FIELD OF THE INVENTION

The present invention relates to a dosing system for dosing a first fluid into a stream of a second fluid, and in particular to a dosing system for dosing a reducing agent into an exhaust pipe of a combustion engine.

BACKGROUND OF THE INVENTION

In order to lower the emission of pollutants, such as nitrogen oxides, from the exhaust gasses of combustion engines, reducing agent, such as e.g. liquefied urea, is often introduced into the exhaust systems. This is typically done by use of a nozzle that ensures atomization of the reducing agent. In some dosing systems, the current amount of reducing agent is determined to correlate to the current demand which depends on e.g. the fuel consumption. The introduction of reducing agents often results in deposits in the form of crystals or amorphous structures building up in the nozzles used to deliver the reducing agent. These deposits grow in size over time and thereby result in a poorer atomization, a poorer control of the delivered amount and eventually a total clogging up of the nozzle outlet. The deposits must therefore be removed from time to time which requires dismantling of parts of the system.

Hence, an improved system and method for removal of nitrogen oxides from exhaust gasses without build-up of deposit would be advantageous.

OBJECT OF THE INVENTION

In accordance with the present invention, it has been realized that formation of deposits particularly occurs when there is no flow of fluid through the nozzle while the exhaust gas and the exhaust pipe are still hot. This may be the case in a number of situations including:

When the actual demand is determined mainly from measurements of the actual fuel consumption, and the fuel consumption is very low e.g. when the vehicle motor brakes. When there are errors in the communication between the vehicle and the dosing system, these errors resulting in a "no dosing" signal.

When the vehicle regenerates the particulate filter which results in increased temperature of the exhaust pipe. The increased temperature may result in formation of deposits from reducing agent present in the nozzle.

When the vehicle engine is switched off but the exhaust pipe is still warm enough to result in deposits being formed; the critical temperature depends on the reducing agent.

Based on the above realizations, an object of the present invention is to provide a dosing system in which the clogging up of the nozzle may be avoided.

It is a further object of the present invention to provide an alternative to the prior art.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a dosing system for dosing a first fluid into a stream of a second fluid, the system comprising a valve, a nozzle having an outlet arranged downstream of the valve, a flow passage through which the first fluid can flow from a reservoir to the nozzle outlet via the valve, - a control system adapted to receive input from one or more sensors and based thereon determine a request value based on which a valve opening period is determined, wherein the valve opening period is different from zero and independent on the request value when the request value is below a predefined threshold value.

The valve preferably has a closed state in which the first fluid cannot flow through the valve and at least one open state in which the first fluid can flow through the valve; the latter is called "the valve opening period". The actual control parameters used may not be the valve opening period but rather the time at which the valve is opened and the time at which the valve is closed. The difference there between is the time opening period.

In addition to the valve opening period, the flow of fluid through the valve may also be controlled by varying other parameters, such as the pressure. A further possibility is to supply the first fluid with pressure waves. The control system may also use further parameters than those measured by the sensors in the determination of the request value. The determinations may be based on predefined relationships between the request value and the further parameters, the relations being determined from e.g. experiments or computer simulations.

The dosing system may further comprise pumping means for pumping the first fluid through the dosing system. Alternatively the reservoir in which the first fluid is stored is pressurized, so that an opening of the valve will result in a flow of first fluid towards the nozzle.

The first fluid may flow out of the nozzle outlet intermittently at least when the request value is below the threshold value. By intermittently is preferably meant that the valve is open for a period of time followed by a period in which it is closed. The opening time and closed time may be of equal or different length. The intermittent flow enables that even small doses of the first fluid can flow through the nozzle under high pressure without unnecessary use of first fluid. A high pressure is used to flush the nozzle and thereby remove possible deposits formed in the nozzle.

In some embodiments of the invention, the nozzle comprises the valve. Hereby a compact and fluid-tight solution may be obtained. Alternatively the nozzle and valve are separate units which are to be connected by any suitable means preferably while ensuring a non-leaking connection.

In preferred embodiments, the nozzle may comprise an atomization device. Such a device is used to provide atomization of first fluid which atomization typically results in an efficient reaction between the first and second fluids. Such an atomization device may comprise at least two converging nozzle channels. Hereby the atomization can be obtained by letting fluid jets flowing through the nozzle channels impinge each other.

The input from the one or more sensors may comprise measures of the temperature of the second fluid and/or measures of the temperature of a wall of a pipe through which the second fluid flows. When the dosing system is arranged on a combustion engine vehicle, the input from the one or more sensors may comprise measures of one or more of the following parameters: the actual fuel consumption, revolutions per minute, and the loading of the engine. Other inputs may be used in addition to those specifically mentioned.

In an embodiment of the invention, the first liquid is a reducing agent, such as liquefied urea, and the second fluid is exhaust gasses from a combustion engine. A detailed description of such an embodiment is given below.

A second aspect of the invention relates to an exhaust system comprising a dosing system as described above, and an exhaust pipe through which the second fluid flows, wherein the nozzle outlet is arranged so as to feed the first fluid into the second fluid.

A third aspect of the invention relates to a method of dosing a first fluid into a stream of a second fluid by use of a dosing system comprising a valve, a nozzle having an outlet arranged downstream of the valve, a flow passage through which the first fluid can flow from a reservoir to the nozzle outlet via the valve, and a control system, the method comprising: measuring one or more parameter values by use of one or more sensors, using the measured values as input to a control system, - determining a request value based on the input, comparing the request value with a predetermined threshold value, and when the request value is below the predetermined threshold value determining a valve opening period which is independent on the request value, and when the request value is above or equal to the predetermined threshold value determining a valve opening period which is dependent on the request value.

The valve opening period and possibly also other parameters, such as the pressure of the first fluid, is determined so that a required amount of the first fluid is fed into the second fluid. What "a required amount" is, is predetermined e.g. from experiments or computer simulations.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

A dosing system according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 shows schematically a dosing system according to the invention arranged in combination with a combustion engine.

Figure 2 shows schematically an overall idea on which the invention is based.

Figure 3 is a flow diagram of a method according to the invention.

Figure 4 illustrates an example of a running scenario for a vehicle provided with a dosing system according to the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

The following description of a preferred embodiment of the invention relates to a combustion system having a dosing system according to the present invention. A schematic illustration of such a combustion system is given in figure 1. In this embodiment the first fluid referred to above is a reducing agent, and the second fluid is exhaust gas from a combustion engine. In a combustion engine vehicle, the exhaust gas leaving the engine 1 comprises nitrogen oxides before it enters into the catalytic system 2. After the exhaust gas has passed through the catalytic system 2, the gas comprises nitrogen and water which is typically discharged to the environment. It is known that this process is significantly improved if a reducing agent, such as liquefied urea, is sprayed into the exhaust gas by use of a nozzle 3 before it enters the catalytic system 2. The reduction agent is stored in a tank 4 and led to the nozzle 3 via a pipe 5 and a valve 6. The tank 4 in which the reducing agent is stored may be pressurized, or the reducing agent may be pumped to the nozzle 3 by a pump (not shown).

The dosing system comprises a valve 6 having a closed state in which the reducing agent cannot flow through the valve 6 and an open state in which the reducing agent can flow through the valve 6. In figure 1 the valve 6 and the nozzle 3 are shown as separate units, but the valve 6 may also be comprised in the nozzle 3. In embodiments where the nozzle 3 and valve 6 are separate units, the valve 6 may be placed anywhere between the tank 4 and the nozzle 3. More than one valve 6 may also be used. The dosing of reducing agent is controlled by a control system 7 receiving input from a number of sensors 8. The embodiment illustrated in figure 1 comprises sensors 8 arranged so that they can measure one or more characteristic(s) of the engine 1, the temperature of the wall of the exhaust pipe 9 and the temperature of the exhaust gas, respectively. The characteristics of the engine 1 may include the fuel consumption, revolutions per minute and the loading of the engine. The monitoring thereof is illustrated schematically by one sensor 8 only although in practice different characteristics may be monitored by different sensors. Other sensors may also be used in addition to or instead of those illustrated. Based on input from the sensors 8, the control system 7 determines the current demand for reducing agent, Q _de m _a n _d which is also called a request value, and the valve 6 is controlled in response thereto so that the determined amount of reducing agent is sprayed into the exhaust gas. The valve 6 is typically controlled in a pulse width modulated (PMW) manner whereby it is possible to ensure sufficient atomisation of the reducing agent even for relatively small demands. A first reason for basing the determination of the amount of reducing agent on the temperature of the wall of the exhaust pipe 9 and/or the temperature of the exhaust gas is that the reaction between most reducing agents and the NOx only takes place to a sufficiently large extent above a certain temperature. In the following, reference is made to a critical temperature, T _cr , which may be either a temperature of the exhaust pipe 9, a temperature of the exhaust gas, or a theoretical value determined from measures of the two first mentioned. Such a theoretical value could e.g. be a weighed average, the weight factors being determined e.g. from tests or simulations. In preferred embodiments of the invention, the temperature of the exhaust gas is determined before and after passing through the catalytic system, and T _cr is the lowest of those. This way of determining T _cr is based on the assumption that when the temperature is above T _cr both before and after the catalytic system 2, it is also above T _cr inside the catalytic system 2. Alternatively or in addition thereto, the temperature could be measured by a sensor placed inside the catalytic system 2. The determination of the needed amount of reducing agent is then based on a known dependence on T _cr . The dependence on T _cr may e.g. be determined from physical tests and/or computer simulations.

In known systems for dosing reducing agent to exhaust gasses, the amount of reducing agent is determined mainly from measures of the fuel consumption, the number of revolutions and the loading of the engine 1. However, in connection with the present invention it has been found that deposits from the reducing agent are also formed in the nozzle 3 when the demand for reducing agent determined from these parameters is very low or zero. This is particularly the case if the valve 6 is leaking resulting in a flow with very low pressure dripping through the nozzle 3. A situation with very little or no flow may also take place when the vehicle motor brakes or even is switched off while the exhaust pipe still has a high temperature.

A second reason for basing the determination of the amount of reducing agent on the temperature of the exhaust gas is therefore that it has been found advantageous to spray reducing agent into the exhaust pipe even when Q _de m _a n _d is zero. When this reducing agent is supplied at high pressure, the nozzle 3 is flushed and thereby cleared from deposits which have built-up inside the nozzle 3. A high pressure without unnecessary use of reducing agent is typically obtained by supplying the reducing agent intermittently. By intermittently is preferably meant that the valve 6 is open for a short period of time followed by a period in which it is closed. The valve opening period may e.g. be 30 ms, and the period between each dose may e.g. be 30 s. The pressure is typically in the order of 5 to 20 bars. With typical nozzle dimensions and pressures used in an exhaust system, a valve opening period of 30 ms typically results in a dose of 0.033-0.05 ml/dose.

In the following two modes of operation are referred to: dosing and purging. "Dosing" is preferably used to designate the periods in which the request value, Qdemand, determined from inputs from the sensors is at or above a predetermined threshold value, Q _cr . In this mode, the current supply of reducing agent typically varies in response to parameters such as the fuel consumption. "Purging" is preferably used to designate the periods in which Q _de m _a n _d determined from inputs from the sensors 8 is below Q _cr . In this mode, the supply of reducing agent is typically supplied intermittently as described above but with a substantially constant amount being supplied during a time period covering one pulse. Figure 2 illustrates schematically the overall difference between a known system and a system according to the present invention. Figure 2. a illustrates a known system having a linear relationship between the demanded amount, Q _de m _a n _d , and the amount supplied through the nozzle, Qoutput- Figure 2.b illustrates in a corresponding way the idea of the present invention that when Q _de m _a n _d is below a threshold value, Q _cr , a constant amount of reducing agent is added to the exhaust gas. As described above, "constant" is not necessarily to be understood as if the valve 6 is constantly open. When the vehicle is running and the determined temperature, T, of the exhaust gas and/or the exhaust pipe (see details of how it may be determined above) is above a critical temperature, T _cr , , Q _de m _a n _d mainly or fully depends on engine parameters such as the fuel consumption. The actual relationships may be linear or have any characteristic which will be well-known to a person skilled in the art.

Tests have been carried out in exhaust systems where liquefied urea was used as reducing agent. It was found that when the engine 1 was switched off but the temperature inside the exhaust pipe 9 was still above around 200 ⁰ C, urea deposits were formed in the nozzle 6. Therefore, in this case T _cr may be set to 200 ⁰ C. However, deposit formation may also start at 170-180 ⁰ C, and the actual T _cr should therefore be adjusted to a specific application.

Steps involved in the application of a method according to the present invention are described in the flow diagram in figure 3. The figure is to be understood so that it should be re-read from the top left corner each time the engine 1 is switched on or off and each time there is a change between idle and running. In the top left corner is shown a determination of whether or not the engine 1 is switched on. It may be necessary to keep supplying reducing agent in purging mode even after the engine 1 has been switched off for as long as the determined temperature, T, is above T _cr in order to keep removing deposits from the nozzle 3 for as long as they may be formed. When the engine 1 is idle, T is also compared to T _cr resulting in purging when T≥T _cr . Both when it is idle and when it is not (i.e. the vehicle is running), the comparison between T and T _cr must be repeated at regular time intervals or constantly, since T is likely to vary over time when the engine 1 is idle due to heating up or cooling down of the exhaust gas and/or the exhaust pipe 9. When the vehicle is running, the first step is again to determine whether T≥T _cr and if it is to start the addition of reducing agent in response to input from the sensors 8. The reason why reducing agent is preferably never added when T<T _cr is that the reaction between most reducing agents and the NOx only takes place to a critical extent above a certain temperature. When the vehicle is running and T≥T _cr , a demanded amount, Qdemand, is determined based on the current fuel consumption and possibly other parameters. As described in relation to figure 2, the dosing system runs in dosing mode when Q _de m _a n _d ≥Q _α - and in purging mode when Q _d emand<Qcr.

Figure 4 illustrates a possible running scenario for a vehicle provided with a dosing system according to the present invention. The dashed line represents the requested dosing, Q _demand , which in the figure is given in ml/hour as shown on the right-hand y-axis. The dash-dot-dot line represents the temperature of the exhaust gas as shown on the left-hand y-axis, and the solid line represents the purging. The engine is assumed to be switched on at time equal to zero, and T _cr is set to 200 ⁰ C. When the temperature reaches T _cr , the dosing system runs in dosing mode where the amount of supplied reducing agent is determined as described above. When Q _de m _a n _d falls to below Q _cr at around 41 min, and T is still above 200 ⁰ C, the system switches to purging mode. This is typically the case when the engine is idle. Around 70 min, Q _de mand increases to above Q _cr , and the system switches to dosing mode. Around 115 min, the system is in purging mode, and T falls below 200 ⁰ C resulting in a situation with no dosing and no purging.

The flow of reducing agent through the nozzle even when the request value is below a predefined threshold value may prevent clogging up of the nozzle for at number of reasons including:

Clearing the nozzle from any deposits formed therein by flushing with reducing agent at high pressure.

Renewing the reducing agent in the nozzle results in a decrease in the concentration inside the nozzle.

Cooling of the nozzle.

In the embodiments illustrated above, the dosing system comprises only one nozzle. However, the system may comprise two or more nozzles, which may e.g. be arranged circumferentially in the wall of the exhaust pipe. The reducing agent may be supplied to all nozzles via one valve, there may be one valve per nozzle, or a number of valves may each be used to control the flow of reducing agent to two or more nozzles.

In addition to the examples given above, a situation in which Q _de m _a n _d is very low or zero is when this is due to errors in the system, such as errors in the transmission of signals from the sensors, resulting in an erroneous Q _de m _a n _d being determined. In a known system this could result in no supply of reducing agent through the nozzle. In a dosing system according to the present invention, such a situation would result in the system running in purging mode. This could mean an insufficient reduction of the pollutants, but at least the nozzle would not clog up.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The present invention has been described in relation to a combustion system having a dosing system according to the claims. However, the invention may also find use on other systems where it is relevant to dose a first fluid into a stream of a second fluid while ensuring that when Qdemand<Qcr, a constant amount of first fluid is dosed independently of input from sensors. An example of a further application is for the exothermic media injected into the diesel particulate filter to burn out the carbon deposits.

The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.