What Is Virtual Tappet?

                  Unlike Aux Engines which have a manual tappet to adjust, there must be a tappet clearance for the Main Engine as well. But with the absence of a rocker arm and spring –how do we adjust the so called TAPPET?

    The answer lies in the fact that there is no real tappet in the main engine.

                  The Exhaust valve is actuated hydraulically by oil from a hydraulic pump and is shut by spring air, when the spring air pressure exceeds the hydraulic oil pressure. This happens when the roller (of the pump that is actuating the hydraulic valve) is resting on the base circle of the cam.

                  There is a valve called a throttle valve that is fitted on the hydraulic pump unit (which actuates the opening of the exhaust valve). 

                 When the system is just put into use the oil is relatively cold. After some time of operation the oil heats up and expands. This additional oil can make the exhaust valve stay open – leading to reduced Pcomp, Pmax, reduced power and burning of the valve seat.

                 In order to prevent the valve being open due to the expansion of hot oil, the throttle valve bleeds off a small quantity of oil each time the valve closes.

                   If the throttle valve bleeds off too much oil then the valve can BOUNCE shut, thus damaging it.This adjustment of the throttle valve bleed-off quantity is called VIRTUAL TAPPET.

                Correct setting of the VIRTUAL TAPPET ensures positive closing of the valve in a smooth manner without bouncing.

About the Author 

Mr.Joel Andrew Pinto is a shrewd and expert sailor, presently working with Anglo Eastern.He exhibits keen interest in innovative technologies as well as in compiling maritime notes. Catch him on Facebook.