Abstract:
A compression ignition engine ( 10 ) has an EGR system operable to provide a first EGR loop ( 46, 52, 54 ) when the engine is lightly loaded and a second EGR loop ( 46, 52, 56 ) when the engine is more heavily loaded. When the first EGR loop is selected, exhaust gas is recirculated from a location upstream of a turbine ( 20 ) of a turbocharger ( 18 ) to a location downstream of the turbocharger compressor ( 22 ). When the second EGR loop is selected, exhaust gas is recirculated from a location downstream of the turbine to a location upstream of the compressor.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to internal combustion engines, especially compression ignition (i.e. diesel) engines. More specifically, the invention relates to a strategy for reducing tailpipe emissions from such engines through the selective use of a low-pressure EGR (exhaust gas recirculation) loop and a high-pressure EGR loop. 
   BACKGROUND OF THE INVENTION 
   The use of EGR as an addition to charge air introduced into the engine cylinders aids in controlling tailpipe emissions, especially NOx and particulates. 
   Because a diesel engine that powers a motor vehicle runs at different speeds and loads depending on various inputs to both the vehicle and the engine that influence engine operation, the nature of the charges created in the cylinders change as engine speed and load change. Exhaust gas recirculation requirements also change with engine speed and load changes. 
   A processor in an engine control system processes data indicative of parameters such as engine speed and engine load to develop control data for controlling constituents of the charges. The data developed is used to control turbocharger boost, engine fueling, and EGR rate. 
   Alternative combustion processes for a compression ignition engine can provide significant reductions in tailpipe emissions, NOx (oxides of nitrogen) and DPM (diesel particulate matter). Examples of alternative combustion processes include Homogeneous Charge Compression Ignition (HCCI), Controlled Auto-Ignition (CAI), Dilution Controlled Combustion Systems (DCCS), and Highly Premixed Combustion Systems (HPCS). 
   SUMMARY OF THE INVENTION 
   Briefly, the present invention relates to a compression ignition engine having two EGR loops that are selectively used to recirculate exhaust gas for reducing NOx (Nitrogen Oxides) and PM (Particulate Matter) emissions. Selection of a particular EGR loop is a function of engine load. 
   While the invention is useful with various turbocharged engines, its use in conjunction with alternative diesel combustion in a turbocharged engine is believed to provide significant reductions in tailpipe emissions by keeping in-cylinder temperatures significantly lower than in comparable engines operating by conventional diesel combustion. 
   In a presently preferred embodiment, a high-pressure EGR loop and a low-pressure EGR loop are provided by one direction control valve and two EGR valves. As a result, the high efficiency of the high-pressure EGR loop may be used to advantage at relatively lower engine loads, and the high EGR rate of the low-pressure EGR loop may be used to advantage at relatively higher engine loads. 
   The strategy for selection of one loop or the other is embodied in the engine control system as a programmed algorithm that is repeatedly executed by a processor. 
   One generic aspect of the present invention relates to a turbocharged compression ignition engine comprising engine cylinders within which combustion occurs to run the engine, an intake system through which charge air is introduced into the cylinders, an exhaust system through which products of combustion from the engine cylinders are exhausted, a turbocharger having a turbine in the exhaust system and a compressor in the intake system, a fueling system for fueling the cylinders, and an exhaust gas recirculation (EGR) system for conveying exhaust gas from the exhaust system to the intake system. 
   The EGR system comprises a direction control valve having an outlet, a first inlet communicated to the exhaust system upstream of the turbine, a second inlet communicated to the exhaust system downstream of the turbine, and an element that selectively communicates the inlets to the outlet to provide exhaust gas from a selected inlet to the outlet. After leaving the direction control valve outlet, exhaust gas passes through a cooler. 
   A first EGR valve and a second EGR valve each has a respective inlet to which exhaust gas that has passed through the cooler is delivered. When the direction control valve is selecting the first inlet, the first EGR valve controls flow of recirculated exhaust gas to a location in the intake system downstream of the compressor while the second EGR valve is closed. When the direction control valve is selecting the second inlet, the second EGR valve controls flow of recirculated exhaust gas to a location in the intake system upstream of the compressor while the first EGR valve is closed. 
   Another generic aspect of the invention relates to a method of controlling exhaust emission during operation of a turbocharged compression ignition engine having engine cylinders within which combustion occurs to run the engine, an intake system through which charge air is introduced into the cylinders, an exhaust system through which products of combustion from the engine cylinders are exhausted, a turbocharger having a turbine in the exhaust system and a compressor in the intake system, a fueling system for fueling the cylinders, and an exhaust gas recirculation system for conveying exhaust gas from the exhaust system to the intake system to aid in limiting in-cylinder combustion temperature. 
   The method comprises selecting between a first EGR loop and a second EGR loop to recirculate exhaust gas, wherein selection of the first EGR loop causes exhaust gas to be recirculated from a location upstream of the turbine to a location downstream of the compressor and selection of the second EGR loop causes exhaust gas to be recirculated from a location downstream of the turbine to a location upstream of the compressor. 
   Another generic aspect relates to an engine for performing the method just described. 
   The foregoing, along with further features and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, now briefly described as follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general schematic diagram of those portions of an exemplary diesel engine relevant to principles of the present invention. 
       FIG. 2  is somewhat schematic cross section view through a portion of one valve that is present in  FIG. 1 . 
       FIG. 3  is somewhat schematic cross section view through a portion of another valve that is present in  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows schematically a portion of an exemplary turbocharged diesel engine  10  operating in accordance with the inventive strategy for powering a motor vehicle. Engine  10  comprises cylinders  12  within which pistons reciprocate. Each piston is coupled to a respective throw of a crankshaft by a corresponding connecting rod. Engine  10  further comprises an intake system  14  and an exhaust system  16 . Turbocharging is provided by a turbocharger  18  having a turbine  20  in exhaust system  16  that operates a compressor  22  in intake system  14 . 
   Intake system  14  further comprises an intercooler  24  downstream of compressor  22  for cooling charge air that has been drawn into intake system  14  and compressed by compressor  22 . From intercooler  24  the charge air is introduced into an engine intake manifold  26  that serves cylinders  12 . Charge air enters each cylinder when a respective intake valve is open during the engine cycle. 
   Engine  10  further comprises a fueling system  28  that comprises fuel injectors for cylinders  12 . The engine also has a processor-based engine control system or unit (ECU)  32  that processes data from various sources to develop various control data for controlling various aspects of engine operation. The data processed by ECU  32  may originate at external sources, such as various sensors  34 , and/or be generated internally. Examples of data processed may include engine speed, intake manifold pressure, exhaust manifold pressure, fuel injection pressure, fueling quantity and timing, mass airflow, and accelerator pedal position, but any particular algorithm that processes data in practice of the invention may not necessarily process data for all of these enumerated parameters. Typically however, a parameter or parameters that are indicative of engine load are processed in the practice of the invention. 
   Engine  10  further comprises an EGR system  36  between exhaust system  16  and intake system  14 . EGR system  36  has a configuration that can provide either low-pressure EGR or high-pressure EGR and comprises a high-pressure inlet  38  upstream of turbine  20  and a low-pressure inlet  40  that is downstream of turbine  20 . 
   In this particular embodiment a DPF (diesel particulate filter)  42  is disposed in the exhaust system downstream of turbine  20 , but before inlet  40 , so that low-pressure exhaust gas at inlet  40  is exhaust gas that has been treated by DPF  42 . 
   Inlet  38  leads to a first port  44  of a directional valve  46 , and inlet  40  to a second port  48  of valve  46 . An outlet port  50  of valve  46  leads to an inlet of an EGR cooler  52 . An outlet of EGR cooler  52  leads to inlet ports of respective EGR valves  54 ,  56 . 
   An outlet of EGR valve  54  leads to intake system  14  between intercooler  24  and intake manifold  26 . An outlet of EGR valve  56  leads to intake system  14  upstream of compressor  22 . EGR valves  54 ,  56  and directional valve  46  are under the control of ECU  32 . 
   Directional valve  46  operates to select either inlet  38  or inlet  40  for communication to the inlet of EGR cooler  52 . 
   When engine  10  runs at lower loads, ECU  32  operates valve  46  to select inlet  38 , keeps EGR valve  56  closed, and operates EGR valve  54  to meter cooled higher pressure exhaust gas to the boosted charge air in intake system  14 . At the relatively lower loads, a major part of the exhaust gas flow passes through turbine  20  and DPF  42  before entering atmosphere. A minor part of the exhaust gas flow passes through directional valve  46 , EGR cooler  52 , and EGR valve  54  to entrain with the boosted charge air. Hence, directional valve  46 , EGR cooler  52 , and EGR valve  54  form a high pressure EGR loop that is active at relatively lower engine loads for controlling exhaust gas recirculation. 
   When engine  10  runs at relatively higher loads, ECU  32  operates valve  46  to select inlet  40 , keeps EGR valve  54  closed, and operates EGR valve  56  to meter cooled lower pressure exhaust gas to the unboosted air entering intake system  14 . At the relatively higher loads, all of the exhaust gas flow passes through turbine  20  and DPF  42 , But before reaching atmosphere, a minor part of the exhaust gas flow passes through directional valve  46 , EGR cooler  52 , and EGR valve  56  to entrain with unboosted air entering intake system  14 . Hence, directional valve  46 , EGR cooler  52 , and EGR valve  56  form a low pressure EGR loop that is active at relatively higher engine loads for controlling exhaust gas recirculation. 
   ECU  32  controls engine fueling by controlling the operation of the fueling system  28 , including controlling the operation of the fuel injectors  30 . The processing system embodied in ECU  32  can process data sufficiently fast to calculate, in real time, the timing and duration of device actuation to set both the timing and the amount of each injection of fuel into a cylinder. Such control capability is used in implementation of a fuel control strategy that provides the low temperature combustion (cool flame) that characterizes alternative diesel combustion processes. The use of high- and low-pressure EGR loops is advantageous when alternative diesel combustion is used to run engine  10  and is believed useful for achieving compliance with certain requirements for reduced NOx (Nitrogen Oxides) and DPM (Particulate Matter) in tailpipe emissions from motor vehicles powered by diesel engines. 
   The present invention can be effective over the full range of engine operating conditions. For example, data that correlates a particular EGR loop with data values for various engine loads is developed from engine tests and stored in memory of ECU  32 . When the engine runs, data values for engine load are processed in conjunction with the stored data to cause the appropriate EGR loop to be selected. The extent to which the particular EGR valve in the selected loop is allowed to open is then controlled by certain processing performed by the control system processor. 
   The present invention can be used for heavy-duty, medium-duty, and light-duty diesel engines, and provides high thermal efficiency. 
   The direction control valve can be a spool valve  46  as shown in  FIG. 2 , or a switch valve  46  as shown in  FIG. 3 . 
   While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the following claims.