US20080083215A1

US20080083215A1 - Standalone thermal energy recycling device for engine after-treatment systems

Info

Publication number: US20080083215A1
Application number: US11/973,457
Authority: US
Inventors: Mi Yan
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-10-10
Filing date: 2007-10-09
Publication date: 2008-04-10

Abstract

A standalone thermal energy recycling device in which exhaust air emitted from an internal combustion engine exchanges heat with that being heated during regeneration of after-treatment devices. By using the thermal energy recycling device, heat generated for regeneration is used to compensate energy loss during heat exchange rather than provide the overall energy needed for boosting exhaust flow temperature to target. Inside the thermal energy recycling device, heat exchanger is bypassed during normal engine operations for decreasing back pressure.

Description

This present application claims priority from U.S. provisional application No. 60/850,459 having the title of Engine Aftertreatment System with Thermal Energy Recycling and filed on Oct. 10, 2006.

FIELD OF THE INVENTION

The present invention relates to devices for effectively boosting and controlling temperature of exhaust gases emitted from an internal combustion engine to facilitate regeneration of after-treatment systems.

BACKGROUND OF THE INVENTION

Reducing oxides of nitrogen (NOx), carbon monoxide (CO), hyrdrocarbon (HC), and particulate matter (PM) in exhaust air from internal combustion engines is required in emission control. DOCs (Diesel Oxidation Catalyst) and three-way catalysts have been broadly used for reducing CO, HC, and NOx, while SCR (Selective Catalytic Reduction), LNT (Lean NOx Trap, a.k.a. NOx adsorber), LNC (Lean NOx Catalyst), and EGR (Exhaust Gas Recirculation) technology are used for achieving low NOx emissions. The PM in exhaust air usually is removed by using a filter, e.g. DPF (Diesel Particulate Filter).
PM filters and LNTs need to be regenerated periodically. For PM filters, especially DPFs, since normal engine out temperature is not high enough to oxidize soot automatically, a number of known methods are used for the regeneration of particulate filters. In addition to passive regeneration methods at low exhaust temperature (˜300° C.), such as using fuel additives and using NOx generated in the engine, the most common one is thermal regeneration, i.e., heating the exhaust gas to a temperature higher than 500° C. (usually 500˜600° C.) and then converting soot matter into gaseous products such as CO₂and H₂O. In LNTs, injecting fuel (HC reductant) into rich exhaust is used for reducing metal nitrate into metal oxide, and heating the exhaust gas to a high temperature (normally higher than 650° C.) together with rich fuel dosing for reducing metal sulfate.
Electrically resistive heating devices, fuel burners, and oxidation catalyst converters have been used for generating heat in thermal regeneration. However, due to high exhaust flow rate, most of regeneration energy is actually used for increasing enthalpy of exhaust air instead of providing enough temperature for soot to be oxidized or for reducing metal sulfate. It usually needs 10 g to 40 g fuel to burn off 1 g soot and the regeneration power usually is 15 kw to 80 kw depending on exhaust flow rate and temperature.
Due to the large power needed in regeneration, resistive heating devices normally cannot be directly used for mobile applications without exhaust flow control, while fuel burners are complex and it is hard to control the regeneration temperature. DOCs are simple and can generate enough heat for regeneration. However, a minimum light-off temperature is required for oxidation reaction. As a result, it is difficult to use DOCs for engines with low exhaust temperature, e.g. engines with two-stage turbochargers. And in transient applications, e.g. vehicles stop and go frequently, when turbo outlet temperature frequently drops below light-off temperature, regeneration cannot be effectively performed.
It is a primary object of the present invention to reuse the energy for boosting exhaust temperature in regeneration. With the reuse of regeneration energy, in addition to better fuel economy, the power needed for regeneration is decreased, and more heating devices can be used to facilitate temperature control.
Another object of the present invention is to facilitate the temperature control for after-treatment systems with DOCs and enable the systems work with low turbo outlet temperature and/or under transient duty cycles.
Yet another object of the present invention is to develop a standalone device that is independent to the after-treatment system and thus can be used with different types of after-treatment systems.
Yet another object of the present invention is to develop a device that can be bypassed during normal operations when the after-treatment system is not in regeneration, therefore, the device has minimum effects on engine backpressure.
Yet another object of the present invention is to decrease after-treatment system cost and elongate system lifetime.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a thermal energy recycling device in an internal combustion engine after-treatment system. In this device, high temperature exhaust air heated for regenerating after-treatment devices, such as DPFs or LNTs, is cycled back and exchanges heat with low temperature exhaust air at inlet. After heat exchange, the heated exhaust air then enters after-treatment system for regeneration, and the cooled exhaust air is emitted into ambient or flows into other after-treatment devices for further treatment.
In an exemplary embodiment of the present invention, the after-treatment system comprises a DOC, a DPF, and a heat exchange device with two ports connected with the outlet of a turbocharger and the inlet of the DOC, and another two connected to the DPF outlet and an exhaust pipe. During regeneration, the exhaust temperature is boosted by oxidizing fuel in the DOC. The high temperatures exhaust air then enter DPF for regeneration. From the downstream of the DPF, the high temperature exhaust air flows back to the heat exchange device where it loses heat to the low temperature exhaust air entering the device from the turbo outlet. The heated exhaust air then flows into the DOC where the new dosing fuel is injected for compensating the heat loss in DOC, DPF, and the heat exchange device. Heat exchange is only used for facilitating after-treatment system regeneration. During normal operations, to decrease engine back pressure, the heat exchange device can be controlled bypassing exhaust air to the after-treatment system.
In accordance to an advantageous feature of the invention, good fuel economy is achieved by burning less fuel in the DOC due to the exhaust flow at DOC inlet has a higher temperature.
In accordance with another advantageous feature of the invention, DOC face plugging risks are mitigated. When exhaust air temperature at DOC inlet is low and dosing time is long, dosing fuel could mix with soot and form a layer at DOC front face blocking exhaust air from passing through. This is called face plugging. High DOC inlet air temperature gained from high DPF outlet temperature reduces the risk of face plugging.
In accordance with another advantageous feature of the invention, DOC size can be decreased, since less dosing and higher DOC inlet temperature enables the after-treatment system use a DOC with lower HC conversion efficiency.
In accordance with another advantageous feature of the invention, the after-treatment system can work with a low exhaust temperature and is insensitive to the variation of exhaust temperature. Once the DOC is able to generate enough heat to meet the target temperature, regeneration is un-interrupted as long as the energy released by burning dosing fuel can compensate the heat loss in DOC, DPF, and the heat exchange device. The heat loss is determined by DOC and DPF size, insulation, and heat exchange efficiency and is not much affected by the turbo outlet exhaust temperature. This feature is especially useful for engines with low exhaust temperature (e.g. engines with two-stage turbocharger). With this feature, regeneration can be started by momentarily creating an exhaust flow with temperature higher than DOC light-off temperature (e.g. by adjusting turbo and EGR, or using an electric heater). Once the DOC is able to generate enough heat to sustain target temperature, the engine can run at its normal mode with low exhaust temperature.
In accordance with another advantageous feature of the invention, due to a positive feedback, the temperature rising time is shortened during dosing. This feature facilitates temperature control.
In accordance with another advantageous feature of the invention, when an external doser is used for injecting fuel, less dosing fuel elongate doser lifetime. Furthermore, since the heat exchange device is a standalone device, the external doser is able to be placed in between the heat exchange device and the DOC. As a result, dosing fuel needs not pass the heat exchange device causing plugging issues and dosing fuel impingement issue is mitigated due to higher exhaust temperature.
In accordance with another advantageous feature of the invention, when regeneration starts, the heat exchange device switches from bypassing mode to heat exchange mode, then a higher engine back pressure is induced. This higher engine back pressure increases engine out exhaust temperature, and thereby facilities regeneration.
In accordance with a further advantageous feature of the invention, safer low exhaust air is emitted to ambient during regeneration due to heat exchange.
In another embodiment of the present invention, the after-treatment system comprises a DOC, a DPF, a SCR (or an LNT), and a heat exchange device with two ports connected between the outlet of a turbocharger and the inlet of the DOC, and another two connected to the DPF outlet and the SCR (or the LNT) inlet. The SCR (or the LNT) is at the very end of the after-treatment system.
In accordance with an advantageous feature of the invention, the heat exchange device intrinsically protects the SCR (or the LNT) from being damaged by thermal runaways in the DPF.
Yet in another embodiment of the present invention, the after-treatment system comprises a DOC, a DPF, an LNT, and a heat exchange device, in which high temperature exhaust air emit from the LNT during desulfation exchanges heat with low temperature exhaust air. In accordance with an advantageous feature of the invention, during desulfation, the temperature dropping is minimized in rich cycle due to the energy exchange between the exchange device and exhaust air, and temperature rising time is shortened in lean cycle. Thereby, desulfation process is more effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates an after-treatment system according to the present invention including a DOC, a DPF, an external doser, and a heat exchange device;

FIG. 1 b illustrates an after-treatment system according to the present invention including a DOC, a DPF, an external doser, and a heat-pump like heat exchange device;

FIG. 1 c shows a heat exchange device with exhaust air bypassing;

FIG. 2 shows an after-treatment system with in-cylinder dosing (post injection) according to the present invention including a DOC, a DPF, and a heat exchange device;

FIG. 3 depicts an after-treatment system according to the present invention including a fuel burner, a DPF, and a heat exchange device;

FIG. 4 shows an after-treatment system according to the present invention including a control valve, a blower, a resistive heating device, a DPF, and a heat exchange device;

FIG. 5 a shows a SCR device installed at the very end of an after-treatment system according to the present invention;

FIG. 5 b shows a LNT device installed at the very end of an after-treatment system according to the present invention;

FIG. 6 a illustrates an after-treatment system according to the present invention including a DPF, an LNT, a DOC, and a heat exchange device; the DOC and DPF connect to the heat exchange device;

FIG. 6 b shows an after-treatment system according to the present invention including a DOC, an LNT, a DPF, and a heat exchange device; the DOC, DPF and LNT all connect to the heat exchange device, and the LNT is in front of the DOC;

FIG. 6 c shows an after-treatment system according to the present invention including a DOC, an LNT, a DPF, and a heat exchange device; the DOC, DPF and LNT all connect to the heat exchange device, and the DOC is in front of the LNT.

DETAILED DESCRIPTION OF THE INVENTION

As depicted in FIG. 1, in an after-treatment system, a standalone heat exchange device 103 is connected with a turbocharger 101 through a pipe 102. Another pipe 104 is used to connect the heat exchange device 103 to a DOC 108, in which the HC, CO, and NOx are oxidized. Particulate matter (PM) generated by the engine is trapped in a DPF 110. A pipe 105 connects the DPF 110 back to the heat exchange device 103, and a pipe 113 conducts exhaust air off the after-treatment system.
When certain amount of PM deposits in the DPF 110, a regeneration process is triggered, and HC injected from a doser 106 is oxidized in the DOC 108 to provide heat for burning off PM in the DPF 110. During the regeneration process, the DOC inlet temperature is measured by a thermistor 107, while the DOC outlet temperature is monitored by using a thermistor 109. To effectively monitor the DOC conversion efficiency and detect thermal runaways inside the DPF, a thermistor 1111 is connected to the outlet of the DPF 110. A relative pressure (deltaP) sensor 112 is used to monitor the DPF pressure drop, which changes with the PM amount in the DPF. In normal after-treatment systems, during regeneration, the high temperature exhaust air from DPF is emitted into ambient air directly, and most of the energy released by oxidizing HC in DOC is wasted. In the present invention, however, the exhaust air from the DPF 110 is conducted back to the heat exchange device 103, in which the high temperature air exchanges thermal energy with the low temperature exhaust air emitted from the turbo 101. The heated exhaust air then goes into the DOC 108, therein its temperature is further boosted for regeneration. In the DOC 108, the amount of energy released by oxidizing fuel is used to compensate the energy loss during the heat exchange, and the heat dissipated in the DOC 109 and the DPF 110, rather than provide the overall energy needed for heating low temperature exhaust air from engine to the target temperature. A variety of heat exchangers, for example, shell and tube heat exchangers, plate heat exchangers, and enthalpy wheels, can be used for the heat exchange device 103. Additionally, a heat-pump like heat exchange device can also be used to deliver the heat generated in regeneration to DOC inlet. As depicted in FIG. 1 b, this device 150 includes a pump 151, a coil 152 at DOC inlet, a coil 153 at DPF outlet, and a connection tube 154 with insulation 155. During regeneration, high temperature exhaust air at DPF outlet heats up the fluid in the coil 153. Then driven by the pump 151, the heated fluid moves into the coil 152 and exchange heat there with low temperature exhaust air.
The heat exchange device 103 may cause higher engine back pressure, especially when heat exchange efficiency is high. To decrease engine back pressure during normal operations, as shown in FIG. 1 c, inside the heat exchange device 103, two control valves 161 or 162 can be used to bypass a heat exchanger 160. When the valves 161 and 162 are on, exhaust air passes through connection pipes 163 and 164 rather than the heat exchanger 160. Thereby, the engine back pressure is decreased.

Better Fuel Economy

The after-treatment system presented in this invention facilities regeneration. Firstly, due to heat exchange, the dosing fuel is used to compensate the energy loss in DOC, DPF, and heat exchange process, rather than provide the overall energy for sustaining regeneration temperature. Therefore, fuel economy is improved.

Less DOC Face Plugging

When exhaust air temperature at DOC inlet is low and dosing time is long, dosing fuel could mix with soot and form a layer at DOC front face blocking exhaust air from passing through. This is called face plugging. High DOC inlet air temperature gained from the high DPF outlet temperature reduces the risk of face plugging.

Smaller DOC Size and Less Hydrocarbon Breakthrough

At steady status, with a given exhaust mass flow rate m_exh ^• and DOC inlet exhaust temperature T₁, the hydrocarbon mass flow rate m_fuel ^• needed for exhaust air to reach a target temperature T_tcan be estimated using the following equation:
m _fuel ^•=(T _t −T ₁)m _exh ^• C _p/(LHVη) (1)
where LHV is the low heat value of the dosing fuel; C_pis the average heat capacity at constant pressure, and η is the HC conversion efficiency of the DOC.
According to the equation (1), to obtain the same target DOC outlet temperature T_t, with a higher DOC inlet temperature T₁, a lower DOC efficiency and thus a smaller size DOC is allowed. In addition, based on the equation (1), the hydrocarbon breakthrough rate S_HC ^• at DOC outlet is given by
S _HC ^• =m _fuel ^•(1−η)=(m _exh ^• C _p/(LHV)[(T _t −T ₁)(1−η)/η] (2)

This equation (2) shows that a higher DOC inlet temperature T₁reduces the hydrocarbon breakthrough at a given conversion efficiency.

Shorter Temperature Response Time

When HC is oxidized in a DOC, since reactions take place at catalyst surface, DOC base and catalyst absorb released energy. As a result, when dosing starts, exhaust air temperature cannot increase immediately. There exists a time lag between fuel dosing and exhaust temperature change. For a given DOC, this time lag is a function of exhaust flow rate. When exhaust flow rate is low, it takes longer time for exhaust temperature to reach target.
With the heat exchange device, a positive feedback is established when dosing starts: exhaust air is heated in DOC and the hot air through DPF then exchanges heat with the low temperature exhaust air in the heat exchange device. The higher temperature DOC inlet exhaust air is then further heated in DOC. This positive feedback shortens the exhaust air temperature rising time, and thus facilitates temperature control.

Insensitive to the Variation of DOC Inlet Temperature

Under some operating conditions, for example, running at low torque, diesel engine generates less exhaust heat, and exhaust air temperature is low. When exhaust air temperature in DOC is lower than catalyst light-off temperature, fuel dosing has to be disabled, otherwise, un-burnt fuel could cause DOC face plugging and hydrocarbon breakthrough. Dosing can only be started when DOC temperature is higher than catalyst light-off temperature. However, limited to heat exchange rate, there is a time lag between fuel dosing and exhaust air temperature change. If DOC inlet exhaust temperature drops below light-off temperature frequently, regeneration cannot be effectively performed.
With the heat exchange device, heated by the exhaust air fed back from the DPF, the DOC inlet temperature can still sustains higher than catalyst light-off temperature even when turbo outlet temperature is low. The high DOC inlet temperature then allows continuous dosing and, therefore, the DOC outlet temperature and DPF outlet temperature are high. Regeneration is un-interrupted as long as the energy released by burning dosing fuel is able to compensate the heat loss in DOC, DPF, and the heat exchange device. This feature is especially useful for engines with low exhaust temperature (e.g. engines with a two-stage turbocharger). With this feature, regeneration can be started by momentarily creating an exhaust flow with temperature higher than catalyst light-off temperature (e.g. by adjusting turbo and EGR, or using an electric heater). Once the DOC is able to generate enough heat to sustain target temperature, the engine can run at its normal mode with low exhaust temperature.

Longer Doser Life

When an external low-pressure doser is used, normally the injector of the doser exposes to exhaust air. After dosing, the fuel that remains in the injector and on the injector surface could be coked and then block the orifice of the injector. As a result, with the same duty cycle, less and less fuel will be injected by the doser at normal fuel pressure. When the maximum achievable dosing rate is lower than that required for reaching regeneration target temperature, the doser needs to be replaced, otherwise the filter cannot be effectively regenerated, and the system may fail.
With the heat exchange device, due to the high DOC inlet temperature, less fueling is needed for reaching regeneration target temperature. Therefore, even the doser deteriorates, as long as it can provide enough dosing fuel to compensate the heat loss in DOC, DPF, and during heat exchange, the system is able to sustain the target temperature for regeneration. Less dosing rate requirement elongates doser life time.

Less Impingement Induced Dosing Fuel Condensation

Limited to the size of the connection pipe at which a doser is installed, normally, when dosing fuel is sprayed out of the doser, some of the fuel droplets may hit the inner wall of the connection pipe. If the inner wall temperature is low, these fuel droplets may condense at the wall surface causing DOC face plugging and exhaust air temperature control issues (e.g., when fuel evaporates with higher temperature exhaust flow, this extra fuel flows into DOC causing exhaust temperature out of control).
The standalone heat exchange device allows doser be installed in between it and the DOC. High temperature exhaust flow emitted from the heat exchange device keeps connection pipe inner wall from being cooled down by ambient temperature. As a result, dosing fuel condensation is mitigated.

Safer Exhaust

High temperature exhaust air during regeneration could cause fire hazard if the regeneration is performed in an inappropriate place, e.g. the exhaust pipe is close to combustible matter. With the heat exchange device, temperature of the exhaust air off the after-treatment system is lowered during heat exchange, and thus less effort is needed to lower exhaust temperature during regeneration.
In addition to external dosing depicted in FIG. 1, the after-treatment system in the present invention can also use in-cylinder dosing, which is achieved by injecting fuel into some cylinders of an engine during expansion stage. As illustrated in FIG. 2, an engine 201 is connected to an after-treatment system similar to the one shown in FIG. 1 a exempt no external doser 106 is included. In this after-treatment system, energy for DPF regeneration is provided by oxidizing unburnt HC from the engine.
FIG. 3 shows an after-treatment system using a fuel burner 301 instead of the DOC 108 for DPF regeneration. In this system, the burner includes a blower 310, a fuel pump 311, and a glow plug 312. The blower 310 provides airflow for burning the fuel injected with the pump 311. The hot air generated in the burner 301 mixes with exhaust flow and the result air flow enters the DPF 110 for regeneration. Exhaust air emitted from the DPF 110 flows through the heat exchange device 103, where low temperature exhaust flow is heated. Compared to an after-treatment system without the heat exchange device 103, in the present system, the fuel burner 301 provides less energy in boosting the exhaust flow temperature to regeneration target temperature.
If an electrical heating means is used, as illustrated in FIG. 4, the energy for regeneration can also be recycled. In this system, an electrical heater 401 is used instead of a DOC 108 for generating hot air. A valve control system 412 is used for controlling exhaust flow during regeneration. During a stationary regeneration, when the engine is not capable to generate exhaust flow, a blower 410 provides airflow through a passage 411. The heat exchange realized in the device 103 lowered the need for electrical power.
If a SCR (Selected Catalyst Reduction) device is connected to the outlet of a DPF, a tighter temperature control is needed, since high temperature generated in a thermal runaway, which is caused by uncontrolled burning of large amount of soot accumulated in the filter, could damage the catalyst in SCR. When the energy exchange device is used, as depicted in FIG. 5 a, where a SCR 500 is connected to the exhaust pipe 113, exhaust temperature at SCR inlet is decreased during heat exchange, and thus the system intrinsically protects the SCR 500 from being damaged. If DPF regeneration is well controlled or thermal runaway is not an issue, the SCR can also be placed in somewhere between the DOC 108 and the heat exchange device 103. In such a system, with fuel dosing, SCR inlet temperature can be controlled in an appropriate range for deNOx reaction at small energy cost.
When an LNT (Lean NOx Trap) 600 is connected to the exhaust pipe 113 (FIG. 5 b), as in the SCR after-treatment system shown in FIG. 5 a, the heat exchange device could protect the LNT from being damaged by thermal runaways in the DPF. If the LNT 600 is connected directly to the DPF 110 as illustrated in FIG. 6 a, however, the heat exchange device is not able to mitigate the effects of thermal runaways in DPF though all other benefits mentioned above can still be provided. In the configurations illustrated in FIG. 6 b and FIG. 6 c, since the DPF 110 is at the very end of the after-treatment system, thermal runaways in the DPF won't affect the performance of the LNT.
In a system with LNT, in addition to filter regeneration, a desulfation process is needed to decompose the sulfate formed due to sulfur in fuel. Usually, high bed temperature (normally higher than 650° C.) and a rich exhaust are needed for desulfation. However, in rich exhaust, due to low oxygen concentration, hydrocarbon cannot be effectively oxidized in catalyst. As a result, the LNT bed temperature drops and desulfation efficiency decreases. When LNT bed temperature is lower than required desulfation temperature, a lean exhaust is needed to increase the exhaust temperature. With the heat exchange device 103 (FIG. 5 a, FIG. 6 a-6 c), the temperature drop is decreased in energy exchange, and temperature rising time is shortened during lean exhaust time. Thereby, the rich exhaust time can be elongated and desulfation process is more effective.

Claims

1. An internal combustion engine after-treatment system comprising:

a filter that traps PM (Particulate Matter) in exhaust air;

a heat generation device that generates heat for the regeneration of said filter;

an energy exchange device in which exhaust air emitted from said internal combustion engine exchanges heat with that heated with said heat generation device;

said filter and said heat generation device are outside said energy exchange device.

2. A system as in claim 1, wherein said heat generation device includes a DOC (Diesel Oxidation Catalyst) device;

3. A system as in claim 2, wherein said heat generation device includes an external doser for fuel injection;

4. A system as in claim 2, wherein said external doser is placed in between said heat exchange device and said DOC;

5. A system as in claim 2, wherein said heat generation device uses in-cylinder post fuel injection;

6. A system as in claim 1, wherein said heat generation device includes a fuel burner;

7. A system as in claim 6, wherein said fuel burner includes a fuel pump and an ignition device;

8. A system as in claim 1, wherein said heat generation device includes an electrical resistive heating device;

9. A system as in claim 1, wherein said energy exchange device includes a heat exchanger through which two air flows exchange heat;

10. A system as in claim 9, wherein said energy exchange device includes an exhaust air bypass apparatus that is controlled to bypass said heat exchanger;

11. A system as in claim 1, wherein said after-treatment system further includes a SCR (Selective Catalytic Reduction) device;

12. A system as in claim 1, wherein said after-treatment system further includes an LNT (Lean NOx Trap) device;

13. A system as in claim 1, wherein said after-treatment system further includes an LNC (Lean NOx Catalyst) device;

14. An internal combustion engine after-treatment system comprising:

an LNT device;

a heat generation device that generates heat for the regeneration of said LNT device;

15. A system as in claim 14, wherein said heat generation device includes a DOC;

16. A system as in claim 15, wherein said heat generation device includes an external doser for fuel injection;

17. A system as in claim 16, wherein said external doser is placed in between said heat exchange device and said DOC;

18. A system as in claim 14, wherein said energy exchange device includes a heat exchanger through which two air flows exchange heat;

19. A system as in claim 18, wherein said energy exchange device includes an exhaust air bypass apparatus that is controlled to bypass said heat exchanger;

20. A system as in claim 14, wherein said after-treatment system further includes a DPF;