GB2249131A

GB2249131A - Variable compression ratio i.c. engine

Info

Publication number: GB2249131A
Application number: GB9023100A
Authority: GB
Inventors: Cedric Paul Davies
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1990-10-24
Filing date: 1990-10-24
Publication date: 1992-04-29
Also published as: GB9023100D0

Abstract

The pistons 10, 12, 14, 16 are mounted in pairs on rocker beams 24, 26. The rocker beams are connected to journals of a crankshaft 38 so that reciprocating movement of the pistons produces rocking of the rocking beams and rotation of the crankshaft. In order to vary the compression ratio of the engine the axis 34 on which the rocker beams are mounted can be moved up and down by means of eccentric journals (56, 58, Fig. 2) on the rocker shaft 36 or hydraulically movable shaft bearing blocks. <IMAGE>

Description

INTERNAL COMBUSTION ENGINE This invention relates to an internal combustion engine in which the compression ratio is variable.

The invention is particularly concerned with an internal combustion engine in which a pair of cylinders are mounted on a rocker beam on opposite sides of the axis of rotation of the beam, with the beam being rocked by a crank drive.

In an engine of this type, for example as described in GB PS 2 134 980, connecting rods are used to connect the pistons to the rocker beam and as a result of the use of a rocker beam the connecting rods deviate only to a small extent from alignment with the cylinder axis so that side thrusts on the pistons are considerably reduced relative to a conventional engine configuration where the connecting rods are connected directly to crank shaft journals.

According to the invention there is provided an internal combustion engine having at least one pair of cylinders, a piston in each cylinder, piston connecting rods connecting the pistons to opposite ends of the rocker beam, the rocker beam being mounted for rocking movement about an axis, a crankshaft and a crankshaft connecting rod fitted between the crankshaft and the rocker beam, the components being arranged so that reciprocating movement of the pistons in the cylinders produces rocking movement of the rocker beam about the axis and movement of the rocker beam about the axis produces rotational motion of the crankshaft, wherein means are provided for moving the rocker beam axis in a direction parallel to the reciprocating motion of the pistons.

Moving the rocker beam axis up and down in this way will directly vary the compression ratio of the engine. The advantages of being able to vary the compression ratio are well known.

A number of different mechanisms can be used for moving the axis. One of the simplest mechanisms would be to mount the axis on an eccentric disc so that turning of the disc varies the height of the axis. Alternatively, the ends of the rocker beam axis could be mounted in bearing blocks which are displaceable, possibly hydraulically, in a direction parallel to the cylinder axis. However other adjustment mechanisms are also possible.

Preferably the position of the rocker beam axis is varied automatically independent on an engine management signal representative of the load which the engine experiences.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a four cylinder internal combustion engine in accordance with the invention; and Figure 2 shows one scheme by means of which the rocker beam axis can be adjusted.

In Figure 1, four pistons 10, 12, 14, 16 are indicated.

For the sake of clarity the cylinders in which these pistons will move have been omitted from the drawing together with the cylinder head and other components which are not essential to an understanding of the invention.

Each piston is associated with a piston connecting rod 18 by means of an upper gudgeon pin 20. The lower end of each connecting rod 18 is connected by means of a lower gudgeon pin 22 to one of two rocker beams 24, 26. Each rocker beam is made up of two triangular plates 28,30 welded together on a trunnion tube 37. The two rocker beams are both freely rotatable on a common shaft 36 which has a centreline 34.

A crankshaft 38 is mounted on a separate axis 40 which is parallel to the axis 34, and crankshaft connecting rods 42, 44 connect the two journals on the crankshaft 38 to the rocker beams 24, 26 through the lower gudgeon pins 32.

In use, the pistons will be driven up and down in their respective cylinders in a known manner by a combustion process in the cylinders. When for example the piston 10 is driven downwards (see arrow 46), the left hand end of the rocker beam 24 is driven downwards and the rocker beam rocks about the axis 34 (see arrow 48). This produces a pushing force on -e connecting rod 42 which leads to a rotation of the crar.kshaft 38 as indicated by the arrow 50.

The timing of the combustion processes in the various cylinders will be controlled so that as the piston 10 is moving down, the piston 12 is moving up (see arrow 52).

This will cause the rocker beam 26 to rock in the opposite direction to the beam 24, and because the crankshaft journal to which the connecting rod 44 is connected is opposite to the journal to which the rod 42 is connected, both motions will contribute to a rotation of the crankshaft 38 in the direction indicated by the arrow 50.

In addition to the operation of the engine in the manner described, it is also possible to raise and lower the axis 34 (see arrow 54) in order to vary the compression ratio of the engine. By raising the axis 34, the clearance volume in the cylinder at the top of the piston stroke will be reduced. As the compression ratio is calculated from the formula Swept volume + Clearance volume Clearance volume an effective reduction in the compression ratio will be achieved. The swept volume will not of course alter.

Because of the geometry of the arrangement as shown in Figure 1, a single adjustment of the height of the axis 34 will produce an eaual effect on the compression ratio of each cylinder.

Figure 2 shows a mechanism by which the position of the axis can be varied. The shaft 36 is mounted between two eccentric journals 56 and 58 which are mounted in fixed bearing blocks 60, 62. An adjusting lever 64 is fitted to the journal 58. in order to vary the compression ratio, the lever 64 is turned to move the axis of the shaft 36 closer towards or further away from the cylinder bores in which the pistons 10, 12, 14, 16 run.

The mounting of pistons in pairs on a rocker beam produces benefits in reduce piston side thrust in the cylinders.

The reduction in the piston/cylinder friction thus arising can be considerable and may lead to a fuel saving of the order of 10%.

Claims

1. An internal combustion engine having at least one pair of cylinders, a piston in each cylinder, piston connecting rods connecting the pistons to opposite ends of the rocker beam, the rocker beam being mounted for rocking movement about an axis, a crankshaft and a crankshaft connecting rod fitted between the crankshaft and the rocker beam, the components being arranged so that reciprocating movement of the pistons in the cylinders produces rocking movement of the rocker beam about the axis and movement of the rocker beam about the axis produces rotational motion of the crankshaft, wherein means are provided for moving the rocker beam axis in a direction parallel to the reciprocating motion of the pistons.

2. An engine as claimed in Claim 1, wherein the rocker bea axis is mounted on an eccentric so that turning of the eccentric varies the height of the axis.

3. .-n engine as claimed in Claim 1, wherein the ends of the rocker beam axis are mounted in bearing blocks which are displaceable, possibly hydraulically, in a direction parallel to the cylinder axis.

4. An engine as claimed in any preceding claim, wherein the position of the rocker beam axis is varied automatically in accordance with an engine management signal representative of the load which the engine experiences.

5. An internal combustion engine, substantially as herein described, with reference to the accompanying drawings.