Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Patent
1992-08-14
1994-02-22
Cross, E. Rollins
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
123 9048, F01L 134, F01L 114
Patent
active
052878309
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a valve control means for controlling the inlet and exhaust valves of an internal combustion engine.
Internal combustion engines for use in, for example, vehicles, must be capable of operation at various engine speeds and loads. The timing of the opening and closing of the intake and exhaust valves must be set to optimise the power output and efficiency of the engine over a reasonable range of speeds and loads.
For example, in a high output, multi-valve, spark ignition four stroke engine which is designed to operate at high engine speeds, it is generally desirable to provide means, such as cams, to control the opening of the inlet valves which preferably have a long valve opening period, in order to maximise the combustible charge drawn into the combustion chambers during the suction strokes of the engine. This has the advantage of improving the volumetric efficiency of the engine, thereby increasing the maximum power and torque outputs of the engine.
However, if such an engine is operated at speeds below that at which maximum power is developed, since the inlet valves are open for a relatively long period, some of the combustible charge drawn into each combustion chamber on its suction stroke can be forced back through the valve before it closes. This effect clearly reduces the volumetric efficiency, and hence the output, of the engine. It also causes uneven engine idling and low speed operation, and also makes exhaust emissions more difficult to control.
It is therefore desirable to additionally provide a valve control mechanism for use only at low engine speeds which has a relatively short operating or opening period.
There have already been a number of proposals for variable valve timing devices in which means are provided for changing the duration of the opening of the valve in an internal combustion engine.
For example in U.S. Pat. No. 4,727,831 a pair of adjacent valves are controlled to operate together by means of rocker shafts and cams. The two valves are normally driven from the camshaft by two low-speed cams (i.e. cams causing the valves to open for a short duration) operating on separate rocker arms for each valve but a third rocker arm is mounted between the two aforesaid rocker arms and is arranged to be driven by a high-speed cam (i.e. a cam causing the valve to open for a long duration). When it is desired to operate the valves via the high-speed cam the third rocker arm is connected to the other two rocker arms so that the valves are both driven via the third rocker arm.
In U.S. Pat. No. 4,475,489 a valve is driven either by a first rocker arm driven by a high-speed cam or a second rocker arm driven by a low-speed cam and means is provided to move the two rocker arms between operative and inoperative positions whereby the valve is driven by either of the rocker arms. There is an overlap between the high-speed and low-speed positions where both rocker arms are driving the valve in order to overcome the problem that if there is no overlap both of the rocker arms will be at intermediate positions at which an undesirable impact takes place between the valve and the rocker arms.
In applicant's co-pending application No. 9003603.9 a value is controlled by a pair of rocker arms which are movable into direct or indirect engagement by high speed or low speed cam means. A locking hydraulic piston arrangement is operable to move a cam follower mounted on one of said rocker arms into engagement with a high speed cam to provide high speed control of the valve. When this arm is retracted the cam follower mounted on the other arm is in sole engagement with a different profile of the cam to provide low-speed control.
GB-A-2017207 illustrates a variable type valve timing mechanism having a tapered finger which in different positions causes different profiles of cam means to engage and control directly or indirectly the tappet mounted on the valve.
According to the present invention there is provided valve control means for an internal combustion engine comprising valve means, cam
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Allen Jeffrey
Dopson Clive
Cross E. Rollins
Group Lotus
Lo Weilun
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