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Mechanical Science Handbooks P2

DIESEL ENGINES pushrods and rocker arms transfer the reciprocating motion generated by the camshaft lobes to the valves and injectors, opening and closing them as needed. The valves are maintained closed by springs. As the valve is opened by the camshaft, it compresses the valve spring. The energy stored in the valve spring is then used to close the valve as the camshaft lobe rotates out from under the follower. Because an engine experiences fairly large changes in temperature (e.g., ambient to a normal running temperature of about 190°F), its components must be designed to allow for thermal expansion. . | Diesel Engine Fundamentals DOE-HDBK-1018 1-93 DIESEL ENGINES pushrods and rocker arms transfer the reciprocating motion generated by the camshaft lobes to the valves and injectors opening and closing them as needed. The valves are maintained closed by springs. As the valve is opened by the camshaft it compresses the valve spring. The energy stored in the valve spring is then used to close the valve as the camshaft lobe rotates out from under the follower. Because an engine experiences fairly large changes in temperature e.g. ambient to a normal running temperature of about 190 F its components must be designed to allow for thermal expansion. Therefore the valves valve pushrods and rocker arms must have some method of allowing for the expansion. This is accomplished by the use of valve lash. Valve lash is the term given to the slop or give in the valve train before the cam actually starts to open the valve. The camshaft is driven by the engine s crankshaft through a series of gears called idler gears and timing gears. The gears allow the rotation of the camshaft to correspond or be in time with the rotation of the crankshaft and thereby allows the valve opening valve closing and injection of fuel to be timed to occur at precise intervals in the piston s travel. To increase the flexibility in timing the valve opening valve closing and injection of fuel and to increase power or to reduce cost an engine may have one or more camshafts. Typically in a medium to large V-type engine each bank will have one or more camshafts per head. In the larger engines the intake valves exhaust valves and fuel injectors may share a common camshaft or have independent camshafts. Figure 10 Diesel Engine Valve Train Depending on the type and make of the engine the location of the camshaft or shafts varies. The camshaft s in an in-line engine is usually found either in the head of the engine or in the top of the block running down one side of the cylinder bank. Figure 10 provides an example