Engine Operation 18 UENR4490-01 Valve System Components The engine lubricating oil enters the lifter (4) through a non-return valve. The engine lubricating oil increases the length of the lifter (4) until all valve lash is removed. If the engine is stationary for a prolonged period the valve springs will cause the lifter (4) to shorten so that when the engine is started engine valve lash is present for the first few seconds. After cranking restores oil pressure the lifter (4) increases in length and removes the valve lash. When load is removed from a lifter (4) during service work by the removal of the rocker shaft the lifter (4) increases in length to the maximum extent.
Refer to Systems Operation, Testing and Adjusting, “Position the Valve Mechanism Before Maintenance Procedures” for the correct procedure. During reassembly of the rocker shaft the engine must be put into a safe position to avoid engine damage. After load is imposed on the lifters by reassembling the rocker assembly, the engine must be left in safe position for a safe period until the lifters have reduced to the correct length. Refer to Disassembly and Assembly, “Rocker Shaft and Pushrod - Install” for the correct procedure. Upward movement of the pushrod against rocker arm (2) results in a downward movement that acts on the valve bridge (1).
This action opens a pair of valves (7) which compresses the valve springs (6). When the camshaft (5) has rotated to the peak of the lobe, the valves are fully open. When the camshaft (5) rotates further, the two valve springs (6) under compression start to expand. The valve stems are under tension of the springs. The stems are pushed upward in order to maintain contact with the valve bridge (1). The continued rotation of the camshaft causes the rocker arm (2), the pushrods (3) and the lifters (4) to move downward until the lifter reaches the bottom of the lobe. The valves (7) are now closed. The cycle is repeated for all the valves on each cylinder.
Illustration 15 g01924293 Valve system components (1) Bridge (2) Rocker arm (3) Pushrod (4) Lifter (5) Camshaft (6) Spring (7) Valve The valve system components control the flow of inlet air into the cylinders during engine operation. The valve system components also control the flow of exhaust gases out of the cylinders during engine operation. The crankshaft gear drives the camshaft gear through an idler gear. The camshaft (5) must be timed to the crankshaft in order to get the correct relation between the piston movement and the valve movement. The camshaft (5) has two camshaft lobes for each cylinder. The lobes operate either a pair of inlet valves or a pair of exhaust valves.
As the camshaft turns, lobes on the camshaft cause the lifter (4) to move the pushrod (3) up and down. The lifter (4) incorporates a hydraulic lash adjuster which removes valve lash from the valve mechanism. The lifter (4) uses engine lubricating oil to compensate for wear of system components so that no service adjustment of valve lash is needed.