Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Reexamination Certificate
2000-03-22
2001-05-08
Lo, Weilun (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
C123S090120, C123S19800E
Reexamination Certificate
active
06227154
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to internal combustion engines of reciprocating piston type and is concerned with the valvegear of such engines. More specifically, the invention relates to valvegear for an engine of reciprocating piston type including a cam mounted to rotate on a camshaft and in contact with a cam piston which is arranged to be moved within a pressure chamber by the cam away from the camshaft to pressurise a fluid within the pressure chamber, an engine valve arranged to be moved into the open position against the force of a restoring spring by a valve piston arranged to be moved by the pressure produced in the pressure chamber, an accumulator piston arranged within an accumulator chamber to be moved against the force of an accumulator spring by the pressure acting in the accumulator chamber, and selectively operable valve means arranged to connect the pressure chamber and the accumulator chamber.
DESCRIPTION OF THE PRIOR ART
The inlet and exhaust valves of internal combustion engines are conventionally operated by respective cams on one or more cam shafts which act on the associated valve either directly or indirectly via a rocker arm and optionally a tappet. It is however also known to operate a valve of an engine by means of a hydraulic fluid contained within a pressure chamber which is exposed to a movable valve piston acting on the valve and to a movable cam piston which is moved cyclically by an associated cam to pressurise the fluid in the space. This increased pressure acts on the valve piston and moves it and thus opens the associated valve.
U.S. Pat. No. 4,671,221 discloses valvegear of this type in which a cam acts on a cam piston to pressurise oil in a pressure chamber. A valve piston, which is coaxial with the cam piston and is loosely connected to a valve stem, is acted on by the oil pressure prevailing in the pressure chamber. Oil is supplied to the pressure chamber via an oil supply conduit from a supply chamber or a spring accumulator which includes an excess pressure valve. The oil supply conduit has two parallel branches. One branch includes a magnetic directional control valve and the other branch includes a check valve. A third branch communicates with the oil supply conduit between the first two branches and the accumulator and is supplied with oil from a reservoir. In use, rotation of the cam produces a cyclical movement of the cam piston and, when the magnetic valve is closed, the pressure in the pressure chamber acts on the valve piston and produces opening and closing movement of the valve. However, if the magnetic valve is opened whilst the cam piston is moving to pressurise the oil in the pressure chamber, oil in the chamber is displaced into the accumulator where it moves an accumulator piston against the force of a restoring spring. As the cam piston moves on its return stroke, any oil in the accumulator is returned to the pressure chamber through one of the branches.
The known valvegear has a number of advantages. Thus it permits the lift of the valve, that is to say the distance by which the valve is lifted away from its seat to be varied. Reduced lift may be desirable e.g. at low engine speed so as to increase the inlet velocity of air into the cylinder and thus to maximise the degree of swirl in the cylinder. It also permits the duration of the lift to be varied, that is to say the length of time for which the valve is open. Ideally the lift duration would be varied with engine speed to optimise trapping efficiencies during the intake stroke. The duration may also be varied to vary the overlap between the intake and exhaust flows, for instance for the purpose of exhaust gas recirculation. The known valvegear also permits the valve, and thus optionally the associated cylinder, to be totally deactivated, that is to say not to open at all.
However, the valve lift and the duration of the valve lift may not be altered truly independently in the known valve gear. Thus if, for instance, the initial movement of the cam causes oil to flow to the accumulator by virtue of the fact that the magnetic valve is open, then initially the valve does not move at all. If the magnetic valve is then closed, the valve starts to open and its lift is less than the maximum value. When the closing ramp of the cam engages the cam piston the valve closes earlier than usual because it has a shorter distance to travel. The change in lift and change in duration are therefore inextricably linked.
In a conventional engine the valve is fully closed as the valve stem or follower moves from contacting the closing ramp of the cam to the base circle of the cam. However, there are circumstances in which it would be desirable for the valve to remain open longer than would normally be the case. This is totally impossible with the known valvegear.
Finally, if the duration of the lift of a valve operated by the known valvegear is reduced, the valve moves along a time/displacement curve which is effectively a sinusoidal curve with the bottom portion cut off. This means that the valve approaches and then strikes against the valve seat at great speed which can result in damage to, and ultimately destruction of, the valve and its seat. There is no way in which this can be prevented in the known valvegear.
It is therefore the object of the present invention to provide valvegear for an engine, particularly for an inlet valve, which overcomes the problems and disadvantages referred to above and which is more versatile than the known valvegear.
SUMMARY OF THE INVENTION
According to the present invention, valvegear of the type referred to above is characterised in that the valve piston is movably arranged in an actuating chamber and that second selectively operable valve means is provided which is connected to control means and is arranged selectively to connect the pressure chamber to the actuating chamber.
The first and second valve means may take various forms, such as hydraulic servo valves, but it is preferred that they are servo valves. It is also preferred that the servo valves are integrated into a single three-port solenoid valve.
Thus the principal distinction of the present invention over U.S. Pat. No. 4,671,221 is that the cam piston and valve piston are not exposed to the same chamber but are exposed to separate chambers, communication between which may be selectively produced or interrupted by a valve, preferably a solenoid valve. Whilst this distinction may superficially appear to be very minor, the advantages it produces are very substantial.
Thus if it is desired to reduce the lift of the inlet valve, this may be done in substantially the same way as in the valvegear of U.S. Pat. No. 4,671,221. If it is desired to vary the duration of the lift of the valve, this may be done by altering the time at which the valve opens in a manner generally similar to that in U.S. Pat. No. 4,671,221. However, it may also be done by varying the time at which the valve closes and this is not possible in the valvegear disclosed in the prior document in which closing of the valve is determined solely by the motion of the associated cam. Thus in the valvegear in accordance with the present invention the communication between the pressure chamber and the actuating chamber may be interrupted once the valve lift has reached a predetermined value, thereby “freezing” the engine valve. The communication between these two chambers may be re-established at any desired time determined by the control means, thereby permitting the valve to close again, and it will be appreciated that the valve may therefore be arranged to close at a time substantially after that at which it would have closed if there had been a direct mechanical connection between the cam and the engine valve or if the cam piston and valve piston were exposed to the same chamber. Accordingly, the valve lift and the duration of the valve lift may be varied truly independently and the duration of the lift may be extended to a time longer than that for which the opening and closing ramps of the cam are in cont
Lo Weilun
Ricardo Inc.
Rosenthal & Osha L.L.P.
LandOfFree
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