exhaust valve. The object being to make the intake valve partly automatic, owing to the fact that but one nicker arm is used to operate both valves. The nicker arm of course replaces the pushrods and the adjustment is incorporated in the ends of the arms. The nicker arm has a roller bearing against the cam. and these two parts are kept in contact by a large spiral spring. The camshaft bearings are divided, but require considerable attention, as a wick feed is resorted to for supplying lubricant. The cam, roller and other parts are exposed so that grit and foreign matter can reach them, and it is a contraction very hard to lubricate due to the intense heat on the cylinder heads.
The combination type of valve arrangement of course em-bodies the features of those just described. Fig. 16 can be applied to the valve in the pocket, while Fig. 17 would apply to the valve in the head or in the pocket when these are placed one above the other.
Various methods are resorted to in fastening the push rod guides into the cylinders or case. They may either be pressed in position, held down in pairs by clamps or bolted down.
Fig. 16. Showing Valve Mechanism of L-Head Motor.
Fig. 17. Showing: the Method of Operating Valve when located in Cylinder Head.
Fig. 18. Overhead Type of Valve Mechanism. Valves set at an Angle.
These various constructions may he applied to most any valve locution. The principal change would come in the crank case, which is dependent upon the number of camshafts that are necessary to operate the valves. Enclosing the valve stems and push-rods is now quite popular.
Fig. 19 illustrates a bottom view of a T-head crank case in which the two camshafts with the respective parts are clearly shown. This view also shows a three-bearing crankshaft with the connecting rods fastened in position on the shaft and in the piston. The case is divided and all bearings are held in position by separate caps, while the earns are of considerable width, as the exhaust camshaft can he moved longitudinally so as to provide a compression release for cranking the motor. This is accomplished through the use of stepped exhaust cams which open the exhaust valve part way during a portion of the compression stroke while cranking the motor. The water pump is located at the front end of the motor and is driven through separate gears meshing with camshaft gears. The oil pump is located on the rear motor arm and is driven through a ratchet and link connection with a valve pushrod.
Fig. 19. Bottom View of a T-Head Crank Case. Showing two Can Shafts.