Patent Publication Number: US-2018045101-A1

Title: A multi-stage exhaust turbocharger system

Description:
TECHNICAL FIELD 
     The present disclosure is concerned with a multi-stage exhaust turbocharger system. More particularly, but not exclusively, the present disclosure is concerned with a parallel-series-sequential, regulated, multi-stage turbocharger for use on an internal combustion engine of a vehicle, with an internal combustion engine so equipped, and with a vehicle having such an engine. Aspects of the invention relate to a system, to an engine and to a vehicle. 
     BACKGROUND 
     An exhaust turbocharger allows a small capacity internal combustion engine to produce the same power as a comparatively large capacity naturally aspirated engine, with fuel efficiency. 
     To further improve performance of internal combustion engines it is known to use turbocharger systems with high and low pressure stages. Such an arrangement can provide good performance over a wide range of exhaust gas flow. One kind of multi-stage turbocharger system comprises two high pressure turbochargers in parallel and one low pressure turbocharger in series with the high pressure turbochargers, with the low pressure turbine downstream of the high pressure turbines. 
     It is known that the power output of a turbocharger may be increased by increasing the aspect ratio ‘A/R’ of the turbine wheel scroll, where A is the entry area or throat area of a turbine and R is the distance of the centroid of this area A from the turbine shaft axis. However, when the A/R ratio is increased, the response time of the turbocharger may be increased, resulting in ‘turbo-lag’, which is noticed by the vehicle driver as a time delay between a demand for acceleration and a corresponding power increase from the engine. 
     It would be desirable to increase the power output of a turbocharger system whilst also minimising the response time of the turbocharger system over a range of engine operating speeds. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided an exhaust turbocharger system, comprising: a first turbocharger having a turbine inlet adapted to be fed directly from an exhaust manifold of an internal combustion engine; a second turbocharger having a turbine inlet adapted to be fed from said exhaust manifold via a first flow control valve; a divided scroll third turbocharger having one scroll in direct communication with the turbine outlet of said first turbocharger and a second scroll in direct communication with the turbine outlet of said second turbocharger; and a second flow control valve having a valve inlet from the respective turbine outlet of each of said first and second turbochargers; the valve outlet of the second flow control valve being for connection to an exhaust. 
     The system may comprise a first variable geometry turbocharger having a turbine inlet adapted to be fed directly from an exhaust manifold of an internal combustion engine. 
     The system may comprise a second variable geometry turbocharger having a turbine inlet adapted to be fed from said exhaust manifold via a first flow control valve. 
     The system may comprise a divided scroll third turbocharger having one turbine scroll in direct communication with the turbine outlet of said first turbocharger and a second turbine scroll in direct communication with the turbine outlet of said second turbocharger; 
     The system may comprise a second flow control valve having a valve inlet from the respective turbine outlet of each of said first and second turbochargers. 
     The valve outlet of the second flow control valve may be for connection to exhaust downstream of the turbocharger system. 
     A turbocharger system according to the invention can provide for effective boosting of the inlet air charge throughout the normal operating range of an internal combustion engine, with reduced turbo lag, and reduced risk of retaining combustion products within the combustion chambers of the engine. 
     The divided scroll third turbocharger may be of any known kind, and for example the scrolls may be arranged axially (side by side) or radially so as to be able to provide a separate and a combined effect on the turbine wheel. 
     In an embodiment a common housing is provided for some or all of the independent turbochargers. This arrangement may reduce flow path connections, and may also reduce overall turbocharger mass to the intent that cold start light-off of the usual exhaust catalyst is not unduly delayed. In an embodiment one or more of the control valves may be provided in such a common housing—that is to say the fixed element(s) of a respective valve may be defined by the housing, and the moving element(s) assembled thereto. 
     Any control valve suitable for use in a turbocharger may be used, for example a spring-closed poppet valve having a respective actuator, for example an electric or pneumatic actuator, for operation thereof under the control of a controller. The controller may typically comprise an electronic control unit having a look-up table, map or algorithm responsive to speed and/or load of the engine to control opening and closing of said control valves in the desired sequence. 
     In an embodiment the compressor wheel of said third turbocharger has an air inlet and an air outlet connected to the valve inlet of a third flow control valve; 
     the third flow control valve has a valve outlet to the valve inlet of a fourth flow control valve; the compressor inlet of said first turbocharger and the compressor inlet of said second turbocharger are connected to the compressor outlet of said third turbocharger;
 
the valve inlet of the fourth flow control valve is connected to the compressor outlet of said second turbocharger;
 
the valve outlet of the fourth flow control valve is adapted to feed an inlet manifold of an internal combustion engine, and
 
the compressor outlet of said first turbocharger is adapted to feed said inlet manifold.
 
     Aspects of the invention are defined in the accompanying claims and also relate to an internal combustion engine of a motor vehicle, which may be a four stroke, reciprocating piston, gasoline engine, and to a wheeled motor vehicle so equipped. 
     According to some, but not necessarily all examples, there is provided an exhaust turbocharger system, comprising first and second independent variable geometry turbochargers in parallel and a third independent relatively low pressure turbocharger in series with the first and second turbochargers. Each independent turbocharger may have a turbine wheel with an associated turbine inlet and turbine outlet, and a connected compressor wheel with an associated compressor inlet and compressor outlet. Each turbine wheel and connected compressor wheel may be rotatable in unison. A plurality of flow control valves may be provided, each control valve comprising a respective valve inlet and a valve outlet. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawing, in which: 
         FIG. 1  shows schematically a turbocharger system according to a first embodiment of the invention. 
     
    
    
     DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
       FIG. 1  shows schematically an arrangement of independent turbochargers of a turbocharger system according to an embodiment of the invention. The independent turbochargers may be incorporated within a common housing or comprise a substantially unitary assembly. 
     An internal combustion engine  10  has an exhaust manifold  12 . A turbocharger system comprises first relatively high pressure turbocharger  30 , a second relatively high pressure turbocharger  40 , a relatively low pressure divided scroll turbocharger  60 , flow control valves V 1 , V 2 , V 3 , and, V 4 , an air inlet  17  with air filter  50 , charge air coolers  52 ,  54 , and an outlet  16  to a vehicle exhaust. 
     The exhaust manifold  12  collects in conventional manner the exhaust gases from the internal combustion engine  10 , which are ducted to the first high pressure turbocharger  30 , comprising a turbine wheel  32  and a compressor wheel  34  coupled for rotation on a common shaft  36 . The turbine wheel  32  has an inlet in direct fluid communication with the exhaust manifold  12 . The turbocharger  30  has a variable geometry turbine of any suitable kind. 
     The second high pressure turbocharger  40  comprises a turbine wheel  42  and a compressor wheel  44  coupled for rotation on a common shaft  46 . The turbocharger  40  also has a variable geometry turbine. A first control valve V 1  comprises an inlet in direct fluid communication with the exhaust manifold  12  and an outlet which is in connected to the inlet for the turbine wheel  42 . 
     A second flow control valve V 2  comprises an inlet which is connected to an outlet of both variable geometry turbine wheels  32 ,  42 , via fluid branches  31 ,  33  and an outlet which is open to an exhaust system  16  of conventional kind. The fluid branches  31 ,  33  may be connected at any point along their respective lengths. 
     The low pressure turbocharger  60  comprises a divided turbine  62  and a compressor  64 , coupled for rotation on a common shaft  66 . The turbine  62  has a first scroll  68  and a second scroll  69 , arranged either side by side axially on the shaft  66 , or circumferentially (meridionally). The scrolls  68 ,  69  have a common turbine outlet in communication with the outlet  16  to the exhaust system. 
     The first high pressure turbine wheel  32 , has an outlet which is in fluid communication with the first scroll  68  of the low pressure turbine  62 , and the second high pressure turbine wheel  42  is in fluid communication with the second scroll  69  of the low pressure turbine  62 . 
     An air filter  50  comprises an air inlet  17 , and an outlet in fluid communication with the inlet of the compressor of the low pressure turbocharger  60 . 
     The low pressure compressor  64  has an outlet which is connected via a fluid duct to a charge air cooler (intercooler)  54 . The charge air cooler  54  has two outlets; the first outlet is connected to the compressor inlet of the first high pressure turbocharger  30  and the second outlet is connected to the compressor inlet of the second high pressure turbocharger  40 . 
     A second charge air cooler  52  comprises two inlets and an outlet. The outlet is adapted to feed compressed air to the internal combustion engine  10 . One of the inlets of the charge air cooler  52  is connected to the outlet of high pressure compressor wheel  34 . The remaining inlet of charge air cooler  52  is connected to the outlet of a fourth flow control valve V 4 . The inlet paths could alternatively be combined upstream of the charge air cooler  52 . 
     The fluid connections to and from the charge coolers  54 ,  52  may be defined within the turbocharger housing and/or by conventional flexible hoses, which may be branched. 
     Control valve V 4  has an inlet which is connected to the outlet of high pressure compressor wheel  44 . A third control valve V 3  has an inlet which is connected to the outlet of low pressure compressor  64 , and an outlet connected downstream of the outlet of the high pressure turbine wheel  44 , as illustrated. The third control valve V 3  provides a bypass for the compressor of the second high pressure turbocharger  40 , as illustrated. 
     In use, the turbocharger has multiple phases of operation which are active according to engine speed, and gas flow in the exhaust manifold. The control valves V 1 -V 4  are sequenced for operation at different flow rates of exhaust gas, as follows. 
     Phase 1 
     At low gas flows, exhaust gas exiting the exhaust manifold  12  is directed to the turbine wheel  32 . The variable geometry mechanism thereof is selected to spool up the turbine wheel at low flow rates so as to provide charge compression via the compressor wheel  34  at low engine speeds. The compressor wheel  34  is driven via the common shaft  36  to compress the inlet gas which has passed through a low pressure charge air cooler  54 . The low pressure charge air cooler  54  is provided with gas from the compressor wheel  64  of the divided turbocharger  60 . The compressed gas exits compressor wheel  34  at a higher pressure and is directed via a high pressure charge air cooler  52  to the air intake manifold (not shown) of the engine  10 . 
     The first turbine scroll  68  of the twin scroll turbocharger  60 , receives gas via passage from the outlet of turbine wheel  32 , which in turn drives the low pressure compressor wheel  64 . 
     During the first phase, the variable geometry mechanism associated with turbine wheel  32  is gradually adjusted to maximize output thereof having regard to exhaust gas flow rate, and will move from a minimum to a maximum condition. Both turbochargers  60  and  30 , provide boost, and thus turbocharging of the internal combustion engine  10  is effected at low to medium rates of exhaust gas flow. Flow control valves V 2  and V 4  are shut in phase 1. Control valve V 3  is open, but the path to the inlet manifold is closed by control valve V 4 ; the output from a spinning compressor wheel  44  is thereby allowed to recirculate, to make it ready for operation as the flow of exhaust gas increases. If necessary a further control valve may be provided to prevent flow from the outlet of turbine wheel  32  to the second turbine scroll  69  via the fluid branches  31 ,  33 . 
     Phase 2 
     As the engine speed increases so does the mass flow rate of exhaust gas. At intermediate gas flow rates at the low/medium transition, the first turbine wheel  32  will approach maximum flow rate as the variable geometry mechanism reaches the limit of adjustment, and accordingly flow control valve V 1  is opened progressively in a controlled manner to supply exhaust gas to the second high pressure turbocharger  40  and to rotate the turbine wheel  42 , thus causing consequential rotation of the second compressor wheel  44 . 
     At a medium flow rate, in addition to the gas flow through turbocharger  30 , flow control valve V 1  is fully open such that gas can flow from the exhaust manifold  12  to turbine wheel  42  of the second high pressure turbocharger  40 . Again the variable geometry mechanism is gradually moved from one end of an operating range to the other, at which substantially the maximum rate of exhaust gas flow can be accommodated. 
     The second turbine scroll  69  of the twin scroll, low pressure turbocharger  60 , receives exhaust gas from the outlet of turbine wheel  42 . The second compressor wheel  44  is accordingly driven via the common shaft  46  to compress the inlet gas which has passed through the low pressure charge air cooler  54 . Valve V 4  is opened and compressed gas is provided to the high pressure charge air cooler  52 , and thence to the inlet manifold; valve V 3  is closed to obviate recirculation. 
     Phase 3 
     As flow rates from the manifold  12  further increase to a maximum, flow control valve V 2  can be opened progressively to prevent back pressure from the low pressure turbocharger  60 ; valve V 2  operates as a wastegate as the flow capacity of the respective turbine wheels  32 ,  42  and  62  is reached. Such flow rates are typically reached at or very close to maximum engine rpm. 
     Mode of Operation 
     A high pressure turbine can provide boost effectively at low mass flow rates and a low pressure turbine can provide boost at high mass flow rates. 
     The divided low pressure turbocharger  60  receives exhaust gas from the parallel high pressure turbochargers  30 ,  40 . Each high pressure turbocharger provides gas to an independent scroll  68 ,  69 . At low gas flow rates i.e. low engine speeds, one of the high pressure turbocharges is disabled by closure of valve V 1 . 
     At low gas flow rates there is an insufficient mass flow rate to drive both high pressure turbines or one low pressure turbine. By disabling one high pressure turbine there is sufficient mass flow to drive the working high pressure turbine effectively; however there is still insufficient mass flow rate to drive a single scroll low pressure turbine with the large throat area that is required to deal with high mass flow rates. 
     By dividing the effective throat of the low pressure turbine into two distinct scrolls, the low pressure turbine can spool up quickly at lower flow rates due to the reduced A/R ratio. At increased flow rates, i.e when both high pressure turbines are functioning, there is sufficient mass flow to drive a large area and thus the effective throat area of the low pressure turbine is substantially increased. At very high mass flow rates (in effect substantially maximum flow rate) the turbines can be bypassed. This is a form of regulated multi-stage operation for the turbocharger system of this embodiment. 
     On the compressor side, the progressive introduction of the turbine wheels  32 ,  42 , and the twin scrolls of the turbine wheel  62  provide for a progressive boosting of inlet air flow as each compressor wheel  34 ,  44 ,  64  becomes effective. Operation of control valves V 1  and V 2  is according to an algorithm or look-up table of an electronic processor, to the intent that the output of the turbocharger system is efficient over the full range of exhaust gas flow rate. The arrangement provides for minimized turbo lag since the respective turbines and turbine scrolls each have a cumulative operating range. The arrangement also provides reduced pumping work for the engine and reduced trapping of combustion residuals in the engine cylinders, which are known to be important factors in the operation of spark-ignition engines. 
     The variable geometry of turbocharger  30 ,  40  (and optionally turbocharger  60 ) are of a known kind and have movable nozzles and/or turbine vanes to permit efficiency to be tuned to a particular rate of exhaust gas flow. Such turbines, whilst being more complex, are not restricted to providing maximum performance at an exhaust gas flow for which a fixed nozzle and fixed turbine blade are optimized. 
     The variable geometry turbochargers  30 ,  40  have the capacity to take substantially the full range of gas flow without choking.