Abstract:
A positive displacement power extraction compensation device is used to start and control the operation of engines. The device includes a positive displacement fixed vane compressor having a rotor connected with a drive shaft, a combustor connected with the compressor and a positive displacement power extraction device also having a rotor connected with a drive shaft. The compressor and power extraction devices are configured to displace unequal volumes of air at a given speed, so that combustion gases from the combustor exert less force on the compressor drive shaft as on the power extraction device drive shaft.

Description:
[0001]    This application claims the benefit of U.S. provisional application No. 60/184,119 filed Jun. 4, 2009. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the substitution of positive displacement devices of a certain type for traditional compressors and turbines in gas turbines. 
       BACKGROUND OF THE INVENTION 
       [0003]    Gas turbines have three basic parts. In its simplest form, the three components are the compressor, the combustor and the turbine. Energy extracted from the turbine is used to drive the compressor, which compresses air so that it may be mixed with fuel and burned in the combustor. The burnt fuel then exits the combustor through the turbine which rotates in response. The turbine drives the compressor and ancillary components. The range of gas turbines from turbojets to turbo shafts is defined by how much energy is extracted as shaft power. 
         [0004]    Turbojets extract as little energy as possible and still run various components. The primary design goal of a turbojet is to produce thrust. The exhaust gas from a turbojet travels extremely quickly and can be used to power a high speed device such as an older war plane. 
         [0005]    Turbo fans and turbo props extract more power from the burning gases with additional turbine stages. The mechanical power is used to turn propulsion fans or propellers. There may be multiple turbines and some may rotate on shafts separate from the compressor shaft. The additional shafts may be used to drive devices at speeds somewhat independent of the primary shaft. A turbo shaft is designed to convert as much of the energy as possible into mechanical energy as shaft horsepower. 
         [0006]    The main difference between a turbo prop and a turbo shaft is nomenclature. The hot gases exiting a turbo prop engine provide very little thrust. Instead, the energy in the burning gases is converted to mechanical energy which spins a much more efficient propeller. The combustor includes various components such as provisions for fuel injection and ignition. For various reasons explained below, it is not feasible to simply replace the compressors and turbines of a gas turbine with positive displacement devices and create a functioning engine. The goal of the present invention is to create a functioning analogous engine. 
         [0007]    All gas turbines are dynamic devices rather than positive displacement devices. They move or are moved by air hitting the blades of the compressor and turbine wheels and reacting. Gases can move slowly though a stopped gas turbine easily. Gas turbines need the gas to move quickly through them to function. The amount of air the compressor of a gas turbine moves increases with the square of its speed. The compressor needs to spin at high speeds in order to compress air but there are factors which limit the operating speed range of a compressor. 
         [0008]    The same factors which confine the operating range of the compressor also apply to the turbine. These factors are related to both the lift coefficient of the blades and the horsepower. The lift coefficient L is defined as 
         [0000]        L= ½ pv   2   AC   L  
 
         [0009]    where 
         [0010]    p is air density, 
         [0011]    v is the velocity of the gas for axial flow devices, 
         [0012]    A is planform area. and 
         [0013]    C L  is the lift coefficient at the desired angle of attack, Mach number and Reynolds number. 
         [0014]    The gas horsepower varies by the cube of the speed for centrifugal fans according to the equation 
         [0000]      GHP 2 =GHP 1 (RPM 2 /RPM 1 ) 3    
         [0015]    where 
         [0016]    GHP is the gas horsepower, and 
         [0017]    RPM is the revolutions per minute of the fan. 
         [0018]    Air velocity, pressure, heat, and the multitude of combinations of different sizes of devices make the design of gas turbines much more complicated than these formulas can begin to predict, but what is common to these devices is that they are efficient over an operating range which is in low multiples of their design speeds. This fact about gas turbines and compressors is what allows them to be started by being driven by a starter motor. The machine can be driven to a speed where the compressor provides enough air pressure for the fuel to be ignited in the combustor, but this speed is designed to be lower than the speed at which the turbine would act as a positive displacement fixed vane compressor and start to create a vacuum in the combustor. Once combustion is sustained, the volume of hot gasses is far greater than the volume being introduced by the compressor. The gasses generated by the combustor drive the turbine. 
         [0019]    The present invention uses a positive displacement compressor, a combustor and a positive displacement expander downstream of the combustor. The expander may drive the compressor just like the turbine drives the compressor in a gas turbine. This type of machine has been called an open cycle engine and falls into to the category of Brayton cycle engines which are characterized by combustion which occurs continuously and at near constant pressure. Gas turbines are Brayton cycle engines which are in widespread use. 
         [0020]    Internal combustion engines based on positive displacement compressors have historically not had constant combustion at constant pressure. Piston engines are examples of positive displacement engines, and the Wankle is another example. These reciprocating machines positively confine the charge gas and reduce its volume and then extract the energy as the volume increases during the expansion cycle. 
         [0021]    It may be noted here that Roots blowers do not actually confine the volume of air they move, but rather they rely on back pressure against the outflow to provide a chamber in which there can be a pressure rise. If a Roots blower vents to atmosphere, no compression takes place. The fixed vane compressor which is used in the present invention shares this characteristic with the Roots blower and in this way is unlike traditional vane machines. 
         [0022]    A common engine type is a hybrid of positive displacement engine and a gas turbine. The turbocharged reciprocating engine is a combination of two complete systems, a reciprocating engine and a gas turbine. The reciprocating engine serves as the combustor for the turbine. The only work the turbine does is to drive the compressor. Shaft power is only taken from the reciprocating engine. This combination is well adapted, with the aid of modern controls, to increase the power density of the reciprocating engine. The power from the exhaust gases is used only to provide greater air flow to the engine so that it can burn more fuel. Not as widely used is turbo compounding which is the use of a turbine in conjunction with a reciprocating engine to provide shaft power and not just compressed intake air. The goal of turbo compounding is to recover energy from the still expanding exhaust gas, and therefore increase shaft power, for a given amount of fuel, and therefore increase efficiency. An increase in power density is best served by turbocharging which makes the engine itself more powerful. Sometimes efficiency is also increased. Power density can be increased by using either a positive displacement device, such as a Roots blower, or a dynamic compressor, either one driven by shaft power from the engine. 
         [0023]    Another hybrid of a dynamic compressor and a reciprocating engine is a supercharged engine where the blower is a centrifugal compressor. The efficiency of a centrifugal compressor is desirable but its operating range is so narrow that mechanical supercharging is almost universally done with a much less efficient positive displacement compressor, like a Roots blower. Because mechanically driven superchargers use shaft power, they do not take advantage of the energy in exhaust gas as a turbocharger does. The benefit of a supercharger is that there is no lag time, and the designer hopes that the overall efficiency of the vehicle can be maintained by having a greater power density and response than a similarly powerful naturally aspirated engine or turbocharged engine respectively. 
         [0024]    The limited operating range of dynamic compressors and turbines contributes to turbo lag in turbocharged engines, limits the use of turbo-compounding so that gas turbines are best suited to applications where their manifold well known advantages outweigh their deficit of a narrow operating speed range. 
         [0025]    A third and more pertinent hybrid of a positive displacement machine and a dynamic compressor is described in the Van Blaricom US patent application publication No. 2008/0087004. This open cycle engine incorporates a positive displacement device in place of the turbine, but uses a centrifugal compressor to supply air to the combustor. There is no reciprocating engine as part of the system. The centrifugal compressor can be replaced by another type of compressor such as an axial or mixed flow compressor. There is a compelling reason for such a hybrid layout, but it comes at a cost of efficiency and limited operating range. Many controls are required to effectively mate a dynamic compressor to a positive displacement device, and examples of these are the use of waste gates on turbochargers and the use of variable geometry turbochargers. 
         [0026]    The preferred embodiment Van Blaricom includes a centrifugal air compressor which supplies air to the combustor with a positive displacement power extraction device placed downstream of the combustor. This arrangement has the advantage that if the components are properly sized, the engine can be started by spinning it with a starter motor. When the engine reaches a predetermined speed, fuel is introduced into the combustor and ignited. Because of the complicated but known characteristics of centrifugal compressors (and the other non-positive displacement compressors mentioned above), the compressor will deliver more air than the power extraction device draws out of the combustion chamber before ignition. Once the fuel is introduced and ignited and combustion is maintained, the volume of the gases drives the power extraction device which in turn drives the compressor. 
         [0027]    Another embodiment suggested but not elaborated upon in Van Blaricom is one where two of the inventive positive displacement devices are used. One of the fixed vane devices is used as a power extraction device which is analogous to the turbine of a gas turbine. The second fixed vane device is the compressor. The second positive displacement device is analogous to the compressor of a gas turbine or the centrifugal compressor in the embodiment described above. Van Blaricom refers to the use of two fixed vane positive displacement devices, but does not explain some of the peculiarities of such a contrivance, nor does Van Blaricom contain information regarding the mechanisms necessary to overcome the corollary complications which result when positive displacement devices are used for both the compressor and the power extraction roles. 
       SUMMARY OF THE INVENTION 
       [0028]    The present invention is used during certain operating regimes and for starting an open cycle engine. The invention compensates for physical differences between positive displacement devices and the compressors and turbines found in gas turbine engines. These physical differences prevent operation of an engine which is similar to a gas turbine but uses positive displacement devices in place of both the compressor section and the turbine section. 
         [0029]    An object of the present invention is to integrate a positive displacement device in the role of primary power extraction device in an engine where the role of compressor is assumed by a similar positive displacement device. This integration is mainly related to the relative size and speeds of the two devices. The integration also requires other mechanisms particular to this system for starting and enhanced transient response. 
         [0030]    To achieve the above objective, the devices need to be connected to each other in such a way that the burning gases in the combustor take the path of least resistance through the power extraction device. If the PED (Power Extraction Device analogous to the turbine) is to drive the PDC (Positive Displacement Compressor analogous to the gas turbine compressor section) directly, then the PED must either be of a greater displacement, or, if they are the same size, the PED must be geared to run at higher revolutions per minute (RPM) than the PDC. Alternatively, the PED could be uncoupled from the PDC or coupled through a variable ratio transmission. The important point is that if the engine were built so that it moves equal volumes of air at a given RPM, the combustion gases exert as much force on the PDC shaft as on the PED shaft, assuming the same efficiency, and such a design would not work. 
         [0031]    A design where the PED is of an effective greater displacement, whether because it displaces more per revolution or because it is geared to run faster than the PDC, poses a significant difficulty when starting the engine. When driven by a starter motor, before combustion, the PED causes a vacuum in the combustion chamber because the PED is moving more air out than the PDC supplies. Combustion cannot begin where a vacuum is being created. 
         [0032]    It is therefore the object of the present invention to arrange the PDC and the PED relative to each other, and to provide a mechanism for overcoming the difficulties of starting and operating a device according to this design. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0033]    Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in the light of the accompanying drawing, in which: 
           [0034]      FIG. 1  is a block diagram of an engine starting and control system; 
           [0035]      FIG. 2  is a detailed schematic view of the engine of  FIG. 1 ; and 
           [0036]      FIG. 3  is a block diagram of a preferred embodiment of an engine starting and control system according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Referring first to  FIG. 1 , there is shown an engine  2  which includes a compressor in the form of a positive displacement fixed vane compressor  4 . The positive displacement fixed vane compressor compresses air supplied to an inlet  6  of the compressor. A fuel injector  8  is also connected with the air inlet for injecting fuel into the air supply. A fluid conduit  10  connected with the compressor  4  delivers output fluid to a combustor  12 . The combustor  12  further receives air for combustion of the fluid. More particularly, a compressor  14  provides pressurized air which is stored in a tank  16 . A valve  18  meters the volume of air delivered from the tank to the combustor during starting of the engine, during periods of transient engine power, or during other periods where a low pressure in the combustor could adversely affect combustion. A fuel injector  20  injects fuel into the combustor and an ignition device  22  is connected with the combustor to initiate combustion of the air and fuel mixture. A fixed vane power extractor  24  is connected with the output of the combustor. The power extractor includes an exhaust outlet  26 . Between the positive displacement fixed vane compressor  4  and the fixed vane power extractor  24  is a drive assembly  28  or other power transfer mechanism such as gears, belts or chains. 
         [0038]    To start the engine, air from the tank  16  is introduced to the combustor  12  via air injection port. Fuel is injected into the combustor through fuel injector  20  and the ignition device  22  ignites the fuel. As combustion occurs, the pressure in the combustor increases. The hot gases from the combustor exit through the power extraction device  24 . A separate starter motor (not shown) may be provided to spin the engine. The air from the supplemental air tank  16  tends to cause the rotors to spin in the correct direction, but delaying ignition until there is both positive pressure in the combustor and correct rotation of the engine is useful in most applications. A brake  30  is connected with the drive assembly  28  to control the speed of the engine  2 . The brake may be of the friction, electric, hydraulic or pneumatic type. 
         [0039]    A controller  32  is connected with the fuel injectors  8 ,  20 , the ignition device  22  and the air tank  16  to control the delivery of fuel and air and the combustion thereof in the combustor to control the speed of the engine. In addition, the controller is connected with the brake  30  to further control engine speed. 
         [0040]    Referring now to  FIG. 2 , the positive displacement fixed vane compressor  4  and the fixed vane power extractor  24  according to  FIG. 1  are shown in more detail. The compressor  4  includes a housing  4   a  in which a fixed vane mechanism rotates. The vane mechanism includes a rotor  4   b  having at least two vanes  4   c  mounted thereon. Air from the inlet  6  is filtered by an air filter  34 . The air is forced by the vanes  4   c  as the rotor rotates within the housing. The rotating vanes intercept a second rotor  4   d  which contains a cutout portion  4   e  for receiving the vanes  4   c  of the first rotor  4   b . The second rotor  4   d  is geared or otherwise timed to rotate in a direction counter to the direction of rotation of the first rotor. The power extractor  24  includes a housing  24   a  which contains a first rotor  24   b  having at least two vanes  24   c  mounted thereon. The rotor housing contains a second rotor  24   d  which contains a cutout portion  24   e  for receiving the vanes  24   c  of the first rotor  24   b . The rotors of the power extractor thus counter rotate as do the rotors in the compressor. 
         [0041]    In  FIG. 2 , the positive displacement fixed vane compressor  4  and the power extraction device  24  have the same displacement. In order for the path of least resistance for the output of the combustor  12  to be through the power extraction device  24 , the positive displacement fixed vane compressor  4  spins more slowly than the power extraction device  24  of the same size. The rotors  4   b ,  24   b  of the compressor  4  and power extractor  24  are connected via the drive assembly  28 . The gear ratio between the compressor and the power extraction device is such that if they are the same size, when they spin the power extraction device spins faster than the compressor. 
         [0042]    The preferred embodiment of the invention will now be described with reference to  FIG. 3 . This embodiment is similar to that of  FIGS. 1 and 2  except that a positive displacement fixed vane compressor  104  is coupled with a positive displacement power extraction device  124  via the combustor. The devices are coupled so that the power extraction device drives the compressor at a 1:1 ratio. More particularly, the fixed vane compressor  4  includes an air inlet  106  which includes an air filter  134  for eliminating contaminants. A fuel injector  108  is connected with the air inlet  106 . The fixed vane compressor is connected with a combustor  112  which receives fuel from a fuel injector  20  and air from an air tank  116  via a valve  118  which regulates the pressure of the air. An ignition device ignites the fuel within the combustor. The output of the combustor is connected with the positive displacement power extraction device  124  having any exhaust outlet  126 . Since the power extraction device of  FIG. 3  has a greater displacement, when there is pressure in the combustion chamber that is greater than the ambient air pressure, the gas in the combustion chamber exits through the power extraction device. This occurs even though it causes the power extraction device to drive the compressor to force air into the combustion chamber against the pressure already there. The exact size ratio depends on the application, but the size relationship will never be reversed. 
         [0043]    A transmission assembly  136  is preferably connected between the positive displacement fixed vane compressor and the positive displacement power extraction device which allows the ratio between the compressor and the power extraction device to be varied. The transmission essentially replaces the drive assembly of  FIGS. 1 and 2 . If desired, a brake  130  can be connected with the transmission, although depending on the design of the transmission, the brake may not be necessary. The transmission may be mechanical, hydraulic, electric or pneumatic. In addition, an auxiliary drive mechanism  138  is connected with the compressor to spin the rotor of the compressor independently of the power extraction device. Operation of the compressor in this manner will generate positive pressure in the combustor. The auxiliary drive mechanism can be a motor or generator to spin the compressor rotor during start up and at other times when more air is required. 
         [0044]    A controller  132  is connected with the fuel injectors  108 ,  120 , the ignition device  122  and the air tank  116  to control the delivery of fuel and air and the combustion thereof in the combustor to control the speed of the engine. In addition, the controller is connected with the brake  130  to further control engine speed and with the auxiliary drive mechanism  138 . 
         [0045]    While the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the inventive concepts set forth above.