Piston for an opposed-piston engine

Abstract

In an opposed-piston engine, a piston has a top land. The piston top land has a non-cylindrical shape which affords more clearance with a piston bore to thrust and anti-thrust sides than to front-facing and rear facing sides.

Claims

1. A combination for an opposed-piston engine, comprising: at least one cylinder; a pair of pistons with end surfaces, which are disposed for reciprocating movement in a bore of the cylinder with their end surfaces facing; each of the pistons comprising: a tubular body with a wristpin boss arrangement; the tubular body capped by a crown having an end surface configured to form a combustion chamber with an end surface of the other piston; the crown having a top land with two first sides, respectively facing in a thrust direction and an anti-thrust direction, and two second sides, respectively facing in an engine-rear direction and facing in an engine-front direction; in which thrust and anti-thrust directions are orthogonal to the engine-front direction and the engine-rear direction; and, in which the two first sides are configured to have a greater clearance with the bore than the two second sides.

2. The combination of claim 1, the crown comprising a ring belt region, wherein the top land is between the ring belt region and the end surface.

3. The combination of claim 2, the ring belt region comprising at least one ring groove, wherein the top land is between a top edge of the at least one ring groove and the end surface.

4. The combination of claim 1, the crown comprising a side surface including an annular ring belt region in which a plurality of ring grooves is formed, in which: a top ring groove of the annular ring belt region is closest to the end surface; the top land comprises a portion of the side surface between the top ring groove and the end surface; and the top land meets the end surface at an edge which defines a periphery of the end surface.

5. The combination of claim 4, wherein the annular belt region is a top belt region of the piston, the piston further comprising a bottom belt region.

6. The combination of claim 5, further comprising at least one injector trench opening through the peripheral edge and having a shape for guiding fuel injected into a bowl formed in the end surface.

7. The combination of claim 1, in which the two second sides have a circular profile with a diameter D1, the two first sides have a circular profile with a diameter D2, and the diameter D1 is greater than the diameter D2.

8. The combination of claim 7, in which the diameter D2 increases clearance to the cylinder bore along the thrust and anti-thrust sides compared to the clearance to the bore along the engine rear and engine front sides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration showing a cut-away view of a pair of pistons in a cylinder of an opposed-piston engine in accordance with the invention.

(2) FIG. 2 illustrates a circumferential profile in a cross section taken along line II-II in FIG. 1.

DETAILED DESCRIPTION

(3) The present invention is directed to a piston for a diesel-fueled two-stroke cycle opposed piston engine (“the opposed-piston engine”). FIG. 1 is a schematic illustration showing a cut-away view of a pair of pistons in a cylinder 10 of the opposed-piston engine, in accordance with the invention. FIG. 2 shows the shape of a circumferential profile of a top land 27 in a cross section taken along line II-II in FIG. 1. These figures are not to scale, and certain dimensions and shapes are exaggerated for clarification. In FIG. 1, 10 indicates a cylinder wall of a diesel-fueled two-stroke cycle opposed-piston engine, 12 indicates the bore of the cylinder, 20 indicates a first piston disposed in the bore 12, and 30 indicates a second piston disposed in the bore 12 in opposition to the first piston 20. Features common to both of the pistons are described with respect to the first piston 20, with the understanding that the piston 30 comprises these features as well. The piston 20 has a longitudinal axis P. 21 indicates a generally tubular body of the piston 20, which is capped at one end by a crown 22. The crown 22 comprises an end surface 23, which is configured to form a combustion chamber with the end surface of the piston 30. The base of the crown 22 has the general shape of the piston body 21. The crown 22 comprises a side surface including an annular ring belt region 24 in which at least one ring groove is formed. Typically, although not necessarily, there is a plurality of ring grooves in the ring belt region; three such ring grooves are shown as an example. Taking the crown 22 as the top of the piston 20, the ring belt region 24 is referred to as the “top ring belt region”. The ring groove 25 closest to the end surface 23 is the top ring groove. The piston 20 has a lower ring belt region 26. The top land 27 of this piston is the portion of the side surface of the crown 22 between the top edge 28 of the top ring groove 25 and the end surface 23. The top land 27 meets the end surface 23 at an edge 29. This edge 29 defines a periphery of the end surface 23, and so is referred to as “the peripheral edge”. An injector trench 31 opens through the peripheral edge 29. The piston body 21 includes a wristpin boss arrangement 32 configured to support a wristpin with an axis A. The top land 27 has the general appearance of a truncated cone of height h, which tapers slightly in radius from the top edge 28 of the top ring groove 25 to the peripheral edge 29.

(4) FIG. 2 is a schematic illustration showing the shape of a circumferential profile of the top land 27 as it would be configured when the piston is cold, before the piston 20, the crown 22, and the top land 27 are deformed by heat and engine dynamics. The circumferential profile corresponds to the outline of a cross section of the top land 27 which is orthogonal to the longitudinal axis P of the piston 20. Representations of the wristpin boss arrangement 32 and the wristpin axis A, which are superimposed on the cross section, extend between an engine-front direction an engine-rear direction that define a longitudinal direction of the engine. Thrust (Th) and anti-thrust (Ath) directions are orthogonal to the engine-front direction and the engine-rear direction.

(5) Referring to FIGS. 1 and 2, the top land 27 includes two sides 45 located on a thrust (Th) side and an anti-thrust (Ath) side of the wristpin boss arrangement 32. Two sides 47 of the top land 27 respectively face the engine-front direction and the engine-rear direction. As the piston 20 slides in the bore 12, it rocks on the wristpin axis A, tilting the Th side 45 of the top land toward the bore 12 during one stroke, and tilting the Ath side 45 of the top land toward the bore 12 during the following stroke. In this manner, carbon accumulating on the top land 27 is packed on the sides 45.

(6) In order to mitigate the effects of carbon buildup on the sides 45, the circumferential profile of the top land 27 has an essentially oval shape, with a geometry which is described by a combination of circular and non-circular profiles, in which the engine rear and engine front sides 47 have a first circular profile with a diameter D1, the thrust and anti-thrust sides 45 have a circular profile with a diameter D2, and the diameter D1 is greater than the diameter D2, i.e., D1>D2. The sides 49 connecting the sides 45 and 47 are continuous, but non-circular. For example, the sides 49 may have a continuously-curved profile that smoothly connects the circular sides 45 and 47; in one embodiment, the sides 45 and 47 may have a double-sine (y=sin 2(x)) profile. In addition, a local relief around an injector trench may be summed with the circular and double sine profiles to create a net shape.

(7) The smaller diameter D2 increases the clearance to the cylinder bore 12 along the thrust and anti-thrust sides 45 compared to the clearance to the bore 12 along the engine rear and engine front sides 47. Due to the piston rocking motion, the thrust and anti-thrust sides 45 typically get closer to bore 12 than the engine rear and engine front sides 47. With a non-cylindrical circumferential profile such as is seen in FIG. 2, it is possible to achieve a more uniform minimum top land clearance around the circumference of the top land over the engine cycle. That is, during engine operation, as the piston 20 deforms in response to thermal loading and engine dynamics, it rocks over between the thrust and anti-thrust directions, and circumferential profile in the thrust/anti-thrust directions can be designed such that the minimum clearance in the thrust/anti-thrust directions is similar to the minimum clearance in the engine-front and engine-rear directions by using a circumferential profile shape similar to what is shown in FIG. 2.