GB2237324A

GB2237324A - Desnodromic valve-actuation mechanism

Info

Publication number: GB2237324A
Application number: GB8924314A
Authority: GB
Inventors: Raymond Joseph Huber
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-10-28
Filing date: 1989-10-28
Publication date: 1991-05-01
Also published as: GB8924314D0

Abstract

Cam discs 4 in halves fixed to flanges 2 on an overhead shaft 1 effect opening and closing of the valve 3 by endless channels 5. Electromagnetically-operated levers 9 may exert a valve closing force to hold the valve 3 on its seat during starting of an i.c. engine. <IMAGE>

Description

DESMODROMIC VALVE ACTUATION WECHANISH This invention relates to desmodromic valve actuation in internal combustion engines.

In normally aspirated engines the route to high power output is high r.p.m. Although rev limits can be raised by lightening or strengthening engine components ultimately, the rev limits are determined by the speeds at which the strength of the engine components can no longer accomodate the forces created by the weight and acceleration of the reciprocating motion.

In practice, improvements in power output derived from increased r.p.m. may be gained at the expense of engine reliability. In the valve gear for example as engine speeds rise so do the inertia forces and the only way to prevent valve float is to increase the strength of the valve springs. This however, increases the load on the valve seats at lower engine speeds and can rapidly hammer them to destruction.

One solution to the problem of valve control at high r.p.m. is desmodromic valve actuation. Whereas on a conventional engine the cam only opens the valve closure being brought about by the action of the spring or springs, desmodromic actuation refers to mechanically closed valves using positive mechanical cam action to shut the valves rather than merely relying on return springs.

According to one aspect of the present invention there is provided a desmodromic valve actuation mechanism comprising a shaft having a number of circular flanges situated at right angles to the shaft axis.

According to another aspect of the present invention there is provided apparatus for opening and closing the valves by means of rotating the shaft. The apparatus includes a number of discs with each disc having a channel or groove in the form of a cam profile. Equally divided into two parts forming base circle and cam lobe respectively each disc is attached to the surface of a shaft flange bringing the two parts together to form a complete cam-shaped channel. For each disc attached to a flange an identical disc is similarly attached to the immediately adjacent flange thus providing a matched pair of inward-facing chaniiels for each valve. The apparatus also includes rectangular shaped valve collars which fit between the discs.On each valve collar there are two pins positioned at right angles to the valve stem winch run in their respective channels in the discs, and each valve collar has Teflon inserts to prevent metal-to-metal contact between the valve collar flanics and the rotating discs. According to another aspect of the present invention there is provided a method of holding the valve securely on its seat including eloctromagnetic control of a lever which bears on the underside of the valve collar.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Fig 1. Shows a sectional view of the overhead shaft with the discs attached and the valve collar with its control lever in position.

Fig 2. Shows a plan view of the valve actuation mechanism indicating the position of the lever and its anchorage point in relation to the valve collar. It also shows the position of the electromagnet in relation to the lever.

Fig 3. Fig 3a shows a sectional view of the valve collar indicating the position of the Teflon inserts. Fig 3b shows a plan view of the valve collar illustrating the use of shims between the upper and lower parts.

Fig 4. Fig 4a shows a plan view of a disc in which the distance between the inner and outer sidewalls of the channel which serve as the bearing surface for the valve-collar pins remains constant throughout. By contrast Fig 4b shows a plan view of a disc in which the outer sidewall or pin bearing surface of the base-circle element of the cam channel is relieved to facilitate valve clearance.

Referring to the drawings the desmodromic valve actuation mechanism comprises a shaft 1 positioned in accordance with normal practice for a direct-acting camshaft but minus the cam lobes. In place of the cam lobes, the shaft I has a pair of circular flanges 2 situated at rightangles to the shaft 1 axis and above each of the valves 3. Two halves of a disc 4 and 4a are attached to the surface of a flange 2 and joined in such a way as to provide a complete cam profile in the form of a channel 5.

An identical two piece disc assembly is attached to the surface of the adjacent flange 2. Removal of both lobe sections 4a of the opposing discs and partial rotation of the shaft 1 enables the protuding pins 6 of the valve collar 7 to be entered into the base circle segment of the discs 4 whereupon the lobe sections 4a are again attached to the discs thus trapping the pins 6 within their respective chaniiels 5. Thereafter the normal rotation of the shaft 1 causes the trapped valve collar pins 6 to follow the route dictated by the twin channels 5 which in turn imparts opening and closing motion to the valves 3.To ensure that the valves remain fully closed for the duration of the requisite valve-closed period for each valve there is a bar 8 situated parallel to the shaft 1 which acts as the anchorage of a forked lever 9 which bears on the underside of the valve collar 7. The forked end of the lever 9 locates in two parallel grooves in the underside of the valve collar 7 and thus prevents transverse motion of the valve collar 7 between the discs 4. An electromagnetic device 10 is mounted above each lever 9 and when activated, each device 10 exerts an upwards force on the lever 9 which acts on the underside of the valve collar to pull the valve onto the valve seat and hold it there until the electromagnetic device is deactivated.The electromagnetic drice is mm activated automatically when the valve collar pins 6 enter the base-circle segment of the cam profile and deactivated when the stipulated valve closed period is completed to permit valve opening under the influence of the camshaped channel to begin. Valve clearances/tolerances are maintained through the use of base circle channels which have some degree of run-out.

As shown in Fig 4 the pin 6 outer bearing surface formed by the channel sidewall is relieved thus the valves are not pulled fully onto their seats by pin 6 contact with the cam channel, but by the forces exerted by the electromagnets' on the levers 9. Further adjustment of valve clearance is possible by the addition or removal of shims between upper and lower sections of the valve collar(s) as shown in Fig 3b.

Claims

1. A desmodromic valve actuation mechanism comprising a shaft fitted with a number of discs each of which has a channel which serves as a cam profile.

2. A mechanism as claimed in claim I wherein the movement of the valves is controlled by both the discs and other mechanisms.

3. A mechanism as claimed in claims 1 and 2 wherein the mechanism comprises electromagnets which act as tolerence springs.

4. A mechanism as claimed in claims 3 wherein the action of the electromagnets is monitored and controlled by the engine management system.

5. A mechanism as claimed in any preceding claim wherein the cam profiles are interchangeable.

6. A desmodromic valve actuation mechanism substantially as described herein with reference to Figs 1 - 4 of the accompanying drawing.