Emissions control system for increasing selective catalytic reduction efficiency

An emissions control system is used with a compression ignition engine capable of producing an exhaust gas stream to be treated by a reduction agent which is mixed with the exhaust gas stream to convert the exhaust gas stream prior to being directed into an SCR catalyst capable of reducing NOx in the exhaust gas stream. An exhaust gas recirculation valve is associated with the compression ignition engine for directing the exhaust gas stream back into the compression ignition engine. An electronic control unit and sensors determine whether the temperature of the exhaust gas stream is at a predetermined temperature, and if so control operation of the valve to allow a portion of the exhaust gas stream to be directed into the SCR catalyst. A bypass valve may also be provided to prevent the exhaust gas stream from entering into the SCR catalyst.

TECHNICAL FIELD

The present invention relates generally to emissions control systems for electronically controlled compression ignition engines, and more particularly to an emissions control system for increasing selective catalytic reduction (SCR) efficiency of a compression ignition engine.

BACKGROUND

Compression ignition engines, such as diesel engines, provide advantages in fuel economy, but produce and emit both NOx (nitrogen oxides) and particulates during normal operation. When primary measures (actions that affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase the output of NOx. Current and proposed regulations and legislation challenge manufacturers to achieve good fuel economy while at the same time require the reduction of the emissions of particulates and NOx.

In order to meet such requirements or restrictions a method known as SCR (selective catalytic reduction) has been used for reducing the emission of NOx. The SCR method consists of injecting gaseous ammonia NH3, ammonia in aqueous solution or aqueous urea, or ammonia supplied from an ammonia generator using a solid source of ammonia such as ammonia carbamate or ammonia carbanate, into the exhaust gas system of the compression ignition engine as a reduction agent. When the temperature of the exhaust gas stream is above a reaction temperature, for example a temperature above 160° C. for aqueous urea, the reduction agent undergoes a hydrolysis process and is decomposed into ammonia and CO2. As the exhaust gas stream is passed through the SCR catalyst the gaseous ammonia reacts with the NOx to reduce the NOx to molecular nitrogen. This reduces or limits the NOx emissions from the compression ignition engine.

Although an SCR catalyst system is effective in reducing NOx emissions above a certain exhaust gas temperature, it has been found that the effectiveness of the SCR catalyst is drastically reduced below this temperature due to several factors. One factor is that the rate of NOx conversion is strongly affected by the temperature of the exhaust gas. The NOx conversion efficiency drops off quickly when the exhaust gas temperature is below the temperature at which the conversion efficiency is 50%. This is known as the catalyst light-off temperature. Consequently, the SCR catalyst is not effective when the engine is operating under light load conditions.

When urea solution is used, another factor which reduces the effectiveness of the SCR catalyst system is the minimum hydrolysis temperature of the urea solution. Whenever a solution of urea is used to supply ammonia to the SCR catalyst, it is important that the temperature of the exhaust gas stream be at or above the minimum hydrolysis temperature, which is 160° C. Below its minimum hydrolysis temperature, the urea solution does not decompose into ammonia at a fast enough rage for a typical engine application. If the unhydrolyzed solution of urea is injected into the SCR catalyst, some of the urea is deposited on the surface of the SCR catalyst as a solid residue, which results in the system clogging or plugging. These conditions occur when an engine is operated under transient conditions, or under light load conditions, and NOx reduction by the SCR catalyst can not be achieved.

A third factor that reduces the efficiency of an SCR catalyst system is that the surface temperature of the catalyst must be at a minimum temperature. Until this temperature is obtained, the solution of urea should not be injected into the catalyst, and the delay in injecting the solution of urea causes the SCR catalyst system to lose efficiency. Low catalyst surface temperature can exist when the engine is coming out of a cold start condition or out of a prolonged period of light load operation.

One other factor that reduces the efficiency of the SCR catalyst system occurs when the engine is operating under a light load or an idle condition, and combustion takes place at low in-cylinder temperatures. Some of the unburned fuel and lube oil can survive the combustion process in the engine and be discharged into the exhaust gas stream as liquid droplets. Since the exhaust gas temperature is low, some of the liquid droplets are deposited on the surface of the SCR catalyst. A subsequent increase in exhaust temperature causes the liquid hydrocarbons to undergo partial oxidation. The residue formed can block the micro pores of the catalyst washcoat, causing the catalyst to reduce efficiency through loss of surface area within the catalyst. Increasing the combustion temperature is an effective way to reduce the amount of liquid hydrocarbons entering into the SCR catalyst.

In view of these factors which decrease the efficiency of the SCR catalyst, it would be advantageous to have a system that could insure that the temperature of the exhaust gas is at or above the minimum hydrolysis temperature for the solution of urea. It would also be desirable to have an emissions control system that ensures that the SCR catalyst does not become clogged with liquid hydrocarbons, or other matter that reduces the capability of the SCR catalyst to operate correctly or properly.

SUMMARY OF THE INVENTION

The present invention relates to an emissions control system for a compression ignition engine capable of producing an exhaust gas stream to be treated by a reduction agent which is mixed with the exhaust gas stream to convert the exhaust gas stream prior to being directed into an SCR catalyst capable of reducing NOx in the exhaust gas stream. The system comprises a sensor for determining the temperature of the exhaust gas stream prior to being mixed with the reduction agent, an exhaust gas recirculation valve associated with the compression ignition engine for directing the exhaust gas stream back into the compression ignition engine, and an electronic control unit connected to the sensor and the exhaust gas recirculation valve for determining whether the temperature of the exhaust gas stream is at a predetermined temperature and if so for controlling operation of the valve to allow a portion of the exhaust gas stream to be directed into the SCR catalyst.

Another aspect of the present invention relates to an emissions control system for a compression ignition engine capable of producing an exhaust gas stream to be treated by a reduction agent which is mixed with the exhaust gas stream to convert the exhaust gas stream prior to being directed into an SCR catalyst capable of reducing NOx emissions in the exhaust gas stream. The system comprises a sensor for determining the temperature of the exhaust gas stream prior to being mixed with the reduction agent, an exhaust gas recirculation valve associated with the compression ignition engine for directing the exhaust gas stream back into the compression ignition engine, a bypass valve associated with the exhaust gas stream and positioned prior to an input of the SCR catalyst, the bypass valve being operated to selectively and alternatively provide the exhaust gas stream to the input of the SCR catalyst or to bypass the SCR catalyst, and an electronic control unit connected to the temperature sensor, the exhaust gas recirculation valve, and the bypass valve for determining whether the temperature of the exhaust gas stream is at a predetermined temperature and if so for controlling operation of the exhaust gas recirculation valve to allow a portion of the exhaust gas stream to be directed to the bypass valve and for controlling operation of the bypass valve to either direct the exhaust gas stream which reaches the bypass valve selectively and alternatively into the SCR catalyst and to bypass the SCR catalyst.

Still another aspect of the invention provides a method for increasing the efficiency of an emissions control system for a compression ignition engine capable of producing an exhaust gas stream to be treated by a reduction agent which is mixed with the exhaust gas stream to convert the exhaust gas stream prior to being directed into an SCR catalyst capable of reducing NOx in the exhaust gas stream, the method providing the steps of determining the temperature of the exhaust gas stream prior to being mixed with the reduction agent; determining whether the temperature of the exhaust gas stream is at a predetermined temperature; and controlling the direction of flow of the exhaust gas stream based upon the temperature of the exhaust gas stream.

Other aspects and advantages of the present invention will be apparent to those skilled in the art upon reading the following detailed description in connection with the drawings and appended claims.

DETAILED DESCRIPTION

Referring now to the drawings,FIG. 1illustrates an emissions control system10constructed according to the present invention. The emissions control system10is used to control the emissions from a compression ignition engine12, such as a diesel engine. The engine12includes an exhaust system14in which an exhaust gas stream, indicated by arrows16, is produced. The exhaust system14includes one or more exhaust gas recirculation (EGR) valves18which are capable of being metered in order to direct all, a portion, or none of the exhaust gas stream16back into the engine12through an intake manifold20.

The exhaust gas stream16is provided toward an ammonia generator system22which is capable of injecting a reduction agent into the exhaust gas stream16. The system22includes a storage tank24for storing a reactant such as urea solution. The urea solution is drawn out of the tank24by a pump26through a conduit28. Although not shown, the tank24may include a fill port to fill the tank24with the solution of urea or with any other known and suitable reactant and suitable sensors which may monitor the temperature, pressure, and amount of reactant within the tank24. The tank24may further include a heating device (not shown) which may be used to heat the contents of the tank24, if required. Again, although not shown, the system22may include other components such as a pressure regulator that insures that the urea solution is maintained at or below a predetermined pressure, and a valve. The urea solution is sprayed from the pump26through a conduit30with a suitable injection nozzle to introduce the urea solution into the exhaust gas stream16.

Once the exhaust gas stream16passes the conduit30and is mixed with the urea solution, the stream16encounters a bypass valve32. Depending upon various conditions which will be discussed the bypass valve32can direct the exhaust gas stream16into either an SCR catalyst34or a bypass pipe36. Once the exhaust gas stream16leaves the SCR catalyst34or the bypass pipe36the stream16is provided through an outlet pipe38into the atmosphere.

The system10further includes an electronic control unit40which is used to control and monitor various operations and functions of the system10and the engine12. The electronic control unit40is capable of monitoring various functions of the engine12by use of sensors42which are associated with the engine12. The sensors42are connected to the electronic control unit40via an electrical connection44. Examples of sensors42which may be employed are an engine speed sensor, an intake manifold air temperature sensor, and an intake manifold pressure sensor, all of which are not shown. A temperature sensor46, which is positioned after the EGR valve18, is used to determine the temperature of the exhaust gas stream16after the stream16flows from the engine12. Alternatively, the temperature can be determined by inference from other sensed data. The sensor46is connected to the electronic control unit40by a electrical wire48.

An NOx sensor50is positioned in the outlet pipe38of the SCR catalyst34and is also connected to the electronic control unit40by a wire52. The NOx sensor50is used to provide a signal indicative of the level of NOx emissions being produced by either the SCR catalyst34or by the engine12. The NOx sensor50is used to continually monitor the level of NOx emissions.

The electronic control unit40is also connected to the EGR valve18via a wire54to control operation of the valve18. The bypass valve32is connected to the electronic control unit40by an electrical connection56. In this manner, the electronic control unit40is able to control the operation of the bypass valve32by sending an appropriate signal over the connection56. The electronic control unit40may also be connected to the pump26by use of a wire58. The electronic control unit40is used to control the operation of the pump26to further control the injection of the solution of urea into the exhaust gas stream16.

The electronic control unit40, also known as a control module or a controller, may take many forms including a computer based system, a microprocessor based system including a microprocessor, a microcontroller, or any other control type circuit or system. The electronic control unit40may include memory for storage of a control program for operating and controlling the emissions control system10of the present invention and other memory for temporary storage of information.

The operation of the emissions control system10is based on the electronic control unit40being able to monitor the status of the engine12and the temperature of the exhaust gas stream16. In particular, there are three conditions which the electronic control unit40must be capable of monitoring and sensing. The first condition occurs when the temperature of the exhaust gas stream16is below the minimum hydrolysis temperature for the solution of urea. The second condition occurs when the temperature of the exhaust gas stream16is above the minimum hydrolysis temperature for the solution of urea and below a temperature which corresponds to efficient NOx reduction. For example the temperature range for this second condition may be between about 160° C. and 250° C. The third condition to be monitored by the electronic control unit40is when the temperature of the exhaust gas stream16is above the temperature for efficient NOx reduction.

With these three conditions in mind, the operation of the system10is as follows. When the electronic control unit40determines that the temperature of the exhaust gas stream16is below the minimum hydrolysis temperature for the solution of urea, the electronic control unit40will operate the EGR valve18to meter a controlled amount of the exhaust gas stream16back into the engine12. Additionally, the bypass valve32may be opened by the electronic control unit40to prevent the flow of the exhaust gas stream16from entering into the SCR catalyst34. Bypassing the exhaust gas stream16from the catalyst34prevents the unburned fuel and lube oil from being depositing on any interior surface of the catalyst34which tends to prolong the life of the catalyst34. Applying exhaust gas recirculation under these conditions reduces NOx formation.

When the second condition is determined by the electronic control unit40, the second condition being that the sensed temperature of the exhaust gas stream16is above the temperature required for hydrolysis of the solution of urea but below the temperature for efficient reduction of NOx emissions from the catalyst34, the electronic control unit40operates the EGR valve18to meter a controlled amount of the exhaust gas stream16back into the engine12which increases the temperature of the exhaust gas stream16. Additionally, the electronic control unit40closes the bypass valve32to allow all of the exhaust gas stream16to enter into the catalyst34. The electronic control unit40may also use the sensors42to determine, directly or by inference, the engine speed, the engine load, the intake air pressure of the engine, and the temperature of the engine to control operation of the generation system22. The effect of the use and operation of the EGR valve18would be to increase the temperature of the exhaust gas stream16which results in a high NOx reduction efficiency. The use of EGR also reduces NOx formation from the combustion process.

The third condition corresponds to the temperature of the exhaust gas stream16being high enough for efficient NOx reduction without the aid of the EGR valve18. During this condition the electronic control unit40would close the EGR valve18to direct all of the exhaust gas stream16toward the SCR catalyst34. Additionally, the bypass valve32would be closed to allow all of the exhaust gas stream16to flow into the catalyst34. This would provide for the highest NOx reduction efficiency. It is also contemplated that EGR valve18can be open during the third condition, to meter a controlled amount of exhaust gas stream16back to engine12, for reducing NOx formation in the combustion process.

Although the system10was described using the bypass valve32, it is possible and contemplated that the system10could function without the bypass valve32. For example, if the bypass valve32was not part of the system10, the exhaust gas stream16would be able to always pass through the SCR catalyst34. The tradeoff of this configuration or construction is that it would allow matter to enter into the catalyst34which may clog the interior of the catalyst34at the expense of not allowing untreated NOx being emitted into the atmosphere.

Referring now toFIG. 2, a flowchart or program100of the software stored in the electronic control unit40and implemented in a preferred embodiment of the present invention is shown. Those skilled in the art can easily and readily develop the specific software code necessary to implement the flowchart or program100by using the specific instructions set associated with the microprocessor or microcontroller selected for use with the electronic control unit40of the present invention.

The program100begins at a step102in which the program100starts or begins operation. The program then continues to a step104in which the temperature sensor46is read to determine the temperature of the exhaust gas stream16. Once the temperature is determined, the program100continues to a step106to decide whether the temperature of the exhaust gas stream16is less than a predetermined temperature. For example, the predetermined temperature may be the temperature at which the solution of urea will undergo the hydrolysis process. If, in the step106, it is determined that the temperature is not less than the predetermined temperature, control of the program continues on to a step108. In the step108another decision is made as to whether the temperature of the exhaust gas stream16is greater than the predetermined temperature. When it is determined that the sensed temperature is greater than the predetermined temperature the program100moves to a next step110. In the step110the electronic control unit40starts urea solution injection and closes the EGR valve18and the bypass valve32. This allows the exhaust gas stream16to enter into the SCR catalyst34. After the valves18and32are closed the program100ends at a return step112.

If in the step106it is determined that the sensed temperature is less than the predetermined temperature, then the program100will branch to a step114. In the step114the electronic control unit40will stop urea solution injection and operate the EGR valve18and the bypass valve32to prevent the exhaust gas stream16from entering into the SCR catalyst34. Once the valves18and32are opened, the control of the program100will pass to the step112. Additionally, if in the step108, for some reason it is determined that the temperature is not greater than the predetermined temperature, the program100will branch to the step112.

INDUSTRIAL APPLICABILITY

It is more expensive to run a compression ignition engine with an SCR catalyst system than it is to run a compression ignition engine without such a system. Due to this increased cost, it is important to operate the SCR catalyst system as efficiently as possible, and to prolong the life of the SCR catalyst. It is possible, under some conditions, to circumvent or bypass the SCR system to reduce operating costs and prolong the life of the SCR catalyst, but at the expense of increased exhaust emissions.

An embodiment of the present invention monitors various operations of emissions control system using 10 sensors such as temperature sensor46, and/or NOx sensor50. The operating conditions of compression ignition engine12are sensed using sensors42. Data regarding sensed conditions from emissions control system10and engine12are supplied to electronic control unit40, to determine if the SCR reaction or process is being completed as efficiently as possible, and to control operations of various system components in response thereto. The present invention can be used to ensure that the exhaust gas stream is at sufficient temperature to effectively complete the SCR reaction. Additionally, optional bypass valve32and bypass pipe36can be used to circumvent SCR catalyst34when conditions are such that use of the system would be ineffective in reducing emissions, or when use of the system could be detrimental to the system. By using an embodiment of the present invention, NOx emissions are reduced in an efficient manner, when using an SCR system in conjunction with a solution of urea as the reactant.