Abstract:
A boost system comprising a turbocharger, a supercharger operable as either a compressor or an expander. Air flows through the turbocharger, optionally through a first charge air cooler CAC 1,  then through the supercharger, a second charge air cooler CAC 2,  and into the engine. At low engine speeds, the supercharger may be used to compress air, which is tempered by CAC 2.  At high engine speeds, the turbocharger has excess capacity, resulting in a hot compressed air stream. The supercharger operates as an expander to cool the air stream and reduce the air pressure and temperature to a desired level. Temperature may be reduced to a level below that desired for combustion; CAC 2  then rewarms the air, thereby storing cooling capacity. A useful embodiment incorporates a turbocharger with a hybrid gas/electric or diesel/electric engine arrangement wherein a supercharger and a starter/generator/motor are disposed on a disconnectable secondary drive powered by the engine.

Description:
RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS 
       [0001]    The present application is a Continuation-In-Part, pursuant to 35 USC §363, of a pending PCT Application, Ser. No. PCT/US09/02364, filed Apr. 15, 2009, the relevant disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to systems for boosting air flow into an internal combustion engine; more particularly, to boosting systems comprising both a turbocharger and a supercharger; and most particularly, to a combined turbocharger/supercharger boosting system to provide high-performance, high-efficiency engine operation over the full range of engine operating conditions and speeds. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the effort to improve vehicle fuel economy while maintaining or improving vehicle power to weight ratios, many automotive OEMs are opting to downsize and turbocharge (“boost”) their new direct injection gasoline engines. This is a continuation of a trend which has already led to nearly 100% penetration of turbocharger systems on automotive diesel engines. As used herein, the terms “boost” and “boosting” should be taken to mean an apparatus and/or method for providing combustion air to an internal combustion engine at a supra-atmospheric pressure. 
         [0004]    One disadvantage of downsized, turbocharged engines is that the available torque at or near idle is very low (sometimes even lower than a naturally-aspirated engine of the same displacement). This leads to the well-known “turbo lag” and to the trend to waste gate, variable geometry turbine, and even two-stage series/sequential turbocharger systems to mitigate such poor low-RPM and transient performance. 
         [0005]    One prior art engine system that overcomes low-RPM and transient performance has been put into volume production by Volkswagen® in a 1.4L “twin-charged” direct injection gasoline engine offered in several of their European models. This engine system uses a high speed ratio supercharger to supplement the turbocharger at low RPM as needed. The system then transitions to turbocharger-only operation, with the supercharger declutched, at high RPM. 
         [0006]    Although, as noted above, the turbocharger is unable to boost an engine well at low RPM, the amount of available exhaust energy at high engine speed and load is often substantially above the level needed for boosting the engine. Therefore, a “waste gate”, as the name implies, is often used at high engine speed and load to waste part of the exhaust energy by diverting a portion of the flow around the turbocharger turbine. A variable geometry turbine is somewhat more efficient, because it can modulate the kinetic energy of the exhaust at the turbine inlet; however, a variable geometry turbine tends to be more expensive and still suffers from some flow losses and thus still involves some level of parasitic energy loss in the system. 
         [0007]    Another important aspect of engine boosting for both gasoline and diesel engines is charge-air cooling. Due to adiabatic compression, the compressed air temperature after a turbo-compressor or supercharger is significantly higher than ambient. An “intercooler”, also known as a charge air cooler (CAC), typically is used to lower the temperature as much as possible to improve the volumetric efficiency and to reduce the propensity of engine “knock”. 
         [0008]    Air-to-air CACs, wherein compressed air is ducted to a heat exchanger at the very front of a vehicle, are widely used. Such an arrangement can offer low compressed air temperatures at the expense of some complexity in packaging and flow losses. 
         [0009]    Coolant-to-air CACs tend be smaller and can be packaged more conveniently in or adjacent to the engine intake manifold to use the engine&#39;s coolant. However, the air temperature can approach only the coolant temperature of the engine, which normally is about 90° C. and in extreme conditions may be as high as 125° C. One coolant-based approach is to use a secondary radiator only for the CAC, with a reservoir, pump and radiator separate from the normal engine cooling loop. This arrangement can produce intercooled charge temperatures of perhaps 40° C. to 70° C. under normal ambient conditions. 
         [0010]    Future engine systems, both gasoline and diesel, will undoubtedly required even higher levels of boost than can be delivered by today&#39;s prior art systems. Staged, twin-turbo systems are known, and even three-turbomachine systems have been proposed for heavy duty vehicles, two for boost and one for energy recovery. 
         [0011]    What is needed in the art is a more efficient “twin-charged” engine boosting system that can have the performance attributes of the aforementioned twin-charger system but with improved exhaust energy recovery, to mitigate the obvious high cost of such a system, and with improved charge cooling, to maintain performance in hot weather conditions and to facilitate the use of elevated levels of exhaust gas recirculation (EGR), consistent with future lower emission standards. 
         [0012]    It is a principal object of the present invention to increase both the performance and the efficiency of an internal combustion engine. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly described, a boost system for an internal combustion in accordance with the present invention comprises a turbocharger driven conventionally by engine exhaust; a supercharger driven by the engine and capable of variable speed operation as either a compressor or an expander; an optional first charge air cooler (CAC 1 ); and a second charge air cooler (CAC 2 ). Combustion air enters the turbocharger, then flows in sequence through optional CAC 1 , the supercharger, CAC 2 , and thence into the engine. 
         [0014]    At low engine and vehicle speeds, when additional boosting is needed, the supercharger is rotated at a high speed relative to the engine. It draws and compresses air, which then is tempered by CAC 2  before passing into the engine. The turbocharger contributes relatively little at very low RPM, the engine boosting coming from action of the supercharger. 
         [0015]    As engine speed and exhaust volume increase, the turbocharger begins to significantly compress the incoming air. The air is tempered principally by CAC 1 , and the supercharger and CAC 2  contribute only modestly to the engine boosting and intercooling. 
         [0016]    At high engine speeds and high exhaust volume, such as when cruising on an interstate highway, the turbocharger capacity may substantially exceed the compressed air requirement, and engine boosting comes exclusively from the turbocharger. In the prior art, the excess capacity is dumped and thereby wasted as via a waste gate. This results in unnecessary backpressure in the engine exhaust. In the present invention, this extra compressive capacity is recovered and stored as follows. 
         [0017]    The speed of the supercharger is reduced such that the supercharger operates as an adiabatic expander to cool the air stream and reduce the intake air pressure to a desired level. Preferably, the air temperature is reduced to a level below that desired for combustion. CAC 2  then serves to rewarm the air by its heat capacity or optionally by the condensation of a refrigerant, thereby storing cooling capacity in CAC 2  for use during the next period of supercharger boosting, and thus improving the cooling of the supercharger output. 
         [0018]    The invention is also useful in an embodiment incorporating a turbocharger with a prior art hybrid gas/electric or diesel/electric engine arrangement wherein a supercharger and a starter/generator/motor are disposed on a disconnectable secondary drive means powered by the engine. This embodiment overcomes efficiency penalties of the prior art arrangement wherein the supercharger alone is responsible for engine boosting at high speed and load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a schematic drawing of a prior art supercharger system, without turbocharger, for stop/start hybrid operation of an internal combustion engine, substantially as disclosed in incorporated reference PCT/US09/02364; 
           [0021]      FIG. 2  is a schematic drawing showing the prior art system in  FIG. 1  adapted as a first embodiment of a twin-charged boosting system in accordance with the present invention; and 
           [0022]      FIG. 3  is a schematic drawing of a generic twin-charged boosting system in accordance with the present invention. 
       
    
    
       [0023]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Various concepts are known in the prior art for recovering energy from engine exhaust. The present invention moves the energy recovery to the air intake, thereby reducing the overall cost of components and sharing components with the supercharger and charge air cooling functions. Additional value is received in improved charge cooling, which improves knock-limited power, and/or reduced power parasitics for accessories such as air conditioning. Cooling of the CAC 2  thermal storage mass in normal driving allows subsequent excellent transient cooling performance because the CAC 2  thermal mass and, optionally, the refrigerant or phase-change material (PCM) keeps air charge temperature low in temporary high-boost supercharged conditions. The option of active cooling using a refrigerant, fuel, or low-temperature loop coolant with a secondary radiator can be useful for extreme hot weather conditions. The charge cooling benefits make the present system attractive for clean diesel engines of the future because very low air charge temperature allows for a desirably very high level of exhaust gas recirculation (EGR). 
         [0025]    A twin-charged engine boost system in accordance with the present invention allows extreme downsizing of an engine, either gasoline or diesel, without compromising performance and efficiency over the entire range of engine load and operating conditions. 
         [0026]    Referring to  FIG. 1 , a prior art supercharger system  10  is shown, for stop/start hybrid operation of an internal combustion engine, substantially as disclosed in incorporated reference PCT/US09/02364. 
         [0027]    System  10  comprises a secondary drive means  12  such as a belt, chain, or direct coupling that is operationally connected to the crankshaft  14  of an internal combustion engine  16 . Secondary drive means  12  may be driven directly via a clutch  18  to permit automatic selective drive of the secondary drive means  12  by the engine control system (not shown). Preferably clutch  18  comprises at least an active (on/off) clutch, and preferably also a passive (so-called “over-running”) clutch. Clutch  18  is mounted directly on the end of crankshaft  14 , along with a pulley damper  20  for driving a primary drive means  22  such as a primary belt or chain at a fixed ratio to engine speed. 
         [0028]    An “over-running” clutch refers to a clutch between two rotatable elements that latches and unlatches with relative rotation of the input and output elements. If the input element, in the present case connected to engine  16 , is at the speed of the output element, connected to supercharger secondary drive means  12 , the clutch latches. If the engine is turning slower than secondary drive means  12 , the clutch freewheels, allowing secondary drive means  12  to run faster than synchronous with engine  16 . Thus, for engine  16  to be driven by system  10  as in starting mode, an additional, on/off clutch is required (also referred to herein as an “active” clutch). The two clutches are not in series but may be on different elements of a planetary gear set, as is known in the prior art. 
         [0029]    In the present discussion, the term “primary” should be taken to mean comprising an apparatus  24  either necessary to the functioning of the engine or which needs to rotate at a fixed ratio to the engine speed, e.g., a water pump. “Secondary” should be taken to mean comprising an apparatus either non-essential to the functioning of the engine or which needs to rotate at a speed independent of engine speed, e.g., supercharger  26 , HVAC compressor  28 , or power steering (not shown) and power brakes (not shown) which may be optionally included in system  10 . Also a belt tensioner  25  and one or more idler pulleys (not shown) may be used (as in the prior art). 
         [0030]    System  10  further includes a low-inertia starter/motor/generator  30  having a wide speed range, driven by the secondary drive means  12 . Supercharger  26  is driven by starter/motor/generator  30 , either directly (preferred) or via an intermediate linkage such as an additional belt (not shown). The starter/motor/generator may be electric, hydraulic, or pneumatic or combinations thereof. The system also includes an energy storage device  34 , such as a battery, ultracapacitor, hydraulic or pneumatic accumulator, or combinations thereof. 
         [0031]    Two-speed (active, plus passive over-running) clutch  18  allows system  10  to turn in three modes: 
         [0032]    1. at a fixed multiple of engine speed (when the engine is “ON” and the overrunning clutch latches), for example when the engine is in “cruise” mode. 
         [0033]    2. at a fixed higher multiple of engine speed (when the active clutch is “ON”), for example, when the engine is being supercharged. 
         [0034]    3. at a variable speed, independent of the engine, when the active clutch is “OFF” and the engine is “OFF” or running at a lower speed than the supercharger system. 
         [0035]    The supercharger device  26  may be a turbo-compressor (centrifugal, axial, or mixed flow, i.e. the cold side of a turbocharger) or a Roots® blower, or scroll or Lysholm® compressor. In the case of a turbo-compressor, a high ratio drive  32  is required to spin the compressor at a very high speed (compared to the secondary belt drive  12 ). This may be with a gear set or a roller traction drive. In the case of the other supercharger technologies, a more moderate step up drive may be used or the device may run at the same speed as its secondary drive. 
         [0036]    The system has a large cost benefit, by using a single starter/motor/generator device  30  (and associated controls) to do multiple functions. For example, in the case of an electric starter/motor/generator, this starter/motor/generator can do: 
         [0037]    engine starting (both for initial cold start and subsequent stop-start cycles), 
         [0038]    steady state (low power) generating to run accessories and keep a SLI battery charged, 
         [0039]    hybrid electric functions (torque assist and regenerative braking) using the ultracapacitor or other high power energy storage  34  to source or sink the required power, 
         [0040]    engine independent boosting, and 
         [0041]    engine independent air conditioning. 
         [0042]    Referring to  FIG. 2 , a first embodiment  110  of a high-performance, high-efficiency twin-charged boosting system in accordance with the present invention for a hybrid-operated engine incorporates all the elements of system  10 , shown in  FIG. 1 . In addition, system  110  comprises a turbocharger  140  driven conventionally by engine exhaust  142  which is then discharged  143 ; a first charge air cooler (CAC 1 )  144 ; and a second charge air cooler (CAC 2 )  146 . Combustion intake air  148  enters turbocharger  140  and is compressed and heated, then flows  150  in sequence through CAC 1   144 , supercharger  26 , CAC 2   146 , and thence into engine  16 . 
         [0043]    By switching or modulating the speed of supercharger  26  and optionally (depending on the type of supercharger) by switching the direction of flow or porting of the supercharger, the function can be switched between operation as a compressor or expander. In system  10  ( FIG. 1 ), for example, supercharger  26  is arranged with an on/off or variable speed accessory drive or clutch  18 . The high speed is defined by operation in highly boosted modes, whereas the freewheeling speed (declutched from the engine) or low speed can be set to have the supercharger act as an expander. The operation as an expander is useful in unboosted conditions (for recovering intake throttling losses) or when the capacity of the turbocharger exceeds the engine flow requirement (for reducing wastegate losses). 
         [0044]    Referring to  FIG. 3 , a non-hybrid, high-performance, high-efficiency twin-charged boosting system  210  in accordance with the present invention for boosting an internal combustion engine  16  comprises a turbocharger  140  driven conventionally by engine exhaust  142  which is then exhausted  143 ; a supercharger  26  driven by engine  16  via at least a two-speed variable drive  220  and capable of variable speed operation, such that supercharger  26  is capable of acting as either a compressor or an expander as described below (appropriate valving not shown); a first charge air cooler (CAC 1 )  144 ; and a second charge air cooler (CAC 2 )  146 . Combustion intake air  148  enters turbocharger  140 , then compressed air  150  flows in sequence through CAC 1   144 , supercharger  26 , CAC 2   146 , and thence into engine  16 . 
         [0045]    Although several types of supercharger devices and valving strategies can be used, a Roots® blower should be considered a preferred technology, because it tends to have high efficiency at modest pressures ratios (where it would normally be used) both as a compressor and expander and the flow direction and porting can remain unchanged. In normal boost operation, the Roots® blower is turned much faster than the engine, forcing air into the intake manifold and thus increasing the manifold pressure. By operating the Roots® blower much slower than the engine, a pressure drop across the Roots® blower causes a reverse to normal torque which may be used to power accessories (not shown), turn the generator (not shown) or to provide a small power increase to the engine (via the accessory belt or a similar chain or gears, not shown). 
         [0046]    A variety of layouts and charge air cooling strategies may be used in accordance with the present invention, but in one aspect of the invention, turbocharger  140  compresses filtered intake air  148 ; and CAC 1   144 , which typically is an air/air CAC, drops the charge air temperature as low as possible. In cruise conditions, for example, with system  210  operating in mid-boost, cruise, or light acceleration modes, turbocharger  140  may compress air  148  to as much as about 3.0 bar. Supercharger  26  then partly expands the charge air to, for example, 2.032 bar, resulting in a lower temperature. By modulating the level of bypass  148 ′ around CAC 1   144 , the resulting temperature can be precisely controlled to match the level required to condense a refrigerant or freeze a phase change material (or both) in CAC 2   146 , thereby storing capacity for future cooling in CAC 2   146  or to supplement vehicle cabin air conditioning. This level can also be chosen so as to not condense water from the ambient humidity, and especially from EGR  154  which may be mixed into the charge air. 
         [0047]    By bypassing  148 ′ all of the flow around CAC 1   144 , hotter gas from turbocharger  140  is expand through supercharger  26 , resulting in somewhat higher energy recovery. Then active cooling of CAC 2   146  may be needed to maintain a low charge temperature to the engine. This can be useful, for example, in certain ambients where active cooling can be achieved with a very low parasitic by circulating refrigerant  152 . 
         [0048]    The use of supercharger  26  as an expander allows very low temperatures of the air charge as admitted to engine  16 , which is attractive for knock-limited power and efficiency, and which helps to match the turbine power and compressor load in the turbocharger, thereby reducing or eliminating the losses associated with a waste gate or VGT. 
         [0049]    When a high level of boost is required, especially at low engine RPM where the turbocharger is inherently inefficient and underpowered, accessory drive  220  can switch back to normal supercharger operation. Because vaporized refrigerant  152  has been condensed and/or phase change material has solidified  152 ′ in the preceding low-load operation, the very low charge air temperatures can be maintained with minimal parasitic losses. If continuous cooling is required, a small amount of refrigerant from a vehicle air conditioning compressor (not shown) can be metered into CAC 2   146 , acting then as an evaporative heat exchanger. 
         [0050]    It will be seen that system  110 ,  210  allows enhanced energy recovery from the turbocharger without the need for extensive additional hardware in the exhaust system. The resulting fuel economy benefits are relatively small but are additive to the more substantial baseline benefits achieved in the highly downsized twin-charger architecture. The fuel economy gains can be achieved with simple hardware changes, such as accessory drive  220 , and thus have an attractive cost/benefit. The improved charge air cooling and flexibility to manage hot and cold ambient conditions is another advantage, allowing the present novel system to be responsive and efficient in different climatic conditions and with varying fuels. 
         [0051]    The ability to control efficiently CAC 2   146  to near the dew point will be useful in future engine systems employing very high EGR flow, needed for both diesel and gasoline engines, with improved ability to meet future extremely low emissions standards without compromising engine efficiency. 
         [0052]    While the invention has been described by reference to various 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 will have full scope defined by the language of the following claims.