Abstract:
A turbocharged, diesel engine has a small catalyst provided upstream of the turbocharger with EGR collected from the exhaust stream downstream of the catalyst and upstream of the turbocharger. By making the catalyst small, it packages into a pipe coupling the manifold to the turbocharger, readily reaches lightoff, and absorbs little exhaust energy, thereby providing acceptable conversion of hydrocarbons and CO, but still allowing fast turbocharger response. In one embodiment, the engine has two cylinder banks, two exhaust manifolds, and two pre-turbo catalysts installed upstream of the turbine.

Description:
BACKGROUND 
     1. Technical Field 
     The present development relates to EGR routing and configuration of aftertreatment devices for a turbocharged diesel engine. 
     2. Background 
     Diesel engine exhaust is generally cooler than exhaust from a gasoline engine because the diesel engine operates with excess air and the cycle is more efficient at most operating conditions, which means there is less rejection of energy to exhaust gases. It is generally desirable to mount the turbine of the turbocharger close to the exhaust manifold so that exhaust energy, which is extracted by the turbine, is at its highest level. Turbocharger lag is partially mitigated by having the turbine located as close to the engine as possible. It is also known that exhaust aftertreatment devices, such as DOCs (diesel oxidation catalysts) and SCR (selective-catalyst reduction) catalysts, operate more efficiently when in a preferred temperature range. In particular, it is important for aftertreatment devices to attain their lightoff temperature as soon as possible following a cold start of the engine. Thus, it is desirable for quick lightoff to place aftertreatment devices as close to the engine as possible so that the aftertreatment devices can process exhaust gases soon after an engine cold start. 
     SUMMARY 
     According to an embodiment of the present disclosure, a multiple-cylinder engine has an exhaust manifold which directs engine exhaust into a pipe leading to the turbocharger; the pipe has a small catalyst fitted within. Inserting the small catalyst into the pipe obviates the need for an additional can that a full-sized close-coupled catalyst would require, which would also entail complicated and bulky plumbing and additional connections. By having a small volume, the catalyst attains its operating temperature rapidly and extracts little energy from the exhaust gases to attain its operating temperature, thereby interfering minimally with supplying exhaust energy directly to the turbine section of the turbocharger. Furthermore, pressure drop across a small catalyst can be minimized by controlling the aspect ratio of the can. The pipe housing the catalyst has an EGR (exhaust gas recirculation) outlet port to provide EGR to the EGR system, which includes: an EGR tube connecting the engine exhaust to the engine intake, EGR valve, and EGR cooler. EGR is extracted upstream of the turbocharger, thus, at high pressure. 
     According to another embodiment, the engine has first and second banks of cylinders, which exhaust to first and second exhaust manifolds, respectively. First and second pipes having first and second catalysts are coupled to the first and second manifolds, respectively, to receive the exhaust gases from the cylinder banks. The turbocharger has first and second turbines on a single shaft supplied exhaust gases through first and second exhaust inlets, which are coupled to the first and second pipes, respectively. Only the first pipe has an EGR outlet port so that the first turbine receives the exhaust gases from the first bank of engine cylinders less what is supplied to the EGR system. The second turbine receives substantially all flow from the second bank of cylinders. 
     In one embodiment, the catalyst is a DOC (diesel oxidation catalyst), which primarily oxidizes unburned hydrocarbons and CO (carbon monoxide). By having a small DOC arranged upstream of the turbocharger, the emissions of hydrocarbons and CO from the tailpipe can be reduced by about half at some operating conditions. Higher conversion efficiencies are achievable with a larger catalyst; however, with concomitant disadvantages of higher back pressure and packaging complications. Another tradeoff is that the turbines extract less energy, thus overall efficiency is harmed, when the back pressure is increased. 
     In one embodiment, a DOC of larger volume than the pre-turbo DOC is provided in the exhaust downstream of the turbocharger. Having a DOC before the turbocharger causes the downstream DOC to attain its lightoff more quickly after engine start, due to exothermic oxidation of hydrocarbons and CO increasing exhaust temperature. Thus, the combination of a pre-turbo DOC combined with a downstream DOC act synergistically to improve conversion efficiency, particularly during cold start. 
     By removing the EGR stream prior to expansion in the turbocharger, the EGR is at high pressure. This allows introduction of EGR gases to the EGR system (in particular an EGR valve and EGR cooler) that have reduced HC levels, mitigating HC deposition issues such as valve sticking and cooler fouling. In some prior art systems, an EGR catalyst is provided to alleviate HC deposition. An advantage of an embodiment of the disclosed configuration is that the pre-turbo catalyst alleviates the HC deposition problem as well as providing gases with fewer HCs to the turbine of the turbocharger and causes the downstream catalyst to lightoff more readily. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a vee engine; and 
         FIGS. 2-6  are schematics showing configurations for turbocharged, diesel engines according to embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. The representative embodiments used in the illustrations relate generally to controlling turbine inlet temperature in a turbocharged, diesel engine. However, this can be applied to any system with an exhaust turbine. Those of ordinary skill in the art may recognize similar applications or implementations consistent with the present disclosure, e.g., ones in which components are arranged in a slightly different order than shown in the embodiments in the Figures. Those of ordinary skill in the art will recognize that the teachings of the present disclosure may be applied to other applications or implementations. 
     Referring to  FIG. 1 , engine  10  is a vee engine having a first bank of cylinders  12  and a second bank of cylinders  14  which are sealed by first cylinder head  16  and second cylinder head  18 , respectively. The combustion chamber is sealed off from the intake manifolds (first is  20  and second is  22 ) by poppet valves. The poppet valves are actuated by camshafts (not shown) to open during predetermined times to allow fresh air to enter the combustion chamber and exhaust gases to be released from the combustion chamber into first and second exhaust manifolds  24  and  26 . In between the cylinder banks  12  and  14  is a valley  28 . 
     In  FIG. 2  a schematic of engine  10  is shown according to an embodiment of the present disclosure. Engine  10  is shown in  FIG. 2  with first cylinder bank  12  separate from second cylinder bank  14 . In reality, they are vee-configured and the separation is shown for convenience in schematically representing the layout. Fresh air flows through throttle valve  28 . About half of the intake air flows to compressor  30   a  of turbocharger  30  and the rest to compressor  32   a  of turbocharger  32 . Compressor  30   a  is coupled to turbine  30   b  via shaft  31 . Compressor  32   a  is coupled to turbine  32   b  via shaft  33 . For schematic representation purposes, the compressors and turbines are shown separated in  FIG. 2 . 
     Continuing with  FIG. 2 , the compressed intake gases are cooled in intercoolers  34  and  36 . Prior to entering intake manifolds  12  and  14 , EGR gases are mixed into the fresh air entering at EGR ports  38  and  40 . The fresh gases and EGR gases enter cylinder banks  12  and  14 . Fuel is directly injected into engine cylinders to initiate combustion. The exhaust gases exiting through first exhaust manifold  24  enter first pipe  42  and exhaust gases exiting through second exhaust manifold  26  enter second pipe  44 . Fitted within pipes  42  and  44  are small catalysts  46  and  48 , respectively. In one embodiment, catalysts  46  and  48  are DOCs. Pipe  42  has an EGR outlet port  50  coupled to an EGR tube  52  and pipe  44  has an EGR outlet port  51  coupled to EGR tube  52 . As illustrated in  FIG. 2 , EGR gases are extracted from both pipes  42  and  44 . In an alternative embodiment, there is no EGR outlet port  51 , and all EGR is supplied from cylinder bank  12  through EGR outlet port  50 . In another alternative, an EGR system is provided on each bank, having two EGR valves and two EGR coolers. 
     EGR outlet ports  50  and  51  are coupled to EGR tube  52 , which has an EGR valve  54  and an EGR cooler  56  disposed therein. Alternatively, EGR cooler  56  is upstream of EGR valve  54 . EGR is recirculated into the intake stream at EGR inlet ports  38  and  40 . 
     In  FIG. 2 , exhaust flowing out of turbines  30   b  and  32   b  tees together before being introduced into DOC  60 , SCR  62 , and DPF (diesel particulate filter)  64 . Alternatively, the order of the SCR and DPF is reversed. In yet another alternative, the gases flowing out of turbines  30   b  and  32   b  remain separated and each exhaust line has a DOC, SCR, and DPF. 
     Also shown in  FIG. 2  is an electronic control unit (ECU)  80 , which has an input/output (I/O)  82 , a microprocessor  84 , called a central processing unit (CPU), which is in communication with memory management unit (MMU)  86 . MMU  86  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  84 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  88 , random-access memory (RAM)  90 , and keep-alive memory (KAM)  92 , for example. KAM  92  may be used to store data while CPU  84  is powered down. The computer readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU  84  in controlling the engine or vehicle into which the engine is mounted. The computer readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  84  communicates with various sensors and actuators via I/O  82 . In  FIG. 2 , ECU  80  controls throttle valve  28  and EGR valve  54 . Exhaust turbines  30   c  and  30   d , in one embodiment, are variable geometry turbines, in which case they are controlled by ECU  80 . Various other sensors  94  and actuators communicate to or are controlled by ECU  80 . Some ECU  80  architectures do not contain MMU  86 . If no MMU  86  is employed, CPU  84  manages data and connects directly to ROM  88 , KAM  90 , and RAM  92 . Of course, more than one CPU  84  can be used to provide engine control and ECU  80  may contain multiple ROM  88 , KAM  90 , and RAM  92  coupled to MMU  86  or CPU  84  depending upon the particulars of the application. 
     In an alternative to  FIG. 2 , engine  100  has cylinder banks  102  and  104 . A turbocharger  106  has two compressor  106   a  and  106   b  as well as turbine  109  on a single shaft  109 . The configuration of engine  100  and turbocharger  106  as separated are shown for illustration purposes. 
       FIG. 2  shows an engine  10  with two banks  12  and  14 . In  FIG. 4 , engine  110  has one cylinder bank  112 . Engine  110  has a turbocharger  130  with one compressor  130   a  and one turbine  130   c . Compressor  130   a  and turbine  130   c  are mechanically coupled by a shaft  132 . Intake air is cooled in intercooler  134  and supplied to intake manifold  120  prior to combusting in engine cylinders. Exhaust travels to exhaust pipe  142  via exhaust manifold  124 . Pipe  142  has a catalyst  146  to treat exhaust gases prior to being expanded in turbine  130   c . Exhaust gases are further processed in DOC  160 , SCR  162 , and DPF  164  prior to exiting the tailpipe. EGR is supplied out of pipe  142  through EGR outlet port  150 . EGR flow rate is controlled by the position of EGR valve  154 . EGR gases are cooled in EGR cooler  156  prior to be introduced into the intake at EGR inlet port  138 . 
     In  FIG. 2 , the intake tees after throttle valve  28  and the exhaust gas streams form one stream after turbines  30   b  and  32   b . Another alternative is shown in  FIG. 5  in which an engine  210  has two cylinder banks  212  and  214  that tee together so that that turbocharger  230  has a single compressor  230   a  and a single turbine  230   c  coupled via a shaft  232 . In such a configuration, a single intercooler  234  and a single pre-turbine catalyst  246  are provided. ECU  280  controls EGR valve  250 , variable geometry turbine  230   c , and throttle valve  228 . Compressor  230   a  and  230   c  are coupled via shaft  232 . Engine  210  has two intake manifolds  220  and  222  and two exhaust manifolds  224  and  226 . DOC  260 , SCR  262 , and DPF  264  are located downstream of turbine  230   c.    
     Yet another alternative is shown in  FIG. 6  in which exhaust gases from two cylinder banks remain separated and pass through catalysts  346  and  348 . EGR is shown in  FIG. 6  as being taken off of a tee downstream of catalysts  346  and  348 . Alternatively, EGR can be taken from the downstream of only one of the branches, e.g., downstream of catalyst  346 . Such an alternative may obviate the need for catalyst  348 . Turbine  330  which is coupled to a compressor (not shown) via shaft  332  has two inlets and one outlet. 
     While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. For example in  FIG. 2 , the two exhaust ducts from turbines  30   c  and  30   d  tee to form one exhaust duct having one having DOC  60 , SCR  62 , and DPF  64 . Alternatively, the two exhaust ducts could remain separated with each having a DOC, SCR, and DPF. Also several alternative configurations are shown in  FIGS. 2 ,  3 , and  4 . However, many more combinations of elements shown in the Figures are possible beyond what is shown explicitly in  FIGS. 2 ,  3 , and  4 . Where one or more embodiments have been described as providing advantages or being preferred over other embodiments and/or over prior art in regard to one or more desired characteristics, one of ordinary skill in the art will recognize that compromises may be made among various features to achieve desired system attributes, which may depend on the specific application or implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described as being less desirable relative to other embodiments with respect to one or more characteristics are not outside the scope of the disclosure as claimed.