Patent Application
20120073268
The subject invention relates to a vehicle exhaust system, and more specifically to a system with a fuel-fired burner to enable NO2 based regeneration of an exhaust system component such as a diesel particulate filter, for example.
Exhaust systems are widely known and used with combustion engines. Some exhaust systems utilize a fuel-fired burner that can be a full range or partial range burner. An active full range burner unit enables regeneration of a diesel particulate filter (DPF) as well as providing exhaust thermal management under various operating conditions. A partial range burner elevates the exhaust temperature of exhaust gas to assist with regeneration of the DPF.
Passive regeneration, i.e. NO2 based regeneration, is advantageous due to the lack of a large exotherm as well as for not incurring a high fuel penalty. For non-EGR (exhaust gas recirculation) engines, such as off road engines having less than 75 horsepower for example, sufficient NO2 is available such that only a passive system would be required for regeneration. However, for most vehicle applications the exhaust gas temperature does not consistently stay above 300 degrees Celsius, which is required to support a system that only uses passive regeneration.
In one example, the partial range burner heats the exhaust gases when possible, or when required, to enable passive regeneration of the DPF. One control strategy activates the burner every time exhaust gas temperatures fall below 300 degrees Celsius. This control strategy is disadvantageous from a fuel conservation perspective. Further, NO2 formation in a DOC to support DPF regeneration can only occur if DOC temperatures exceed approximately 250 degrees Celsius, hydrocarbon (HC) and carbon monoxide (CO) concentrations levels are limited, and NOx levels are sufficient.
A control strategy for a fuel-fired burner considers the various aforementioned factors to provide NO2 based regeneration in a fuel efficient manner.
In one example, the vehicle exhaust system includes a partial range fuel-fired burner, a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF) assembly, and a controller. In another example, the system includes a partial range fuel-fired burner and a catalyzed DPF. In another example, the system includes a full range fuel-fired burner with DOC/DPF assembly or a catalyzed DPF.
In one example, a method of operating a fuel-fired burner in a vehicle exhaust system includes monitoring at least one engine operating condition, monitoring exhaust gas temperature, and communicating engine operating condition information and the exhaust gas temperature to a controller. The controller includes a control strategy to identify when the fuel-fired burner should be activated to achieve a desired reference temperature to increase NO2 levels sufficiently to regenerate the diesel particulate filter. The controller generates the control signal to activate the fuel-fired burner to raise exhaust gas temperature to the desired reference temperature only when the control strategy identifies that the fuel-fired burner should be activated.
The control strategy can take various forms. For example, the control strategy could include one or more of a look-up table which outputs the desired reference temperature as a function of engine operating conditions, and a steady-state model or a transient model that outputs the desired reference temperature as a function of exhaust back-pressure and estimated exhaust oxygen flowrate.
In another example, the control strategy includes continuously monitoring a pressure drop across a diesel particulate filter, continuously monitoring exhaust temperature, comparing the pressure drop to a look-up table of pressure drop versus an engine operating condition, and comparing the exhaust temperature to a threshold temperature. The fuel-fired burner is then selectively activated to increase NO2 levels sufficiently to regenerate the diesel particulate filter only when predetermined pressure and temperature criteria are met.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
The fuel-fired burner 16 includes an air/fuel supply system 26 that is selectively activated to inject/spray a mixture of air and fuel into the combustion chamber 20. The mixture is sprayed into existing exhaust gases within the combustion chamber 20 and an igniter 28 then ignites the fuel to increase heat. In one example, the igniter 28 comprises one or more electrodes, however, other types of igniters could also be used. Further, an airless fuel supply could also be used where only fuel would be injected/sprayed and then ignited.
The fuel-fired burner 16 is selectively activated by a controller 30 to elevate the exhaust temperature of exhaust gas to increase NO2 based regeneration, i.e. passive regeneration of the DPF 14. The controller 30 includes a control strategy for the fuel-fired burner 16, which considers various factors to provide the NO2 based regeneration in a fuel efficient manner.
The controller 30 includes various electronic components that cooperate to provide a electronic control unit to control an electromechanical system. For example, the controller 30 may include, amongst other electronic components typically included in such units, a processor and a memory device. The processor can comprise one or more microprocessors or microcontrollers, for example. The memory device can comprise a programmable read-only memory device (PROM) including erasable PROM's (EPROM, EEPROM), for example. The memory device is provided to store instructions in the form of one or more software routines and/or algorithms, which when executed by the processor, allow the controller 30 to control operation of the fuel-fired burner 16 using a specific control strategy.
In one example, a method of operating the fuel-fired burner 16 includes monitoring at least one engine operating condition, monitoring exhaust gas temperature, and communicating engine operating condition information and the exhaust gas temperature to the controller 30. Examples of engine operating conditions that are monitored include engine speed, engine load, mass flow rate, temperature, etc. The controller 30 utilizes the control strategy to identify when the fuel-fired burner 16 should be activated to achieve a desired reference temperature to increase NO2 levels sufficiently to regenerate the DPF 14. The controller 30 generates a control signal to selectively activate the fuel-fired burner 16 to raise exhaust gas temperature to the desired reference temperature only when the control strategy identifies that conditions require the fuel-fired burner 16 to be activated.
The control strategy can take various forms. For example, the control strategy could include one or more of a look-up table which outputs the desired reference temperature as a function of engine operating conditions, and a steady-state model or a transient model that outputs the desired reference temperature as a function of exhaust back-pressure and estimated exhaust oxygen flowrate. Each of these will be discussed in more detail below.
In another example, the control strategy includes continuously monitoring a pressure drop across the DPF 14 via pressure sensors P1, P2 and continuously monitoring exhaust temperature with a temperature sensor T. The pressure drop is compared to a look-up table of pressure drop versus a specified engine operating condition. The current pressure drop is compared to a predetermined pressure threshold and the exhaust temperature is compared to a predetermined threshold temperature. The fuel-fired burner 16 is then selectively activated to increase NO2 levels sufficiently to regenerate the DPF only when predetermined pressure and temperature criteria are met.
Specifically, the controller 30 only activates the fuel-fired burner 16 if the following criteria are met: 1) the pressure drop exceeds the predetermined pressure threshold; (2) the exhaust temperature is below 300 degrees Celsius; and (3) the rate of pressure increase exceeds a certain rate threshold. The fuel-fired burner 16 is turned off when the pressure drop falls below the predetermined pressure threshold and/or the exhaust temperature from the engine E increases above 300 degrees Celsius. As the output from the fuel-fired burner 16 is low in hydrocarbon species, the DOC 12 is selective for the NO to NO2 reaction required for passive regeneration.
NO2 formation in the DOC 12 to support DPF regeneration can only occur if DOC temperatures exceed approximately 250 degrees Celsius, hydrocarbon (HC) and carbon monoxide (CO) concentrations levels are limited, and NOx levels are sufficient. The control strategies utilized by the controller 30 function to schedule the desired reference/outlet temperature of the partial range burner, i.e. fuel-fired burner 16, to manage the conversion of NO2, HC, and CO.
One proposed control strategy utilizes a look-up table that outputs a mapped reference or desired outlet temperature of the fuel-fired burner 16 as a function of one or more engine operating conditions, such as engine speed, load, etc. Based on the determined outlet temperature of the fuel-fired burner 16, the controller 30 activates the fuel-fired burner 16 to inject the fuel/air mixture until the outlet temperature is achieved, and then the fuel-fired burner 16 is shut off. Once the desired outlet temperature is reached, NO2 levels are sufficient for passive regeneration of the DPF 14.
Another proposed control strategy utilizes a steady-state and model-based control scheme that outputs a reference or desired outlet temperature as a function of exhaust back-pressure measured by one or more pressure sensors and as a function of estimated exhaust oxygen flow rate. This steady-state and model-based controls scheme schedules the operating temperature of the fuel-fired burner 16 such that that it is operated at output temperatures of 250 degrees Celsius or greater as a function of estimated exhaust oxygen by mass flow rate and measured exhaust back-pressure, and limited by a pre-defined exhaust oxygen velocity threshold. This offers the benefit that a reference temperature map look-up table, such as that discussed above, would not be required for each different engine. Also, this strategy has the effect that calibration effort is reduced as a consequence of being applicable for controllers for any partial range burner associated with the engine.
This steady-state and model based strategy includes an algorithm stored within the controller 30 which compiles data based on steady-state engine operating conditions. The controller analyzes the data and then generates the control signal to activate the fuel-fired burner 16 by injecting the fuel only or the air/fuel mixture until the desired outlet temperature is reached. Once the temperature is reached the controller 30 turns off the fuel-fired burner 16.
Another proposed control strategy utilizes a transient and model-based control scheme that outputs a reference or desired outlet temperature as a function of exhaust back-pressure and estimated exhaust oxygen flow rate. This transient and model-based control scheme comprises a steady-state model, as discussed above, and a pre-filter 40 (
In each of the control strategies, the controller 30 issues a control signal to selectively activate the fuel-fired burner 16 to raise exhaust gas temperatures to promote NO2 based regeneration as needed. As shown in
Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
