The present invention relates to emissions control in compression-ignited internal combustion engines; more particularly, to systems for injecting urea into diesel exhaust to scavenge nitrogen oxides and rejuvenating a diesel particulate filter; and most particularly, to a system for heating and liquefying a storage tank solution of urea at normally sub-freezing urea-solution temperatures.
To scavenge oxides of nitrogen (NOx) from the exhaust of compression-ignited (CI) engines, and especially diesel engines, urea injection systems are commonly in use in the prior art. An aqueous urea solution is injected into the hot exhaust pipe, where urea is hydrolyzed into ammonia ahead of a selective catalytic reduction (SCR) converter. Ammonia reacts with NOx trapped on the catalyst face to form N2, CO2, and H2O, thereby lowering the level of noxious emissions in the exhaust.
A serious problem in the prior art is that at temperatures below about −11° C., the urea solution can freeze. Thus, a thermal heating system and method are required to thaw the solid solution into a liquid solution (or to keep the solution from freezing) to permit a pump to draw solution for delivery into an engine emissions abatement system.
A typical prior art urea supply system comprises a relatively small reservoir tank module from which liquid urea solution is dispensed into a diesel engine exhaust system, and a larger storage tank in which the tank module is immersed. The tank module contains a resistance heater that can liquefy suitable quantities of solution in a short time upon engine start-up under cold conditions, as is required to meet government air pollution standards. Solution in the surrounding storage tank then is heated by transfer of heat through the walls of the heated reservoir tank module.
It is an important operating requirement that the storage tank be able to re-supply the reservoir tank module within a short time after starting of the engine. In prior art systems when the solution in the storage tank is frozen, meeting this requirement can be difficult because of limited heat flow through the walls of the reservoir tank module, which typically is formed of a plastic polymer having relatively low thermal conductivity.
U.S. Pat. No. 6,387,336 discloses generally that an electric heating system and/or a heating device that uses waste heat of the engine coolant and/or the exhaust gas can be used to heat the frozen urea.
Published US Patent Application No. US 2008/0092531 discloses use of hot engine coolant to heat the frozen urea in both the storage tank and the supply line from the reservoir tank module to the point of injection of urea into the exhaust system. The is specification describes a separate coolant conduit embedded in the urea tank for heating the tank, and a double pipe construction surrounding the urea feed pipe for heating the urea flowing through the feed pipe. The inside of the double pipe arrangement provides a flow path for the urea while a jacketing-surrounding outer pipe carries the heated coolant, which may flow counter to the direction of flow of the urea. Also disclosed is the use of heat from a proximate exhaust pipe, which heat may be used to heat a non-jacketed urea feed pipe from a storage tank to the emissions abatement system. Alternatively, exhaust gas may be passed through the jacket of a double-walled feed pipe between the storage tank and the point of injection. It is not disclosed or suggested to pass exhaust gas through a heat exchanger disposed within the stored solution in the storage tank.
What is needed in the art is an improved storage system for urea solution wherein frozen urea solution in a urea storage tank may be liquefied at a rate sufficient to maintain replenishment of the reservoir tank module.
It is a principal object of the present invention to provide a reliable flow of liquid urea solution at ambient temperatures below the freezing point of the solution.
Briefly described, a system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures comprises a reservoir tank module disposed in a storage tank. The reservoir tank module preferably includes a level sensing apparatus, inlet and outlet ports for supplying and withdrawing urea solution, and at least one heating element. The walls of the reservoir tank module are preferably immersed in urea solution contained in the storage tank, which solution is partially heated by passage of heat through the walls of the reservoir tank module.
In accordance with the present invention, additional heat for melting frozen urea solution in the storage tank is derived from waste heat in engine exhaust gas and is added to the system by passing a portion of the exhaust gas stream through a gas/liquid heat exchanger disposed within the solution in the storage tank. After being passed through the heat exchanger, the cooled exhaust gas may be conveniently disposed of either by being returned to the tailpipe or by feeding the exhaust gas into the engine intake manifold in a method of exhaust gas recirculation (EGR).
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
The exemplifications set out herein illustrate currently-preferred embodiments of the present invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring to
In a first method for replenishing the reservoir tank module in accordance with the present invention, (and referring now to
Preferably, bypass flow line 150 for exhaust gas portion 130 passes through reservoir tank module 116 where some auxiliary heating of solution 20 occurs by heat loss from line 150.
Referring now to
While the invention has been described by reference to specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.