Engine aftertreatment system with exhaust lambda control

Abstract

An aftertreatment device for reducing nitrogen oxides (NOx), particulate matter (PM), hydrocarbon (HC), and carbon monoxide (CO) generated by a compression-ignition (CI) engine. In this device, lean exhaust air generated in the CI engine is converted to rich exhaust air, and energy used for the conversion is recycled using an energy recovery device. The result rich exhaust air then pass through an oxidation catalyst, where NOx is reduced with CO and HC.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

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FIELD OF THE INVENTION

This present application claims priority from U.S. provisional application No. 60/922,152 having the same title as the present invention and filed on Apr. 7, 2007.

The present invention relates to systems and methods for reducing exhaust emissions from internal combustion engines, more particularly, to apparatus and methods for reducing Nitrogen Oxides (NOx), Carbon Monoxide (CO), Hydrocarbon (HC), and Particulate Matter (PM) from compression ignition engines.

BACKGROUND OF THE INVENTION

Internal combustion engines are subject to limits for exhaust emissions. In addition to improving in-cylinder designs, using Exhaust Gas Recirculation (EGR), and better controlling combustion, an aftertreatment device is normally needed for reducing pollutants, which include Nitrogen Oxides (NOx), Carbon Monoxide (CO), Hydrocarbon (HC), and Particulate Matter (PM), to required levels. In spark ignitions (SI) engines, fuel and air can be pre-mixed stoichiometrically, therefore, not much PM is seen in exhaust air, while CO, HC, and NOx are major pollutants. However, in a compression ignition CI engine, due to heterogeneous fuel-air mixing, PM and NOx are major components in its pollutants, while CO and HC are relatively insignificant.

In CI engines, PM and NOx emissions have strong relations to peak combustion temperature. High peak combustion temperature decreases PM generation while increases NOx emission, and low peak combustion temperature affects emissions reversely. Consequently, in using EGR for adjusting peak combustion temperature, a tradeoff needs to be made between PM level and NOx emission. When both of PM and NOx need to be controlled, normally, two methods are used with an aftertreatment device. One is tuning NOx emission low, and using a high efficiency filter for removing PM. The other one is tuning PM level low, and using lean NOx removing technology, such as urea/ammonia Selective Catalytic Reduction (SCR), Lean NOx Trap (LNT)/NOx Absorber (NAC), and Lean NOx Catalyst (LNC), for controlling NOx emission. In the first method, since PM level is high, the filter needs to be regenerated periodically. The regeneration normally is achieved by heating up the filter to 400° C. to 600° C., and the heating energy is provided by burning fuel in an oxidation catalyst or a burner. Fuel penalty for filter regeneration depends on engine operating conditions and NOx emission level. When a low NOx emission level is required, e.g. according to US2010 standard, NOx emission cannot be over than 0.2 g/bhp·hour, fuel penalty could be a limiting factor for using the particulate filter method.

The other method needs to remove NOx from lean exhaust air. As oxygen, NOx is also an oxidant. Therefore, a selective environment must be created more favorably for reactions reducing NOx, since oxygen concentration is much higher than that of NOx. Among all technologies used in reducing NOx in lean exhaust air, SCR has the highest conversion efficiency, and thus is used broadly. However, a difficulty in developing selective catalyst is that there exist a tradeoff between conversion efficiency and selectivity. A catalyst with high selectivity normally has poor conversion efficiency. As a result, to have high selectivity, a device with a large volume is needed when high conversion efficiency is required.

Though SCR technology needs not dosing fuel, the hydrolysis of urea, which is used in generating ammonia for SCR reactions, is endothermic and needs extra energy. If this energy is provided by burning more fuel in engine, this fuel penalty could be 3% of total engine fuel consumption, depending on operating conditions. Additionally, urea is consumed in reducing NOx. The overall cost of urea consumption and fuel penalty for urea hydrolysis is comparable with cost of fuel penalty in using particulate filter. Combining the particulate filter method and lean NOx reducing method could achieve the best aftertreatment performance. However, the cost is system complexity and fuel penalty.

Different from that in CI engines, in SI engines, when air-fuel ratio is controlled at stoichiometric level, NOx could have a higher or comparable concentration as oxygen. As a result, even in an oxidation catalyst without selectivity, reductant is able to remove NOx from exhaust. This type of catalyst usually is called three-way catalyst, since it uses CO and HC as reductant in removing NOx, consequently, all three pollutants are removed from exhaust.

Compared to SI engines, the lean combustion nature of CI engines creates lean exhaust air, which causes the difficulties in using reductants in exhaust air to reduce NOx. Accordingly if the lean exhaust of a CI engine is converted to rich exhaust, an oxidation catalyst can be used to reduce NOx with reductants. It is a goal of the present invention to provide a means for reducing NOx and other pollutants in lean exhaust air by converting the lean exhaust air to rich exhaust air without significantly sacrificing fuel economy. Furthermore, it is a goal of the present invention to use solely fuel in exhaust air aftertreatment.

BRIEF SUMMARY OF THE INVENTION

In the present invention, a new technology of reducing exhaust pollutants in a CI engine is developed. In this technology, oxygen is firstly removed and then an oxidation catalyst is used for reducing NOx, CO, and HC from exhaust air.

Normally, due to the lean combustion nature, air-fuel ratio in CI engines cannot be stoichiometric. In one embodiment of this invention, oxygen left in exhaust air is removed by using a fuel reactor in which fuel injected during expansion (in-cylinder late injection) or provided by a dedicated doser reacts with oxygen. The fuel reactor act as an air-fuel ratio controller, which adjusts the lambda value of the exhaust air close to 1, thus an oxidation catalyst can be used for effectively reducing NOx, CO and HC. Compared to SI engines, CI engines have better fuel economy: usually CI engines are 30% or more efficient than SI engines. Therefore, it is not economic if the dosing fuel is just used for reducing pollutants from exhaust though comparatively there could also be around 6% fuel penalty or equivalent fuel penalty when using other types of aftertreatment devices such as LNT and SCR.

Heat generated in exhaust lambda control needs to be recovered. Both turbines and heat exchange devices can be used for energy recovery and the energy recovery efficiency determines overall fuel penalty. Ideally, if the energy recovery efficiency is higher than engine efficiency, there will be no fuel penalty in using the fuel reactor.

In another embodiment, oxygen in exhaust air is removed by using an oxygen sorption device, through which oxygen is separated from exhaust. The result rich exhaust air then passes through an oxidation catalyst where NOx, CO and HC are reduced. Once the oxygen sorption device reaches a saturation level, a regeneration process is triggered. During the regeneration, oxygen adsorbed and/or absorbed in the device is removed and the device is ready for the next sorption process. A wheel structure and/or a valve-controlled structure can be used for continuous operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an aftertreatment system with an oxygen-removing device;

FIG. 2 illustrates an embodiment of the aftertreatment depicted in FIG. 1 with a fuel reactor for exhaust lambda control and a turbine as an energy recovery device;

FIG. 3 shows the aftertreatment system of FIG. 2 further including a heat exchanger;

FIG. 4 depicts an aftertreatment with multiple fuel reactors and multiple turbines;

FIG. 5 is a block diagram of the aftertreatment system depicted in FIG. 2 further including a soot filter;

FIG. 6 illustrates another embodiment of the aftertreatment depicted in FIG. 1 with an oxygen sorption device;

FIG. 7 shows the oxygen sorption device of FIG. 6 with a wheel structure;

FIG. 8 shows an aftertreatment of FIG. 6 controlled by using valves.

DETAILED DESCRIPTION OF THE INVENTION

As depicted in FIG. 1, an engine system includes an engine 101, an oxygen-removing device 102 and an oxidation catalyst 103. The oxygen-removing device 102 is used for enriching exhaust air emitted by the engine 101. The result exhaust from the device 102 has a very low oxygen concentration. In the oxidation catalyst 103, NOx in the rich exhaust reacts with CO and HC, and thereby the pollutants are removed.

An embodiment of the oxygen-removing device is shown in FIG. 2. A fraction of the exhaust air from an engine 201 goes back to intake manifold trough an EGR system. The rest of the exhaust air goes into a fuel reactor 202. Therein HC fuel provided in in-cylinder late injection or through a fuel doser 203 reacts with oxygen in the lean exhaust emitted from the engine 201. The heated exhaust air then passes through a turbo-charger including a turbine 204, where heat energy in the exhaust air is partially recovered and used in compressing fresh air. The result exhaust air from the turbine 204 goes through a catalyst 205, where NOx reacts with CO and HC, and the treated air is emitted to ambient or to a soot filter (not shown in FIG. 2) for further removing PM. In the system, for better controlling the HC concentration in reducing NOx, or actively controlling the regeneration of the soot filter, an extra doser 206 could be installed between the turbine 204 and the catalyst 205. In addition to compressing fresh air, when the turbo-charger is replaced with a turbo-generator, in which the turbine 204 is used to drive an alternator, the recovered energy can be converted to electric energy. The turbo-generator is especially useful in a hybrid vehicle.

The reactor can also improve aftertreatment performance at cold-start. When engine starts, the exhaust pressure and temperature is not enough to effectively drive turbo-charger. As a result, large amount of PM could be generated. The reactor can be used for increasing the exhaust temperature and thus improves the transient performance of the turbo-charger and burns PM in exhaust air.

Heat released in the reactor increases exhaust temperature. Suppose the overall temperature gained by exhaust is T_g. When fueling rate in lambda control is small compared to exhaust mass flow,

T_g=m_fuel^·*LHV/(C_p*m_exh^·)  (1)

where m_fuel^· is the fuel injection mass flow rate in lambda control, LHV the low heat value of fuel, C_p the specific heat at constant pressure, and m_exh^· is the exhaust mass flow.

To control the exhaust lambda value at 1, the fueling rate can be calculated using the following equation:

$\begin{array}{cc}\stackrel{.}{m_{\mathrm{fuel}}}=\left(\frac{\stackrel{.}{m_{\mathrm{exh}}}}{AF_0}\right)*\left(1-\frac{1}{{\lambda }_1}\right)& \left(2\right)\end{array}$

where λ₁ is the engine out lambda value, and AF₀ is the stoichiometric air fuel ratio.

Based on equation (1) and (2), the exhaust temperature increase across the reactor is

$\begin{array}{cc}{T}_g=\left(1-\frac{1}{{\lambda }_1}\right)*\mathrm{LHV}/\left(C_p*{\mathrm{AF}}_0\right)& \left(3\right)\end{array}$

According to the equation (3), the temperature gained by the exhaust air is determined by the lambda value of engine out exhaust air. When the air fuel ratio is high, a very high temperature can be generated. Consequently, the lambda value of engine out exhaust air needs to be carefully controlled, otherwise, a complex and expensive reactor and turbo that can work at very high temperature are needed. In addition to tuning EGR fraction, a heat exchanger or multi-stage turbine can be used for lowering the temperature at turbine inlet.

As depicted in FIG. 3, a heat exchanger can be used in between the fuel reactor 202 and the turbine 204 for decreasing the temperature of the exhaust air passing through it. A heat pump (not shown in the figure) can be used with the heat exchanger for recovering the heat energy.

Another method for lowering the turbine inlet exhaust temperature is using multi-stage turbines. As shown in FIG. 4, a second stage turbine 403 is positioned at the downstream of the turbine 204. A fuel reactor 402 is used in between these two turbines for further lambda control. A doser 401 can be used for flexibly controlling the temperature of exhaust air passing through the turbine 403. By using the second turbine 403, the exhaust temperature at upstream of the turbine 204 can be decreased by controlling lambda value higher than 1.0. The exhaust temperature at downstream of the turbine 204 is lowered since much of the heat energy is converted back to mechanical energy therein. In the fuel reactor 402, the exhaust temperature is increased again and the lambda value is further lowered. The result exhaust air passes through the turbine 403 for energy recovering. More turbines can be used for flexibly distributing heat generated in lambda control, if engine back pressure, cost and recover efficiency allow.

The exhaust air with lambda controlled at stoichiometric level flows into an oxidation catalyst, where HC and CO in the exhaust react with NOx and generate N₂, CO₂, and H₂O. To remove PM in the exhaust air, referring to FIG. 5, a soot filter 502 is installed in between the turbine 204 and a catalyst 503. A catalyzed soot filter (CSF) can be more efficient in removing PM, and other pollutants.

Normally the soot filter 502 needs to be regenerated after a period of time. During regeneration, the exhaust lambda value at the inlet of the soot filter 502 cannot be controlled below 1.0, otherwise, soot in the filter is not able to be effectively removed, since oxygen in the exhaust is not enough for soot oxidation. A doser 501 can be used for further controlling lambda during filter regeneration, in which the fueling injected from the doser 501 reacts with the oxygen left in the regeneration in the front area of the catalyst 503 for lowering lambda to stoichiometric level.

Through turbines, heat energy is recovered into mechanical energy or electric energy. When the energy recovery efficiency is η_r, we can define the fuel penalty r_p as the ratio of the net fuel loss in lambda control and the overall fueling, i.e.:

$\begin{array}{cc}{r}_p=\stackrel{.}{m_{\mathrm{fuel}}}*\frac{{\eta }_e-{\eta }_r}{\stackrel{.}{m_{\mathrm{fuel}}}+\stackrel{.}{m_{\mathrm{fuel\_e}}}}& \left(4\right)\end{array}$

where m_fuel—e^· is the fueling mass flow rate in engine control and η_e is the engine energy efficiency. According to equations (2) and (4), the fuel penalty can be calculated in using the following equation:

$\begin{array}{cc}{r}_p=\left(1-\frac{1}{{\lambda }_1}\right)\left({\eta }_e-{\eta }_r\right)& \left(5\right)\end{array}$

The equation (5) shows that the fuel penalty actually is determined by the engine out exhaust lambda value and the difference between the energy recovery efficiency and the engine efficiency. As an example, if λ₁=1.4, then to have a fuel penalty of 5%, which is normally the value of an RPF system, assuming engine energy efficiency is 40%, the required energy recovery efficiency will be only 22.5%. If a turbine system has an energy recovery efficiency higher than 40%, there will be no fuel penalty.

In another embodiment of the present patent, referring to FIG. 6, an oxygen sorption device 602 is connected to a turbo-charger 601. Exhaust air flows through the device 602, where oxygen in the exhaust flow is absorbed and/or adsorbed and thereby, lambda is controlled to stoichiometric level. The result exhaust air then flows into a catalyst 603, therein NOx reacts with the HC and CO in the exhaust and then is reduced. Hydrocarbon level in the exhaust can be controlled by either using in-cylinder late injection, or using an external doser 605. The clear rich exhaust from the catalyst 603 is emitted to ambient, and a fraction of this exhaust is fed back to the oxygen sorption device 602 for device regeneration. To decrease the energy consumed in regeneration, a valve 604 is used for controlling airflow.

The structure of an embodiment of the oxygen sorption device 602 is depicted in FIG. 7. This device includes a rotating apparatus 701 driven by actuator 702, a working area 703 and a regeneration area 704 both having oxygen sorption materials. Firstly the working area 703 in the device 602 is in the exhaust stream absorbing and/or absorbing oxygen from exhaust air, and thus the lambda is controlled at stoichiometric level. When the oxygen sorption material in the working area 703 reaches its saturation level, the actuator 702 is energized and drives the rotating apparatus 701 moving the working area 703 to the position of the regeneration area 704 and turning the regenerated area 704 into the exhaust stream for oxygen sorption. The oxygen sorption material in the regeneration area (previous working area) is then regenerated in the rich air fed back from the outlet of the catalyst 603 (the rich air flow rate is controlled by the valve 604). The process repeats for continuous oxygen level control.

A variety of materials can be used for absorbing and/or adsorbing oxygen. Among them, perovskite-related oxides has a good oxygen sorption capacity at temperature range of 200° C. to 400° C., and can be regenerated at temperature at 600° C. [Kusaba, H., Sakai, G., Shimanoe, K., Miura, N., Yamazoe, N., Solid State Ionics, 152-153 (2002)689-694]. Extra energy is needed in regenerating the oxygen absorption material and in rotating the device. This part of energy contributes to the overall fuel penalty for exhaust aftertreatment.

In addition to the rotating device, a valve-controlled system can also be used for removing oxygen in exhaust air. In such a system, as depicted in FIG. 8, two oxygen sorption devices: devices 802 and 804 are used together with two control valves 801 and 803 for oxygen level control. At beginning, the control valve 801 is off and the control valve 803 is on. Exhaust flow from the turbocharger 601 passes through the device 804 and has oxygen removed therein. The result exhaust then goes into the catalyst 603 and NOx is reduced by HC and CO. An HC doser 805, which in FIG. 8 is positioned in between the oxygen removing devices and the catalyst, can be used for flexibly controlling the reactions. When the device 804 is saturated, the control valve 803 is shut off and the control valve 801 is turned on. The device 802 is then used for passing exhaust air through and the device 804 is regenerated for next cycle. The two oxygen control devices work alternatively in continuous oxygen level control.

For better removing NOx, referring to FIG. 1, the catalyst 103 may include an LNT. In this system, when the lambda is not controlled at stoichiometric level during some transient operations, the LNT then is able to remove NOx in exhaust air. When lambda is back to stoichiometric level, the LNT is regenerated by dosing with HC.

One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments which are presented in this description for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow. It is noted that equivalents for the particular embodiments discussed in this description may practice the invention as well.

Claims

1. An emission control apparatus for an engine comprising:

at least one combustion device for converting lean exhaust air emitted from said engine into rich exhaust air;

at least one energy conversion device for recovering heat energy generated in said combustion devices;

a catalyst in which NOx in said rich exhaust air is reduced through reactions with hydrocarbon and carbon monoxide;

2. The apparatus of claim 1, wherein said combustion device is a fuel burner that includes a combustion chamber and an ignition means.

3. The apparatus of claim 1, wherein said combustion device is an oxidation catalyst.

4. The apparatus of claim 1, wherein said energy recovery device includes a turbine.

5. The apparatus of claim 1, further comprises at least one external hydrocarbon doser for controlling hydrocarbon concentration in exhaust air.

6. The apparatus of claim 1, further comprises at least one heat exchanger.

7. The apparatus of claim 1, wherein said catalyst further includes lean NOx trap.

8. An emission control apparatus for an engine comprising:

at least one oxygen sorption device for converting lean exhaust air emitted from said engine into rich exhaust air;

a catalyst in which NOx in said rich exhaust air is reduced through reactions with hydrocarbon and carbon monoxide;

9. The apparatus of claim 8, further comprises at least one external hydrocarbon doser for controlling hydrocarbon concentration in exhaust air.

10. The apparatus of claim 8, wherein said oxygen sorption device includes at least two sections. Oxygen sorption materials in these sections work sequentially in removing oxygen in exhaust air.

11. The apparatus of claim 10, wherein said sections include a working section and a regeneration section. Oxygen sorption material in the regeneration section uses rich exhaust air generated from the working section.

12. The apparatus of claim 8, wherein perovskite-related oxides are included as an oxygen sorption material in said oxygen sorption device.

13. The apparatus of claim 8, wherein said catalyst further includes lean NOx trap.

14. A method of controlling exhaust emission of an engine, wherein the control system has an oxygen concentration control device, a catalyst facilitating reactions of NOx with HC and CO, and a dosing means providing HC, comprising:

converting lean exhaust air into rich exhaust air by using said oxygen concentration control device;

controlling exhaust air temperature to a level appropriate for the reactions of NOx with HC and CO in said catalyst;

controlling HC level in exhaust air by using said dosing means to a level appropriate for reducing NOx in exhaust air.

15. The method of claim 14, wherein said control system further includes a heat energy recovery device.

16. The method of claim 15, further includes recovering the heat energy generated in converting lean exhaust air into rich exhaust air through said energy recovery device;

17. The method of claim 14, further includes controlling HC level in exhaust air to a level appropriate for using said catalyst to remove the oxygen left in the process converting lean exhaust air to rich exhaust air using said oxygen concentration control device.

18. The method of claim 17, further includes controlling exhaust air temperature to a level appropriate for reactions between oxygen and HC.

19. The method of claim 14, wherein said oxygen concentration control device includes at least two sections.

20. The method of claim 19, further includes controlling said sections working sequentially.