This invention is concerned with a piston for use in an engine or motor, the piston comprising a crown and a skirt projecting from the crown.

For the purposes of this specification, a piston is considered to have a longitudinal axis disposed generally centrally of the crown and the skirt, which axis extends along the cylinder in which the piston reciprocates. Although pistons are not generally truely cylindrical about their central axis, the term "circumferential" is used herein to indicate a direction generally around the longitudinal axis in a plane extending normal to said longitudinal axis.

The skirt of a piston has two thrust surfaces disposed on opposite sides of the longitudinal axis of the piston. These thrust surfaces engage the wall of the cylinder and guide the piston in its motion. It is known from GB Patent Specification 2104188 to provide the skirt of a piston with pads on each of two thrust surfaces thereof, the pads being arranged to transmit lateral thrust forces between the piston and the wall of a cylinder in which the piston reciprocates. The pads are provided with ramps at their circumferentially-extending edges to cause lubricant to be forced over the pad by hydro-dynamic action during reciprocation of the piston. Thus, not only is there a reduction in the contact area between the skirt and the wall of the cylinder but also there is a consequential reduction of the viscous losses in the oil lubricating the skirt. This effectively reduces friction and thereby increases the efficiency of the engine or motor.

It is an object of the present invention to provide pistons in which the pads on the thrust surfaces of the

skirt are arranged to optimise the reductions in friction.

The invention provides a piston for use in an engine or motor, the piston comprising a crown and a skirt projecting from the crown, the skirt having two substantially continuous thrust surfaces disposed on opposite sides of a longitudinal axis of the piston, at least one of the thrust surfaces having pads projecting therefrom to transmit lateral thrust forces between the piston and the wall of a cylinder in which the piston reciprocates, the pads being integral with the remainder of the thrust surface and having ramps at their circumferentially-extending edges to cause lubricant to be forced over the pad by hydro-dynamic action during reciprocation of the piston, characterised in that there are two such pads on said at least one of the thrust surfaces and each pad extends circumferentially substantially completely across the thrust surface and has the ratio of its circumferential width to its axial length substantially greater than unity.

In a piston according to the invention, by comparison with the piston known from GB 2104188, the same total area of pad can be achieved but with pads having substantially less axial length. This enables better oil film formation to be achieved and gives the possibility of shorter piston skirts.

Preferably, the ratio of the circumferential width to the axial length of each pad is at least 4:1, more preferably at least 6:1.

In order to augment the supply of oil, apertures through the skirt may be provided between the two pads. Such apertures may be in the form of a row of holes extending parallel to said pads.

A piston according to the invention may have two similar pads on its other thrust surface so that both thrust faces are the same. Alternatively it may be advantageous if the thrust surfaces have different arrangements of pads to suit the different conditions experienced thereby. For example, one thrust surface may have two pads as described above while the other has three pads as described in GB 2104188 in relation to Figure 2 thereof. Furthermore, it may be advantageous if the two pads on a thrust surface differ dimensionally from each other.

Where the skirt is not a full skirt, i.e., does not occupy the full circumference of the piston, it is possible for the shorter edges of the pads to correspond with edges of portions of the skirt, thereby avoiding the necessity for machining side ramps as described in GB Patent Specification 2104188.

The pads, preferably, project by between 12 and 125 microns beyond the remainder of the skirt and the ramps are, preferably, inclined at an angle of less than 2 degrees.

There now follow detailed descriptions, to be read with reference to the accompanying drawings, of two pistons which are illustrative of the invention.

In the drawings:

Figure 1 is a side elevational view of a first illustrative piston; and

Figure 2 is a side elevational view of a second illustrative piston.

The first illustrative piston 10 is for use in an internal combustion engine and comprises a crown 12 provided with the usual grooves 14 for the reception of

piston rings. The piston 10 also comprises a skirt 16 projecting from the crown 12 and providing support bosses (not shown) in which a gudgeon pin can be mounted to connect the piston to a connecting rod (not shown) . The skirt 16 is, in this case, a partial skirt. In other words, it occupies only a proportion of the circumferential extent of the piston, this proportion being made up of two diametrically opposed portions. In particular, the skirt 16 provides two substantially continuous thrust faces 20 which are disposed on opposite sides of the longitudinal axis 22 of the piston 10. Projecting from each thrust surface 20 are two pads 24 which are arranged to transmit lateral thrust forces between the piston 10 and the wall of a cylinder in which the piston reciprocates (not shown) .

Each pad 24 is integral with the remainder of the thrust surface 20, i.e. they are cast and machined from the same piece of metal. Each pad 24 extends circumferentially substantially completely across the thrust surface 20. Furthermore, the ratio of the circumferential width W to the axial length L of each of the pads 24 is substantially greater than unity, being in this case approximately 6. Each pad is also provided with ramps 25 at its circumferentially-extending edges in the same manner as is described in GB Patent Specification No. 2104188 to cause lubricant to be forced over the pad by hydro-dynamic action during reciprocation of the piston. The pads 24 are also provided with ramps 27 at their axially edges as is also described in GB 2104188. It will be observed that the pads 24 are mutually similar and arranged one above the other. The pads 24 project approximately 100 microns beyond the remainder of the thrust surface 20 and the ramps 25 and 27 are inclined at angles of approximately 1.5 degrees.

Between the pads 24, apertures through the skirt 16 are provided in each thrust surface 20 in the form of a row of holes 26 extending parallel to the pads 24. These holes

26 have the purpose of enabling lubricant to enter the space between the pads 24 so that it can augment the oil left on the cylinder wall after passage of the leading pad

24.

It will be observed that, as a result of the natural rocking action of the piston during reciprocation in the cylinder, the thrust forces on the pads 24 will be unequal for most of the engine operating cycle. The pads 24 are located in such a way as to minimise the extent of the rocking action.

The second illustrative piston 40 shown in Figure 2 has a crown 42 with grooves 44, and a partial skirt 46. The piston 40 has two pads 54 on each of two thrust surfaces 50 thereof. These pads 54 extend circumferentially substantially completely across the thrust surface 50. The ratio of the circumferential width to the axial length of each of the pads 54 is substantially greater than unity but this ratio differs between the upper and lower pads 54 because the pads 54 extend right to the edges of the portion of the skirt 46 on which they are formed so that, because of a taper in the skirt 46, the upper pad 54 has a greater circumferential width than the lower pad 54. Because they extend to the edges of the skirt portions, the pads 54 are not provided with ramps as described in GB 2104188 at their axial edges but do have such ramps 55 at their circumferential edges. Holes 56, similar to the holes 26, are provided between the pads 54 to augment the oil supply.