A split-cycle air hybrid engine with improved efficiency is disclosed in which the centerline of a compression cylinder is positioned at a non-zero angle with respect to the centerline of an expansion cylinder such that the engine has a V-shaped configuration. In one embodiment, the centerlines of the respective cylinders intersect an axis parallel to, but offset from, the axis of rotation of the crankshaft. Modular crossover passages, crossover passage manifolds, and associated air reservoir valve assemblies and thermal regulation systems are also disclosed.

FIELD

The present invention relates to split-cycle engines and in particular to split-cycle air hybrid engines having a V-shaped configuration.

BACKGROUND

For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well-known Otto cycle (i.e., the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.

A split-cycle engine as referred to herein comprises:

a crankshaft rotatable about a crankshaft axis;

a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;

an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and

a crossover passage (port) interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.

U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Scuderi and U.S. Pat. No. 6,952,923 granted Oct. 11, 2005 to Branyon et al., both of which are incorporated herein by reference, contain an extensive discussion of split-cycle and similar-type engines. In addition, these patents disclose details of prior versions of an engine of which the present disclosure details further developments.

Split-cycle air hybrid engines combine a split-cycle engine with an air reservoir and various controls. This combination enables a split-cycle air hybrid engine to store energy in the form of compressed air in the air reservoir. The compressed air in the air reservoir is later used in the expansion cylinder to power the crankshaft.

A split-cycle air hybrid engine as referred to herein comprises:

a crankshaft rotatable about a crankshaft axis;

a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;

an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft;

a crossover passage (port) interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween; and

an air reservoir operatively connected to the crossover passage and selectively operable to store compressed air from the compression cylinder and to deliver compressed air to the expansion cylinder.

U.S. Pat. No. 7,353,786 granted Apr. 8, 2008 to Scuderi et al., which is incorporated herein by reference, contains an extensive discussion of split-cycle air hybrid and similar-type engines. In addition, this patent discloses details of prior hybrid systems of which the present disclosure details further developments.

Referring toFIG. 1, an exemplary prior art split-cycle air hybrid engine is shown generally by numeral10. The split-cycle air hybrid engine10replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder12and one expansion cylinder14. The four strokes of the Otto cycle are “split” over the two cylinders12and14such that the compression cylinder12, together with its associated compression piston20, perform the intake and compression strokes, and the expansion cylinder14, together with its associated expansion piston30, perform the expansion and exhaust strokes. The Otto cycle is therefore completed in these two cylinders12,14once per crankshaft16revolution (360 degrees CA) about crankshaft axis17.

During the intake stroke, intake air is drawn into the compression cylinder12through an intake port19disposed in the cylinder head33. An inwardly-opening (opening inward into the cylinder and toward the piston) poppet intake valve18controls fluid communication between the intake port19and the compression cylinder12.

During the compression stroke, the compression piston20pressurizes the air charge and drives the air charge into the crossover passage (or port)22, which is typically disposed in the cylinder head33. This means that the compression cylinder12and compression piston20are a source of high pressure gas to the crossover passage22, which acts as the intake passage for the expansion cylinder14. In some embodiments two or more crossover passages22interconnect the compression cylinder12and the expansion cylinder14.

The volumetric (or geometric) compression ratio of the compression cylinder12of the split-cycle engine10(and for split-cycle engines in general) is herein referred to as the “compression ratio” of the split-cycle engine. The volumetric (or geometric) compression ratio of the expansion cylinder14of the split-cycle engine10(and for split-cycle engines in general) is herein referred to as the “expansion ratio” of the split-cycle engine. The volumetric compression ratio of a cylinder is well known in the art as the ratio of the enclosed (or trapped) volume in the cylinder (including all recesses) when a piston reciprocating therein is at its bottom dead center (BDC) position to the enclosed volume (i.e., clearance volume) in the cylinder when said piston is at its top dead center (TDC) position. Specifically for split-cycle engines as defined herein, the compression ratio of a compression cylinder is determined when the XovrC valve is closed. Also specifically for split-cycle engines as defined herein, the expansion ratio of an expansion cylinder is determined when the XovrE valve is closed.

Due to very high volumetric compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater) within the compression cylinder12, an outwardly-opening (opening outwardly away from the cylinder and piston) poppet crossover compression (XovrC) valve24at the crossover passage inlet25is used to control flow from the compression cylinder12into the crossover passage22. Due to very high volumetric compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater) within the expansion cylinder14, an outwardly-opening poppet crossover expansion (XovrE) valve26at the outlet27of the crossover passage22controls flow from the crossover passage22into the expansion cylinder14. The actuation rates and phasing of the XovrC and XovrE valves24,26are timed to maintain pressure in the crossover passage22at a high minimum pressure (typically20bar or higher at full load) during all four strokes of the Otto cycle.

At least one fuel injector28injects fuel into the pressurized air at the exit end of the crossover passage22in correspondence with the XovrE valve26opening, which occurs shortly before the expansion piston30reaches its top dead center position. The air/fuel charge enters the expansion cylinder14shortly after the expansion piston30reaches its top dead center position. As the piston30begins its descent from its top dead center position, and while the XovrE valve26is still open, a spark plug32, which includes a spark plug tip39that protrudes into the cylinder14, is fired to initiate combustion in the region around the spark plug tip39. Combustion is initiated while the expansion piston is between1and30degrees CA past its top dead center (TDC) position. More preferably, combustion is initiated while the expansion piston is between 5 and 25 degrees CA past its top dead center (TDC) position. Most preferably, combustion is initiated while the expansion piston is between 10 and 20 degrees CA past its top dead center (TDC) position. Additionally, combustion may be initiated through other ignition devices and/or methods, such as with glow plugs, microwave ignition devices, or through compression ignition methods.

During the exhaust stroke, exhaust gases are pumped out of the expansion cylinder14through an exhaust port35disposed in the cylinder head33. An inwardly-opening poppet exhaust valve34, disposed in the inlet31of the exhaust port35, controls fluid communication between the expansion cylinder14and the exhaust port35. The exhaust valve34and the exhaust port35are separate from the crossover passage22. That is, the exhaust valve34and the exhaust port35do not make contact with the crossover passage22.

With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, volumetric compression ratio, etc.) of the compression and expansion cylinders12,14are generally independent from one another. For example, the crank throws36,38for the compression cylinder12and the expansion cylinder14, respectively, may have different radii and may be phased apart from one another such that top dead center (TDC) of the expansion piston30occurs prior to TDC of the compression piston20. This independence enables the split-cycle engine10to potentially achieve higher efficiency levels and greater torques than typical four-stroke engines.

The geometric independence of engine parameters in the split-cycle engine10is also one of the main reasons why pressure is maintained in the crossover passage22as discussed earlier. Specifically, the expansion piston30reaches its top dead center position prior to the compression piston reaching its top dead center position by a discreet phase angle (typically between 10 and 30 crank angle degrees). This phase angle, together with proper timing of the XovrC valve24and the XovrE valve26, enables the split-cycle engine10to maintain pressure in the crossover passage22at a high minimum pressure (typically 20 bar absolute or higher during full load operation) during all four strokes of its pressure/volume cycle. That is, the split-cycle engine10is operable to time the XovrC valve24and the XovrE valve26such that the XovrC and XovrE valves are both open for a substantial period of time (or period of crankshaft rotation) during which the expansion piston30descends from its TDC position towards its BDC position and the compression piston20simultaneously ascends from its BDC position towards its TDC position. During the period of time (or crankshaft rotation) that the crossover valves24,26are both open, a substantially equal mass of gas is transferred (1) from the compression cylinder12into the crossover passage22and (2) from the crossover passage22to the expansion cylinder14. Accordingly, during this period, the pressure in the crossover passage is prevented from dropping below a predetermined minimum pressure (typically 20, 30, or 40 bar absolute during full load operation). Moreover, during a substantial portion of the intake and exhaust strokes (typically 90% of the entire intake and exhaust strokes or greater), the XovrC valve24and XovrE valve26are both closed to maintain the mass of trapped gas in the crossover passage22at a substantially constant level. As a result, the pressure in the crossover passage22is maintained at a predetermined minimum pressure during all four strokes of the engine's pressure/volume cycle.

For purposes herein, the method of opening the XovrC24and XovrE26valves while the expansion piston30is descending from TDC and the compression piston20is ascending toward TDC in order to simultaneously transfer a substantially equal mass of gas into and out of the crossover passage22is referred to herein as the Push-Pull method of gas transfer. It is the Push-Pull method that enables the pressure in the crossover passage22of the split-cycle engine10to be maintained at typically20bar or higher during all four strokes of the engine's cycle when the engine is operating at full load.

As discussed earlier, the exhaust valve34is disposed in the exhaust port35of the cylinder head33separate from the crossover passage22. The structural arrangement of the exhaust valve34not being disposed in the crossover passage22, and therefore the exhaust port35not sharing any common portion with the crossover passage22, is preferred in order to maintain the trapped mass of gas in the crossover passage22during the exhaust stroke. Accordingly, large cyclic drops in pressure, which may force the pressure in the crossover passage below the predetermined minimum pressure, are prevented.

The XovrE valve26opens shortly before the expansion piston30reaches its top dead center position. At this time, the pressure ratio of the pressure in the crossover passage22to the pressure in the expansion cylinder14is high, due to the fact that the minimum pressure in the crossover passage is typically 20 bar absolute or higher and the pressure in the expansion cylinder during the exhaust stroke is typically about one to two bar absolute. In other words, when the XovrE valve26opens, the pressure in the crossover passage22is substantially higher than the pressure in the expansion cylinder14(typically in the order of 20 to 1 or greater). This high pressure ratio causes initial flow of the air and/or fuel charge to flow into the expansion cylinder14at high speeds. These high flow speeds can reach the speed of sound, which is referred to as sonic flow. This sonic flow is particularly advantageous to the split-cycle engine10because it causes a rapid combustion event, which enables the split-cycle engine10to maintain high combustion pressures even though ignition is initiated while the expansion piston30is descending from its top dead center position.

The split-cycle air-hybrid engine10also includes an air reservoir (tank)40, which is operatively connected to the crossover passage22by an air reservoir tank valve42. Embodiments with two or more crossover passages22may include a tank valve42for each crossover passage22, which connect to a common air reservoir40, or alternatively each crossover passage22may operatively connect to separate air reservoirs40.

The tank valve42is typically disposed in an air tank port44, which extends from the crossover passage22to the air tank40. The air tank port44is divided into a first air tank port section46and a second air tank port section48. The first air tank port section46connects the air tank valve42to the crossover passage22, and the second air tank port section48connects the air tank valve42to the air tank40.

The volume of the first air tank port section46includes the volume of all additional recesses which connect the tank valve42to the crossover passage22when the tank valve42is closed. Preferably, the volume of the first air tank port section46is small (e.g., less than approximately 20%) relative to the volume of the crossover passage22. More preferably, the first air tank port section46is substantially non-existent, that is, the tank valve42is most preferably disposed such that it is flush against the outer wall of crossover passage22.

The tank valve42may be any suitable valve device or system. For example, the tank valve42may be a pressure-activated check valve, or an active valve which is activated by various valve actuation devices (e.g., pneumatic, hydraulic, cam, electric or the like). Additionally, the tank valve42may comprise a tank valve system with two or more valves actuated with two or more actuation devices.

The air tank40is utilized to store energy in the form of compressed air and to later use that compressed air to power the crankshaft16, as described in aforementioned U.S. Pat. No. 7,353,786 to Scuderi et al. This mechanical means for storing potential energy provides numerous potential advantages over the current state of the art. For instance, the split-cycle engine10can potentially provide many advantages in fuel efficiency gains and NOx emissions reduction at relatively low manufacturing and waste disposal costs in relation to other technologies on the market such as diesel engines and electric-hybrid systems.

The air hybrid split-cycle engine10can be run in a normal operating mode (referred to as the engine firing (EF) mode or as the normal firing (NF) mode) and four basic air hybrid modes. In the EF mode, the engine10functions normally as previously described in detail herein, operating without the use of its air tank40. In the EF mode, the tank valve42remains closed to isolate the air tank40from the basic split-cycle engine10.

In the four hybrid modes, the engine10operates with the use of its air tank40. The four hybrid modes are:

1.Air Expander (AE) mode, which includes using compressed air energy from the air tank40without combustion;

2.Air Compressor (AC) mode, which includes storing compressed air energy into the air tank40without combustion;

3.Air Expander and Firing (AEF) mode, which includes using compressed air energy from the air tank40with combustion; and

4.Firing and Charging (FC) mode, which includes storing compressed air energy into the air tank40with combustion.

In the split-cycle engine10, the compression and expansion cylinders12,14are positioned in-line with each other and share a common cylinder head33in which the crossover passage22is formed. Additionally, the common head33must include several cooling passages (not shown) to enable engine coolant to be pumped through the head33to remove heat from the compression cylinder12, the expansion cylinder14, and the crossover passage22. Because the crossover passage22is formed integrally with the cylinder head33, it is very difficult to independently control the temperature of the crossover passage22(and the fluid therein) relative to the cylinders12,14.

Also, the relative lack of available space in the cylinder head33imposes undesirable size and shape restrictions on the crossover passage(s)22and the air reservoir control valve(s)42. For example, the crossover passage22or the first air tank port section46, which connects the valve42to the crossover passage22, may have to be curved in order to avoid breaking through or getting too close to the various cooling passages. The curved crossover passages would then be longer than necessary, which would increase heat losses therein and decrease efficiency. The curved first tank port section46would undesirably combine with the volume of the crossover passage to decrease pressure in the crossover passage and also decrease efficiency. Moreover, the common head may become so crowded that it may become very difficult (if not virtually impossible) to connect a tank valve42to the crossover passage22without breaking through or coming too close to some of the cooling passages.

Still further, the casting process that is typically used to form the crossover passage22in the cylinder head33leaves behind manufacturing artifacts that disrupt air flow in the crossover passage22and undesirably limit the shape and size of the crossover passage(s)22. Accordingly, there is a need for improved split-cycle engine configurations.

SUMMARY

A split-cycle air hybrid engine with improved efficiency is disclosed in which the centerline of a compression cylinder is positioned at a non-zero angle with respect to the centerline of an expansion cylinder such that the cylinders of the engine have a V-shaped configuration. The centerlines of the respective cylinders do not actually form a “V”, as they do not typically intersect with each other. Rather, the centerlines are usually spaced apart from one another in the axial direction of the crankshaft (i.e., to accommodate the thickness of the respective crank throws for each cylinder). When viewed along the axis of rotation of the crankshaft, however, the centerlines have the appearance of a “V.” In one embodiment, the centerlines of the respective cylinders intersect with the axis of rotation of the crankshaft such that the apex of the V is formed at the axis of rotation of the crankshaft.

In another embodiment, one or both of the compression cylinder and the expansion cylinder have a centerline that is “offset,” meaning the centerline does not intersect with the axis of rotation of the crankshaft. In this embodiment, it is preferable that the centerlines of the cylinders intersect with a line (i.e., the line on which the apex of the V is formed) that is located below the axis of rotation of the crankshaft (i.e., located on the side opposite the cylinders relative to the axis of rotation of the crankshaft). The line on which the apex of the V is formed can optionally be parallel to the axis of rotation of the crankshaft. Modular crossover passages, crossover passage manifolds, thermal regulation systems, and associated air reservoir valve assemblies are also disclosed.

In one aspect of at least one embodiment of the invention, a V-shaped split-cycle air hybrid engine is provided that includes a compression cylinder having a centerline that is positioned at a non-zero angle with respect to the centerline of an expansion cylinder. In one embodiment, the non-zero angle is in a range of about 10 degrees to about 120 degrees. The non-zero angle can also be selected from the group consisting of about 30 degrees, about 45 degrees, and about 60 degrees.

In another aspect of at least one embodiment of the invention, a split-cycle engine is provided that includes a first cylinder head coupled to a compression cylinder, a second cylinder head coupled to an expansion cylinder, and at least one crossover passage formed externally to the first and second cylinder heads and configured to selectively transfer fluid between the first and second cylinder heads.

In one embodiment, the engine is an air hybrid engine and the at least one crossover passage includes an air reservoir valve for selectively placing an air reservoir in fluid communication with the first or second cylinder heads. The at least one crossover passage can include first and second crossover passages, each having an associated crossover compression valve and a crossover expansion valve. The crossover compression valves and the crossover expansion valves can be outwardly opening. In one embodiment, the air reservoir valve is outwardly opening.

In another aspect of at least one embodiment of the invention, a split-cycle air hybrid engine is provided that includes a crankshaft that rotates about a crankshaft axis and a compression cylinder having a centerline offset from the crankshaft axis that intersects an offset axis, the offset axis being parallel to the crankshaft axis and offset therefrom. The engine also includes an expansion cylinder having a centerline that intersects the offset axis, and the centerline of the compression cylinder is positioned at a non-zero angle with respect to the centerline of the expansion cylinder when viewed along the offset axis.

In another aspect of at least one embodiment of the invention, a split-cycle air hybrid engine is provided that includes a crankshaft that rotates about a crankshaft axis, a first cylinder that is offset such that a centerline of the first cylinder does not intersect the crankshaft axis, and a second cylinder having a centerline, wherein the centerline of the first cylinder is positioned at a non-zero angle with respect to the centerline of the second cylinder. The first cylinder can be a compression cylinder or the first cylinder can be an expansion cylinder. In one embodiment, the second cylinder is offset such that a centerline of the second cylinder does not intersect the crankshaft axis.

In another aspect of at least one embodiment of the invention, a split-cycle engine is provided that includes a first cylinder head coupled to a compression cylinder, a second cylinder head coupled to an expansion cylinder, and a thermally regulated crossover manifold configured to selectively transfer fluid between the first and second cylinder heads. The manifold includes at least one insulated crossover passage and at least one cooled crossover passage. In one embodiment, the manifold includes a plurality of valves configured to selectively divert fluid through either the at least one cooled crossover passage or the at least one insulated crossover passage depending on an operating condition of the engine. The engine can also include one or more fluid jackets through which engine coolant flows, the one or more fluid jackets being disposed in proximity to the at least one cooled crossover passage An insulative material can also be provided and that is disposed around the at least one insulated crossover passage. In one embodiment, the insulative material is a ceramic. The insulated crossover passage can also be heated.

DETAILED DESCRIPTION

FIGS. 2-4illustrate one exemplary embodiment of a split-cycle air hybrid engine200according to the present invention. The engine200generally includes an engine block202, a crankshaft204rotating about a crankshaft axis (or axis of rotation)228, first and second cylinder heads206,208, first and second crossover passages210,212, and an air reservoir214.

As shown inFIG. 3, the engine block202defines at least one compression cylinder216and at least one expansion cylinder218. As shown, the centerlines of the compression and expansion cylinders216,218are positioned at a non-zero angle A relative to each other such that the engine200is oriented in a V-shaped configuration when viewed along the crankshaft axis228. The angle A can be between about 0.1 degrees and about 180 degrees, between about 5 degrees and about 150 degrees, between about 10 degrees and about 120 degrees, between about 15 degrees and about 90 degrees, between about 30 degrees and about 60 degrees, between about 10 degrees and about 30 degrees, between about 60 degrees and about 90 degrees, and/or between about 45 degrees and about 55 degrees. For example, the angle A can be 0.1 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, 90 degrees, 105 degrees, 120 degrees, 150 degrees, 165 degrees, or 180 degrees. In the illustrated embodiment, the compression and expansion cylinders216,218are oriented at an angle A of about 54 degrees with respect to each other.

It will be appreciated that the engine200can include virtually any number of compression and/or expansion cylinders, and that the number of compression cylinders need not necessarily be equal to the number of expansion cylinders. In this embodiment, the engine200includes one compression cylinder and one expansion cylinder. The four strokes of the Otto cycle are “split” over the compression and expansion cylinders such that the compression cylinder216contains the intake and compression strokes and the expansion cylinder218contains the expansion and exhaust strokes. The Otto cycle is therefore completed in the compression and expansion cylinders216,218once per crankshaft revolution (360 degrees CA).

Upper ends of the cylinders216,218are closed by the respective cylinder heads206,208. The compression and expansion cylinders216,218receive for reciprocation a compression piston220and an expansion (or “power”) piston222, respectively. The first cylinder head206, the compression piston220and the compression cylinder216define a variable volume compression chamber224in the compression cylinder216. The second cylinder head208, the expansion piston222and the expansion cylinder218define a variable volume combustion chamber226in the expansion cylinder218.

Having separated cylinder heads206,208oriented in the V-shaped configuration allows for better access to the crossover passages210,212, which makes it easier to attach the air reservoir valve260thereto, thereby facilitating the construction of the air reservoir214.

This configuration also avoids the necessity of forming the crossover passages in the common cylinder head33, which, as discussed in detail below, enables independent thermal control of the crossover passages relative to the compression and expansion cylinders. The V-shaped configuration of engine200enables a substantial portion of the crossover passages210,212to be located outside of the first and second cylinder heads206,208, as, for example, in a separate crossover passage manifold (not shown). Accordingly, separate cooling passages can be designed for just the crossover passages, making the area around the crossover passages more open and accessible. This means that the crossover passages can be made straighter and shorter, which would cut down on heat loss and increase engine efficiency. Additionally, one or more air reservoir valves260can be more easily fitted to the crossover passages210,212and connected to the air reservoir214with little structural problems in hitting or coming too close to the cooling passages. Moreover, the connection to the air reservoir214can be made straight and the air reservoir valve(s)260can be mounted flush against the outer surface of the crossover passages210,212to further increase crossover passage pressure and engine efficiency.

The crankshaft204is journaled into the engine block202for rotation about the crankshaft axis228and includes axially displaced and angularly offset first and second crank throws230,232, having a phase angle therebetween. The first crank throw230is pivotally joined by a first connecting rod236to the compression piston220and the second crank throw232is pivotally joined by a second connecting rod238to the expansion piston222to reciprocate the pistons220,222, respectively, in their respective cylinders216,218in a timed relation determined by the angular offset of the crank throws230,232and the geometric relationships of the cylinders216,218, the crankshaft204, and the pistons220,222. Alternative mechanisms for relating the motion and timing of the pistons220,222can be utilized if desired.

The cylinder heads206,208include various passages, ports and valves suitable for accomplishing the desired purposes of the split-cycle air hybrid engine200. In the illustrated embodiment, a first, compression-side cylinder head206is provided that includes an inwardly-opening intake valve240for controlling fluid flow between an intake port242and the compression cylinder216. The cylinder head206also includes first and second outwardly-opening poppet crossover compression (XovrC) valves244,246at the inlets of the respective crossover passages210,212, respectively, for controlling fluid flow between the compression cylinder216and the crossover passages210,212.

During the intake stroke, intake air is drawn through the intake port242and into the compression cylinder216via the intake valve240. During the compression stroke, the compression piston220pressurizes the air charge and drives the air charge into the crossover passages210,212which act as intake passages for the expansion cylinder218.

The illustrated engine200also includes a second, expansion-side cylinder head208. The head208includes first and second outwardly-opening poppet crossover expansion (XovrE) valves248,250at the outlets of the respective crossover passages210,212which control fluid flow between the crossover passages210,212and the expansion cylinder218. The head208also includes an inwardly-opening poppet exhaust valve252for controlling fluid flow between the expansion cylinder218and an exhaust port254.

One or more fuel injectors (not shown) inject fuel into the pressurized air at the exit ends of the crossover passages210,212in correspondence with the opening of the XovrE valves248,250respectively. Alternatively, or in addition, fuel can be injected directly into the expansion cylinder218and/or directly into one or both of the crossover passages210,212. The fuel-air charge fully enters the expansion cylinder218shortly after the expansion piston222reaches its TDC position. As the piston222begins its descent from its TDC position, and while one or more of the XovrE valves248,250are still open, one or more spark plugs (not shown) are fired to initiate combustion (typically between 10 to 20 degrees CA after TDC of the expansion piston222). The spark plug(s) are mounted in the cylinder head208with electrodes extending into the combustion chamber226for igniting air fuel charges at precise times by an ignition control (not shown). It should be understood that the engine200can also be a diesel engine and can be operated without a spark plug. Moreover, the engine200can be designed to operate on any fuel suitable for reciprocating piston engines in general, such as hydrogen or natural gas.

After the spark plug is fired, the XovrE valves248,250are closed before the resulting combustion event enters the crossover passages210,212. The combustion event drives the expansion piston222downward in a power stroke. Exhaust gases are pumped out of the expansion cylinder222and through the exhaust port254via the exhaust valve252during the exhaust stroke.

The crossover passages210,212can have a variety of configurations. While the illustrated engine200includes two crossover passages210,212, it can also have only a single crossover passage or can have more than two crossover passages.

The illustrated crossover passages210,212generally include an elongated hollow flow tube with mounting flanges256formed on either end for mounting the crossover passages210,212to the cylinder heads206,208. The crossover passages210,212also include at least one air reservoir valve assembly258that houses at least one air reservoir valve260(seeFIG. 3), as discussed in further detail below. In the illustrated embodiment, the crossover passages210,212have a generally circular cross-section, although virtually any cross-sectional shape can be used without departing from the scope of the present invention. For example, the crossover passages can have an ellipsoidal cross-section. The crossover passages210,212can be generally straight as shown or can include one or more curves or bends. In one embodiment, the crossover passages are sized and shaped such that they have different internal volumes to accommodate flow for different engine load ranges. For example, the crossover passage210could be sized to have approximately half the volume of the crossover passage212. Accordingly, the smaller volume passage210could be used primarily for the lower third of the engine load range, the larger volume passage212could be used primarily for the middle third of the engine load range, and the combined passages210,212could be used primarily for the upper third of the engine load range.

The air reservoir valve assemblies258of the crossover passages210,212control fluid flow between the crossover passages210,212and the air reservoir214. The air reservoir214is sized to receive and store compressed air energy from a plurality of compression strokes of the compression piston220, and facilitates operation of the engine200in any of a variety of air hybrid modes, as explained below. It will be appreciated that each crossover passage210,212can be coupled to its own respective air reservoir and/or can be coupled to a single shared air reservoir214as shown.

The valves in the engine200(i.e., the intake valve240, the XovrC valves244,246, the XovrE valves248,250, the exhaust valve252, the air reservoir valves260, etc.) are typically actuated by camshafts (not shown) having cam lobes for respectively actuating and engaging the valves either directly or via one or more intermediate elements. Each valve can have its own cam and/or its own camshaft, or two or more valves can be actuated by common cams and/or camshafts. Alternatively, one or more of the valves can be mechanically, electronically, pneumatically, and/or hydraulically actuated variably.

The engine200is capable of operating in any of the aforementioned air hybrid modes (i.e., AE, AC, AEF, and FC modes).

In existing split-cycle engines, the respective centerlines of the expansion and compression cylinders are generally parallel to one another and intersect the axis of rotation of the crankshaft, as shown inFIG. 1. In the engine200ofFIG. 3, the centerline262of the compression cylinder216and the centerline264of the expansion cylinder218, while not parallel to one another, do intersect with the rotational axis228of the crankshaft204. This need not always be the case, however. In other words, one or both of the compression cylinder and the expansion cylinder can be “offset,” meaning that their centerlines do not intersect the axis of rotation of the crankshaft. In such embodiments, it is preferable that the centerlines of the cylinders intersect with a line (i.e., the line on which the apex of the V is formed) that is located below the axis of rotation of the crankshaft (i.e., located on the side opposite the cylinders relative to the axis of rotation of the crankshaft). The line on which the apex of the V is formed can optionally be parallel to the axis of rotation of the crankshaft. For example,FIG. 5illustrates a split-cycle air hybrid engine200′ in which the centerlines262′,264′ of the compression and expansion cylinders216′,218′ do not intersect with the crankshaft axis228′. Rather, the centerlines262′,264′ intersect with an offset axis266′ that is parallel to the crankshaft axis228′ but offset therefrom. This advantageously reduces friction between the piston skirt and the cylinder wall. In addition, this allows for the angle A′ of the V-shaped engine block202′ to be reduced, which in turn allows for shorter crossover passages210′,212′. With the shorter crossover passages210′,212′, there is less pressure drop and thermal loss across the passages which increases engine efficiency. A variety of offsets (i.e., distances between the crankshaft axis228′ and the offset axis266′) can be used without departing from the scope of the present invention.

FIGS. 6-7illustrate one embodiment of an air reservoir valve assembly258according to the present invention. As shown, the valve assembly258generally includes a longitudinal tubular portion268configured to be placed in-line with a crossover passage (i.e., the crossover passages210,212). In one embodiment, the valve assembly258is formed integrally with the crossover passage. Alternatively, the crossover passage can include first and second portions, each coupled to respective ends of the longitudinal tubular portion268of the valve assembly258. The tubular portion268includes a valve seat270for forming a sealing engagement with the head272of an air reservoir valve260. In the illustrated embodiment, the air reservoir valve260is an outwardly-opening (i.e., opening outwardly away from the interior of the tubular portion268) poppet valve having a valve head272and a valve stem274. The valve stem274extends through a transverse portion276of the valve assembly258that extends up and away from the tubular portion268. Fluid communication between the interior of the transverse portion276and the interior of the tubular portion268is selectively established by actuating the air reservoir valve260. The end of the transverse portion276opposite from the tubular portion268is coupled to an air reservoir (not shown), either directly or via one or more intermediate structures, such as tubes, valves, etc.

The valve stem274extends through a sidewall of the transverse portion276in a slidable arrangement such that linear motion can be imparted thereto by a cam or other valve actuator disposed outside of the transverse portion276. A sealing feature is provided as known in the art to permit the valve stem274to slide with respect to the transverse portion276without permitting pressurized fluid in the transverse portion276to escape around the surface of the valve stem274. It will be appreciated that a variety of other valve and/or housing types can be used to selectively place the air reservoir in fluid communication with one or more crossover passages.

As noted above, forming the crossover passages external to the cylinder head advantageously permits independent thermal regulation of the crossover passages.FIG. 8illustrates one embodiment of a split-cycle air hybrid V-shaped engine300in which a thermal control system is employed to regulate the temperature of the crossover passages depending on various engine operating parameters. As shown, the engine300includes a thermally regulated crossover passage manifold378in which four crossover passages380,382,384,386are formed. It will be appreciated that the use of such a crossover passage manifold is not limited to V-shaped split-cycle engines, and that the manifolds described herein can also be used with traditional inline split-cycle engines. Each passage in the manifold378has its own air reservoir valve assembly358. Again, the number of illustrated crossover passages and air reservoir valves is merely exemplary, and any number of crossover passages and/or air reservoir valves can be used without departing from the scope of the present invention. The crossover passages380,382share a common XovrC valve344and a common XovrE valve348. Likewise, the crossover passages384,386share a common XovrC valve346and a common XovrE valve350. In other embodiments, each crossover passage includes its own unique XovrC and/or XovrE valve, or a single XovrC or XovrE valve is shared by more than two crossover passages.

FIG. 9illustrates a cross-sectional view of the crossover manifold378. As shown, the ends of the manifold378are bolted to the first and second cylinder heads306,308. The manifold378includes first and second XovrC inlets388,390through which fluid flow is controlled by the XovrC valves344,346, respectively. The manifold378also includes first and second XovrE outlets392,394through which fluid flow is controlled by the XovrE valves348,350, respectively. Adjustable ball valves391,395are disposed in the manifold inlets388,390respectively, and adjustable ball valves393,397are disposed in the manifold outlets392,394, respectively. The configurations of the ball valves391,393are adjustable to selectively direct fluid entering the inlet388through either the crossover passage380or the crossover passage382. Similarly, the configurations of the ball valves395,397are adjustable to selectively direct fluid entering the inlet390through either the crossover passage384or the crossover passage386. Any of a variety of means known in the art can be employed to change the configuration of the ball valves391,393,395,397, including mechanical, hydraulic, electromagnetic, and/or pneumatic actuators. In addition, the illustrated ball valves are only one exemplary type of valve that can be employed in the present invention, and a person having ordinary skill in the art will appreciate that any of a variety of known valve types can be used without departing from the scope of the present invention. The valves391,393,395,397can optionally be two-position valves. In one embodiment, the switch between crossover passages can occur over a plurality of engine cycles (i.e., dozens, hundreds, etc.), which means that the valves391,393,395,397need not necessarily be fast-actuating and can instead be of a slower, more durable or inexpensive variety.

The crossover passages380,384include features for generally maintaining or increasing the temperature of fluid disposed therein or passing therethrough. In the embodiment ofFIG. 9, the crossover passages380,384are encased in a thermal insulation396configured to maintain engine heat within the crossover passages380,384. Any of a variety of insulative materials can be used for this purpose, including without limitation ceramics, Kevlar, plastics, composites, and the like. In addition, the crossover passages380,384can be vacuum-lined (i.e., can be disposed within an outer tube in which a vacuum is generated). The engine300can also optionally include active heating elements. For example, high-temperature exhaust gasses can be routed through air passages formed alongside the crossover passages380,384, or can be used to heat oil or other fluid which can then be pumped through fluid jackets disposed adjacent to the crossover passages380,384. In one embodiment, the crossover passages380,384can be wrapped in an electric heating coil.

The crossover passages382,386include features for generally decreasing the temperature of fluid disposed therein or passing therethrough. As illustrated, fluid jackets398are formed in the manifold378in close proximity to the crossover passages382,386. Engine coolant or other fluid is routed through the fluid jackets398to cool the crossover passages382,386. The cooled crossover passages382,386can also include other cooling mechanisms, such as heat sinks or fans and can optionally be formed from materials such as aluminum that are known to dissipate heat quickly.

The engine300also includes a thermal control computer (not shown) and any of a variety of associated sensors, thermostats, actuators, and/or other controls to facilitate precise temperature control.

In operation, the ball valves391,393,395,397are selectively actuated such that fluid flowing from the compression cylinder to the expansion cylinder is either insulated, heated, or cooled as needed to improve the efficiency of the engine300. For example, when the engine300is first started and has not yet reached operating temperature, the valves391,393,395,397are placed in a first configuration, as shown inFIG. 9, such that the fluid compressed in the compression cylinder is routed through the insulated crossover passages380,384, and heated and/or insulated before entering the expansion cylinder. The flow of fluid in this configuration is indicated by the illustrated arrows. This configuration is also used when the engine300is operating under low loads (e.g., when the engine is operating below about 70% of full load). By heating and/or insulating the incoming air charge before it reaches the expansion cylinder, crossover passage pressures are maintained at a high level, thereby improving overall efficiency.

When the engine300is operating at high load (e.g., when the engine is operating above about 70% of its rated load), it is desirable to cool the air charge before it enters the expansion cylinder to prevent premature combustion and to improve output power. Accordingly, the valves391,393,395,397are placed in a second configuration, as shown inFIG. 10, to route the fluid compressed in the compression cylinder through the cooled crossover passages382,386. The flow of fluid in this configuration is indicated by the illustrated arrows. By cooling the incoming air charge before it reaches the expansion cylinder, the temperature and pressure of the air charge is reduced which advantageously prevents pre-ignition and knocking. The cooled crossover passages382,386can optionally have no air reservoir valve358, since it may not be desirable to operate in an air hybrid mode under the conditions in which the cooled crossover passages382,386are used.

FIG. 11illustrates another embodiment of a split-cycle air hybrid engine400in which a thermal control system is employed to regulate the temperature of the crossover passages depending on various engine operating parameters. The engine400is substantially identical to the engine300discussed above with respect toFIGS. 8-10, except that the manifold478of the engine400has only three crossover passages480,484,499. In other words, whereas the engine300includes two cooled crossover passages382,386, the engine400instead has a single cooled crossover passage499. Thus, as shown inFIG. 12, the engine400includes first and second insulated crossover passages480,484and a central cooled crossover passage499. It will be appreciated that the engine400could alternatively have first and second cooled crossover passages and that the insulated crossover passages could instead be merged into a single passage.

In operation, the engine400operates in substantially the same way as the engine300described above. During low load and/or low speed operation, or during engine start-up/warm-up, a series of valves491,493,495,497are configured as shown inFIG. 12to direct fluid from the compression cylinder through the insulated crossover passages480,484to insulate or heat the fluid before it enters the expansion cylinder. During high load and/or high speed operation, the valves491,493,495,497are configured as shown inFIG. 13to direct fluid from the compression cylinder through the central, cooled crossover passage499, thereby cooling the fluid before it enters the expansion cylinder.

The engines200,200′,300,400disclosed herein are configured to operate reliably over a broad range of engine speeds. In certain embodiments, engines according to the present invention are capable of operating up to a speed of at least about 4000 rpm, and preferably at least about 5000 rpm, and more preferably at least about 7000 rpm.

Although the invention has been described by reference to specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. For example, one or more of the crossover valves or the air reservoir valves can be inwardly-opening. There can also be more than four crossover valves, and more than two crossover passages. In addition, the engines disclosed herein need not necessarily be air hybrid engines, but rather the V-shaped configuration can be applied to non-hybrid split-cycle engines as well. These changes are only exemplary, and other changes may be made without departing from the scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.