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.
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.
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
In
Continuing with
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
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Yet another alternative is shown in
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
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