Abstract:
An intake manifold ( 16 ) of an internal combustion engine ( 10 ) is charged to superatmospheric pressure by operating a first compressor ( 44 ) to compress fresh air diluted by exhaust gas into the intake manifold while operating a second compressor ( 24 C) to compress undiluted fresh air into the intake manifold through a device ( 40 ) that allows flow in a direction from the second compressor into the intake manifold but not in an opposite direction.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to internal combustion engines, such as diesel engines for propelling motor vehicles, that have charging devices for creating superatmospheric pressure, i.e. boost) in an intake manifold and that use exhaust gas recirculation (EGR) as a component of a tailpipe emission control strategy. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    One EGR strategy for a turbocharged diesel engine comprises the use of a high-pressure EGR loop that has a pierce point to the engine exhaust system at, or close to, an exhaust manifold. An EGR valve controls the flow of recirculated exhaust gas to a mixer in the engine intake system where the exhaust gas mixes with fresh air that has been compressed by a turbocharger compressor before the mixture enters an intake manifold serving engine cylinders. 
         [0003]    By metering a controlled amount of exhaust gas into the fresh air, the EGR valve effectively dilutes the air so that in-cylinder temperature rise resulting from combustion is limited from the rise which would occur in the absence of such dilution. As a consequence, the quantity of oxides of nitrogen (NOx) in the exhaust gas that results from combustion is also limited. 
         [0004]    Because exhaust gas for a high-pressure EGR loop is sourced directly from the engine exhaust manifold without any appreciable cooling, it is quite hot, and that compels the recirculation path that contains the EGR valve to provide sufficient heat rejection for enabling the exhaust gas that mixes with fresh air to be suitably cooled before it entrains with the fresh air in the intake system. 
         [0005]    In order to provide adequate cooling, the recirculation path typically comprises a gas-to-liquid heat exchanger functioning as an EGR cooler. The greater the amount of cooling needed, the larger the heat rejection capacity of the EGR cooler, and if the EGR valve is upstream of the EGR cooler, the valve is exposed to high-temperature exhaust coming off the exhaust manifold. Use of an EGR cooler in the recirculation path also creates a potential for condensation of constituents in the gas that can eventually lead to fouling of EGR components. 
         [0006]    Another EGR strategy for a turbocharged diesel engine comprises the use of a low-pressure EGR loop that has a pierce point to the engine exhaust system that is downstream of a turbocharger turbine. An EGR valve still controls the flow of recirculated exhaust gas to a mixer in the engine intake system, but the mixer is upstream of the turbocharger compressor, requiring that the compressor to be sized large enough to handle not only the fresh air mass but also that of the added EGR. For large amounts of EGR, this raises a potential turbocharger mismatch issue and can affect turbocharger response. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    This disclosure relates to an internal combustion engine comprising engine cylinders within which combustion of fuel occurs to operate the engine, an air intake system comprising an intake manifold through which air enters the engine cylinders to support the combustion of fuel, and an exhaust system for conveying combustion-created exhaust gas from the engine cylinders. 
         [0008]    The air intake system further comprises a fresh air entrance leading to mutually parallel first and second flow paths to the intake manifold. 
         [0009]    The first flow path comprises a compressor of a turbocharger having a turbine operated by exhaust gas being conveyed through the exhaust system and a heat exchanger through which air compressed by the compressor passes. 
         [0010]    The second flow path comprises an initial portion that places the fresh air entrance in communication with a first inlet port of a valve that further comprises a second inlet port, an outlet port, and a mechanism for selectively setting relative proportions of respective flows that have entered the first inlet port and the second inlet port in a combined flow leaving the outlet port. 
         [0011]    An EGR flow path conveys exhaust gas from the exhaust system to the second inlet port of the valve, and the second flow path comprises a final portion that comprises a positive displacement pump for compressing the flow from the outlet port of the valve and a heat exchanger through which compressed flow from the positive displacement pump passes to the intake manifold. 
         [0012]    The disclosure also relates to a method of charging an intake manifold of an internal combustion engine to superatmospheric pressure comprising: operating a first compressor to compress fresh air diluted by exhaust gas into the intake manifold while operating a second compressor to compress undiluted fresh air into the intake manifold through a device that allows flow in a direction from the second compressor into the intake manifold but not in an opposite direction. 
         [0013]    The foregoing summary is accompanied by further detail of the disclosure presented in the Detailed Description below with reference to the following drawings that are part of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram showing a diesel engine and components relevant to the present disclosure. 
           [0015]      FIG. 2  is a drawing showing more detail of a portion of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows an engine  10  having structural components assembled together to form engine cylinders  12  within which combustion of fuel occurs to operate a kinematic mechanism comprising pistons, connecting rods, and a crankshaft. Fresh air for supporting combustion of fuel is delivered to cylinders  12  through an air intake system  14  that comprises an intake manifold  16  serving cylinders  12 . 
         [0017]    An exhaust system  18  comprises an exhaust manifold  20  at which combustion-created exhaust gases from cylinders  12  enter the exhaust system for conveyance to a tailpipe  22  through which they pass into the surrounding atmosphere. Exhaust gases leaving exhaust manifold  20  pass through a turbocharger  24  before passing through one or more exhaust after-treatment devices  26  and then tailpipe  22 . Examples of exhaust after-treatment devices are a diesel particulate filter (DPF) and a diesel oxidation catalyst (DOC). 
         [0018]    Turbocharger  24  is shown as a wastegate type turbocharger comprising a turbine  24 T shunted by a wastegate  24 W that sets respective quantities of exhaust gases that pass through and that by-pass turbine  24 T. The exhaust gas quantity that passes through turbine  24 T operates turbine  24 T and consequently a turbocharger compressor  24 C which is disposed in air intake system  14  and to which turbine  24 T is mechanically coupled. 
         [0019]    Air intake system  14  comprises a fresh air entrance  28  leading to mutually parallel first and second flow paths  30 ,  32  respectively. Before it splits into these respective flow paths, the entering airflow passes through an air filter  34  and then a mass airflow measuring device  36 . 
         [0020]    Flow path  30  comprises compressor  24 C and a charge air cooler (CAC)  38  through which air compressed by the compressor passes. CAC  38  is a heat exchanger that can be an air-to-liquid type or an air-to-air type for cooling the compressed air. A check valve  40  allows airflow from CAC  38  to move forward into intake manifold  16  but prevents reverse flow from intake manifold  16 . 
         [0021]    Flow path  32  comprises a valve  42 , a supercharger  44 , and an intercooler  46 , in that order, for conveying flow to intake manifold  16 . Supercharger  44  comprises a positive displacement rotary pump, such as a screw machine type pump, that is mechanically driven by torque obtained from an external shaft end  48  of the engine crankshaft.  FIG. 1  shows an example of a mechanical drive  50  as a belt drive that couples respective sheaves on the shaft end  48  and supercharger pump. Intercooler  46  is a heat exchanger through which the flow that has been compressed by supercharger  44  passes, and is portrayed by way of example as an air-to-liquid type heat exchanger. 
         [0022]    An exhaust gas recirculation flow path  52  has an entrance  54  at a pierce point to exhaust system  18  that is downstream of after-treatment devices  26 . From there, flow path  52  continues through an EGR cooler  56  to valve  42 . 
         [0023]    Valve  42  comprises a first inlet port  42 A, a second inlet port  42 B, and an outlet port  42 C. An initial portion of flow path  32  communicates fresh air to inlet port  42 A while a final portion that contains supercharger  44  and intercooler  46  communicates outlet port  42 C to intake manifold  16 . 
         [0024]    Valve  42  is shown in more detail in  FIG. 2  to comprise a Y-body  58  having convergent passages  60 ,  62  from inlet ports  42 A,  42 B respectively. From the convergence of passages  60 ,  62 , a passage  64  extends to outlet port  42 C. 
         [0025]    Y-body  58  provides for respective flows that have entered inlet port  42 A and inlet port  42 B to approach each other along imaginary vectors V 1  and V 2  at an acute angle  66 . Valve  42  further comprises an internal mechanism in the form of a blade  68  that is selectively positionable over an angular range  70  about an axis  72  for selectively setting relative proportions of respective flows that have entered inlet port  42 A and inlet port  42 B to form a combined flow leaving outlet port  42 C. 
         [0026]    Blade  68  is effective to set the relative proportions by channeling the respective flows from vectors V 1  and V 2  with various degrees of restriction depending on the angular position of the blade about axis  72  which is perpendicular to a plane containing vectors V 1  and V 2 . The positioning of blade  68  about axis  72  is controlled by an actuator  74  that acts on blade  68  via a linkage  76 . At the midpoint of angular range  70 , each approaching flow is partially restricted by blade  68 . As blade  68  swings in either direction, it increasingly restricts one approaching flow while decreasing restriction of the other. 
         [0027]    Valve may be considered a swing gate type control valve that creates small pressure loss while being capable of modulating the outlet flow over a range extending from 0% EGR to 100% EGR. 
         [0028]    The disclosed arrangement may provide improved fuel economy because of improved boost-to-exhaust back pressure relationship, may meet relevant specifications without two-stage turbocharging, and without variable geometry turbocharging that is sometimes used to control exhaust manifold pressure for EGR control. Because the entire exhaust gas flow leaving exhaust manifold  20  flows through turbocharger  24 , improved turbocharger response also becomes possible. Recirculated exhaust gas is relatively clean and cool because it is sourced downstream of the turbine and after-treatment devices. 
         [0029]    An engine  10  operates, supercharger  44  pumps a controlled combination of relatively cooler, relatively cleaner EGR and fresh air through intercooler  46  into intake manifold  16  in parallel with a conventional wastegate turbocharger  24  that is exclusively forcing fresh air through charge air cooler  38 . Turbocharger  24  is sized to supply a majority of the supercharged air in intake manifold  16  at most engine operating conditions, particularly those beyond light load and low speed. Supercharger  44  may supply a majority of the charge air in intake manifold and an appropriate quantity of relatively cooler and cleaner recirculated exhaust gas at light load/low speed engine operation, and that allows the engine to have improved transient response.