GB2228533A

GB2228533A - I.c.engine valve gear

Info

Publication number: GB2228533A
Application number: GB8830261A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1988-12-24
Filing date: 1988-12-24
Publication date: 1990-08-29
Also published as: WO1990010141A1; GB8830261D0

Abstract

A twin cam engine is described having a primary and a secondary inlet valve (118, 120) for each engine cylinder and means (144) for varying the opening phase of the secondary inlet valves (120) relative the primary inlet valves (118). The first of the overhead camshafts (110) is provided with cams (150, 152, 164) for all the exhaust valves (114, 116) and the primary inlet valves (118) and the second camshaft (112) is formed with cams (158) for only the secondary inlet valves (120), whereby phase shifting of the second camshaft (112) relative to the first camshaft (110) causes a phase shift between the secondary inlet valves and the primary inlet valves without affecting the exhaust event.

Description

INTERNAL COMBUSTION ENGINE The invention relates to a twin cam engine having a primary and a secondary inlet valve for each engine cylinder and means for varying the opening phase of the secondary inlet valves relative the primary inlet valves.

It is has been proposed in copending European Patent Application No. 87306843.1, that by allowing the inlet valve to remain open during part of the compression stroke of a four stroke engine, it is possible to reduce the trapped mass of the charge and thereby achieve part load performance without throttling the intake passage.

This can be achieved in the case of an engine with two intake valves per cylinder by introducing a major phase shift between the two inlet valves.

In known combustion engines of the type having three or four valves per cylinder operated by twin camshafts, it is usual for there to be an inlet camshaft controlling all the inlet valves and an exhaust camshaft for the exhaust valves. If in such a lay out, one wishes to vary the phase of the secondary inlet valves relative to the primary inlet valves, then concentric cams are required which makes for a complicated and an expensive construction. The strength of the camshafts is also impaired tending to reduce the engine reliability.

According to the present invention, there is provided a twin cam engine having a primary and a secondary inlet valve for each engine cylinder and means for varying the opening phase of the secondary inlet valves relative the primary inlet valves, wherein the first of the overhead camshafts is provided with cams for all the exhaust valves and the primary inlet valves and the second camshaft is formed with cams for only the secondary inlet valves, whereby phase shifting of the second camshaft relative to the first camshaft causes a phase shift between the secondary inlet valves and the primary inlet valves.

It is possible for the ports to be of conventional lay out, that is to say with all the inlet valves lying to one side of the engine and the exhaust valves on the other. In this case, the cams on the first camshaft can act on the primary inlet valves by way of rockers.

Such a configuration is however difficult to achieve because of packaging problems and it is preferred that the valves on one side of the engine should be the secondary valves operated directly by the second camshaft and that both the exhaust valves and the primary inlet valves should be arranged along the other side of the engine for operation by the first camshaft.

In the latter case, the intake ports for the primary valves should preferably cross the engine centre line to the same side as the intake ports for the secondary inlet valves in order to enable conventional inlet and exhaust manifolds to be employed. The primary and secondary valve of an individual cylinder may, if necessary, share the same duct but alternatively it is possible to provide separate ducts for each valve opening on to the same side of the engine.

It is preferred to provide only a single exhaust valve per cylinder as the provision of four valves makes location of the spark plug troublesome. It is however possible to provide two exhaust valves per cylinder and in this case the exhaust valves preferably share a common exhaust port which may be bent to afford space for the spark plug.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic representation of the cylinder head of a first embodiment of the invention, Figure 2 is a similar view of a second embodiment of the invention, and Figure 3 shows an embodiment having four valves per cylinder.

Figure 1 shows the port arrangement of one cylinder of a multi-cylinder engine having twin overhead camshafts 10 and 12. The camshaft 10 passes over the valves of the exhaust ports 14 and the primary intake ports 16. The camshaft 12 on the other hand passes over the second intake ports 18 and over the spark plug holes 20. In this embodiment, all the valves are operated directly by cams mounted on the respective camshafts 10 and 12, the primary intake valves and the exhaust valves being operated by cams on the camshaft 10 and the secondary intake valves being operated by the cams on the camshaft 12.

If it is desired to use late inlet valve closing in order to reduce the engine volumetric efficiency, then it is only necessary to introduce a phase shift between the two camshafts. The camshafts 10 and 12 can be solid and there is no necessity to resort to hollow camshafts.

It is possible either to alter the phase of both camshafts relative to the engine crankshaft or only the phase of the second camshaft 12. In the latter case the exhaust timing remains constant and the effect of the relative phase shift is only to alter the duration of the intake valve opening in each cycle.

It is usual to have in an engine an exhaust manifold on one side and an intake manifold on the other. It is therefore preferred that both intake ports should enter the cylinder head from the same side of the engine. In the case of the embodiment of Figure 1, the intake ports branch off from the same intake duct 22. In the embodiment of Figure 2, on the other hand, two separate intake ducts 24 and 26 lead to the respective intake ports 16, 18 from the same side of the engine. It will also be noted that in Figure 2 the two intake ports are of unequal size.

In the case of the embodiment of Figure 3, each of the cylinders has two exhaust valves 30, 32 and two intake valves 34, 36. The spark plug 38 is arranged centrally between all four ports. The exhaust ports of each cylinder lie on opposite sides of the engine centre line as do the intake ports. One of the camshafts in this embodiment must control both exhaust ports and the primary intake ports while the other camshaft controls only the secondary intake ports. It is not feasible to introduce a significant phase shift the operation of the exhaust valves and for this reason both exhaust valves must be controlled by the same camshaft. Because the exhaust valves are arranged on opposite sides of the engine centre line, one of the exhaust valves must be operated indirectly, such as by means of a rocker.

In the three valve embodiments described above, the valves have always been acted upon directly by the cams and the port arrangement has been modified in order to permit one intake valve and one exhaust valve to be operated by cams on the same camshaft This however is rt essential and it is alternatively possible to resort to a conventional port arrangement with one or two exhaust valves on one side of the engine centre line and two intake valves on the other and to operate one set of intake valves indirectly through the action of rockers following intake cams on the exhaust camshaft. Such an arrangement does result in packaging problems and is not the most preferred from the point of view of ease of manufacture.

Claims

1. A twin cam engine having a primary and a secondary inlet valve for each engine cylinder and means for varying the opening phase of the secondary inlet valves relative the primary inlet valves, wherein the first of the overhead camshafts is provided with cams for all the exhaust valves and the primary inlet valves and the second camshaft is formed with cams for only the secondary inlet valves, whereby phase shifting of the second camshaft relative to the first camshaft causes a phase shift between the secondary inlet valves and the primary inlet valves.

2. An engine as claimed in claim 1, wherein the ports are arranged with all the inlet valves lying to one side of the engine centre line and the exhaust valves on the other and wherein cams on the first camshaft act on the primary inlet valves by way of rockers.

3. An engine as claimed in claim 1, wherein the primary intake valves are arranged on same side of the engine centre line as the, or one set of, exhaust valves and are directly operated by cams mounted on the first camshaft which includes cams for the exhaust valves.

4. An engine as claimed in claim 3, wherein the intake ducts for the primary valves cross the engine engine centre line to the same side as the intake ducts for the secondary inlet valves.

5. An engine as claimed in claim 4, wherein the primary and secondary intake ports have separate intake ducts opening onto the same side of the cylinder head.

6. A twin cam engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.