Abstract:
A compound engine system includes a rotary engine with rotating chambers, a compressor section in successive communication with the rotating chambers, and a turbine section in successive communication with the rotating chambers. The turbine section has an output shaft. The output shaft and the engine shaft are drivingly engaged to each other and wherein the turbine section has a power output corresponding to from 20% to 35% of a total power output of the compound engine system. A method of compounding power in a compound engine system is also discussed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/701,809 filed May 1, 2015, which is a continuation of U.S. application Ser. No. 13/272,738 filed Oct. 13, 2011, which claims priority on provisional U.S. application No. 61/512,570 filed Jul. 28, 2011, the entire contents of both of which are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The application relates generally to a compound engine system including a rotary internal combustion engine. 
       BACKGROUND OF THE ART 
       [0003]    Rotary engines, such as for example Wankel engines, use the eccentric rotation of a piston to convert pressure into a rotating motion, instead of using reciprocating pistons. In these engines, the rotor includes a number of apex or seal portions which remain in contact with a peripheral wall of the rotor cavity of the engine throughout the rotational motion of the rotor to create a plurality of rotating chambers when the rotor rotates. 
         [0004]    In a never-ending quest to achieve greater power output, Wankel engines have relatively low rotor recess volume in order to achieve the high volumetric expansion ratio required for such increased power output. However, such engines may not be fully optimized for use in turbocompounding systems, and thus room for improvement exists. 
       SUMMARY 
       [0005]    In one aspect, there is provided a compound engine system comprising: a rotary engine having a stator body defining an internal cavity and a rotor body engaged to an engine shaft, the rotor body sealingly engaged within the cavity to provide rotating chambers of variable volume; a compressor section in successive communication with the rotating chambers; and a turbine section in successive communication with the rotating chambers, the turbine section having an output shaft; wherein the output shaft and the engine shaft are drivingly engaged to each other and wherein the turbine section has a power output corresponding to from 20% to 35% of a total power output of the compound engine system. 
         [0006]    In another aspect, there is provided a method of compounding power in a compound engine system, the method comprising: driving an engine shaft with a rotor of a rotary engine, the rotary engine having rotating chambers of variable volume; feeding compressed air from a compressor section to the rotating chambers; flowing exhaust from the rotating chambers to a turbine section to drive an output shaft of the turbine section; and compounding power of the engine shaft and of the output shaft, wherein the turbine section has a power output corresponding to from 20% to 35% of a total power output of the compound engine system. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    Reference is now made to the accompanying figures in which: 
           [0008]      FIG. 1  is a block diagram of a compound engine system; 
           [0009]      FIG. 2  is a schematic cross-sectional view of a rotary internal combustion engine which can be used in a system such as shown in  FIG. 1 ; 
           [0010]      FIG. 3  is a schematic, partial peripheral view of a rotor of the engine of  FIG. 2 ; and 
           [0011]      FIG. 4  is a schematic, partial cross-sectional view of the rotor of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring now to  FIG. 1 , a compound engine system  8  is schematically shown. The system  8  includes a compressor  11  and a turbine  13  which are connected by a shaft  15 , and which act as a turbocharger to one or more rotary engines  10 . The compressor  11  may be a single-stage or multiple-stage centrifugal device and/or an axial device. A rotary engine  10 , or a plurality of rotary engines, receives compressed air from the compressor  11 . The air optionally circulates through an intercooler  16  between the compressor  11  and the rotary engine(s)  10 . 
         [0013]    The exhaust gas exiting the rotary engine  10  is supplied to the compressor turbine  13  and also to a power turbine  17 , the turbines  13 ,  17  being shown here in series, i.e. with the exhaust gas flowing first through one of the two turbines where the pressure is reduced, and then through the other turbine, where the pressure is further reduced. In an alternate embodiment (not shown), the turbines  13 ,  17  are arranged in parallel, i.e. with the exhaust gas being split and supplied to each turbine at same pressure. In another alternate embodiment, only one turbine is provided. 
         [0014]    Energy is extracted from the exhaust gas by the compressor turbine  13  to drive the compressor  11  via the connecting shaft  15 , and by the power turbine  17  to drive an output shaft  19 . The output shaft  19  may be connected via a gear system  21  to a shaft  22  connected to the rotary engine(s)  10 . The combined output on the shafts  19 ,  22  may be used to provide propulsive power to a vehicle application into which the system  8  is integrated. This power may be delivered through a gearbox (not shown) that conditions the output speed of the shafts  19 ,  22  to the desired speed on the application. In an alternate embodiment, the two shafts  19 ,  22  may be used independently to drive separate elements, e.g. a propeller, a helicopter rotor, a load compressor or an electric generator depending whether the system is a turboprop, a turboshaft or an APU (Auxiliary Power Unit). 
         [0015]    Although not shown, the system  8  also includes a cooling system, including a circulation system for a coolant to cool the outer body of the rotary engine (e.g. water-ethylene, oil, air), an oil coolant for the internal mechanical parts of the rotary engine, one or more coolant heat exchangers, etc. 
         [0016]    The compound engine system  8  may be as described in Lents et al.&#39;s U.S. Pat. No. 7,753,036 issued Jul. 13, 2010 or as described in Julien et al.&#39;s U.S. Pat. No. 7,775,044 issued Aug. 17, 2010, the entire contents of both of which are incorporated by reference herein. 
         [0017]    The rotary engine  10  forms the core of the compound cycle engine system  8 . Referring to  FIG. 2 , an embodiment of the rotary engine  10 , known as a Wankel engine, is schematically shown. The rotary engine  10  comprises an outer body  12  having axially-spaced end walls  14  with a peripheral wall  18  extending therebetween to form a rotor cavity  20 . The inner surface of the peripheral wall  18  of the cavity  20  has a profile defining two lobes, which is preferably an epitrochoid. 
         [0018]    An inner body or rotor  24  is received within the cavity  20 . The rotor  24  has axially spaced end faces  26  adjacent to the outer body end walls  14 , and a peripheral face  28  extending therebetween. The peripheral face  28  defines three circumferentially-spaced apex portions  30 , and a generally triangular profile with outwardly arched sides. The apex portions  30  are in sealing engagement with the inner surface of peripheral wall  18  to form three rotating working chambers  32  between the inner rotor  24  and outer body  12 . The geometrical axis of the rotor  24  is offset from and parallel to the axis of the outer body  12 . 
         [0019]    The working chambers  32  are sealed. Each rotor apex portion  30  has an apex seal  52  extending from one end face  26  to the other and protruding radially from the peripheral face  28 . Each apex seal  52  is biased radially outwardly against the peripheral wall  18  through a respective spring. An end seal  54  engages each end of each apex seal  52 , and is biased against the respective end wall  14  through a suitable spring. Each end face  26  of the rotor  24  has at least one arc-shaped face seal  60  running from each apex portion  30  to each adjacent apex portion  30 , adjacent to but inwardly of the rotor periphery throughout its length. A spring urges each face seal  60  axially outwardly so that the face seal  60  projects axially away from the adjacent rotor end face  26  into sealing engagement with the adjacent end wall  14  of the cavity. Each face seal  60  is in sealing engagement with the end seal  54  adjacent each end thereof. 
         [0020]    Although not shown in the Figures, the rotor  24  is journaled on an eccentric portion of a shaft and includes a phasing gear co-axial with the rotor axis, which is meshed with a fixed stator phasing gear secured to the outer body co-axially with the shaft. The shaft rotates the rotor  24  and the meshed gears guide the rotor  24  to perform orbital revolutions within the stator cavity. The rotor  24  performs three rotations for each orbital revolution. Oil seals are provided around the phasing gear to impede leakage flow of lubricating oil radially outwardly thereof between the respective rotor end face  26  and outer body end wall  14 . 
         [0021]    During one orbital revolution, each chamber varies in volumes and moves around the stator cavity to undergo the four phases of intake, compression, expansion and exhaust, these phases being similar to the strokes in a reciprocating-type internal combustion engine having a four-stroke cycle. 
         [0022]    The engine includes a primary inlet port  40 , shown here as being defined in the end wall  14 ; in an alternate embodiment, the primary inlet port  40  may be defined through the peripheral wall  18 . The primary inlet port  40  is in communication with the exhaust of the compressor  11  through an intake duct  34  which is defined as a channel in the end wall  14 . The primary inlet port  40  delivers air to each of the chambers  32 , and a fuel injection port  36  (see  FIG. 4 ) is also provided for delivering fuel into each chamber  32  after the air therein has been compressed. Fuel, such as kerosene (jet fuel) or other suitable fuel, is delivered into the chamber  32  such that the chamber  32  is stratified with a rich fuel-air mixture near the ignition source and a leaner mixture elsewhere, and the fuel-air mixture may be ignited within the housing using any suitable ignition system known in the art (e.g. spark plug, glow plug). 
         [0023]    The engine also includes an exhaust port  44 , shown here as being defined through the peripheral wall  18 ; in an alternate embodiment, the exhaust port  44  may be defined through the end wall  14 . The exhaust port  44  communicates with the inlet of at least one of the turbines  13 ,  17 . 
         [0024]    The rotary engine  10  operates under the principle of the Miller or Atkinson cycle, with its compression ratio lower than its expansion ratio. For example, the ratio obtained by dividing the volumetric compression ratio by the volumetric expansion ratio may be between 0.3 and 0.8, and more particularly about 0.4-0.5. Accordingly, the primary inlet port  40  is located further away (i.e. measured as a function of piston rotation) from the exhaust port  44  when compared to a rotary engine having compression and expansion ratios that are equal or approximately equal to one another. The angle of the primary inlet port  40 , relative to the angle of the exhaust port  44 , can then be determined to achieve a desired peak cycle pressure given the inlet air pressure. The position of the primary inlet port  40  may vary between the 7 o&#39;clock position up to the 10 o&#39;clock position. In the embodiment shown, the primary inlet port  40  extends between the 8 o&#39;clock and the 9 o&#39;clock positions. 
         [0025]    In the embodiment shown, the primary inlet port  40  is spaced from the exhaust port  44  so that the rotor  24  prevents communication therebetween in all rotor positions. In an alternate embodiment, the primary inlet port  40  and exhaust port  44  may be in momentary communication with each other throughout the revolution of the rotor  24 . 
         [0026]    The rotary engine  10  may also include a secondary inlet port or purge port  42  also in communication with the exhaust of the compressor  11 . The purge port  42  is shown here as being defined through the end wall  14  and communicating with the same intake duct  34  as the primary inlet port  40 ; alternately, the purge port  42  may be defined through the peripheral wall  18 , and/or be defined independently of the primary inlet port  40 . The purge port  42  is located rearwardly of the primary inlet port  40  and forwardly of the exhaust port  44  along the direction R of the rotor revolution and rotation. The purge port  42  is located such as to be in communication with the exhaust port  44  through each of the chambers  32  along a respective portion of each revolution, to effectively purge each of the chambers  32 . In an alternate embodiment, the purge port  42  may be omitted, particularly but not exclusively when the inlet port  40  and exhaust port  44  are in momentary communication with each other. 
         [0027]    Referring to  FIGS. 3-4 , the peripheral face  28  of the rotor  24  includes a recess  38  defined therein between each pair of adjacent apex portions  30 . The recess  38  defines part of the volume of the corresponding chamber  32 ; when the chamber  32  is at its minimum volume, for example at Top Dead Center, the recess  38  defines a significant part of the volume of the chamber  32 . 
         [0028]    Typical Wankel engines have relatively low rotor recess volume in order to have a high volumetric expansion ratio for a generally higher power output. However, a low recess volume limits the combustion volume which in turn may limit the amount of fuel burned, the rotational speed and the quality of combustion, especially for Wankel engines used with heavy fuel. It has been discovered that it is possible to increase the volume of the recess  38  above the usual volume seen in typical Wankel engines while having an acceptable power output of the system  8 . In a particular embodiment, the volume of each recess  38  corresponds to between 5% and 15% of the displacement volume of the corresponding chamber  32  of the rotor  24 , with the displacement volume being defined as the difference between the maximum and minimum volume of one chamber  32 . In another particular embodiment, the volume of each recess  38  is at least 6% and at most 11% of the displacement volume. In a further particular embodiment, the volume of each recess  38  corresponds to about 8 to 10% of the displacement volume. 
         [0029]    The recess  38  may be defined as a single, dual or multiple pocket(s) in the peripheral face, which together define the recess volume. The shape of the recess  38  may be different than that of the particular embodiment shown. 
         [0030]    The increased volume of the recess  38  allows for a reduced volumetric compression ratio, which may improve combustion stability and efficiency. A higher combustion volume when the rotor  24  comes near Top Dead Center may allow the rotary engine  10  to burn more fuel as more air is available, and as such turn the rotary engine faster and increase the power density. The increased ratio of volume to wall surface may also reduce heat losses which tend to quench the flame. The increased combustion chamber volume may also allow flexibility in injection spray design. 
         [0031]    However, the increased volume of the recess  38  correspondingly lowers the expansion ratio and as such would tend to lower the power output of the rotary engine when used alone. However, in the compound system  8 , the lower expansion ratio of the rotary engine  10  is compensated by the expansion within the turbines  13 ,  17 . 
         [0032]    In a particular embodiment, the expansion ratio of the turbines  13 ,  17  is selected such that the turbine section provides a power output corresponding to from 20% to 35% of the total power output of the compound engine system  8 . In a particular embodiment, this may be achieved by having an expansion ratio in the turbine section which is similar to the boost compression pressure ratio, i.e. the compression pressure ratio of the compressor  11 . 
         [0033]    The increased power output of the turbine section may provide increased power for a given air mass flow, which may result in a smaller, lighter and more efficient engine at a given power. The low volumetric compression ratio of the rotary engine  10  may help heavy fuel (e.g. diesel, kerosene (jet fuel), equivalent biofuel) to remain at a pressure low enough to prevent self-ignition which may help ensure that the cycles runs with direct injection with a source of ignition, may save structural weight, and may reduce internal leakages. 
         [0034]    Although the rotary engine  10  with the increased volume recess may assist in permitting for the large volume and improved combustion in non-intercooled systems such as described in U.S. Pat. No. 7,775,044, it may also be employed in other suitable systems, such as shown in U.S. Pat. No. 7,753,036 with or without intercooling, assuming suitable expansion ratios are selected. With intercooled systems, use of the larger recess volume may indeed facilitate stable combustion and hence improve such intercooled systems. 
         [0035]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention(s) disclosed. For example, the present teachings may be applied to any suitable rotary engine, such as a rotary vane pumping machine or other suitable engine, and is thus not limited in application to Wankel engines. Other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.