Patent Application
20110030349
This invention relates to internal combustion engines, especially motor vehicle engines that utilize urea dosing for after-treatment of engine exhaust.
The performance of a diesel engine after-treatment system in converting NOx to other chemical products by selective catalytic reduction (SCR) relies on the presence of ammonia in the exhaust stream. Dosing engine exhaust by injection of aqueous urea, an ammonia-based reductant, into the exhaust stream at a location upstream of an SCR catalyst is one way to introduce ammonia into the exhaust system.
For promptly commencing the conversion of NOx in engine exhaust gas to other chemical products through catalytic action upon engine starting, a urea dosing system needs to become effective in as short a time as possible. A known design practice places a urea injector at a location in the engine exhaust system where it can spray urea solution into the exhaust stream ahead of the SCR catalyst with the objective of completely evaporating the solution by the time it reaches the catalyst. Incomplete evaporation can lead to undesired consequences such as the formation of solid deposits in the exhaust system and poor performance of the after-treatment system.
When a “cold” engine is started in warm ambient temperatures, aqueous urea stored in an on-board urea tank is in the liquid phase and therefore sufficiently fluid for pumping to the urea injector.
Because the urea injector pierces the exhaust system, it begins to absorb heat from the passing exhaust gases essentially as soon as the engine starts. That is typically not objectionable, at least until such time as it becomes necessary to limit injector temperature due to exposure to significantly elevated exhaust gas temperatures. Those extremely high temperatures can occur when a diesel particulate filter (DPF) located upstream of an injector is being regenerated. In order to limit injector temperature rise it is known to circulate liquid coolant from the engine cooling system through internal coolant passages in the injector. Depending on relative temperatures of engine exhaust gas and engine coolant, the circulation of engine coolant may be controlled in any suitably appropriate way such as by a control valve, or it may be left uncontrolled and therefore essentially continuous.
The use of engine coolant for thermal management of a urea injector may also extend to thermal management of the urea tank and a supply pump that pumps solution from the tank to the injector. Thermal management of the pump and the tank is important because in a motor vehicle such as a truck, the latter two components are typically mounted on the motor vehicle chassis where the urea solution is continually exposed to ambient temperature. In cold ambient temperatures near and below about 12° F., the solution in the tank, the pump, and associated conduits can freeze while in torrid ambient temperatures, the solution can become slush, significantly reducing its effectiveness when injected into the after-treatment system.
Certain governmental regulations applicable to certain motor vehicles require that when a “cold” engine is started in ambient temperatures sufficiently cold that urea solution in the tank and/or associated conduits and components is completely and/or partially frozen, the after-treatment system must become effective within certain time constraints. Hence thawing of frozen urea that might otherwise adversely impact regulatory compliance is essential.
It has been proposed to immerse a heating element in the urea tank and to flow engine coolant through it in order to hasten thawing of frozen solution so that liquid solution can be sucked out of the tank by the supply pump and conveyed to the injector for spraying into the exhaust. Such a tank comprises several ports including a coolant inlet port, a coolant outlet port, a urea suction port, a urea backflow port, and a vent port.
The immersed heating element is disposed in heat exchange relationship with the contents of the tank to form a segment of a coolant flow path that runs through the urea dosing system, the in-tank segment running between the tank's coolant inlet port and the tank's coolant outlet port. Engine coolant from the engine cooling system enters the tank via the tank's coolant inlet port and leaves via the tank's coolant outlet port. After leaving the tank, the coolant flow path that runs through the dosing system may pass through a coolant passageway in the supply pump before returning to the engine.
The urea suction port of the tank, which is typically at or near the top of the tank, is placed in fluid communication with a suction inlet of the supply pump via a supply conduit. A pick-up tube extends downward within the tank from the suction port to terminate in an entrance near the bottom of the tank. When the supply pump operates, the venting of the tank allows the pump to draw solution from the bottom of the tank into and through the pick-up tube, and then through the supply conduit. A urea outlet port of the pump is placed in fluid communication with the urea injector via an injector supply conduit to provide for the solution drawn from the tank through the supply pump to be conveyed to the injector. A backflow conduit extends from the pump to the backflow port of the tank to return excess solution to the tank.
The inventors have observed a failure of one proposed urea dosing system to comply with applicable criteria for thawing frozen urea and have discovered a cause for that deficiency. The result of their discovery has led them to devise a construction for accelerating thawing by a more efficient transfer of heat from engine coolant to urea solution in the dosing system.
Consequently, the present invention relates generally to an improvement for thermal management of a dosing system that delivers a dosing fluid, or agent, to an engine exhaust after-treatment system, especially an improvement for quick-heating certain dosing system components in a motor vehicle where those components and/or conduits associated with them are exposed to ambient temperatures that at times are sufficiently low to freeze dosing fluid in the dosing system.
The invention is effective to more efficiently transfer engine coolant heat to dosing fluid, thereby accelerating the thawing of frozen solution in sub-freezing ambient temperatures, an important factor for achieving compliance of an after-treatment system with relevant governmental regulations.
The disclosed embodiment of the invention is a urea dosing system that introduces aqueous urea into the after-treatment system upstream of an SCR catalyst that serves to promote chemical reaction of the injected reductant with NOx in engine exhaust gas to convert the latter to other chemical products before the exhaust enters the atmosphere.
The invention, as particularly applied to respective conduits through which coolant enters and urea solution leaves a urea tank, addresses certain situations that may occur after the engine has been shut down in cold ambient conditions, a specific example being the accumulation and eventual freezing of droplets of urea solution and water condensation in the conduit that serves as a urea pick-up tube.
A general aspect of the invention relates to an internal combustion engine comprising combustion chambers within which fuel is combusted to operate the engine, a cooling system, including a coolant pump, for circulating liquid engine coolant through the engine, an exhaust after-treatment system through which products of combustion are conveyed from the combustion chambers to atmosphere, and a dosing system for introducing dosing fluid into the exhaust after-treatment system for use in an exhaust gas after-treatment process carried out in the after-treatment system.
The dosing system comprises a dosing fluid conduit having a thermally conductive wall for conveying dosing fluid toward a point of introduction into the after-treatment system and a coolant circuit through which liquid engine coolant circulates in heat exchange relationship with at least a portion of the dosing system. A portion of the coolant circuit comprises a coolant conduit through which engine coolant is conveyed and which has a thermally conductive wall disposed side-by-side and physically associated with the thermally conductive wall of the dosing conduit to form a thermal conduction path for conductive heat transfer from relatively hotter coolant in the coolant conduit to relatively cooler dosing fluid in the dosing conduit.
Another general aspect of the invention relates to a method of heating urea solution in solid or liquid phase in a urea supply conduit which has a thermally conductive wall and through which urea solution in liquid phase is delivered from a tank to a point of use in an engine exhaust after-treatment system through which products of combustion are conveyed from engine combustion chambers to atmosphere.
The method comprises circulating liquid engine coolant through a coolant conduit that has a thermally conductive wall placed side-by-side and in physical association with the thermally conductive wall of the urea supply conduit to form a thermal conduction path for heat transfer between coolant in the coolant conduit and urea in the urea supply conduit.
The foregoing, along with further features and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes a drawing, now briefly described as follows.
One such after-treatment device is an SCR catalyst 20. A urea injector 22 is mounted on a portion of exhaust system 14 upstream of catalyst 20 with its outlet, or nozzle, arranged to spray aqueous urea into the exhaust system for entrainment with, and evaporatively mixing throughout, engine exhaust gas coming from cylinders 16. Catalyst 20 promotes a chemical reaction between the reductant and NOx in the exhaust gas that converts substantial amounts of NOx to other products before the exhaust gas passes into the atmosphere.
A tank 24 holds a supply of aqueous urea and is suitably vented through a vent port (not shown) to allow solution to be sucked out via a urea outlet port 26. A conduit 28 extends from port 26 to an inlet port 30 of a supply pump module 32. A conduit 34 extends from an outlet port 36 of supply pump module 32 to an inlet 38 of injector 22.
When supply pump module 32 operates, it draws solution from tank 24 through conduit 28 and pumps the solution through conduit 34 to injector 22, with a backflow conduit 40 returning excess solution to tank 24.
Engine 10 further comprises a liquid cooling system 42 through which engine coolant is circulated by a pump 44. Two conduits 46, 48 provide for pump 44 to circulate engine coolant through a coolant passageway in injector 22. Three more conduits 50, 52, 54 provide for pump 44 to circulate engine coolant through the coolant passageway of a heating element 55 that runs through the interior of tank 24 in heat exchange relationship with solution in the tank and then through a coolant passageway in pump module 32.
Conduit 50 connects to a coolant inlet port 56 of tank 24. Conduit 54 connects a coolant outlet port 58 of tank 24 to a coolant inlet port 60 of pump module 32. Conduit 52 returns coolant from a coolant outlet port 62 of pump module 32 to engine cooling system 42.
The suction side of pump 44 acts through conduits 48 and 52 to apply suction to a coolant outlet port of injector 22 and to port 62.
The suction applied to the coolant outlet port of injector 22 is effective to draw coolant from the engine through conduit 46, a coolant passage or passages in the body of injector 22, and back to the engine via conduit 48. The suction applied to coolant outlet port 62 is effective to draw coolant from the engine through conduit 50, through the coolant passageway in tank 24 that includes heating element 55, through conduit 54, through the coolant passageway in pump module 32, and then back to the engine via conduit 52.
Heating element 55 is essentially a tube having a geometry appropriate for the geometry of the tank interior. For example, segment 66 may extend downward almost to the bottom wall of the tank where it merges with a generally horizontal bottom segment that runs laterally either straight or curved to a vertical exit segment that extends upward to back wall 64 although the bottom and exit segments are not shown in
In accordance with principles of the invention, heat is transferred more efficiently from relatively warmer engine coolant entering tank 24 at inlet port 56 to relatively cooler urea solution (either frozen or liquid) in pick-up tube 68 by fabricating them from material of good thermal conductivity and by physically associating their thermally conductive walls side-by-side so as to form a thermal conduction path for heat transfer between coolant in segment 66 and urea in pick-up tube 68.
Creation of the thermal conduction path can be accomplished in different ways, several of which are shown in
It should be noticed in all of these examples that heat transfer is performed entirely by conduction through solid material, a method that transfers heat significantly more rapidly than alternative methods. Hence, quick heating of urea solution in either solid (frozen) or liquid phase occurs.
While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the following claims.
