Split-cycle air-hybrid engine with air tank valve

CROSS-REFERENCE TO RELATED APPLICATIONS

p-0002This application claims the priority of U.S. Provisional Application No. 61/313,831 filed Mar. 15, 2010, U.S. Provisional Application No. 61/363,825 filed Jul. 13, 2010, and U.S. Provisional Application No. 61/365,343 filed Jul. 18, 2010.

TECHNICAL FIELD

p-0003This invention relates to split-cycle engines and, more particularly, to such an engine incorporating an air-hybrid system.

BACKGROUND OF THE INVENTION

p-0004For 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 (or inlet), compression, expansion (or power) and exhaust strokes) are contained in each piston/cylinder combination of the engine. Each stroke requires one half revolution of the crankshaft (180 degrees crank angle (CA)), and two full revolutions of the crankshaft (720 degrees CA) are required to complete the entire Otto cycle in each cylinder of a conventional engine.

p-0005Also, 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.

p-0006A split-cycle engine as referred to herein comprises:

p-0007a crankshaft rotatable about a crankshaft axis;

p-0008a 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;

p-0009an 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

p-0010a 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.

p-0011U.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.

p-0012Split-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.

p-0013A split-cycle air-hybrid engine as referred to herein comprises:

p-0014a crankshaft rotatable about a crankshaft axis;

p-0015a 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;

p-0016an 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;

p-0017a 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

p-0018an 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.

p-0019U.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.

p-0020A split-cycle air-hybrid engine can be run in a normal operating or firing (NF) mode (also commonly called the Engine Firing (EF) mode) and four basic air-hybrid modes. In the EF mode, the engine functions as a non-air hybrid split-cycle engine, operating without the use of its air reservoir. In the EF mode, a tank valve operatively connecting the crossover passage to the air reservoir remains closed to isolate the air reservoir from the basic split-cycle engine.

p-0021The split-cycle air-hybrid engine operates with the use of its air reservoir in four hybrid modes. The four hybrid modes are: <ul><li id="ul0001-0001" num="0000"><ul><li id="ul0002-0001" num="0021">1) Air Expander (AE) mode, which includes using compressed air energy from the air reservoir without combustion;</li><li id="ul0002-0002" num="0022">2) Air Compressor (AC) mode, which includes storing compressed air energy into the air reservoir without combustion;</li><li id="ul0002-0003" num="0023">3) Air Expander and Firing (AEF) mode, which includes using compressed air energy from the air reservoir with combustion; and</li><li id="ul0002-0004" num="0024">4) Firing and Charging (FC) mode, which includes storing compressed air energy into the air reservoir with combustion. <br /> However, further optimization of these modes, EF, AE, AC, AEF and FC, is desirable to enhance efficiency and reduce emissions. </li></ul></li></ul>

SUMMARY OF THE INVENTION

p-0022The present invention provides a split-cycle air-hybrid engine in which the use of the Engine Firing (EF), the Air Expander (AE), the Air Compressor (AC), the Air Expander and Firing (AEF), and the Firing and Charging (FC) modes are optimized for potentially any vehicle in any drive cycle for improved efficiency.

p-0023More particularly, an exemplary embodiment of a split-cycle air-hybrid engine in accordance with the present invention includes a crankshaft rotatable about a crankshaft axis. A compression piston is 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 piston is 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 interconnects the compression and expansion cylinders. The crossover passage includes a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween. An air reservoir is 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. An air reservoir valve selectively controls air flow into and out of the air reservoir. The engine is operable in one or more of an Engine Firing (EF) mode, an Air Expander (AE) mode, an Air Compressor (AC) mode, an Air Expander and Firing (AEF) mode, and a Firing and Charging (FC) mode. In the EF mode, the air reservoir valve is kept closed during the entire rotation of the crankshaft. In the AE and AEF modes, the air reservoir valve is kept open for a duration that is at least as long as a duration of the XovrE valve opening event. In the AC and FC modes, the air reservoir valve is selectively opened and closed during a single rotation of the crankshaft.

p-0024A method of operating a split-cycle air-hybrid engine is also disclosed. The split-cycle air-hybrid engine includes a crankshaft rotatable about a crankshaft axis. A compression piston is 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 piston is 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 interconnects the compression and expansion cylinders. The crossover passage includes a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween. An air reservoir is 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. An air reservoir valve selectively controls air flow into and out of the air reservoir. The engine is operable in one or more of an Engine Firing (EF) mode, an Air Expander (AE) mode, an Air Compressor (AC) mode, an Air Expander and Firing (AEF) mode, and a Firing and Charging (FC) mode. The method in accordance with the present invention includes the following steps: in the EF mode, keeping the air reservoir valve closed during the entire rotation of the crankshaft to isolate the air reservoir; in the AE and AEF modes, keeping the air reservoir valve open for a duration that is at least as long as a duration of the XovrE valve opening event to allow for use of stored compressed air; and in the AC and FC modes, selectively opening and closing the air reservoir valve during a single rotation of the crankshaft to allow for flow of compressed air into the air reservoir for storage of compressed air.

p-0025These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0026In the drawings:

p-0027<figref idrefs="DRAWINGS">FIG. 1</figref> is a lateral sectional view of an exemplary split-cycle air-hybrid engine in accordance with the present invention;

p-0028<figref idrefs="DRAWINGS">FIG. 2</figref> is a lateral view of an exemplary air tank valve of the split-cycle air-hybrid engine; and

p-0029<figref idrefs="DRAWINGS">FIG. 3</figref> is a perspective view of the air tank valve of <figref idrefs="DRAWINGS">FIG. 2</figref>.

DETAILED DESCRIPTION OF THE INVENTION

p-0030The following glossary of acronyms and definitions of terms used herein is provided for reference.

In General

p-0031Unless otherwise specified, all valve opening and closing timings are measured in crank angle degrees after top dead center of the expansion piston (ATDCe).

p-0032Unless otherwise specified, all valve durations are in crank angle degrees (CA).

p-0033Air tank (or air storage tank): Storage tank for compressed air. <br /> ATDCe: After top dead center of the expansion piston. <br /> Bar: Unit of pressure, 1 bar=10⁵N/m²<br /> Compressor: The compression cylinder and its associated compression piston of a split-cycle engine. <br /> Expander: The expansion cylinder and its associated expansion piston of a split-cycle engine. <br /> Flow control valve(s): Device(s) inserted into the pipework which can control the flow in that pipework. <br /> Reed valve: A pressure activated valve where the control element is a flexible plate which seals against a fixed housing and blocks flow in a forward direction. When pressure builds up on the reverse side of the plate, the plate deflects and opens, allowing flow in the reverse direction. <br /> Tank valve: Valve connecting the Xovr passage with the compressed air storage tank. <br /> VVA: Variable valve actuation. A mechanism or method operable to alter the shape or timing of a valve's lift profile. <br /> Xovr (or Xover) valve, passage or port: The crossover valves, passages, and/or ports which connect the compression and expansion cylinders through which gas flows from compression to expansion cylinder. <br /> XovrC (or XoverC) valves: Valves at the compressor end of the Xovr passage. <br /> XovrE (or XoverE) valves: Valves at the expander end of the crossover (Xovr) passage.

p-0034Referring to <figref idrefs="DRAWINGS">FIG. 1</figref>, an exemplary split-cycle air-hybrid engine is shown generally by numeral <b>10</b>. The split-cycle air-hybrid engine <b>10</b> replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder <b>12</b> and one expansion cylinder <b>14</b>. A cylinder head <b>33</b> is typically disposed over an open end of the expansion and compression cylinders <b>12</b>, <b>14</b> to cover and seal the cylinders.

p-0035The four strokes of the Otto cycle are “split” over the two cylinders <b>12</b> and <b>14</b> such that the compression cylinder <b>12</b>, together with its associated compression piston <b>20</b>, perform the intake and compression strokes, and the expansion cylinder <b>14</b>, together with its associated expansion piston <b>30</b>, perform the expansion and exhaust strokes. The Otto cycle is therefore completed in these two cylinders <b>12</b>, <b>14</b> once per crankshaft <b>16</b> revolution (360 degrees CA) about crankshaft axis <b>17</b>.

p-0036During the intake stroke, intake air is drawn into the compression cylinder <b>12</b> through an intake port <b>19</b> disposed in the cylinder head <b>33</b>. An inwardly opening (opening inwardly into the cylinder and toward the piston) poppet intake valve <b>18</b> controls fluid communication between the intake port <b>19</b> and the compression cylinder <b>12</b>.

p-0037During the compression stroke, the compression piston <b>20</b> pressurizes the air charge and drives the air charge into the crossover passage (or port) <b>22</b>, which is typically disposed in the cylinder head <b>33</b>. This means that the compression cylinder <b>12</b> and compression piston <b>20</b> are a source of high-pressure gas to the crossover passage <b>22</b>, which acts as the intake passage for the expansion cylinder <b>14</b>. In some embodiments, two or more crossover passages interconnect the compression cylinder <b>12</b> and the expansion cylinder <b>14</b>.

p-0038The geometric (or volumetric) compression ratio of the compression cylinder <b>12</b> of split-cycle engine <b>10</b> (and for split-cycle engines in general) is herein commonly referred to as the “compression ratio” of the split-cycle engine. The geometric (or volumetric) compression ratio of the expansion cylinder <b>14</b> of split-cycle engine <b>10</b> (and for split-cycle engines in general) is herein commonly referred to as the “expansion ratio” of the split-cycle engine. The geometric 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.

p-0039Due to very high compression ratios (e.g., to 1, 30 to 1, 40 to 1, or greater) within the compression cylinder <b>12</b>, an outwardly opening (opening outwardly away from the cylinder) poppet crossover compression (XovrC) valve <b>24</b> at the crossover passage inlet <b>25</b> is used to control flow from the compression cylinder <b>12</b> into the crossover passage <b>22</b>. Due to very high expansion ratios (e.g., to 1, 30 to 1, 40 to 1, or greater) within the expansion cylinder <b>14</b>, an outwardly opening poppet crossover expansion (XovrE) valve <b>26</b> at the outlet <b>27</b> of the crossover passage <b>22</b> controls flow from the crossover passage <b>22</b> into the expansion cylinder <b>14</b>. The actuation rates and phasing of the XovrC and XovrE valves <b>24</b>, <b>26</b> are timed to maintain pressure in the crossover passage <b>22</b> at a high minimum pressure (typically 20 bar or higher at full load) during all four strokes of the Otto cycle.

p-0040At least one fuel injector <b>28</b> injects fuel into the pressurized air at the exit end of the crossover passage <b>22</b> in correspondence with the XovrE valve <b>26</b> opening, which occurs shortly before expansion piston <b>30</b> reaches its top dead center position. The air/fuel charge enters the expansion cylinder <b>14</b> when expansion piston <b>30</b> is close to its top dead center position. As piston <b>30</b> begins its descent from its top dead center position, and while the XovrE valve <b>26</b> is still open, spark plug <b>32</b>, which includes a spark plug tip <b>39</b> that protrudes into cylinder <b>14</b>, is fired to initiate combustion in the region around the spark plug tip <b>39</b>. Combustion can be initiated while the expansion piston is between 1 and 30 degrees CA past its top dead center (TDC) position. More preferably, combustion can be initiated while the expansion piston is between 5 and degrees CA past its top dead center (TDC) position. Most preferably, combustion can be 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.

p-0041During the exhaust stroke, exhaust gases are pumped out of the expansion cylinder <b>14</b> through exhaust port <b>35</b> disposed in cylinder head <b>33</b>. An inwardly opening poppet exhaust valve <b>34</b>, disposed in the inlet <b>31</b> of the exhaust port <b>35</b>, controls fluid communication between the expansion cylinder <b>14</b> and the exhaust port <b>35</b>. The exhaust valve <b>34</b> and the exhaust port <b>35</b> are separate from the crossover passage <b>22</b>. That is, exhaust valve <b>34</b> and the exhaust port <b>35</b> do not make contact with, or are not disposed in, the crossover passage <b>22</b>.

p-0042With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, volumetric compression ratio, etc.) of the compression <b>12</b> and expansion <b>14</b> cylinders are generally independent from one another. For example, the crank throws <b>36</b>, <b>38</b> for the compression cylinder <b>12</b> and expansion cylinder <b>14</b>, respectively, may have different radii and may be phased apart from one another such that top dead center (TDC) of the expansion piston <b>30</b> occurs prior to TDC of the compression piston <b>20</b>. This independence enables the split-cycle engine <b>10</b> to potentially achieve higher efficiency levels and greater torques than typical four-stroke engines.

p-0043The geometric independence of engine parameters in the split-cycle engine <b>10</b> is also one of the main reasons why pressure can be maintained in the crossover passage <b>22</b> as discussed earlier. Specifically, the expansion piston <b>30</b> reaches 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 valve <b>24</b> and the XovrE valve <b>26</b>, enables the split-cycle engine <b>10</b> to maintain pressure in the crossover passage <b>22</b> at 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 engine <b>10</b> is operable to time the XovrC valve and the XovrE valve <b>26</b> such that the XovrC and XovrE valves are both open for a substantial period of time (or period of crankshaft rotation) during which the expansion piston <b>30</b> descends from its TDC position towards its BDC position and the compression piston <b>20</b> simultaneously ascends from its BDC position towards its TDC position. During the period of time (or crankshaft rotation) that the crossover valves <b>24</b>, <b>26</b> are both open, a substantially equal mass of air is transferred (1) from the compression cylinder <b>12</b> into the crossover passage <b>22</b> and (2) from the crossover passage <b>22</b> to the expansion cylinder <b>14</b>. 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 engine cycle (typically 80% of the entire engine cycle or greater), the XovrC valve <b>24</b> and XovrE valve <b>26</b> are both closed to maintain the mass of trapped gas in the crossover passage <b>22</b> at a substantially constant level. As a result, the pressure in the crossover passage <b>22</b> is maintained at a predetermined minimum pressure during all four strokes of the engine's pressure/volume cycle.

p-0044For purposes herein, the method of having the XovrC <b>24</b> and XovrE <b>26</b> valves open while the expansion piston <b>30</b> is descending from TDC and the compression piston <b>20</b> is ascending toward TDC in order to simultaneously transfer a substantially equal mass of gas into and out of the crossover passage <b>22</b> is referred to herein as the Push-Pull method of gas transfer. It is the Push-Pull method that enables the pressure in the crossover passage <b>22</b> of the split-cycle engine <b>10</b> to be maintained at typically 20 bar or higher during all four strokes of the engine's cycle when the engine is operating at full load.

p-0045As discussed earlier, the exhaust valve <b>34</b> is disposed in the exhaust port <b>35</b> of the cylinder head <b>33</b> separate from the crossover passage <b>22</b>. The structural arrangement of the exhaust valve <b>34</b> not being disposed in the crossover passage <b>22</b>, and therefore the exhaust port <b>35</b> not sharing any common portion with the crossover passage <b>22</b>, is preferred in order to maintain the trapped mass of gas in the crossover passage <b>22</b> during the exhaust stroke. Accordingly, large cyclic drops in pressure are prevented which may force the pressure in the crossover passage below the predetermined minimum pressure.

p-0046XovrE valve <b>26</b> opens shortly before the expansion piston <b>30</b> reaches its top dead center position. At this time, the pressure ratio of the pressure in crossover passage <b>22</b> to the pressure in expansion cylinder <b>14</b> is 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 XovrE valve <b>26</b> opens, the pressure in crossover passage <b>22</b> is substantially higher than the pressure in expansion cylinder <b>14</b> (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 expansion cylinder <b>14</b> at 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 split-cycle engine <b>10</b> because it causes a rapid combustion event, which enables the split-cycle engine <b>10</b> to maintain high combustion pressures even though ignition is initiated while the expansion piston <b>30</b> is descending from its top dead center position.

p-0047The split-cycle air-hybrid engine <b>10</b> also includes an air reservoir (tank) <b>40</b>, which is operatively connected to the crossover passage <b>22</b> by an air reservoir (tank) valve <b>42</b>. Embodiments with two or more crossover passages <b>22</b> may include a tank valve <b>42</b> for each crossover passage <b>22</b>, which connect to a common air reservoir <b>40</b>, or alternatively each crossover passage <b>22</b> may operatively connect to separate air reservoirs <b>40</b>.

p-0048The tank valve <b>42</b> is typically disposed in an air reservoir (tank) port <b>44</b>, which extends from crossover passage <b>22</b> to the air tank <b>40</b>. The air tank port <b>44</b> is divided into a first air reservoir (tank) port section <b>46</b> and a second air reservoir (tank) port section <b>48</b>. The first air tank port section <b>46</b> connects the air tank valve <b>42</b> to the crossover passage <b>22</b>, and the second air tank port section <b>48</b> connects the air tank valve <b>42</b> to the air tank <b>40</b>. The volume of the first air tank port section <b>46</b> includes the volume of all additional ports and recesses which connect the tank valve <b>42</b> to the crossover passage <b>22</b> when the tank valve <b>42</b> is closed.

p-0049The tank valve <b>42</b> may be any suitable valve device or system. For example, the tank valve <b>42</b> may be an active valve which is activated by various valve actuation devices (e.g., pneumatic, hydraulic, cam, electric or the like). Additionally, the tank valve <b>42</b> may comprise a tank valve system with two or more valves actuated with two or more actuation devices.

p-0050Air tank <b>40</b> is utilized to store energy in the form of compressed air and to later use that compressed air to power the crankshaft <b>16</b>, as described in the 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 engine <b>10</b> can 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.

p-0051By selectively controlling the opening and/or closing of the air tank valve <b>42</b> and thereby controlling communication of the air tank <b>40</b> with the crossover passage <b>22</b>, the split-cycle air-hybrid engine <b>10</b> is operable in an Engine Firing (EF) mode, an Air Expander (AE) mode, an Air Compressor (AC) mode, an Air Expander and Firing (AEF) mode, and a Firing and Charging (FC) mode. The EF mode is a non-hybrid mode in which the engine operates as described above without the use of the air tank <b>40</b>. The AC and FC modes are energy storage modes. The AC mode is an air-hybrid operating mode in which compressed air is stored in the air tank <b>40</b> without combustion occurring in the expansion cylinder <b>14</b> (i.e., no fuel expenditure), such as by utilizing the kinetic energy of a vehicle including the engine <b>10</b> during braking. The FC mode is an air-hybrid operating mode in which excess compressed air not needed for combustion is stored in the air tank <b>40</b>, such as at less than full engine load (e.g., engine idle, vehicle cruising at constant speed). The storage of compressed air in the FC mode has an energy cost (penalty); therefore, it is desirable to have a net gain when the compressed air is used at a later time. The AE and AEF modes are stored energy usage modes. The AE mode is an air-hybrid operating mode in which compressed air stored in the air tank <b>40</b> is used to drive the expansion piston <b>30</b> without combustion occurring in the expansion cylinder <b>14</b> (i.e., no fuel expenditure). The AEF mode is an air-hybrid operating mode in which compressed air stored in the air tank <b>40</b> is utilized in the expansion cylinder <b>14</b> for combustion.

p-0052The air tank valve <b>42</b> may be a fully controllable variably actuated valve that can be kept closed, held open, or selectively opened and closed at any desired timing. In the EF mode, the air reservoir valve <b>42</b> is kept closed during the entire rotation of the crankshaft <b>16</b> to isolate the air reservoir <b>40</b> from the rest of the engine. In the AE and AEF modes, the air reservoir valve <b>42</b> is kept open for a duration (in CA degrees) that is at least as long as a duration of the XovrE valve <b>26</b> opening event, to allow for use in the expansion cylinder <b>14</b> of previously stored compressed air. In a specific embodiment, in the AE and AEF modes, the air reservoir valve <b>42</b> may be kept open during the entire rotation of the crankshaft <b>16</b>. In the AC and FC modes, the air reservoir valve <b>42</b> is selectively opened and closed during a single rotation of the crankshaft <b>16</b> to allow for flow of compressed air into the air reservoir <b>40</b> in order to store the compressed air for later use.

p-0053In the EF mode, the compression piston <b>20</b> draws in and compresses inlet air for use in the expansion cylinder <b>14</b>. The compressed air from the compression cylinder <b>12</b> is admitted to the expansion cylinder <b>14</b> with fuel, at the beginning of an expansion stroke, which is ignited, burned and expanded on the same expansion stroke of the expansion piston <b>30</b>, transmitting power to the crankshaft <b>16</b>, and the combustion products are discharged on the exhaust stroke. Since compressed air is neither stored in nor released from the air tank <b>40</b> in the EF mode, the air tank valve <b>42</b> is closed.

p-0054In the AE mode, compressed air stored in the air tank <b>40</b> is admitted to the expansion cylinder <b>14</b>, at the beginning of an expansion stroke. Since in this mode the air tank valve <b>42</b> is kept open at least as long as the XovrE valve <b>26</b>, air flow into the expansion cylinder <b>14</b> is controlled by the XovrE valve. The air is expanded on the same expansion stroke of the expansion piston <b>30</b>, transmitting power to the crankshaft <b>16</b>, and the (expanded) air is discharged on the exhaust stroke.

p-0055In the AEF mode, compressed air stored in the air tank <b>40</b> is admitted to the expansion cylinder with fuel, at the beginning of an expansion stroke. Since in this mode the air tank valve <b>42</b> is kept open at least as long as the XovrE valve <b>26</b>, flow of the air/fuel mixture into the expansion cylinder <b>14</b> is controlled by the XovrE valve <b>26</b>. The air/fuel mixture is ignited, burned and expanded on the same expansion stroke of the expansion piston <b>30</b>, transmitting power to the crankshaft <b>16</b>, and the combustion products are discharged on the exhaust stroke.

p-0056In the AC mode, the compression piston <b>20</b> draws in and compresses inlet air. The compressed air is then stored in the air tank <b>40</b> by selectively opening and then closing the air tank valve <b>42</b>.

p-0057In the FC mode, the compression piston <b>20</b> draws in and compresses inlet air for use in the expansion cylinder <b>14</b> during a single rotation of the crankshaft <b>16</b>. Some of the compressed air from the compression cylinder <b>12</b> is admitted to the expansion cylinder <b>14</b> with fuel, at the beginning of an expansion stroke, which is ignited, burned and expanded on the same expansion stroke of the expansion piston, transmitting power to the crankshaft, and the combustion products are discharged on the exhaust stroke. The air tank <b>40</b> is also charged with compressed air during the same single rotation of the crankshaft <b>16</b> by selectively opening and then closing the air tank valve <b>42</b>.

p-0058In an exemplary embodiment shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>, the air tank valve <b>42</b> is an outwardly opening poppet valve disposed in the air reservoir port <b>44</b> and connected to the crossover passage <b>22</b>. The air tank port <b>44</b> has an angular bend (i.e., elbow) allowing the stem <b>43</b> of the valve <b>42</b> to extend vertically from the valve head. The angular bend is shown as a generally right-angle bend, but may be an S-curve or other similarly shaped elbow. A pneumatic, hydraulic, electric or mechanical valve actuation device <b>45</b> or the like may be disposed at the distal end of the stem <b>43</b>. While the air tank valve <b>42</b> is exemplified as an outwardly opening poppet valve, one of ordinary skill in the art would realize that the air tank valve may be one or more of, or a combination thereof, the following valve types: an inwardly opening poppet valve, a rotary valve, a sleeve valve, a pintle valve, or the like, and may include a pressure activated check valve (such as a reed valve) in the combination.

p-0059Although 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. 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.