Patent Publication Number: US-2020284187-A1

Title: Twin Scroll Turbocharger with Waste Heat Recovery

Description:
This application relates to Provisional Application No. 62/386,901 having a filing date of Dec. 14, 2015, Provisional Application No. 62/391,136 having a filing date of Apr. 19, 2016 and Provisional Application No. 62/493,881 having a filing date of Jul. 18, 2016. 
    
    
     BACKGROUND OF THE INVENTION 
     Engine downsizing is viewed by US and foreign automobile manufacturers as one of the best options for improving passenger car and light duty truck fuel economy. While this strategy has already demonstrated a degree of success, downsizing and fuel economy gains are currently limited by the ability of low-cost turbocharging systems to provide high boost pressures over a wide range of engine speeds. 
     At low engine speeds turbochargers are generally unable to deliver high boost pressures because of surge and insufficient turbine power. When boost pressures become too great the inertial force of the air exiting the compressor is overcome by the high pressure air downstream. When this occurs the pressurized air downstream of the compressor surges back upstream through the compressor. The compressor inducer first stalls, and then pressurized downstream air surges upstream. 
     The boost pressures that can be sustained without encountering surge decrease as engine speed decreases, because the air mass flow volume and therefor air speed through the compressor also decreases. The lower speed air is less able to hold back the pressurized air downstream of the compressor. 
     A smaller turbocharger can be used for attaining higher boost pressures at low engine speed, because the smaller turbocharger has a smaller outlet area and a higher air flow velocity. The turbine must also produce enough power from the exhaust gas to drive the compressor. A smaller turbine can produce more shaft power from the smaller flow of exhaust gas at low engine speeds, the increased shaft power being needed to produce higher boost pressures with or without surge. The problem with using a smaller turbocharger, however, is that maximum engine power is reduced. If maximum power is reduced, the fuel economy benefits from engine downsizing cannot be attained. 
     A number of technologies can provide higher boost pressures at low engine speeds, however those technologies are expensive. Technologies that provide high boost pressure over a wide range of engine speeds but that are relatively expensive include combining turbo and supercharging; electrically driven turbochargers; variable geometry turbochargers and sequential turbocharging. 
     Dual scroll or twin scroll turbochargers are beneficial but do not provide a large enough gain to meet future engine needs. The dual scroll when applied to cylinder engines. The first benefit is separating the combustion pulses between the cylinders. The second benefit is a smaller exhaust manifold volumes upstream of the turbine. The smaller manifold volume increases the impulse blow down energy of the exhaust gas entering the turbine, leading to improved turbocharger response at low engine speeds. 
     Some gain in boost pressure can be realized by routing exhaust from all of the cylinders into both of the scrolls of a twin scroll turbine at high engine speeds, and then closing off one of the scrolls to attain higher boost pressure at low engine speeds. This approach has limited benefit. 
     Anti-lag system technology commonly referred to as ALS is another approach used for attaining high boost pressures over a broad range of speeds. The anti-lag system technology includes a bypass duct for routing compressed air from downstream of the compressor into the exhaust manifold upstream of the turbine. The engine is run with a rich fuel air mixture ratio, and the excess fuel explodes in the exhaust manifold upstream of the turbine on contact with the bypass air. The technology has a number of problems when considered for passenger cars and light duty trucks, including noise, emissions compliance and reduced turbocharger life. The exploding gas can also blow back through the bypass duct into the engine air intake system. 
     The AMX Leclerc French main battle tank employs another approach. The AMX Leclerc has an 8-cylinder 1500 horse power hyperbar diesel engine. The hyperbar system integrates a Turbomeca TM 307B gas turbine in the engine, acting both as a turbocharger and an auxiliary power unit. A fuel and air mixture is combusted in a combustor upstream of the turbine to increase turbine power output. Employing a gas turbine combustor in a passenger car or light duty truck would be prohibitively expensive, and introduce safety concerns. 
     Accordingly, an objectives of the current invention is to provide a low cost turbocharging system capable of providing high boost pressures over a wide range of engine speeds. Another objective is to meet safety needs for commercial use of the technology in passenger cars and light duty trucks. Another objective is for the turbocharging system to have a high efficiency in order to maximize vehicle fuel economy. Yet another objective is to minimize exhaust gas emissions from engines employing the turbocharging technology. 
     SUMMARY OF THE INVENTION 
     According to the present invention, bypass air from downstream of the compressor is directed into a heat exchanger that draws heat from the exhaust gas of the engine. The bypass air does not include fuel, and instead is heated by the exhaust gas in the heat exchanger. The bypass duct enables air mass flow through the compressor to be increased, thereby preventing compressor surge at low engine speeds. The turbocharge   entry scroll. The   ss air is fed into the first scroll after being heated in the heat exchanger, and the engine exhaust gas is fed into the second scroll. Both turbine scrolls are used at low engine speeds for maximizing engine torque. Use of two scrolls enables the blowdown impulse energy of the exhaust gas to be retained within the exhaust manifold prior to entry into the turbine. In more detail the exhaust gas does not backflow into the bypass duct and loose working pressure. Engine efficiency is also increased by using the exhaust energy to heat the bypass air instead of combusting additional fuel. There is no explosive combustion of fuel and bypass air upstream of the turbine that would create noise and decrease turbocharger life. The heated bypass air flowing into the turbine increases turbine power leading to higher turbocharger boost pressure ratios. 
     A control valve may optionally be used to open both scrolls for exhaust flow, and close of the bypass duct at high engine speeds where use of the bypass duct is not needed. Flow of exhaust gas into both of the scrolls at high engine speeds enables higher maximum engine power output levels to be attained. In more detail, the first scroll is used for the bypass air during low speed engine operation, and the second scroll is used for exhaust gas at all engine speeds. Use of separate scrolls for the bypass air and exhaust gas prevents exhaust pressure waves from propagating into the bypass duct and Intake air system of the engine. At high engine speeds the first scroll or bypass scroll may be closed. Optionally the bypass scroll may be unused during high engine speeds, or a valve may be used to open the bypass scroll for receiving exhaust gas so that both the first and second scroll receive exhaust gas during high speed engine operation. Use of both scrolls for exhaust gas enables higher engine power levels to be achieved. 
     The present invention enables high boost pressures to be attained across a wide range of engine speeds. Yet another advantage of the present invention is that it does not include a combustor or a secondary fuel delivery system. The fueling needs can be fully met with the existing fuel injection system. A significant advantage of the present invention is that use of waste heat recovery through the exhaust gas heat exchanger provides improved engine efficiency relative to other engines operating at similar speed and brake mean effective pressure. Another advantage of the present invention is that the heated bypass air and exhaust gas from the engine enter the turbine through separate scrolls. Use of separate scrolls prevents the exhaust gas from back flowing into the bypass duct. Another advantage of the turbocharging system is that it has a relatively low cost. An expensive heat exchanger is not required for effective operation of the turbocharger system. High boost pressures can be attained with the present invention across a wide engine speed range. Prospective applications for the technology include long haul diesel trucks where steady state fuel economy can be improved through the waste heat recovery, and light duty cars and trucks where fuel economy can be significantly improved   zing. 
     In current production engines it is common to employ a rich fuel to air mixture during high load engine operating conditions in order to internally cool the engine and suppress detonation. According to an embodiment of the present invention, under high loads the fuel injection system of the engine injects excess fuel for combustion. Enrichment levels are similar to current production engines. The unburned excess fuel in the exhaust gas combines with the bypass air and undergoes catalytic combustion in the catalytic converter. The catalytic combustion increases the temperature of the exhaust gas entering the heat exchanger, causing the temperature of the bypass air entering the turbine to become even hotter. The increase of bypass air temperature increases the power output of the turbine causing the compressor to spin faster and generate higher boost pressures. In addition to increasing turbocharger boost pressure, hydrocarbon emissions are reduced because the added air provides the needed oxygen for complete combustion of the fuel. The turbine is upstream and isolated from the catalytic combustion. Effective waste heat recovery in the heat exchanger minimizes the degree and frequency of enrichment. 
     Advantages of the present invention include increased boost pressure at low engine speeds without compromising maximum engine power output; increased engine efficiency at some engine power levels due to the waste heat recovery of the heat exchanger; lower hydrocarbon emissions during high load conditions due to bypass air being provided for complete combustion of the fuel during rich engine operating conditions; and a relatively low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is intended to diagrammatically illustrate air flow through a turbocharger compressor according to the present invention, and in more detail  FIG. 1  is intended to diagrammatically illustrate a portion of the present invention. 
         FIG. 2  is intended to diagrammatically illustrate an extended performance range turbocharging system according to the present invention. 
         FIG. 3  is similar to  FIG. 2  but includes a turbine entry valve. 
         FIG. 4  is similar to  FIG. 3  but shows turbine entry valve with a different setting. 
         FIG. 5  is similar to  FIG. 2  but shows a single entry turbine scroll. 
         FIG. 6  is similar to  FIG. 5  but has a rich fuel to air ratio. 
         FIG. 7  is a section view of a twin scroll turbocharger having a turbine clearance volume. 
         FIG. 8  is similar to  FIG. 3  but without a heat exchanger. 
         FIG. 9  is similar to  FIG. 4  but without a heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is intended to diagrammatically illustrate air flow through a turbocharger compressor according to the present invention, and in more detail  FIG. 1  is intended to diagrammatically illustrate a portion of the present invention. Air mass flow is shown on the horizontal axis of the  FIG. 1  diagram, and compressor pressure ratio in shown on the vertical axis of the  FIG. 1  diagram. Constant efficiency compressor contour lines  2  are plotted in the  FIG. 1  diagram, with contour line  4  indicating the area of highest compressor operating efficiency. The surge limit line  6  indicates the maximum pressure ratio that can be achieved by the compressor for a given air mass flow rate. Compressor operating conditions to the left of surge limit line  6  will encounter surge, and are therefore unacceptable. Operating conditions to the right of surge limit line  6  will not encounter surge, and may be used within the operating speed limits of the turbocharger. The maximum speed limit line  8  of the compressor is shown generally above contour lines  2 . According to the present invention, Point  10  is intended to indicate air flow and pressure ratio needs for a representational internal combustion engine operating at a low speed and a high brake mean effective pressure ratio. Point  10  is located to the left of surge limit line  6  and will therefore encounter surge. The compressor cannot be operated at point  10  because of surge. Point  12  is intended to indicate air flow and pressure needs for the same representational internal combustion engine plus air flow being directed to a bypass duct. The bypass duct will be described in more detail with reference to  FIG. 2 . Point  12  shows the combined air mass flow through the engine and the bypass duct. Point  12  is located to the right of surge limit line  6  and will therefore provide stable and acceptable compressor performance according to the present invention. 
       FIG. 2  is intended to diagrammatically illustrate an extended performance range turbocharging system  14  according to the present invention. Turbocharging system  14  includes a turbocharger  16  having a turbine  18  and a compressor  20 , having a compressor inlet  22  and compressor outlet  24 . Turbocharging system  14  further includes compressor inlet air  26 , compressor inlet air  26  being in compressor inlet  22 . Turbocharging system  14  further includes an internal combustion engine  28  having one or more combustion cylinders  30 . Turbocharging system  14  further includes and an intake air passageway  32  connecting compressor  20  to internal combustion engine  28 , and compressor outlet air  34 , compressor outlet air  34  being inside intake air passageway  32 . Intake air passageway  32  may have one or more branches or ducts  36  for delivery of compressor outlet air  34  to combustion cylinders  30 . 
     Turbocharging system  14  further includes an upstream exhaust gas passageway  38  connecting internal combustion engine  28  to turbine  18  for providing a flow path from internal combustion engine  28  to turbine  18 , and exhaust gas  40 , exhaust gas  40  being inside upstream exhaust gas passageway  38 . Upstream exhaust gas passageway  38  may have one or more branches or ducts  42  for delivery of exhaust gas  40  from combustion cylinders  30  to turbine  18 . Turbine  18  also has a turbine outlet flow passageway  44 . 
     Turbocharging system  14  further includes a bypass air passageway  46  connecting intake air passageway  32  to turbine  18  for providing a flow path from intake air passageway  32  to turbine  18  outside of internal combustion engine  28 . Turbocharging system  14  further includes bypass air  48 , bypass air  48  being inside bypass air passageway  46 . Bypass air passageway  46  also includes a bypass valve  50  located between intake air passageway  32  and turbine  18 . Referring now to  FIGS. 3 and 4 , bypass valve  50  may optionally be integrated into a turbine entry valve  62 . Bypass air  48  may optionally include some exhaust gas, for example from exhaust gas recirculation. Bypass air  48  may optionally also include water. 
     According to the present invention, bypass air passageway  46  further includes a heat exchanger  68  for heating bypass air  48  with the hot exhaust gas of the engine. Heating bypass air  48  upstream of turbine  18  increases turbine power, thereby reducing turbo lag and increasing compressor pressure ratio. 
     Bypass air passageway  46  has a bypass fuel to air mixture ratio  54  in heat exchanger  68 . According to the present invention, the bypass fuel to air mixture ratio  54  is equal to zero in order to prevent combustion of bypass air  48  upstream of exhaust gas passageway  38  for maximizing safety and minimizing system cost. 
     Preferably, according to the present invention, bypass air  48  and exhaust gas  40  are noncombustible upstream of turbine  18  in order to prevent large explosions from occurring in upstream exhaust gas passageway  38 . Combustion of fuel and air in upstream exhaust gas passageway can damage turbocharger  16  and upstream exhaust gas passageway  38 . 
       FIG. 2  shows internal combustion engine  28  having a first turbine inlet setting  56 . Upstream exhaust gas passageway  38  and bypass air passageway  46  are separated in first turbine inlet setting  56 . Bypass air  48  is separated from exhaust gas  40  so that oxygen from bypass air  48  cannot mix with unburned fuel  84  in exhaust gas  40 . Bypass air  48  is separated from unburned fuel  84  to prevent combustion or significant explosions from occurring in upstream exhaust gas passageway  38 . 
     Bypass air  48  is considered separate from unburned fuel  84  provided that any mixing of bypass air  48  with unburned fuel  84  is minor or small. Minor mixing of bypass air and unburned fuel  84  may occur downstream of upstream exhaust gas passageway  38 , for example in the turbine inlet clearance volume  17 , shown in  FIG. 7 . This mixing and potential combustion is both minor and occurs downstream of the upstream exhaust gas passageway  38 . 
       FIG. 2  shows turbine  18  having a first scroll or volute  58  for flow of exhaust gas  40  into turbine  18 , and bypass air passageway  46  having a second scroll or volute  60  for flow of bypass air  48  into turbine  18 . First scroll  58  is separated from second scroll  60  for limiting or preventing mixing of exhaust gas  40  with bypass air  48  upstream of turbine  18 , and also to maximizing the blowdown impulse energy of the exhaust gas    8 . 
     Referring now to  FIG. 7 , turbocharger  16  further includes a turbine wheel  21 , a turbine inlet clearance volume  17  and a center divider or patrician wall  23  between first scroll  58  and second scroll  60 . Turbine inlet clearance volume  17  is the clearance volume between the turbine wheel  21  and the center divider  23 . In more detail turbine clearance volume  17  extends outward from the turbine wheel  21  into contact with the center divider  23 . Twin scroll turbochargers include a turbine inlet clearance volume  17  as an assembly tolerance and to improve aerodynamic flow into the turbine wheel  21 . Turbine clearance volume  17  encircles the turbine wheel inlet and is diagrammatically illustrated with a dashed line in the section view drawing. 
     For multi or twin scroll turbines, turbine  18  includes the swept volume of the turbine wheel  21  plus the turbine inlet clearance volume  17  between the turbine wheel  21  and the center divider  23 . Turbine  18  is being defined to include clearance volume  17  because there is brief contact between exhaust gas  40  and bypass air  48  within clearance volume  17 , and this contact should not diminish the scope and validity of the present invention as claimed. In single scroll turbines there is no center divider  23  or clearance volume  17 , and in this case turbine  18  includes only the swept volume of turbine wheel  21 . 
     Referring now to  FIG. 2 , in one embodiment of the present invention second scroll  60  is a dedicated scroll  61  for bypass air  48  only. 
     Referring now to  FIGS. 3 and 4 , in another embodiment of the present invention, turbocharging system  14  further includes a turbine entry valve  62 . 
     Turbine entry valve  62  has a first position  64 . Bypass air passageway  46  is in fluid communication with second scroll  60  in first position  64  for flow of bypass air  48  into turbine  18  through second scroll  60 . First position  64  is generally used for providing high boost pressure from turbocharger  16  at low engine speeds. 
     Turbine entry valve  62  has a second position  66 . Bypass air passageway  46  is closed to second scroll  60  in second position  66 , and exhaust gas  40  is in fluid communication with first scroll  58  and second scroll  60 . Second position  66  is generally used for providing high boost pressures from turbocharger  16  at higher engine speeds. First position  64  combined with second position  66  provides a turbocharging system able to deliver high boost pressures over a wide range of engine speeds. 
     In more detail, turbine entry valve  62  has a first position  64 . Bypass air passageway  46  is in fluid communication with second scroll  60  in first position  64  for flow of bypass air  48  into turbine  18  through second scroll  60 , and for flow of exhaust gas  40  into first scroll  58 . First position  64  is generally used for providing high boost pressure from turbocharger  16  at low engine speeds. Turbine entry valve  62  has a second position  66 . Upstream exhaust gas passageway  38  is in fluid communication with first scroll  58  and second scroll  60  in second position  66  for flow of exhaust gas  40  into first scroll  58  and second scroll   is generally used   oviding high boost pressures from turbocharger  16  at high engine speeds. First position  64  combined with second position  66  provides a turbocharging system able to deliver high boost pressures over a wide range of engine speeds. 
     Referring now to  FIG. 3 , bypass valve  50  has an open position in first position  64  for flow of bypass air  48  into turbine  18  through turbine entry valve  62 . Bypass valve  50  may optionally regulate flow of bypass air  48  into turbine  18 . Bypass valve  50  may provide superior flow rate control of bypass air  48 , and may be located in a cool location of bypass air passageway  46  for minimizing cost and maximizing reliability. Bypass valve  50  may optionally not be used. Flow of bypass air  48  into turbine  18  may optionally be controlled directly by turbine entry valve  62 . Referring now to  FIG. 4 , bypass valve  50  is closed in second position  66  for preventing flow of bypass air  48  into turbine  18 . 
     Turbine entry valve  62  is diagrammatically illustrated in  FIGS. 3 and 4 . Turbine entry valve  62  may be similar in design to a turbocharger waste gate valve, or exhaust flow valves currently used in commercially available engines, or an alternative type of turbine entry valve may be used according to the present invention. 
       FIGS. 2, 3 and 4  show heat exchanger  68  located in turbine outlet flow passageway  44  for heating of bypass air  48  according to the present invention. Heating of bypass air  48  increasing the power output of turbine  18 , and the more powerful turbine reduces turbo lag and increases the compressor pressure ratio, and in more detail increases the boost pressure of compressor outlet air  34 . 
     Optionally bypass air  48  can be heated by a heat exchanger located upstream of turbine  18 , that receives heat from the exhaust gas  40  in upstream exhaust gas passageway  38 , but preferably heat exchanger  68  is located downstream of turbine  18  for maximizing turbocharger performance. 
     According to an embodiment of the present invention, a catalytic converter  70  is used for both reducing pollutants and increasing turbocharger performance. Referring now to  FIGS. 2, 3 and 4 , turbocharging system  14  further includes a catalytic converter  70 . Catalytic converter  70  is located in turbine outlet flow passageway  44  upstream of heat exchanger  68 . In more detail, heat exchanger  68  is located downstream of catalytic converter  70  in turbine outlet flow passageway  44 . 
     Turbocharging system  14  further including fuel injection  72  and an engine fuel to air ratio  74  in internal combustion engine  28 . Turbocharging system  14  further includes a first engine setting  76 , first engine setting  76  has a rich engine fuel to air ratio  78 . Exhaust gas  40  in upstream exhaust gas passageway  38  includes unburned fuel  84  in first engine setting  76  due to the rich engine fuel to air ratio  78 . The unburned fuel  84  combines with bypass air  48  and combusts upstream of heat exchanger  68 , thereby increasing the temperature of the heat exchanger and in turn bypass air  48 . Heating of bypass air  48  increasing the power output of turbine  18 , and the more powerful turbine reduces turbo lag and increases the boost pre   tlet air  34 . 
     Catalytic converter  70  enhances or accelerates combustion of unburned fuel  84  and bypass air  48  in a process referred to as catalytic combustion. Catalytic combustion increases the temperature of heat exchanger  68  and bypass air  48  according to the present invention, thereby increasing turbine power and compressor boost pressure. 
     Internal combustion engine  28  has a tailpipe  80  and exhaust pollutants  82  in tailpipe  80  from combustion of fuel in internal combustion engine  28 . Turbine  18  has a turbine outlet  19  having unburned fuel  84  from combustion of the rich engine fuel to air ratio  78  in combustion cylinders  30 . According to an embodiment of the present invention, catalytic converter  70  has an optimal catalytic converter inlet fuel to air ratio  86  for minimizing exhaust pollutants  82 , and rich engine fuel to air ratio  78  is richer than optimal catalytic converter inlet fuel to air ratio  86 . Turbine outlet  19  includes rich engine fuel to air ratio  78 , and according to the present invention addition of bypass air  48  provides optimal catalytic converter inlet fuel to air ratio  86  for minimizing exhaust pollutants  82 , thereby increasing the temperature of bypass air  48  upstream of turbine  18  for increasing the performance range of turbocharger  16  and thereby minimizing exhaust pollutants. 
     Referring now to  FIG. 5  according to an embodiment of the present invention bypass air passageway  46  includes heat exchanger  68 . Heat exchanger  68  is located in turbine outlet flow passageway  44  for heating of bypass air  48 . Heating of bypass air  48  increasing the power output of turbine  18 , and the more powerful turbine reduces turbo lag and increases the boost pressure of compressor outlet air  34 . Internal combustion engine  28  further includes fuel injection  72  and internal combustion engine  28  has an engine fuel to air ratio  74 . Turbocharging system  14  has a second engine setting  90  according to the present invention. Second engine setting  90  has a lean engine fuel to air ratio  92 . Lean engine fuel to air ratio  92  may be used for a compression ignition diesel engine, or for a spark ignition engine, or for an engine having an alternative method of ignition. Exhaust gas  40  has a second fuel to air mixture ratio  94  for second engine setting  90 , second fuel to air mixture ratio  94  effectively being zero. In more detail effectively all of the fuel is combusted in internal combustion engine  28  due to the excess air of lean fuel to air ratio  92 , and accordingly the fuel to air mixture ratio of exhaust gas  40  is zero. Exhaust gas  40  having no fuel content combines with bypass air  48  upstream of single turbine inlet scroll  88 , bypass air  48  also having no fuel content. Bypass valve  50  is used for control of bypass air into turbine  18 . Optionally the regulator  98  shown in  FIG. 6  may be used instead of bypass valve  50 . Optionally turbine  18  has a single turbine inlet scroll  88  and a bypass valve  50  for control of bypass air into single turbine inlet scroll  88 . A twin or multi scroll turbine may also be used according to the present invention. 
     According to the preferred embodiment of the present invention, heat exchanger  68  is located in turbine outlet flow passageway  44  for heating of bypass air  48 . Heating of bypass air  48  increasing the power output of turbine  18 , and the more powerful turbine reduces turbo lag and increases the compressor pressure ratio and the boost press   t air  34 . Additiona   pstream exhaust gas passageway  38  further includes a first scroll  58  for flow of exhaust gas  40  into turbine  18 , and bypass air passageway  46  further includes a second scroll  60  for flow of bypass air  48  into turbine  18 , first scroll  58  being separated from second scroll  60  for limiting mixing of exhaust gas  40  with bypass air  48  upstream of turbine  18 , for maximizing the blowdown impulse energy of the exhaust gas flowing into turbine  18 . According to one embodiment of the present invention, second scroll  60  is a dedicated scroll  61  for bypass air  48  only. According to another embodiment of the present invention, turbocharging system  14  further includes a turbine entry valve  62 . Turbine entry valve  62  has a first position  64 . Bypass air passageway  46  is in fluid communication with second scroll  60  in first position  64  for flow of bypass air  48  into turbine  18  through second scroll  60 . First positon  64  is generally used for providing high boost pressure from turbocharger  16  at low engine speeds. Turbine entry valve  62  has a second position  66 . Bypass air passageway  46  is closed to second scroll  60  in second position  66 , and exhaust gas  40  is in fluid communication with first scroll  58  and second scroll  60 . Second position  66  is generally used for providing high boost pressures from turbocharger  16  at higher engine speeds. First position  64  combined with second position  66  provides a turbocharging system able to deliver high boost pressures over a wide range of engine speeds. In more detail, turbine entry valve  62  has a first position  64 . Bypass air passageway  46  is in fluid communication with second scroll  60  in first position  64  for flow of bypass air  48  into turbine  18  through second scroll  60 , and for flow of exhaust gas  40  into first scroll  58 . First position  64  is generally used for providing high boost pressure from turbocharger  16  at low engine speeds. Turbine entry valve  62  has a second position  66 . Upstream exhaust gas passageway  38  is in fluid communication with first scroll  58  and second scroll  60  in second position  66  for flow of exhaust gas  40  into first scroll  58  and second scroll  60 . Second position  66  is generally used for providing high boost pressures from turbocharger  16  at higher engine speeds. First position  64  combined with second position  66  provides a turbocharging system able to deliver high boost pressures over a wide range of engine speeds. 
     Referring now to  FIG. 6 , according to another embodiment of the present invention, heat exchanger  68  is located in turbine outlet flow passageway  44  for heating of bypass air  48 , and internal combustion engine  28  includes fuel injection  72  and an engine fuel to air ratio  74 . Internal combustion engine  28  further includes a first engine setting  76 , first engine setting  76  having a rich engine fuel to air ratio  78 , and unburned fuel  84 , unburned fuel  84  being in exhaust gas  40 . Unburned fuel  84  and bypass air  48  combine and combust or partially combust upstream of turbine  18 , thereby reducing turbo lag and increasing boost pressure. Heating of bypass air  48  in heat exchanger  68  reduces the amount of fuel needing to be burned upstream of turbine  18  for providing high boost pressures and reduced turbo lag. According to the present invention, heating of the bypass air in heat exchanger  68  enables the amount of combustion taking place in upstream exhaust gas passageway  38  to be small enough so as    18  or upstream   ust gas passageway  38 . Heat exchanger  68  may optionally be located upstream of turbine  18 , and in more detail heat exchanger  18  may draw heat from exhaust gas  40  upstream of turbine  18 . In some embodiments of the present invention, bypass air passageway  46  further including a regulator  98  for preventing backflow of bypass air  48  in bypass air passageway  46 . Regulator  98  may be a check valve, a rotating valve, a positive displacement regulator such as a positive displacement pump, a roots blower, a poppet valve, or another functional regulator for preventing backflow of air in bypass air passageway  46 . Regulator  98  may be driven by an electric motor for regulating the flow rate of bypass air  48 . 
     Referring now to  FIGS. 2 through 5 , according to another embodiment of the present invention bypass air passageway  46  further has an extended flow period  52  for providing bypass air  48  to turbine  18  over an extended period of time. The extended flow period  52  is preferably greater than or at least twenty seconds for minimizing turbo lag and providing high boost pressure at low engine speeds. Referring now to  FIG. 5 , the extended flow period may be much longer, for example in long haul diesel truck engines. A sustained load extended flow period  53  is preferably at least 2 minutes long, and in some cases may last for over ten minutes. The sustained flow can improve diesel engine fuel economy under some driving conditions. Flow may be controlled by a constant flow valve, by a pulse width modulated valve, or by other means. According to the present invention, the average flow rate will be considered for pulse width modulated flow and other modulated flow control systems when calculating the extended flow period. Using the average flow rate is necessary for valves that turn on and off many times to control flow, because these systems could be inaccurately assumed to have a very short flow period. The flow rate of bypass air  48  may be controlled with bypass valve  50  or regulator  98 . 
     Referring now to  FIGS. 2 through 6 , preferably turbocharging system  14  includes an intercooler or after cooler  96  for cooling compressor outlet air  34 . Preferably bypass air passageway  46  receives compressor outlet air  34  upstream of intercooler  96 . Bypass air  48  is drawn from upstream of intercooler  96  in order to maximize the temperature of bypass air  48  for maximizing the power output of turbine  18 . 
     Referring now to  FIGS. 8 and 9 , the cost of turbocharging system  14  can be minimized by eliminating heat exchanger  68 . According to an embodiment of the present invention, turbocharging system  14  for an internal combustion engine  28  includes a turbocharger  16  having a compressor  20 , having a compressor outlet  24  and an intake air passageway  32  connecting compressor outlet  24  to internal combustion engine  28 , and compressor outlet air  34 , compressor outlet air  34  being in intake air passageway  32 . Turbocharger  16  further including a turbine  18 , and turbine  18  has a turbine outlet flow passageway  44 . Turbocharging system  14  has an upstream exhaust gas passageway  38  connecting internal combustion engine  28  to turbine  18  for providing a flow path from internal combustion engine  28  to turbine  18 . Exhaust   passageway  38 . T   arging system  14  further includes a bypass air passageway  46  connecting intake air passageway  32  to turbine  18  for providing a flow path from intake air passageway  32  to turbine  18  outside of internal combustion engine  28 , and bypass air  48 , bypass air  48  being in bypass air passageway  46 . According to the present invention, upstream exhaust gas passageway  38  further includes a first scroll  58  for flow of exhaust gas  40  into turbine  18 , and bypass air passageway  46  further includes a second scroll  60  for flow of bypass air  48  into turbine  18 . First scroll  58  is separated from second scroll  60  for limiting mixing of exhaust gas  40  with bypass air  48  upstream of turbine  18 , for maximizing the blowdown impulse energy of the exhaust gas flowing into turbine  18 . 
     A bypass valve  50  is optionally used for control of bypass air into second scroll  60 . Second scroll  60  may optionally be a dedicated scroll  61  for bypass air  48  only as shown in  FIG. 2 . 
     Referring again to  FIGS. 8 and 9 , turbocharging system  14  may optionally include a turbine entry valve  62 . Turbine entry valve  62  has a first position  64 . Bypass air passageway  46  is in fluid communication with second scroll  60  in first position  64  for flow of bypass air  48  into turbine  18  through second scroll  60 , and for flow of exhaust gas  40  into first scroll  58 . Turbine entry valve  62  has a second position  66 . Upstream exhaust gas passageway  38  is in fluid communication with first scroll  58  and second scroll  60  in second position  66  for flow of exhaust gas  40  into first scroll  58  and second scroll  60 . First position  64  is generally used for providing high boost pressure from turbocharger  16  at low engine speeds. Second position  66  is generally used for providing high boost pressures from turbocharger  16  at higher engine speeds. First position  64  combined with second position  66  provides a turbocharging system able to deliver high boost pressures over a wide range of engine speeds. Elimination of heat exchanger  68  enables system cost to be minimized. 
     Referring now to  FIGS. 1 and 3 , exhaust gas passageway  38  has an exhaust gas mass flow rate  100 , and bypass air passageway  46  has a bypass air mass flow rate  102 . Preferably, according to the present invention the ratio of bypass air mass flow rate  102  to exhaust gas mass flow rate  100  is at least 0.12, for providing needed air for reducing hydrocarbon emissions in catalytic converter  70 , thereby providing low tailpipe emissions and increased boost pressure from turbocharger  16 . 
     Bypass air flow can be increased for further increasing turbocharger boost pressure and engine brake mean effective pressure at low engine speeds. A ratio of bypass air mass flow rate  102  to exhaust gas mass flow rate  100  of at least 0.25 provides maximum boost pressure from turbocharger  16  at low engine speed.