Claims:CLAIMS
We Claim:

1. An exhaust gas treatment (EGT) system (100) for a naturally aspirated internal combustion engine (102) , said EGT system (100) comprising:
- a Diesel Oxidation Catalyst DOC (106) disposed in exhaust path 104 of said engine (102) by closely coupling said DOC (106) with exhaust port of said engine (102)
- a DPF (108) disposed downstream of said DOC (106)
- an SCR (110) disposed downstream of said DPF (108)
- a differential pressure sensor disposed at inlet and outlet of said DPF (108) to determine fuel injection quantity
2. An exhaust gas treatment (EGT) system (100) according to claim 1 wherein said Diesel Particulate Filter 108 and SCR catalyst (110) are functionally separated
3. An exhaust gas treatment (EGT) system (100) according to claim 1 wherein said DPF (108) has optimized thermal mass
4. An exhaust gas treatment (EGT) system (100) according to claim 1 wherein said DPF 108 is operated in continuous regeneration mode
5. An exhaust gas treatment (EGT) system (100) according to claim 1 wherein power density of said engine is maintained low
6. An exhaust gas treatment (EGT) system (100) according to claim 1 uses closed loop air mass control to enable high SCR performance
7. An exhaust gas treatment (EGT) system (100) according to claim 1 wherein said DOC (106) is directly connected to said engine (102)
, Description:Complete Specification:

The following specification describes and ascertains the nature of this invention and the manner in which it is to be performed.
Field of the invention
[001] This invention relates to the field of exhaust gas treatment systems. The invention relates in specific to an exhaust gas treatment system for a small, naturally aspirated diesel engine.

Background of the invention

[002] Exhaust gas treatment for internal combustion engines are known in the prior arts. The exhaust gas treatment (EGT) systems are used to reduce the exhaust emissions coming out of the internal combustion engine. The EGT system of diesel engines typically reduces hydro carbons, carbon monoxide, nitrogen oxide and particulate matter.

[003] The prior art. US 2010/0242447 A1 discloses a method and control system for a selective catalytic reduction (SCR) catalytic converter and a diesel particulate filter (DPF) includes a DPF control module that determines a particulate matter (PM) load progress of the DPF and generates a DPF regeneration request based on the PM load progress.

Brief description of the accompanying drawing

[004] Different modes of the invention are disclosed in detail in the description and illustrated in the accompanying drawing:

[005] Fig. 1 illustrates an EGT system according to one embodiment of the invention.

Detailed description of the embodiments

[006] FIG. 1 illustrates a typical EGT system 100. Shown in fig. 1 is an internal combustion engine 102, an exhaust path 104, a diesel oxidation catalyst DOC 106, a diesel particulate filter DPF 108, a selective catalytic reduction SCR component 110, manifold air flow sensors 112 disposed across an air filter 113, temperature sensors, a tank 114 to store reducing agent etc. An engine control unit ECU 116 controls the operations of the EGT system. The engine 102 is shown only as representation and no. of cylinders may vary.

[007] The engine shown in fig. 1 is a 1 or 2 cylinder naturally aspirated diesel engine. During the suction stroke, the air enters into the cylinder. During the compression stroke, the air gets pressurized and when the piston is close to top dead center, the fuel is injected into the cylinder. The fuel and air combust in the cylinder and the power is generated. The combustion generates the exhaust gases and during the exhaust stroke, the exhaust gases are pushed out of the engine through the exhaust port of the engine into the exhaust path.

[008] The exhaust gases coming out of the engine comprise like NOx, HC, Sulphur oxide etc. The exhaust gases also contain particulate matter and soot. All of these are harmful if released into the atmosphere. These need to be reduced to meet the emission standards before the exhaust is released into the atmosphere.

[009] To reduce the harmful materials in the exhaust gases to meet the emission standards, the EGT is used. The EGT typically comprises DOC, DPF and SCR.

[010] Doc: The diesel oxidation catalyst (DOC) is an after treatment component that is designed to convert carbon monoxide (CO) and hydrocarbons into carbon dioxide (CO2) and water. Platinum group metals PGM including platinum, palladium, and rhodium are commonly used in DOC. Catalytic converters comprise a monolith honeycomb substrate which is coated with the PGM metal compounds and packaged into a stainless steel container. The honeycomb is made either of ceramics or stainless steel foil. Its structure of many small parallel channels presents high catalytic contact area to the exhaust gases. As the hot gases flow through the channels and contact the catalyst, several exhaust pollutants are converted into harmless substances. In the DOC exhaust gases are optimized so that they can be treated further in other EGT components.

[011] DPF: The diesel particulate filter (DPF) captures and stores exhaust soot in order to reduce harmful emissions from the exhaust gases. The trapped soot periodically needs to be emptied by the way of burning it. The burning of the soot is called regeneration process. The regeneration process may be active regeneration or a passive regeneration. In the active regeneration the engine control unit ECU detects the suit accumulated in the filter, then the injection timings are adjusted so that higher temperatures are generated in the exhaust gases. Because of this higher temperature, the soot is burnt off. The passive regeneration typically takes place during high load operation. This typically causes higher temperature of the exhaust gases.

[012] SCR: The Selective Catalytic Reduction component is used to reduce the level of nitrogen oxide in the exhaust gases by chemical reactions in the catalyst element with a reducing agent. The reducing agent which is typically urea solution is stored in the tank 114 and injected into the exhaust gases passing through the SCR component. The water in the urea solution is evaporated as the solution is injected into the hot exhaust gases. The high temperature results in thermal decomposition of the urea into ammonia (NH3) and carbon dioxide (CO2). The NOx gases are converted into nitrogen (N2) and water (H2O), as they react with the ammonia at a catalytic surface. The SCR uses platinum, zeolite metals as base for the catalysts. The ECU controls the SCR component by injecting appropriate amount of urea into the exhaust path. A NOx sensor 118 provides information to the ECU regarding amount of NOx coming out of the exhaust.

[013] To meet the emission regulations, the EGT system makes use of the DOC, DPF and SCR.

[014] In some of the prior arts the EGT systems, also include exhaust gas recirculation (EGR) technique. As the temperature in the cylinder increases, the oxygen and nitrogen combine to form NOx in the cylinder. To reduce the NOx formed by the high temperature, a controlled amount of exhaust gases coming out of the engine is re-circulated into the engine. An EGR valve controls the amount of re-circulation of exhaust gases. The re-circulation of burnt gases will reduce the oxygen available for the combustion in the engine. This results in lower heat release and lower temperature in the cylinder thereby reducing the NOx formation. But the EGR technique has disadvantages. Since EGR reduces the available oxygen in the cylinder, the production of particulates is increased when EGR is activated. Lower combustion also results in lower power generation.

[015] The invention proposes a technique to reduce the harmful materials in the exhaust gases, without using the EGR. The invention proposes to use the heat already available in the exhaust gases to burn the soot in the DPF. The invention uses the exhaust heat to heat up the SCR component quickly on the starting of the engine. The improvements suggested by the invention are as below.

[016] EGR component is eliminated. To reduce soot entry rates, the high NO2 in the exhaust is used effectively, enabling “slow” (continuous) soot burning in the DPF via Continuous Regeneration Technique CRT®. This in turn will reduce share of active regeneration of particulate filter and subsequent oil dilution. An EGR usage would increase the soot entry rate into the DPF, reduce the NO2 amount in the EGT and hence subsequently the CRT effect as well as a higher frequency of active regeneration. As a prior arts the active regeneration of the DPF is initiated via diesel fuel injection late after top dead center enriching the hot EGT gases with unburned fuel. The unburned fuel hits the precious metal enriched surface of the DOC and leads to an exothermal oxidization of the Hydrocarbons and a temperature increase of the DPF. The oil dilution happens when the late injected fuel hits the cold cylinder liners and admixes with lubrication oil.

[017] Close coupling of high efficient SCR system to ensure fast heat up.

[018] Well adopted DPF geometry to reduce heat capacity of the diesel particulate filter. DPF is designed to that extend, that it would be large enough to capture as much as possible Particulate matter but on the hand has to be limited not to increase the thermal mass of the entire EGT to that extend, that the subsequent SCR will not heat up and hence will show a low or no NOx conversion performance.

[019] Close loop air mass control is necessary to control the oxygen content and the burning process of the particulate matter in the DPF during active regeneration.
Due to very high pulsation of air mass in the air inlet of single cylinder naturally aspirated engine air mass sensing concept based on usage of differential pressure sensor is used.

[020] The system uses a naturally aspirated (NA) small Diesel engine to target stringent emission limits. Key element for a good NOx conversion performance is the fast heat up of the SCR. Following System design elements are supporting a rapid heat up of the SCR:
• The thermal lower combustion efficiency of a NA engine
• The close couple design of the EGT due to non-availability of the TC as a heat sink and the EGR pipe.

[021] To reduce particulate matter load that is typically higher in NA engines compared to turbo charged engines, no EGR is used. The elimination of EGR itself enables very close coupling of the Exhaust Gas Treatment System to the engine exhaust port. This results in fast heat up of the SCR component and in turn high NOx conversion efficiency.

[022] The EGT components are placed close to the exhaust port of the engine as there is no turbocharger and intercooler required for the small engines compared to the large engines. Power density is kept low by design of the engine and in absence of EGR the soot entry rate is on a comparable low level for NA engines – enabling in turn also a reduced size and thermal mass of the DPF, enabling fast heat up of the SCR component.

[023] The combustion is optimized for low particulate matter. The fuel injection is controlled by the ECU based on the differential pressure sensed across the DPF.
The closed coupled design of the EGT system enables a comparable high EGT operation temperature.

[024] The invention proposes a simple EGT system for a single cylinder or two cylinder NA engine by eliminating the EGR, placing the EGT components close to the exhaust port of the engine, making use of the exhaust heat to burn the soot in continuous regeneration mode and to heat up the SCR component quickly. The engine is operated with low power density. The DOC is directly connected to the engine exhaust port. The EGR is eliminated.