Swirl-conserving combustion chamber construction for opposed-piston engines

A combustion chamber construction for opposed-piston engines includes an elongated, bilaterally symmetrical shape referenced to a major axis and a pair of injection ports located on the major axis when the pistons are near respective top center positions. The combustion chamber is defined between a bowl in the end surface of a first piston of a pair of pistons and mirrored ridges protruding from the end surface of a second piston of the pair. Each ridge includes a central portion that curves toward a periphery of the end surface of the second piston and which transitions to flanking portions that curve away from the periphery. The ridge configuration imparts a substantially spherical configuration to a central portion of the combustion chamber where swirling motion of charge air is conserved.

BACKGROUND

The field includes opposed-piston engines in which a combustion chamber is defined between end surfaces of pistons disposed in opposition in the bore of a ported cylinder. More particularly, the field includes opposed-piston engines with combustion chamber constructions that promote complex, turbulent bulk motion in charge air admitted into the cylinder.

A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which a pair of pistons is disposed in opposition in the bore of a cylinder for reciprocating movement in opposing directions. PerFIG. 1, an opposed-piston engine includes at least one cylinder10with a bore12and longitudinally-displaced intake and exhaust ports14and16machined or formed therein. (In some aspects, when the term “cylinder” is used in this application, it refers also to a cylinder liner.) One or more fuel injectors17are secured in injector ports (ports where injectors are positioned) that open through the side surface of the cylinder. Two pistons20,22according to the prior art are disposed in the bore12with their end surfaces20e,22ein opposition to each other. For convenience, the piston20is denominated as the “intake” piston because of its proximity to the intake port14. Similarly, the piston22is denominated as the “exhaust” piston because of its proximity to the exhaust port16.

Operation of an opposed-piston engine with one or more ported cylinders (cylinders with one or more of intake and exhaust ports formed therein) such as the cylinder10is well understood. In this regard, a power stroke commences when, in response to combustion, the opposed pistons move away from respective top center (TC) positions where they are at their innermost positions in the cylinder10. While moving from TC, the pistons keep their associated ports closed until they approach respective bottom center (BC) positions where they are at their outermost positions in the cylinder. The pistons may move in phase so that the intake and exhaust ports14,16open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.

For example, presume the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BC positions in a sequence in which the exhaust port16opens as the exhaust piston22moves through BC while the intake port14is still closed so that combustion gasses start to flow out of the exhaust port16. As the pistons continue moving away from each other, the intake piston20moves through BC causing the intake port14to open while the exhaust port16is still open. As the pistons reverse direction, the exhaust port closes first, followed by the intake port.

A compression stroke commences when the pistons reverse direction and move from BC toward TC positions. As the pistons move away from their BC positions their movements are phased such that the port openings overlap to promote scavenging. In scavenging, a charge of pressurized air is forced into the cylinder10through the open intake port14, driving exhaust gasses out of the cylinder through the open exhaust port16. Typically, the charge of fresh air is swirled as it passes through ramped openings of the intake port14. With reference toFIG. 1, the swirling motion (or simply, “swirl”) is a generally helical movement of charge air that circulates around the cylinder's longitudinal axis and moves longitudinally through the bore of the cylinder10. PerFIG. 2, as the pistons20,22continue moving toward TC, the ports close and the swirling charge air remaining in the cylinder is compressed between the end surfaces20eand22e. As the pistons near their respective TC locations in the cylinder bore, fuel40is injected into the compressed charge air30, between the end surfaces20e,22eof the pistons. As injection continues, the swirling mixture of air and fuel is increasingly compressed in a combustion chamber32defined between the end surfaces20eand22e. When the mixture reaches an ignition temperature, fuel ignites in the combustion chamber, initiating another power stroke by driving the pistons apart toward their respective BC locations.

The geometries of the intake port openings and the cylinder of an opposed-piston engine provide a very effective platform for generation of a strong bulk fluid motion of the charge air in the form of swirl that promotes both removal of exhaust gasses (scavenging) and the movement of fuel to air (air/fuel mixing). However, charge air motion that is dominated by swirl can produce undesirable effects during combustion. For example, during combustion in a cylindrical combustion chamber defined between flat piston end surfaces, swirl pushes the flame toward the cylinder bore, causing heat loss to the (relatively) cooler cylinder wall. The higher velocity vectors of swirl occur near the cylinder wall, which provides the worst scenario for heat losses: high temperature gas with velocity that transfers heat to the cylinder wall and lowers the thermal efficiency of the engine. The peripheries of the piston end surfaces also receive a relatively high heat load, which causes formation of a solid residue of oil coke that remains in the piston/cylinder interface when lubricating oil breaks down at high engine temperatures.

Accordingly, it is desirable to maintain the benefits provided by swirl while mitigating its undesirable effects as combustion begins. At the same time, it is desirable to continue to promote turbulence in the charge air motion in order to encourage a homogeneous mixture of fuel and air, which in turn, produces more complete and more uniform ignition than would otherwise occur.

These advantages have been achieved in two-stroke opposed-piston engines by provision of shapes in the opposing end surfaces of the pistons that generate additional components of bulk air turbulence in the combustion chamber. In this regard, certain opposed-piston combustion chamber constructions include bowls that promote squish flow from the periphery of the combustion chamber in a radial direction of the cylinder toward the cylinder's axis. In some aspects, squish flow can be inwardly directed as when a high pressure region at the peripheries of the piston end surfaces causes charge air to flow to a lower-pressure region generated by a bowl formed in at least one piston end surface. For example, U.S. Pat. No. 1,523,453 describes a pair of opposed pistons having depressions formed in their heads which form a pear-shaped combustion chamber when the pistons are adjacent each other. The larger end of the chamber is substantially closed and the smaller end is open to permit injection of fuel into the chamber by an injection valve in the cylinder wall.

A number of recently-disclosed opposed piston designs have been directed to generation of tumble in bulk motion of charge air. For example, grandparent U.S. application Ser. No. 13/066,589 describes formation of an ellipsoidally-shaped combustion chamber between projecting curved ridges in the adjacent end surfaces of opposed pistons. The curved ridges are identical, but mutually inverted by 180°. The end surfaces interact with swirl and squish flows to produce tumble at the narrow ends of the combustion chamber, near the bore surface of the cylinder. The wider central portion of the combustion chamber preserves swirl. Priority application Ser. No. 13/843,686 describes an improvement to this mutually-inverted ridge configuration in which the central portion of the combustion chamber has a pronounced spherical aspect that preserves more swirl than the mainly ellipsoidal shape. An ellipsoidally-shaped combustion chamber formed between opposed pistons having non-identical, but complementary end surface shapes is described in the grandparent PCT application. In this construction, a concave bowl is formed in one end surface. The opposing end surface has a convex projection in which a bilaterally-tapered, diametrical cleft is formed between mirrored, continuously curved ridges. When the end surfaces are adjacent, the convex projection is received in the concave bowl and the combustion chamber is defined principally by the cleft. Bordering squish regions are formed on either side of the chamber by opposing convex/concave end surface portions.

The pistons described in the grandparent and parent US applications are subjected to significant thermal challenges. Both of the opposing pistons have highly contoured end surfaces in which the heat load falls heavily on the curved ridges. The intake piston is afforded some thermal relief by the passage of charge air over its end surface during scavenging. But this construction requires a piston cooling construction with a thermal capacity designed to adequately cool the exhaust piston. These challenges are mitigated by the combustion chamber construction of the grandparent PCT application, in which the piston with the concave bowl is placed in the exhaust side of the cylinder and the piston with the cleft-defining mirrored ridges is placed in the intake side. The concave bowl lacks projecting ridges, which makes it easier to cool in spite of its exposure to outflowing exhaust gases, while the cooling effect of in-flowing charge air is delivered to the mirrored ridges during scavenging. However, the continuously-curved configurations of the ridges define a combustion chamber shape that lacks the enhanced swirl-conserving effects.

SUMMARY

It is therefore an objective to provide a combustion chamber for an opposed-piston engine that produces a complex, turbulent bulk motion in charge air, while preserving swirl in a central portion of the chamber and rationalizing the thermal aspects of chamber construction.

The objective is achieved by piston constructions which utilize a bowl/mirrored ridge construction that distributes the combustion chamber volume between a central, largely spherical (or spheroidal) volume which holds most of the heat of combustion and lateral spray clearance channels that are mutually aligned along an injection axis and disposed on respective sides of the central volume. The bowl construction is provided on the end surface of one piston of a pair of opposed pistons and the mirrored ridge construction is provided on the end surface of the other piston of the pair. Preferably, the bowl construction is provided on the end surface of an exhaust piston of a pair of opposed pistons and the mirrored ridge construction is provided on the end surface of an intake piston of the pair.

The central, largely spherical, volume of the combustion chamber preserves much of the swirl in the bulk motion of the charge air when the combustion chamber is formed between the end surfaces of opposed pistons. The conserved swirl acts jointly with tumble flows to create a generally spherical (or spheroidal) bulk air flow field when the pistons approach minimum separation.

In some aspects of the preferred construction, paired opposing pistons have complementary opposing end surfaces. A generally concave bowl formed in one end surface receives a generally convex protrusion of the opposing end surface when the pistons are adjacent. An elongated cleft with a widened central portion formed in the convex protrusion extends in a diametrical direction of the end surface, is symmetrically shaped in cross-section with respect to a plane of symmetry, and has a shape in plan that tapers bi-directionally from a generally spherical center to narrower end. When the pistons are at or near TC, the cleft is positioned adjacent the generally concave bowl, and there defines an elongated, tapered combustion chamber with a central, largely spherical (or spheroidal) volume and lateral curved spray clearance channels that are mutually aligned along an injection axis and disposed on respective sides of the central space.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the construction to be described, an opposed-piston engine includes at least one cylinder with a bore and longitudinally-separated exhaust and intake ports formed or machined in the sidewall of the cylinder. A pair of pistons is disposed in opposition in the bore of the cylinder, and a combustion chamber is defined between the opposing end surfaces of the pistons as the pistons move through respective TC positions. A circumferential area includes a periphery on each of the end surfaces. The combustion chamber structure includes a cavity or space in the bore that is defined by the opposing end surfaces of the pistons. In plan, the combustion chamber has an elongated trench-like shape with a widened central portion. The elongated combustion chamber shape extends in a diametrical direction of the cylinder. The combustion chamber has diametrically opposing openings through which fuel is injected.

During operation of the internal combustion engine, as the pistons approach TC, one or more squish zones direct flows of compressed air (called “squish flows”) into the combustion chamber. This process is referred to as “generating squish”.

The construction to be described produces a bulk fluid motion in the combustion chamber due to the interaction of the swirling flow of charge air in the cylinder with the piston end surfaces as the pistons move towards TC. The bulk fluid motion includes elements of swirl, squish, and tumble.

In the following descriptions, “fuel” is any fuel that can be used in an opposed-piston engine. The fuel may be a relatively homogeneous composition, or a blend. For example, the fuel may be a gaseous fuel, a liquid fuel, or any other fuel ignitable by compression ignition. The descriptions contemplate injection of fuel into a compressed gas in a combustion chamber when opposed pistons are at or near TC locations. In some aspects, injection may occur earlier in the compression stroke. The gas is preferably pressurized ambient air; however, it may include other components such as exhaust gases or other diluents. In any such case, the gas is referred to as “charge air.”

FIGS. 3A and 3Billustrate a pair of pistons50and60with complementary piston end surface structures for defining a combustion chamber having a central, largely spherical (or spheroidal) volume and lateral spray clearance channels that are mutually aligned along an injection axis and disposed on respective sides of the central space. InFIGS. 4A and 4B, the end surfaces are seen in plan; inFIGS. 5A and 5Bthe end surface are seen in elevational section. The piston50has a crown51with an end surface52including a periphery53surrounding a bowl54with a concave surface55curving away from the periphery53toward the interior of the piston50. Opposed notches56formed in the end surface52open through the periphery53into the bowl54. The other piston60has a crown61with an end surface62including a periphery63surrounding a convex portion64protruding outwardly from the interior of the piston60. An elongated cleft65extending in a diametrical direction of the piston60is formed in the convex portion64. Opposed notches66aligned along an injection axis67and formed in the end surface61open through the periphery63into respective ends of the cleft65. As per the description to follow, the concave bowl54receives the convex portion64and covers the elongated cleft65to define a combustion chamber therewith.

As best seen inFIGS. 4B and 5B, the cleft65has an elongated, bilaterally symmetrical shape referenced to a plane of symmetry that contains the injection axis67and the longitudinal axis68of the piston60. The bilaterally symmetrical shape is defined between mirrored ridges69protruding outwardly from the end surface of the piston60. Viewing the end surface62in plan, as inFIG. 4B, each ridge includes a central portion72that curves outwardly (toward the periphery63) and transitions to flanking portions74that curve inwardly (away from the periphery63). The outward/inward variation in the curvature of the mirrored central portions defines a largely spherical (or spheroidal) volume75in the central portion of the cleft65and lateral spray clearance channels77that are mutually aligned along the injection axis67and disposed on respective sides of the central volume75.

The end surface constructions thus far described provide flexibility in management of piston thermal loads. Preferably, the piston50is placed on the exhaust side of a cylinder. The exhaust piston tends to be hotter because of the longer exposure to high temperature exhaust gases during the engine operating cycle. The concave bowl54has a smaller surface area for heat transfer than the highly contoured end surface of the piston60; there is, therefore, a lower heat flux through the end surface52than through the end surface62. As best seen inFIG. 5A, the piston50has an internal cooling structure that includes an annular gallery57and a central gallery58through which liquid coolant circulates. Preferably, the piston60is placed on the intake side of the cylinder where the generally convex shape, the cleft, the protruding surfaces, and the larger surface area of the end surface62are exposed to the cooling effects of the intake flow of charge air when the intake port opens as the piston60moves through BC. The protruding surfaces64are difficult to cool and tend to bear the greatest thermal load. And so, asFIG. 5Bshows, the piston60is provided with an internal cooling structure that includes an annular gallery87and a central gallery88through which liquid coolant circulates. With reference toFIG. 4B, the wider central portion of the combustion chamber achieved by the enlarged spherical center portion75of the cleft65minimizes the interaction between the flame and the wall of the piston60to minimize heat transfer. This larger central portion75is achievable without increasing the combustion volume thanks to the reduction of the angle of the spray clearance channels77resulting from the inwardly-curving ridge portions74.

Refer now toFIGS. 6A and 6Bfor an understanding of the operation of an opposed-piston engine equipped with opposed pistons constructed as shown inFIGS. 3A and 3B. The engine includes one or more ported cylinders. Each cylinder120includes a bore121and longitudinally-separated intake and exhaust ports124and126. InFIG. 6A, the two pistons50and60are shown near respective BC locations within the bore121of a ported cylinder120.FIG. 4Bshows the pistons50and60near TC locations in the bore121. The pistons50and60are rotationally oriented in the bore so as to align the end surfaces52and62in complement; that is to say, the notches56of the end surface52are aligned with the notches66of the end surface62, and each notch pair56,66is positioned in alignment with an injector port125that opens through the sidewall of the cylinder120. Charge air enters the cylinder120through the intake port124as exhaust products flow out of the cylinder through the exhaust port126. For purposes of scavenging and air/fuel mixing, the charge air is caused to swirl as it passes through the intake port. As perFIG. 6A, when the pistons50and60move away from BC in a compression stroke, the intake and exhaust ports are closed and the swirling charge air130is increasingly compressed between the end surfaces52and62. With reference toFIG. 6B, as the pistons50and60approach TC, a combustion chamber200is defined between the end surfaces52and62. The combustion chamber200has a cavity defined between the central portion of the concave surface55and the elongated cleft65.

Referring toFIG. 6B, as the pistons50and60approach TC positions, squish flows135of compressed charge air flow into the combustion chamber200from the peripheries of the end surfaces, through opposing curved squish regions defined between concave-convex surface portions55,64. At the same time, compressed air nearer the longitudinal axis of the cylinder120continues to swirl. The interactions of swirl and squish flow produce tumble at each end of the combustion chamber200. Each tumble component has a motion that circulates around the injector axis. That is to say, the tumble circulates around a diameter of the cylinder120that is generally collinear with the injection axis.

With reference toFIGS. 6B and 7, as the pistons50and60move through their respective TC locations, the adjacent concave-convex end surfaces and form a combustion chamber cavity having an elongated, bilaterally symmetrical shape referenced to an injector axis and a pair of injection ports located on the injector axis. Opposing pairs of notches56,66(seeFIG. 6A) in the end surfaces and define injection ports that open into the combustion chamber200at opposing positions of the combustion chamber.

With reference toFIGS. 6A, 6B, and 7, fuel123is injected through injection ports125in the cylinder120into the combustion chamber200by opposed injectors127. The fuel123travels through the lateral spray clearance channels77into the central portion75where it encounters and mixes with a largely spherical flow field140of swirling charge air130. The tumble flows result from interactions of swirl and squish as described in the parent and grandparent applications. The tumble flows145circulate around the injector axis67and act jointly with the swirl130in the combustion chamber200to create the spherical flow field140. As injection continues, the swirling mixture of air and fuel is increasingly compressed in the combustion chamber. When the mixture reaches an ignition temperature, fuel ignites in the combustion chamber.

Although principles of piston and combustion chamber constructions have been described with reference to presently preferred embodiments, it should be understood that various modifications can be made without departing from the spirit of the described principles. Accordingly, the patent protection accorded to these principles is limited only by the following claims.