Combustion chamber for internal combustion engine

A geometry for an IC combustion chamber is disclosed that increases the air-fuel mixing efficiency within the chamber. The piston head includes a squish surface portion for generating a squish jet, and a bowl portion that cooperatively with the cylinder head defines a combustion volume. A horizontal channel extends between the squish surface portion and the bowl portion, and is adapted to provide a high-momentum jet into the bowl portion. A transverse channel may also be provided that directs the squish flow toward the horizontal channel, thereby generating a toroidal vortex therein. A premix fuel injector may inject fuel near the upstream end of the horizontal channel. A converging nozzle portion near the distal end of the horizontal channel may increase the flow speed into the bowl portion. A chamber fuel injector is disposed near the downstream end of the horizontal channel.

BACKGROUND

It is well known that improved air-fuel mixing provides advantages in reduced knock, improved emission characteristics and improved combustion efficiency in internal combustion (“IC”) engines. In addition to improving the completeness of the fuel combustion, improvements in mixing of the fuel and air allow the engine designer to utilize greater engine compression ratios without producing engine “knock” and, therefore, may further improve engine efficiency.

Improved air-fuel mixing in an IC engine may be achieved by providing a suitable geometry in the combustion chamber, defined generally by the piston and cylinder head, such that during the compression stroke the piston will generate a transverse fluid motion and thereby generate additional turbulence in the combustion chamber. For example, it is known to use a so-called “squish jet” geometry, wherein the piston head and corresponding cylinder head are designed such that one or more peripheral portions of the piston head extend upwardly to substantially fill a corresponding portion of the combustion chamber volume when the piston is near the top of the compression stroke, while recessed portions of the piston head define the actual combustion volume. As the peripheral piston head portion approaches the cylinder head, the air or air-fuel mixture therebetween is pushed out, producing a generally transverse fluid flow in the combustion chamber that increases turbulence and enhances air-fuel mixing.

A prior art example disclosing the use of squish jets to increase turbulence of the air-fuel mixture is U.S. Pat. No. 5,103,784 (hereby incorporated by reference, in its entirety), which discloses a combustion chamber wherein the piston and/or cylinder head have a compression face that defines a bowl portion, and a plurality of squish jet channels arranged about the bowl portion. The channels provide a flow path directing the compressed air-fuel mixture toward the bowl portion. U.S. Pat. No. 6,199,544 (hereby incorporated by reference, in its entirety) discloses a similar apparatus wherein angled squish areas impel the air-fuel mixture toward the center of the combustion chamber. See also, U.S. Pat. No. 6,237,579 and U.S. Pat. No. 6,708,666 (hereby incorporated by reference, in their entirety).

However, prior art squish jet designs do not provide optimal mixing between the air and fuel, and therefore additional improvements in such mixing remain a desirable goal.

SUMMARY

An internal combustion engine is disclosed having a combustion chamber cooperatively defined by the cylinder, piston head and cylinder head. The piston head includes a squish surface portion that is adapted to create a squish jet during the compression stroke of the piston. The piston head further includes a bowl portion, and a horizontal channel (i.e., along a plane generally perpendicular to the cylinder axis) that extends from the squish surface portion to the bowl portion.

In an embodiment of the invention a combustion chamber is provided, defined by a cylinder, cylinder head and piston head. The piston head includes a recessed bowl portion that is generally aligned with an inverted bowl portion defined in the cylinder head. The piston head also includes a squish surface portion, and a horizontal channel that extends from the squish surface portion to the recessed bowl portion. The squish jet is thereby directed by the horizontal channel to the recessed bowl portion.

In different embodiments, a fuel injector may be provided near the upstream end of the horizontal channel, near the downstream end of the horizontal channel, and/or directly into the chamber defined by the bowl shaped portions of the piston and cylinder heads.

In an embodiment of the invention, during operation the Damkohler number in the combustion chamber is less than one.

In an embodiment of the invention, during operation the Stokes number in the combustion chamber is less than one.

DETAILED DESCRIPTION

A method and apparatus is disclosed that incorporates squish jet mixing to generate a high-momentum jet into a combustion chamber volume, and optionally includes confined duct mixing, to improve mixing performance, thereby reducing emissions and improving combustion efficiency.

A key objective for improving engine efficiency and decreasing emissions of NOx and other undesirable products from internal combustion engines is to improve the air-fuel mixing efficiency in the combustion chamber of a reciprocating engine. In contemplating methods for increasing mixing efficiency, the present inventor considered research relating to fundamentals of confined mixing, and postulated that better mixing can be achieved by directing a squish jet air flow into a transverse duct or channel, and thence into the combustion chamber, generating a single, high-momentum jet flow into the combustion chamber to establish a pair of counter-rotating vortices therein. Fuel may be injected into the combustion chamber near the entry of the high-momentum jet, such that the counter-rotating vortices very rapidly mix the fuel and air in the combustion chamber. In a current embodiment of the invention, the mixing is further enhanced using a vortex generator such as an oblique jet that is directed at an angle into the duct to form a toroidal vortex that substantially fills the duct. Fuel may be injected near the entrance of the duct, for example using a second fuel injector, such that the toroidal vortex in the duct pre-mixes the injected fuel with the air.

It is known that in conventional IC engines, most of the circulation in the air received from the intake process is attenuated by the time the piston reaches top dead center in the compression stroke. Therefore, it is not effective to attempt to induce vortex formation during the intake stroke. For this reason, engine designers have developed a type of combustion chamber with a geometry that includes so-called “squish surfaces” that generate “squish jets” during the piston compression stroke, and in particular near the end of the compression stroke. The piston squish surfaces are typically disposed at or near the periphery of the piston, and project upwardly to approach nearer the corresponding cylinder head surface, such that gasses near the squish surfaces are preferentially displaced, imparting a transverse momentum to the gas in a desired direction.

A novel aspect of the combustion chamber described herein is that a squish jet geometry is provided that produces a squish jet air stream within the combustion chamber that is directed into a horizontal channel (i.e., a channel in a plane generally perpendicular to the axis of the cylinder), and is then directed as a single jet into the portion of the combustion chamber where combustion is intended to occur. Preferably, the single jet is directed near the center of the combustion chamber. A vortex generator may be provided to generate vortices in the duct. In a current embodiment, the vortex generator comprises an air stream that is directed at an oblique angle to the horizontal channel. To provide pre-mixing of fuel and air, fuel may also be injected into the horizontal channel, providing a very good air-fuel mixing environment.

Preferably, the horizontal channel has an aspect ratio of about two, such that a pair of counter-rotating confined vortices is formed in the channel to provide a greatly enhanced mixing environment therein. If premixing is desired, one or more fuel injectors meter fuel into the channel, downstream of the oblique air stream, and the confined vortices aid in mixing the fuel and air within the horizontal channel, producing a relatively uniform air-fuel mixture.

Refer now toFIG. 1, which shows a portion of an internal combustion engine100, including an engine block102, a cylinder head104sealingly fixed at an upper end of the engine block102, the engine block102defining a plurality of cylinders106(one shown), each cylinder106having a piston108disposed for reciprocating therein as indicated by the arrow90. Conventional piston rings118provide a sliding interface between the cylinder106and the piston108. The piston108is shown generally at the uppermost portion of its stroke, i.e., at top dead center.

The cylinder head104, cylinder106, and piston108cooperatively define a combustion chamber150that changes in volume as the piston108reciprocates within the cylinder106. The combustion chamber150is described in more detail below. In this exemplary IC engine100one or more inlet ports112are provided for selectively admitting air into the combustion chamber150, and one or more outlet ports (not shown) are similarly provided to permit expulsion of the products of combustion and other gasses in the combustion chamber150. A conventional valve114and optional ignition initiator such as a glow plug116are also shown, as are well-known in the art. In the exemplary IC engine100shown, a premix fuel injector110and a chamber fuel injector111near the centerline of the combustion chamber150are also provided. It is contemplated that in some suitable embodiments only one of the premix fuel injector110and the chamber fuel injector111would be used.

Particular details of IC engines that are not related to the novelty of the present invention, and are well-known in the art, are not described in detail herein, in order to provide clarity to the description of the novel aspects of the present invention. It will be appreciated that a typical IC engine may include multiple inlet and outlet ports with corresponding valves, an electronic control unit for controlling the amount and timing of the fuel injection, and the like. The timing and control of the inlet and outlet valves may be controlled in a conventional manner such as by utilizing a rotating cam mechanism, and the work may be extracted from the engine, for example, using conventional connecting rods and a crank shaft mechanism.

Refer now also toFIG. 2, which shows a perspective view of the piston108, andFIG. 3, which shows a cross-sectional side view of a portion of the IC engine100. The piston108includes a generally cylindrical skirt or wall120that engages the piston rings118, and a portion above the rings118defining the piston head122. The piston head122includes a recessed bowl portion124, that is positioned to generally align with an inverted bowl portion105formed in the cylinder head104(FIG. 1). Although the bowl portion124is illustrated as a generally semi-spherical volume, it is contemplated that the bowl portion124may be alternatively shaped to optimize engine performance and/or manufacturability, for example having a shallower, more ovoid shape and/or including one or more flat portions.

The piston head122further includes an upwardly extending squish surface portion126disposed generally toward one side of the periphery of the piston head122. As will be appreciated from the figures, the squish surface portion126is shaped such that as the piston108approaches the top dead center position shown inFIG. 1, the air and other gasses disposed over the squish surface portion126are forced inwardly, imparting a momentum to the gasses that will facilitate air-fuel mixing in the combustion chamber150.

A horizontal duct or channel130extends generally from a proximal end near the squish surface portion126to a distal end opening to the combustion chamber bowl portion124. It will be appreciated that “horizontal” as used herein is intended to refer to the plane perpendicular to the axis of the cylinder106. In the embodiment shown inFIGS. 1-3a vortex generator comprising an angled or transverse channel132extends from an upper surface134of the squish surface portion126to intersect with the horizontal channel130. It is contemplated that the present invention may be practiced without a vortex generator, or with a vortex generator other than a transverse channel132. Although in the current embodiment the horizontal channel130and the transverse channel132are generally rectangular in cross-section, it is contemplated that other channel geometries may be used, and may provide further advantages. As seen most clearly inFIGS. 1 and 3, the premix fuel injector110is provided near the upstream or proximal end of the horizontal channel130.

Referring now also to the plan view ofFIG. 4, it will be appreciated that as the piston108moves toward the top dead center position, the squish air displaced by the squish surface portion126of the piston head122will be at least partially directed into the transverse channel132, as indicated by arrow92, and therethrough to the horizontal channel130. The air stream then proceeds toward the bowl portion124via the horizontal channel130. Airflow from the transverse channel132into the horizontal channel130will produce a toroidal vortex in the horizontal channel130, as indicated by arrow94. The fuel from the premix fuel injector110entering the horizontal channel130is very rapidly mixed with the air entering from the transverse channel132, as the air-fuel mixture approaches the bowl portion124. The air-fuel mixture is then expelled as a high-momentum jet96into the bowl portion124, generating a pair of strong, counter-rotating vortices that are extremely efficient mixers. Additional fuel from the chamber fuel injector111, as well as fuel in the high-momentum jet96is very rapidly mixed with the air in the bowl portion124.

In a preferred embodiment, the width142of the transverse channel132is smaller than the width140of the horizontal channel130, and the horizontal channel130is approximately twice as wide as it is deep, to accommodate and facilitate the generation of a strong pair of counter-rotating vortices.

As seen most clearly inFIG. 3, in the disclosed embodiment, the horizontal channel130in the piston head122includes a first lip portion136, and directly thereabove the cylinder head104includes a second lip portion138, the first and second lip portions136,138cooperatively defining a converging nozzle that increases the velocity of the air-fuel mixture ejected from the horizontal channel130.

As discussed above, the piston108may alternatively be formed without the transverse channel132and/or without the premix fuel injector110such that the high-momentum jet94expelled into the bowl portion124comprises essentially inlet air, and fuel is added only through the chamber fuel injector111. AlthoughFIG. 3shows the chamber fuel injector111on one side of the combustion chamber150, in an embodiment without the transverse channel132the chamber fuel injector111may alternatively be positioned to inject fuel into the downstream end of the horizontal channel130. Injecting the fuel near the downstream end of the horizontal channel130would improve the fuel distribution between the pair of counter-rotating vortices96(FIG. 4) that are generated in the combustion chamber150, to further improve air-fuel mixing. It will also be appreciated that placing the fuel injector near the downstream end of the horizontal channel130will minimize any tendency of the fuel droplets to centrifuge onto the channel walls, thereby relaxing constraints on the size of the fuel droplets. The chamber fuel injector111is positioned to inject fuel into the combustion chamber150near the exit end of horizontal channel130to provide the best opportunity for the injected fuel to mix with the air prior to combustion.

The preferred embodiment of the present invention includes only a single horizontal channel130generating a single high-momentum jet stream into the bowl-shaped portion of the combustion chamber150. This configuration is believed to be optimal for producing the desired air-fuel mixing because the single jet will produce a strong pair of counter rotating vortices in the combustion chamber.

The following discussion is provided to explain some considerations regarding designing an optimal implementation of the present invention. A Damkohler number is a dimensionless number used in fluids engineering to relate the chemical reaction timescale to mass transport time, and is generally defined to be the ratio of the vortex rotation period to the chemical ignition delay time. In order to achieve good mixing prior to combustion it is desirable that the Damkohler number be less than about one, i.e., that the vortex rotation period in the combustion chamber be less than the chemical ignition delay time.

It is also desirable that the fuel droplets be sufficiently small so that they are not centrifuged out of the vortices onto nearby walls prior to combustion. The Stokes number may be defined as the ratio of the droplet characteristic inertial time scale to the vortex rotation period. It is preferred that the Stokes number be less than about one to avoid significant centrifuging of the droplets.

In the current combustion chamber the final composition of the mixed fluid is controlled by achieving the mixing within a confined channel and/or chamber, whose size is essentially equal to that of the vortex pair or toroidal vortex responsible for the mixing (i.e., the vortices are volume-filling). Within a few rotations, the composition of a vortex core becomes quite uniform. For a symmetric vortex pair, both vortices are essentially identical in composition, as is the entire toroidal vortex. Thus, if a vortex pair or toroidal vortex fills the duct or chamber, the fluid will quickly mix to a uniform composition throughout the entire volume. On the other hand, if the vortices do not fill the duct or chamber, the composition of the mixed fluid will vary widely.

The final mixture composition is selected by controlling the flows of fuel and air into the confined duct or chamber. By combining the confined mixing and metered reactant flows, the mixed fluid will produce a relatively well-mixed air-fuel mixture, as is optimal for fuel economy and emission reduction.

In order to achieve large, volume-filling vortices the momentum of the squish flow is exploited to stir the reactants. It is contemplated that alternatively a separate source of high-pressure air may be used instead of squish flow, such as a separate piston-cylinder assembly, a turbocharger, a supercharger, a compressor, or the like.

In the embodiment wherein the toroidal vortex is formed in the horizontal channel130, it is contemplated that other vortex generator means may alternatively be used, such as bends in the channel, vane-type vortex generators, or the like.

The optimum aspect ratio of the duct or channel is approximately two, so that the aspect ratio of each vortex is approximately one. The optimum shape of the mixing chamber portion defined between the bowl shaped portions of the cylinder head and the piston head is approximately a sphere or oblate spheroid, such that the aspect ratio of the core of the toroidal vortex is about one.

A second embodiment for a combustion chamber250for an internal combustion engine200according to the present invention is shown inFIGS. 5A and 5B, which show in simplified form a cross-sectional side view of a portion of a second engine similar to the first engine100. For brevity and clarity, only those aspects of the second embodiment that are different from the first embodiment will be described. InFIG. 5Athe piston208is shown before top dead center, and includes a horizontal channel230, and a vortex generator such as transverse channel232, similar to the horizontal and transverse channels130,132described above. However, in this embodiment the squish surface portion226of the piston208includes a vertical valve surface226, that is disposed to approximately align with a projecting portion205of the cylinder head204, such that the squish flow from the squish surface portion226is substantially cut off for a portion of the compression stroke. As the piston208moves upwardly towards top dead center, the vertical valve surface226will pass the projecting portion205, opening the flow path for the squish jet, thereby automatically timing the squish flow.

A third embodiment of a piston308for an internal combustion engine according to the present invention is shown inFIGS. 6,7, and8.FIG. 6shows a perspective view of the piston308,FIG. 7shows a plan view of the piston308andFIG. 8shows a side view of the piston308. In this third embodiment, the rest of the combustion chamber defined by the cylinder and cylinder head (not shown) are substantially the same as that described above, and for clarity and brevity are not described again. In this embodiment, the piston308includes a piston head322having a central bowl portion324with a pair of oppositely disposed horizontal channels330, each horizontal channel330having an associated transverse channel332extending from a peripheral squish surface portion326. In this embodiment, the piston308reciprocating in the cylinder (not shown) will produce a pair of squish jets into the central bowl portion324. Although two oppositely-disposed horizontal channels330are shown, it will be appreciated that additional channels330,332may be provided to generate more than two air-fuel jet streams, and/or the horizontal channels330may be asymmetrically disposed.