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The reciprocating piston petrol engine
Published in M.J. Nunney, Light and Heavy Vehicle Technology, 2007
Its operating cycle begins each time an engine valve closes, at which point the tappet dwells upon the base circle of the cam and the plunger return spring extends the tappet assembly to absorb any clearances that exist in the valve train. As any extension of the tappet will create a depression beneath the non-return valve of the plunger, the valve opens so that oil under pressure from the engine lubrication system is admitted to the tappet compression chamber (Figure 1.101a). This is formed between the closed end of the barrel and the underside of the plunger. As soon as the cam begins to lift the tappet, the increased pressure on the trapped oil maintains the non-return valve closed, so that the column of oil behaves like a rigid strut to transmit the opening load developed between the cam and the engine valve (Figure 1.101b). There is an intentional slight leakage of oil from the compression chamber, which takes place between the plunger and its operating bore in the tappet barrel. This controlled leakage is known as tappet leak-down, and its purpose is to ensure that the engine valve always returns fully to its seating, as once again the tappet returns to the base circle of the cam ready for the next operating cycle.
Tribology of Automotive Components
Published in Ahmed Abdelbary, Extreme Tribology, 2020
In an engine, the opening and closing of valves is invariably controlled by cam/tappet mechanism. This mechanism usually consists of two moving elements: The rotating cam and the tappet (follower) mounted on a fixed frame. The cam is driven from the crankshaft in order to ensure synchronization of the valve motion with the combustion cycle and piston movement. In general, there are different types of surface contact between a cam and a follower and, according to surface contact, they are classified into roller, flat, knife edge and spherical follower, as shown in Fig. 7.11.
Experimental investigation of Te addition on chilled cast iron tappet manufacturing process
Published in Materials and Manufacturing Processes, 2021
The tappet, alternatively termed as cam follower or valve lifter, is the part that runs on the camshaft and is made to move vertically following the rotational profile of the cam. The tappet drives a long, thin pushrod that actuates the rocker arm, making the valves go downwards to open them. Being the most important functional part of an engine, the quality of the tappet has become a critical issue for engine performance. Typically the tappet surface undergoes many defects such as pointed pitting, desquamation, corrosion due to substantial surface contact, high sliding speed between the camshaft lobe and cam lifter, and high temperature caused by the mechanical movement. These defects result in the catastrophic failure of tappet material.[1]