GB1498460A - Hydraulic valve lifter for internal combustion engine - Google Patents

Abstract

1498460 Hydraulic tappet NISSAN MOTOR CO Ltd 30 Dec 1975 53199/75 Heading F2K A conventional self-adjusting I.C. engine hydraulic tappet assembly 10 is constructed to provide a controlled amount of lost-motion between the tappet 10 and an engine valve (not shown) so as to vary the lift and opening period imparted to the valve from a cam 12 in response to engine speed and hence the engine oil pressure in the tappet. As shown, the tappet assembly 10 includes an auxiliary chamber the volume of which is variable in response to engine oil pressure and hence engine r.p.m. and is determined by a chamber formed between the seating 32 of a main chamber balance, piston 84 and a floating piston 76 the position of which is controlled by the engine oil pressure acting in the main chamber 50, to downwardly bias the floating piston 76, in opposition to a return spring 88 acting beneath the floating piston 76 to upwardly bias the piston 76 towards its seating position on the balance piston 84, as shown, the volume of the auxiliary chamber thus varying from zero, as shown, when the engine speed and hence oil pressure is low, to the length (NIL to d) of the floating piston bore 28 when the engine speed and oil pressure is increased and the floating piston 76 is downwardly moved by such higher oil pressure to a position balanced in the bore 28 between the engine oil pressure and the return spring 82, the arrangement thus enabling the hydraulic tappet to operatively vary the opening and closing times and the lift of the inlet and exhaust valve of the engine cylinders in response to engine speed whereby the engine achieves its maximum performance efficiency at maximum speed and reduces the valve overlap period to zero during idling (Fig. 2). Operation.-With the engine idling and thus producing low engine oil pressure and with the tappet 10 seated on the cam base circle, i.e. with the associated engine valve closed, the floating piston 76 is upwardly seated by the spring 82 against the balance piston 84 which is itself seated on face 32 (as shown in Fig. 3), the engine oil pressure in the main chamber 50 being overridden by the spring 82. As the cam 12 commences to lift the tappet 10 the valve operating resistance reacts through the usual push-rod 16 and its seating plunger 42, 46 to further pressurize the main chamber 50 and thus overcome the spring 82 and downwardly move the floating piston 76 whereby the volume of the main chamber 50 becomes partially transferred to form the auxiliary chamber and thus proportionately lower the push-rod plunger 42 in the tappet body such that part of the cam lift is absorbed as lost motion in the tappet so that the valve lift is reduced and the valve opening and closing times, i.e. the opening periods reduced to provide zero overlap (see Full line in Fig. 2). At maximum engine speed, the engine oil pressure is increased to maximum pressure such that even when the tappet is seated on the cam base circle the spring 82 is rendered ineffective and the floating piston is maintained downwardly seated against face 30a with the plunger 42 (as before in Fig. 3) providing zero back-lash on the closed engine valve push-rod 16. As the cam commences to lift the push-rod the full cam lift is thus imparted to the push-rod whereby the maximum valve lift is effected and the valve timings modified to increase the opening period and provide maximum valve overlap as shown in dotted lines in Fig. 2. At engine speeds between idling and maximum the floating piston 76 partially downwardly moves between the seatings 32 and 30a when the tappet is on the cam base circle so that as in the engine idling condition when the cam starts to lift the tappet the increased reaction load pressurizes the main chamber 50 to further move the floating piston the remaining downward distance to its lower seating 30a to thus reduce the effective cam lift to proportionately vary the valve lift and timings, as before, in accordance with the engine speed, such lifts and timings being appropriately positioned between the full and dotted valve envelopes of Fig. 2 according to the particular engine speed.