a illustrates an after-treatment system according to the present invention including a DOC, a DPF, an external doser, and a heat exchange device;
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;
c shows a heat exchange device with exhaust air bypassing;
a shows a SCR device installed at the very end of an after-treatment system according to the present invention;
b shows a LNT device installed at the very end of an after-treatment system according to the present invention;
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;
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;
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.
As depicted in
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
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
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.
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.
At steady status, with a given exhaust mass flow rate mexh• and DOC inlet exhaust temperature T1, the hydrocarbon mass flow rate mfuel• needed for exhaust air to reach a target temperature Tt can be estimated using the following equation:
m
fuel
•=(Tt−T1)mexh•Cp/(LHVη) (1)
where LHV is the low heat value of the dosing fuel; Cp is 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 Tt, 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 SHC• at DOC outlet is given by
S
HC
•
=m
fuel
•(1−η)=(mexh•Cp/(LHV)[(Tt−T1)(1−η)/η] (2)
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.
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.
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.
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.
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
If an electrical heating means is used, as illustrated in
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
When an LNT (Lean NOx Trap) 600 is connected to the exhaust pipe 113 (
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 (
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.
Number | Date | Country | |
---|---|---|---|
60850459 | Oct 2006 | US |