Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2002-04-26
2003-06-24
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S276000, C060S286000, C060S295000, C060S303000
Reexamination Certificate
active
06581374
ABSTRACT:
This invention relates generally to nitrogen oxide (NOx) emissions produced by internal combustion engines in a vehicle and more particularly to a system for controlling reduction of the NOx emissions by means of a selective catalytic reduction (SCR) method.
The invention is particularly applicable to and will be described with specific reference to a control system for regulating the supply of an external reductant, ammonia, to a reducing catalyst in an SCR system taking into account the effect of NOx transient emissions produced in vehicles powered by diesel engines. However, those skilled in the art will recognize that the control system has broader applications and may be applied to SCR systems using other reductants such as fuel oil or hydrocarbons as well as SCR systems used in other mobile internal combustion engine applications, such as gasoline engines employing “lean burn” techniques.
INCORPORATION BY REFERENCE
The following patents and publications are incorporated by reference herein and made a part hereof:
1) U.S. Pat. No. 4,403,473, to John R. Gladden, dated Sep. 13, 1983, entitled: “AMMONIA/FUEL RATIO CONTROL SYSTEM FOR REDUCING NITROGEN OXIDE EMISSIONS”;
2) SAE Paper No. 952493, by H. Luders, R. Backes, and G. Huthwohl, FEV Motoremtechnik and D. A. Ketcher, R. W. Horrocks, R. G. Hurley and R. H. Hammerle, Ford Motor Co., dated Oct. 16-19, 1995, entitled: “AN UREA LEAN NOx CATALYST SYSTEM FOR LIGHT DUTY DIESEL VEHICLES” (See page 7);
3) SAE Paper No. 921673, by J. Walker, Ortech and B. K. Speronello, Engelhard Corp., dated Sep. 14-17, 1992, entitled: “DEVELOPMENT OF AN AMMONIA/SCR NOx REDUCTION SYSTEM FOR A HEAVY DUTY NATURAL GAS ENGINE”;
4) U.S. Pat. No. 5,606,855, to Naoki Tomisawa, dated Mar. 4, 1997, entitled: “APPARATUS AND METHOD FOR ESTIMATING THE TEMPERATURE OF AN AUTOMOTIVE CATALYTIC CONVERTER”; and,
5) U.S. Pat. No. 5,490,064, to Minowa et al., dated Feb. 6, 1996, entitled: “CONTROL UNIT FOR VEHICLE AND TOTAL CONTROL SYSTEM THEREFOR”.
None of the material cited above form any part of the present invention. The material is incorporated by reference herein so that details relating to SCR systems such as the operation of the SCR systems with ammonia or hydrocarbon reductants, the metering of the reductants, principles and control of engine operation etc., need not be set forth or described in detail herein.
BACKGROUND
This invention is directed to the removal of nitrogen oxides (NOx) from the exhaust gases of internal combustion engine, particularly diesel engines, which operate at combustion conditions with air in large excess of that required for stoichiometric combustion, i.e., lean. Unfortunately, the presence of excess air makes the catalytic reduction of nitrogen oxides difficult. Emission regulations impose a limit on the quantity of specific emissions, including NOx, that a vehicle can emit during a specified drive cycle, such as i) for light duty trucks, an FTP (“federal test procedure”) in the United States or an MVEG (“mobile vehicle emissions group”) in Europe or ii) for heavy duty trucks, a Heavy Duty Cycle in the United States or an ESC (European Steady State Cycle) or ETC (European Transient Cycle) in Europe. The regulations are increasingly limiting the amount of nitrogen oxides that can be emitted during the regulated drive cycle.
There are numerous ways known in the art to remove NOx from a waste gas. This invention is directed to a catalytic reduction method for removing NOx. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Conventionally, there are three ways to treat vehicular exhaust to reduce NOx. The first method is non-selective catalyst reduction (NSCR). The second way is selective non-catalytic reduction (SNCR) and the last method is selective catalyst reduction (SCR). This invention relates to SCR systems.
In diesel engines, sufficient NOx reduction to meet current regulations has been achieved by combustion modifications in the diesel engine by, for example, incorporating EGR. Projected emission levels are such that combustion and engine modifications will not be sufficient to meet the more stringent levels. Because of excess oxygen present in diesel exhaust gases, the opportunity for NOx reduction under rich or stoichiometric air/fuel is not possible. SCR is a technology that has been shown effective in removing NOx from oxygen rich exhaust. A number of SCR systems have been developed which, because of infrastructure concerns, have used diesel fuel or diesel oil as the reductant source. Unfortunately, as of this date, an HC reducing catalyst has not yet been developed which has sufficient activity and is effective over the entire operating range of the diesel engine.
A common nitrogen oxide reducing agent, long used in industrial processes, is ammonia. NOx reducing catalysts have been developed which are effective over the operating range of the engine. Despite the infrastructure concerns relating to the use of urea in a mobile application as well as the potentially dangerous risks of ammonia break-through or slip, ammonia SCR systems are becoming the favored choice for mobile applications to meet the more stringent NOx emissions. This is, among other reasons, because of the high NOx conversion percentages possible with ammonia coupled with the ability to optimize the combustion process for maximum power output with minimum fuel consumption.
Notwithstanding what may be said to be inherent advantages of an ammonia based SCR system, the control systems to date have been excessively complicated and/or ineffective to control the SCR system when the impact of NOx transient emissions on the SCR system is considered. As will be shown below, if the transient NOx emissions can not be adequately reduced by the SCR control system, then stringent NOx emission regulations will not be met.
Early patents controlled ammonia metering by considering the emissions to be controlled at steady state conditions. For example, U.S. Pat. No. 4,403,473 to Gladden (Sep. 13, 1983) considered NOx emissions at various speed ranges and concluded that a linear relationship exists between fuel flow and NOx. (Earlier Gladden U.S. Pat. No. 4,188,364, Feb. 12, 1980 concluded that ammonia catalyst adsorbed ammonia at temperatures lower than 200° C. and desorbed at temperatures between 200-800° C., the SCR system should operate at higher temperatures to achieve complete reaction between ammonia and NOx.) Thus, in Gladden '473, the fuel mass flow is sensed and NH
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throttled at a percentage of fuel flow provided the temperature of the gases in the catalytic converter are within a set range. This basic control concept is used today in most mobile, ammonia SCR systems. For example, U.S. Pat. 5,116,579 to Kobayashi et al. (May 26, 1992) additionally measures the humidity of intake air and one or more operating parameters of engine power, intake air temperature, fuel consumption and exhaust gas temperature to set an ammonia ratio control valve. The molar ratio of ammonia to NOx is set at less than one (sub-stoichiometric) to minimize ammonia slip.
Typically the reductant is pulse metered into the exhaust gas stream in a manner similar to that used for operating conventional fuel injectors. In U.S. Pat. No. 4,963,332 to Brand et al. (Oct. 16, 1990), NOx upstream and downstream of the catalytic converter is sensed and a pulsed dosing valve controlled by the upstream and downstream signals. In U.S. Pat. No. 5,522,218 to Lane et al. (Jun. 4, 1996), the pulse width of the reductant injector is controlled from maps of exhaust gas temperature and engine operating conditions such as engine rpm, transmission gear and engine speed.
As noted, the industrial art has long used ammonia in SCR systems to control NOx emissions typically by set point control loops such as shown in U.S. Pat. No. 5,047,220 to Polcer (Sep. 10, 1991) in which a downstream NOx sensor is used to generate a trim signal in the control loop. The industrial art has also recognized that changes
Calabrese John Lawrence
Genderen Mathijs Van
Grimston Karl Richard
Patchett Joseph A.
Rice Gary Wayne
Denion Thomas
Engelhard Corporation
Negin Richard A.
Tran Binh
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