Method for a fluid delivery system

Technical Field

0001The present invention relates to a fluid delivery system for delivering a metered dose of fluid from a supply tank to a downstream vessel or chamber. In particular, but not exclusively, the invention relates to fluid delivery system including a pump apparatus for delivering a metered dose of reagent fluid, such as urea, to an exhaust gas pipe of an internal combustion engine or other equivalent device. The invention also relates to a method of determining the total volumetric delivery of a fluid to the downstream vessel or chamber and/or to a method of determining a remaining level of said fluid in the fluid supply tank.

Background to the Invention

0002Internal combustion engines emit undesirable pollutants in their exhaust stream. One such pollutant is nitrogen oxides, comprising nitrogen monoxide and nitrogen dioxide (hereinafter referred to simply as "NOx"). NOx is generated from automobile engines such as diesel engines, and other combustion devices. Accordingly, exhaust system devices are coupled to the engine to limit and/or remove the pollutants from the exhaust system. Technologies have been and continue to be developed to attenuate these emissions.

0003NOx may be cleaned from exhaust gases of internal combustion engines through the use of catalysis. In addition to removing NOx, other catalysts may be employed to also remove unburned hydrocarbons (HC) and carbon monoxide (CO). When the engine is operated with a lean air/fuel ratio, the catalyst is efficient at removing the HCs and COs because of the extra oxygen in the exhaust gas. However, the extra oxygen tends to inhibit the removal of NOx.

0004In the case of exhaust gas from gasoline engines, NOx is usually removed by using so-called three-way catalysts. Also, in the case of large, stationary combustion apparatus, such as internal combustion engines for cogenerators, metal oxide catalysts such as V₂O₅ are used, and ammonia is introduced into the exhaust gas stream, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced.

0005Conventional Selective Catalytic Reduction (SCR) of NOx involves injection of a typically aqueous urea solution or reductant into the exhaust system ahead of the SCR catalyst. Common reductants include aqueous urea in conjunction with selective catalytic reduction, and perhaps hydrocarbon diesel fuel, for the supplemental heat necessary to initiate particulate trap regeneration. As used herein the term "urea" is meant to encompass urea in all of its commercial forms, including those containing: ammelide; ammeline; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids, including sulfuric acid and phosphoric acid; ammonium salts of organic acids, including formic and acetic acid; biuret; cyanuric acid; isocyanic acid; melamine and tricyanourea.

0006The reductant dosing system is required to accurately meter the reductant into the exhaust system, while being robust in service and responsive to the engine or after treatment control system. Accordingly, most prior art dosing systems have been designed for delivery of reductant in a two fluid system, namely the reductant and a supply of pressurized air. Such systems make use of a simple low pressure metering pump for delivering the reductant into a mixing chamber where it co-mingles with pressurized air from an onboard source. This mixture is conducted through a pipe to the remote dosing location in the exhaust where it exits through a simple atomizing nozzle into the exhaust stream. A typical air atomizing pressure for this type of system might be 2 bar. By way of example, <patcit id="pcit0001" dnum="EP1149991A"><text>EP 1 149 991</text></patcit> describes a two fluid system of the aforementioned type.

0007Since urea is not essential to the functioning of the engine, it is possible to operate the engine even when the supply of urea is low or has been depleted altogether. However, such a condition is unacceptable, not least because the engine then no longer complies with statutory regulations for emissions levels. It is therefore known to provide the urea tank with a urea level sensor (not shown) so that imminent exhaustion of the supply can be forewarned. <patcit id="pcit0002" dnum="US6063350A"><text>US 6063350</text></patcit>, for example, describes a reagent dosing system in which such a level sensor is used for this purpose.

0008Aqueous urea is a common reagent in such systems, but the characteristics of the fluid are problematic in certain respects. For example, aqueous urea can form hard salt crystals which clog mechanisms and disrupt regular operation. For this reason, fluid level sensors of the mechanical type, which employ a float on a pivoting arm connected to a variable resistance coil, are unsuited for use in a urea supply tank. Typically, therefore, a more expensive urea level sensor is employed that is immune from clogging.

0009It is one object of the invention to provide a system for use in delivering reagent in an SCR exhaust gas cleaning system of an internal combustion engine in which the requirement for a separate reagent level sensor is removed altogether.

Summary of the Invention

0010According to a first aspect of the present invention, there is provided a fluid delivery system comprising a pump apparatus for delivering a metered dose of fluid from a supply tank to a downstream vessel. The pump apparatus comprises a pump plunger which is operable to perform a pumping stroke under the control of an electromagnetic actuator, including a solenoid, to effect delivery of the fluid, and a control unit for supplying an input signal to the solenoid to initiate a current flow to the solenoid and thereby initiate movement of the pump plunger. The fluid delivery system further includes an electronic device which provides an output signal when movement of the pump plunger stops at the end of its pumping stroke, and a timer is provided for determining a time difference between the input signal being supplied to the solenoid and the output signal being output by the electronic device. A processor compares the time difference signal with a predetermined time difference and determines, as a result of the comparison, whether or not the pump has performed a valid pumping stroke in which an intended volume of fluid is displaced.

0011In another aspect, the invention relates to a method for determining an actual total number of valid pumping strokes performed by the pump apparatus of the fluid delivery system.

0012The pump apparatus includes a pump plunger for performing a pumping stroke of fixed displacement/stroke.

0013By calculating the actual total number of valid pumping strokes, it is possible to determine with accuracy the total volumetric delivery of fluid to the downstream vessel. If the level of fluid within the supply tank is known at start of pumping (i.e. an initial condition), it is therefore possible to determine with accuracy the remaining level of fluid within the supply tank.

0014The invention is particularly suitable as a reagent dosing system of an internal combustion engine. The system may be used in a two fluid system of the type described previously, in which reagent is delivered to an engine exhaust chamber (the downstream vessel) by the pump apparatus where it is mixed with pressurised air. In such systems the pressure of reagent delivered by the pump apparatus tends to be relatively low. In single fluid systems no source of pressurised air is provided, in which case the pump apparatus may be arranged to deliver a higher pressure of reagent fluid to the exhaust chamber. By way of example, the invention may be implemented in a reagent dosing system of the type described in <patcit id="pcit0003" dnum="US20040093856A1"><text>US Patent Publication No. 2004-0093856-A1</text></patcit>.

0015The invention may also be used for fluids other than a reagent. It may be adapted for use as a fuel injection system on board a remotely piloted or robotized vehicle of the type powered by an internal combustion engine, for example in which a computation of fuel used and thus fuel remaining on board may be used to establish mission duration.

0016In one embodiment of the system, the pump apparatus includes an armature coupled to the pump plunger which is attracted to the solenoid when a current is supplied thereto, thereby causing the pump plunger to move.

0017In one embodiment of the system, the processor forms a part of the control unit but, alternatively, it may be a separate component.

0018The processor may include a counter for counting the number of valid pumping strokes so as to determine a total volumetric quantity of fluid delivered to the downstream vessel since an initial condition.

0019Conveniently, the processor is operable to determine a value for a remaining level of fluid within the supply tank based on (i) the total volumetric quantity of fluid delivered to the downstream vessel since the initial condition and (ii) a fluid level in the supply tank at the initial condition.

0020It is convenient for the initial condition to be the point at which the supply tank is at full level. For example, if the supply tank is a replaceable canister having a known full level, the canister may be replaceable each time the processor determines that the supply tank has depleted below a predetermined amount. Replacement of the canister defines the initial condition.

0021In one embodiment the electronic device is a glitch detect circuit for determining when movement of the pump plunger has stopped at the end of its pumping stroke.

0022By way of example, the glitch detect circuit determines when movement of the pump plunger has stopped at the end of the pumping stroke by monitoring a change in current decay through the solenoid.

0023Normal valid pumping strokes in which a full charge of reagent is pressurised and discharged through its corresponding nozzle would be expected to occur within a defined time window. Any glitch detections within this window could be considered valid and the window position/width may be varied in dependence upon friction, temperature, system voltage, wear or other factors. The glitch detect circuit may therefore monitor glitch events against a valid event window, with glitch events outside this window being deemed invalid pumping strokes and glitch events inside the window being deemed valid pumping strokes.

0024In another aspect of the invention, there is provided a method for determining the total number of valid pumping strokes performed by a pump apparatus of a fluid delivery system for delivering a metered dose of fluid from a supply tank to a downstream chamber or vessel. The method may comprise driving a pump plunger of the pump apparatus to perform a pumping stroke under the control of an electromagnetic actuator, including a solenoid, to effect delivery of the fluid, supplying an input signal to the solenoid to initiate a current flow to the solenoid and thereby to initiate movement of the pump plunger, and counting the number of input signals so as to determine a total number of pumping strokes that are initiated. A halt to movement of the pump plunger at the end of the pumping stroke is detected and an output signal is generated to indicate said halting of pump plunger movement. A time difference signal is determined between the input signal being supplied to the solenoid and the output signal being generated, and the time difference signal is compared with a predetermined time difference representative of a valid pumping stroke. As a result of the comparison, it is determined whether or not the pump plunger has performed a valid pumping stroke and, if it is determined that the pump plunger has not performed a valid pumping stroke, the pumping stroke is discounted from the total number of pumping strokes so as to determine an actual number of valid pumping strokes which have contributed to depletion of fluid within the supply tank.

0025Pumping strokes that are initiated, but which do not contribute a full delivery of fluid to the total dose to the downstream chamber, are not considered to have contributed to depletion of the supply tank. By discounting invalid pumping strokes from the total stroke count, an accurate determination can be made of when the supply tank has depleted, or is within a predetermined tolerance of being depleted.

0026The method may further comprise determining a value for the volume of fluid delivered by a single, valid pumping stroke, and using the actual number of valid pumping strokes and the value for the volume of fluid delivered by a single, valid pumping stroke to determine the remaining level of fluid within the supply tank.

0027The method may still further comprise determining a value for the volume of fluid delivered by a single, valid pumping stroke, and using the actual number of valid pumping strokes and the value for the volume of fluid delivered by a single, valid pumping stroke to determine the total volume of fluid delivered since an initial condition.

0028Conveniently, the initial condition is equivalent to the supply tank being at a predetermined known full level.

0029Conveniently, the step of determining the value for the volume of fluid delivered by a single, valid pumping stroke, includes interrogating a control unit of the fluid delivery system to look-up a pre-stored value of said volume.

0030Conveniently, the step of providing an output signal when movement of the pump plunger has stopped at the end of its pumping stroke includes using a glitch detect circuit to determine when movement of the pump plunger has stopped.

0031For example, a halt to movement of the pump plunger at the end of the pumping stroke may be determined by monitoring a change in slope of current flow through the solenoid or by detecting a glitch or discontinuity in said current flow. (In other words, the method may include determining when movement of the pump plunger has stopped by monitoring a change in slope of current flow through the solenoid or a discontinuity in said current flow).

0032Conveniently, the method may include providing an alert signal if it is determined that the remaining level of fluid within the supply tank drops below a predetermined level.

0033The invention relates, in another aspect, to a reagent dosing system for an internal combustion engine including a fluid delivery system as set out in the first aspect of the invention for delivering a metered dose of fluid from a reagent supply tank to an exhaust passage of the engine. The invention relates in a further aspect to a fuel injection system for an engine including a fluid delivery system as set out in the first aspect of the invention for delivering an accurate volume of fuel to an injection nozzle of the engine.

Brief Description of the Drawings

0034The invention will now be described, by way of example only, with reference to the following Figures in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> is a schematic illustration of an exhaust system including a reagent delivery system in accordance with a first embodiment of the present invention;</li><li><figref idref="f0002">Figure 2</figref> is a cross sectional view of a metering pump for use in the reagent delivery system of <figref idref="f0001">Figure 1</figref>;</li><li><figref idref="f0003">Figure 3</figref> is a schematic diagram of a fluid delivery system of one embodiment of the present invention, including a metering pump as in <figref idref="f0002">Figure 2</figref> together with a glitch detect circuit and an engine control unit, for determining a number of valid plunger strokes of the metering pump;</li><li><figref idref="f0003">Figure 4</figref> is a schematic diagram of a processor of the engine control unit of the system in <figref idref="f0003">Figure 3</figref>;</li><li><figref idref="f0004">Figure 5</figref> is a flow diagram to illustrate a method of one embodiment of the invention for determining the supply tank level of a source of reagent in the fluid delivery system of <figref idref="f0003">Figure 3</figref>; and</li><li><figref idref="f0005">Figure 6</figref> shows logic pulse, drive current and plunger motion traces with respect to time and illustrates valid and invalid pumping events.</li></ul>

Detailed Description of the Drawings

0035Referring to <figref idref="f0001">Figure 1</figref>, an exhaust system for an internal combustion engine includes an exhaust pipe 10 having an inlet end 12 in communication with an exhaust of an engine (not shown) and an outlet end 14 in communication with atmosphere. The exhaust pipe is provided with a diesel particulate filter, located approximately at position X, and a selective catalytic reduction (SCR) catalyst located approximately at position Y. A mixing section 16, intermediate the particulate filter and the SCR catalyst, accommodates a nozzle of an injection device 18 for delivering a reagent into the mixing section 16 of the exhaust pipe 10.

0036The injection device 18 is positioned so as to deliver an atomised spray of reagent to the exhaust flow as it flows between the filter and the catalyst. A pump apparatus in the form of a metering pump 20 delivers a known quantity of reagent to the injection device 18 through a supply pipe 22 in a known time under the control of an engine control unit (ECU) 24. The ECU 24 includes a microprocessor 26 loaded with an algorithm to control operation of the metering pump 20 in dependence on various operating parameters. The metering pump 20 communicates with the supply tank 28 of reagent fluid via an inlet supply pipe 30. Typically, the reagent is aqueous urea. In other systems (not shown) the diesel particulate filter is downstream of the SCR catalyst, in which case the metering pump 20 delivers reagent to a mixing section upstream of the SCR catalyst and diesel particulate filter.

0037The metering pump 20 is shown in more detail in <figref idref="f0002">Figure 2</figref>, and includes a pump plunger 32 which is slidable within a stepped bore of a pump housing 34 under the control of a solenoid-controlled actuator, referred to generally as 36. The actuator 36 includes a solenoid and stator (not visible), to which a current is supplied to generate a magnetic field, and an armature 38 coupled to the pump plunger 32 at its upper end. The solenoid and stator are housed within a solenoid housing 40 which abuts the pump housing 34 at its upper end. The frame and structure surrounding the solenoid, the upper end of the pump plunger 32 and a stationary magnetic pole (not shown) provide a flux path to focus the magnetic field. The armature 38 reacts to the magnetic field and is attracted to the stationary pole, thereby causing the pump plunger 32 to move within the pump housing 34.

0038The pump plunger 32 includes a flange 42 at its lower end, the upper surface of the flange defining an abutment surface for a biasing component in the form of a spring 44. The spring 44 serves to urge the pump plunger 32 in a downward direction into a position in which the lower surface of the flange 42 abuts the end of the bore in the pump housing 34 when the solenoid of the actuator 36 is de-energised. When energized, the armature 38, the pump plunger 32 and the flange 42 move upwards until the upper surface of the flange 42 abuts a step in the bore of the pump housing 34, thereby arresting its motion. By controlling the current supplied to the solenoid, movement of the pump plunger 32, back and forth within the bore, can therefore be controlled. Because the pump plunger 32 operates between two hard stops described, it is therefore a fixed stroke device providing a fixed and predetermined displacement.

0039The metering pump 20 draws aqueous urea from the supply tank 28 through the inlet supply pipe 30 which delivers the urea through an inlet check valve 46 to a pump chamber 48 defined within the stepped bore of the pump housing 34. As the pump plunger 32 is moved up and down by the actuator 36, the pressure of fluid within the pump chamber 48 is increased and is dispersed under pressure into the outlet supply pipe 22, via an outlet valve 50 and, hence, to the injection device 18.

0040By way of example, the solenoid may be an E-core type of solenoid having a short stroke and providing high force with a correspondingly high injection pressure. As an alternative to using a solenoid-actuated pump plunger 32, a piezoelectrically-actuated pump plunger may be used. A further suitable metering pump is the reagent pump disclosed in <patcit id="pcit0004" dnum="EP1878920A"><text>EP1878920</text></patcit>.

0041In normal use, the metering pump 20 is operated so as to deliver a known volume of urea to the injection device 18, and therefore the exhaust stream, in a known time and in such a manner that appropriate proportionality is maintained between engine NOx production and the reagent delivered. This may be achieved by varying the actuation cyclic frequency or repetition rate of the pump plunger 32 accordingly. Thus, by counting and totalizing the number of strokes of the pump plunger 32 it is possible to determine the total flow through the pump 20 in unit time. Furthermore, if the volumetric capacity of the supply tank 28 is known, and the displacement volume of the metering pump 20 is known, it is possible to determine the number of strokes of the pump plunger 32 that will exhaust the supply tank 28.

0042For example, if the volumetric capacity of the supply tank 28 is 100 litres and the displacement volume of the pump 20 is 4.5 cubic millimetres, the number of strokes of the pump plunger 32 required to exhaust the supply tank 28 is given by: <maths id="math0001"><math display="block"><mn>100</mn><mspace width="1em" /><mi>x</mi><mspace width="1em" /><mfenced><mn>1000</mn><mspace width="1em" /><mi>x</mi><mspace width="1em" /><mn>1000</mn></mfenced><mo>=</mo><mn>100</mn><mo>,</mo><mn>000</mn><mo>,</mo><mn>000</mn><mspace width="1em" /><mi>cubic millimetres</mi><mo>/</mo><mn>4.5</mn><mo>=</mo><mn>22</mn><mo>,</mo><mn>222</mn><mo>,</mo><mn>222</mn><mspace width="1em" /><mi>strokes</mi><mn>.</mn></math><img file="EP2017444A2_D0001.tif" /></maths>

0043The remaining volumetric content of the supply tank 28 may also be deduced by referencing the totalized number of valid pump plunger strokes at any point in time to the number above. For example, if the total valid stroke count is 18, 500, 000, the remaining volumetric content of the supply tank 28 will be: <maths id="math0002"><math display="block"><mn>22</mn><mo>,</mo><mn>222</mn><mo>,</mo><mn>222</mn><mo>-</mo><mn>18</mn><mo>,</mo><mn>500</mn><mo>,</mo><mn>000</mn><mo>=</mo><mn>3</mn><mo>,</mo><mn>722</mn><mo>,</mo><mn>222</mn><mspace width="1em" /><mi>x</mi><mspace width="1em" /><mn>100</mn><mo>/</mo><mn>22</mn><mo>,</mo><mn>222</mn><mo>,</mo><mn>222</mn><mo>=</mo><mn>16.75</mn><mo>%</mo><mn>.</mn></math><img file="EP2017444A2_D0002.tif" /></maths>

0044Since the remaining volumetric content of the supply tank 28 can be calculated using the aforementioned technique, it therefore becomes possible to eliminate the expensive reagent level sensor that is commonly provided to monitor the remaining level of reagent within the tank. Alternatively, this technique may be used to supplement the information from a low-cost low resolution reagent tank level sensor.

0045The inventor has, however, now identified a problem with the aforementioned method, namely that not all strokes of the pump plunger 32 necessarily give rise to displacement of the full theoretical volumetric discharge. Such an effect leads to inaccuracies in the total volumetric displacement through the pump 20.

0046One example of where such a situation may occur is if the reagent fluid within the supply tank 28 changes phase to a gas due to elevated temperature. Another example is where the temperature of reagent fluid within the supply tank 28 drops to below about -11° Celsius, the temperature at which the reagent fluid freezes. Should freezing occur, it is possible that the ECU 24 may demand a discharge of urea at a time when the system is inoperable. In such circumstances, activation of the metering pump 20 can be used to accelerate the defrosting process as described in co-pending European patent application No. <patcit id="pcit0005" dnum="EP06253638A"><text>06253638.8</text></patcit> (Delphi Technologies, Inc.), or intermittent pumping may be commanded throughout the intermediate phase between frozen and normal operation. In this event, simply counting the number of pump plunger strokes will give a misleading representation of the total volumetric displacement of the pump 20 during this period.

0047Not only does frozen reagent fluid within the supply tank 28 give rise to an error in the total volumetric displacement of the pump 20, but it can also lead to a mismatch between the demanded urea dose based on NOx production. The present invention recognises these problems and provides a modification to the aforementioned pumping stroke counting method so as provide an alert that such an error or mismatch has occurred.

0048The invention utilises a means for counting the number of strokes for which the pump plunger 32 completes a full valid stroke, by, for example, using a glitch detection method. Glitch detection techniques are known in fuel injection systems, for example as described in granted US patents <patcit id="pcit0006" dnum="US5668476A"><text>US 5668476</text></patcit>, <patcit id="pcit0007" dnum="US5959825A"><text>US 5959825</text></patcit> and <patcit id="pcit0008" dnum="US5803049A"><text>US 5803049</text></patcit>. Another glitch detection technique is described in <patcit id="pcit0009" dnum="US4140084A"><text>US 4140084</text></patcit>.

0049Referring to <figref idref="f0003">Figure 3</figref>, the apparatus for counting the number of full strokes for the pump includes a glitch sensor circuit 54 and a timer circuit or device 56. The ECU 24 includes a processor 26 and a current drive circuit 27 and receives a number of signals 29 from the engine. The signals 29 may include a NOx sensor output signal and an ammonia sensor output signal. In response to the signals 29, the ECU 24 determines a demanded urea dose based on a model, look-up tables or data maps stored in the ECU 24. Once the demanded urea dose is determined, the processor 26 of the ECU 24 calculates the number of pumping strokes and/or an actuation frequency required for the pump 20 to deliver the demanded dose and outputs a first energisation signal to the current drive circuit 27 which delivers a current 58 to the solenoid 36a of the actuator. Each time an energisation signal is output to the current drive circuit 27 to cause a drive current 58 to be supplied to the solenoid 36a, the ECU 24 provides a simultaneous signal 60 to the timer device 56 which records the time at which the signal 60 was received.

0050When the drive current 58 is output to the solenoid 36a, the resulting current flow through the solenoid 36a causes a magnetic field to attract the armature 38, thereby causing the pump plunger 32 to move upwardly within the pump housing 34 against the force of the spring 44. This is referred to as the plunger pumping stroke. At the end of the upward stroke the current is removed and the plunger 32 performs the downward stroke under the action of the spring 44. This is referred to as the plunger return stroke. The pumping stroke of the plunger is of fixed stroke length due to the flange 42 engaging the blind end of the bore in the pump housing 34 at the start of the stroke, and engaging with the step in the bore in the pump housing 34 at the other end of the stroke.

0051Just before the pump plunger 32 reaches the end of its pumping stroke, with the upper surface of the flange 42 just spaced from the step in the pump housing bore, the drive current is removed and current flow through the solenoid 36a decays at a relatively low rate. Just before the current falls to zero, and before the flange 42 of the pump plunger 32 has moved into engagement with its stop (i.e. the step in the pump housing bore), a drive current is re-applied to the winding again for a short time. Just after this current is removed, movement of the pump plunger 32 comes to a stop and, at this instant, a small glitch or discontinuity can be seen in the drive current waveform. The current waveform is monitored by the glitch detect circuit 54, which provides an output signal 62 to indicate when the discontinuity has occurred.

0052The output signal 62 is provided to the timer circuit 56, where a time difference is measured between (i) the signal 62 to indicate that the pump plunger has stopped moving and (ii) the signal 60 to indicate that movement of the pump plunger was initiated.

0053A time difference signal 64 representing the difference between the timing of the signals 60, 62 is provided to the ECU 24 where the processor 26 compares it with a predetermined value for the time difference expected for a pump plunger 32 having completed a normal pumping stroke (referred to as a valid pumping stroke). During the pumping event, a pressure develops within the chamber 48, and ultimately within the supply pipe 22, due to movement of the pump plunger 32. This pressure is a reflection of the characteristic of the injection device 18. In normal operation, the time taken from the application of the drive current 58 to the actuator 36 to cause movement of the plunger 32, build-up of the pressure in the pump chamber 48, and delivery of the glitch detect signal 62 will be a finite time, for example of the order of one millisecond. If the computed time difference 64 is greater or less than the expected time difference, a decision must be made as to whether that pumping stroke should be counted as valid, or whether the pumping stroke should be discounted as invalid. For example, if the calculated time difference is not within a pre-determined tolerance of the expected time difference for a normal pumping stroke, the pumping event is deemed invalid.

0054By way of example, if the supply tank 28 were to become depleted or if reagent in the supply pipe 30 or pump chamber 48 were to change from liquid to gas phase, the pump 20 would not be able to generate the expected pressure in the pump chamber 48 with the result that the resistance to travel as seen by the plunger 32 and the actuator 36 would be less than for a normal pumping stroke. In this case, the time interval between the timer 56 receiving the signal 60 and the glitch detect signal 62 would be shorter than normal (i.e. shorter than for a normal pumping stroke). Conversely, if the temperature of the pump 20 and the reagent were below its freezing point or otherwise clogged with salt crystals, then the plunger 32 might not move at all, or only very slowly, despite a drive current 58 being supplied to the actuator. In this case the time interval between the timer 56 receiving the signal 60 and the glitch detect signal 62 would be longer than normal. In both of these examples, a comparison of the measured time difference signal 64 with a predetermined value for the time difference signal which represents a normal pumping event provides a means of determining whether or not the pumping event is valid and should be counted.

0055Other glitch detect methods may be employed to determine the end of the plunger stroke whilst using the same principle of monitoring the current drive waveform (for example, as described in the above-referenced patents). For example, a change in the slope of current decay or current flow through the solenoid may be used to identify the end of plunger movement. Also, instead of monitoring the end of the pumping stroke using a glitch detection technique, it is also possible to monitor the end of the plunger return stroke to determine whether a valid pumping event has occurred.

0056In order to calculate a total volumetric displacement of the pump apparatus since an initial condition (e.g. since the supply tank was at full level), the aforementioned method is carried out to count the number of pumping strokes, except that if the difference signal 64 output from the timer circuit 56 indicates an invalid pumping stroke, the totalized count is not incremented. By knowing the actual pump volume displacement of a single, valid pumping stroke, the total volume displacement of the pump since an initialized condition can then be determined by multiplying the actual pump volume displacement of a single, valid pumping stroke by the actual number of valid pumping strokes that have been counted.

0057The glitch detect circuit 54 is therefore used to eliminate from the total pumping stroke count any plunger stroke which is not valid i.e. which would not have caused a full volumetric displacement of reagent to the injection device 18. In this way, inaccuracies in the stroke of the plunger 32, for example due to problems at low temperatures, which would otherwise give rise to inaccuracies in the total flow calculation can be eliminated or substantially removed.

0058<figref idref="f0003">Figure 4</figref> shows the components of the processor 26 of the ECU 24 for determining the total volume displacement of the pump. The time difference signal 64 is input to the processor 26 where it is received by a comparator 66 which compares the time difference signal 64 with a pre-stored time difference expected for a normal pumping stroke. If the time difference signal 64 is within a predetermined tolerance of the pre-stored time difference, the comparator 66 outputs a valid count signal 68 to a counter unit 70. If the comparator 66 determines that the time difference signal is not within a predetermined tolerance of the pre-stored time difference, an invalid count signal 82 is output to an emissions on-board diagnostic module (OBD Module) 84, which records that an invalid pumping stroke has occurred. Knowledge of invalid pumping strokes may be provided to the OBD module 84 to aid in the identification of ineffective or malfunctioning operation of the reagent dosing system.

0059The counter unit 70 delivers a total count output signal 72 to a calculation unit 74 which calculates the total pump volume delivered since the initial condition (e.g. since the supply tank 28 was at full level). The calculation unit 74 also compares the total count with the known number of total pumping strokes required to deplete the supply tank 28 fully and provides an output signal 76 to a tank content indicator 78.

0060The tank content indicator 78 provides an indication of the remaining level of reagent within the supply tank 28. The remaining level of reagent within the supply tank 28 can therefore be determined by subtracting the calculated value for total pump delivery volume from the level of reagent at the initial condition.

0061It is not necessary for the calculation unit 74 to calculate the remaining tank contents after each valid pumping stroke (e.g. after each count is incremented by the counter unit 70), and instead a count request signal 80 is output from the calculation unit 74 to the counter unit 70 at a set frequency (e.g. every 5 minutes) to initiate the total count signal 72 being provided to the calculation unit 74.

0062The processor also includes a non-volatile memory (not shown) to which the current total count of valid pumping strokes is stored. It is important that the memory is non-volatile so that, for example, in the event of disconnection of the battery of the vehicle in which the system is used, the total count of valid pumping strokes is not lost.

0063In order to use the calculation of the number of valid plunger strokes to determine the remaining level of fluid in the supply tank 28, it is necessary to have an accurate indication of the level of fluid at the initial condition (e.g. when the supply tank was full). This may be achieved, for example, by using a replaceable canister for the supply tank 28, which is replaced each time there is an indication that the canister has depleted below a certain level. By using a replaceable canister, the initial volume of fluid within the canister is always known accurately at the start of delivery (i.e. the initial condition referred to above). The calculation unit 74 therefore also receives a re-fill input signal 86 which provides an indication to the comparator that the canister has been replaced and the total pump delivery since the initialising condition should be reset to zero.

0064If it is determined that the level of fluid within the supply tank 28 drops below a predetermined level, an alert signal 88 is provided by the ECU 24, as shown in <figref idref="f0003">Figure 3</figref>, to provide an indication that action needs to be taken by the operator to re-fill or replace the supply tank 28.

0065<figref idref="f0004">Figure 5</figref> is a flow diagram to illustrate the aforementioned method steps for providing an indication of the level of the supply tank to the tank content indicator 78.

0066One potential difficulty with the aforementioned counting method is that the actual displacement of the metering pump 20 may be slightly less or more than the specified value due to manufacturing tolerances. Over the course of depleting the supply tank 28, any such discrepancy between the actual pump volume displacement and the specified pump volume displacement may therefore accumulate a significant error both in the total volumetric delivery and thus in the remaining tank content computation.

0067In order to overcome this potential difficulty, a modification to the aforementioned method may be used which includes running the metering pump 20 during manufacture through its range of operation and over a known number of strokes so that its actual volume displacement can be measured and compared to the theoretical volume displacement, and an adjustment or offset applied to the inrunning calculation.

0068<figref idref="f0005">Figure 6</figref> shows a logic pulse trace, drive current trace and plunger motion trace with respect to time.

0069The top trace 100 shows the "logic signal" issued by the ECU 24 to the solenoid drive circuit 27. This is typically a 5 volt TTL signal. The second trace 102 illustrates the drive current 58 applied to the solenoid 36a. The third trace 104 shows the pumping plunger (32) motion moving from its default un-energised position up to its full stroke position under the influence of the solenoid actuator 36a.

0070Timing line "A" indicates the start of logic which thus initiates the time clock 56. Timing line "B" indicates the glitch detection point (glitch 106 in trace 102) for an invalid pumping event in which the stroke was made against no resistance (other than the return spring 44). Timing line "C" indicates the glitch detect point (glitch 108 in trace 102) for a typical valid pumping stroke, with C1 and C2 being the extent of the valid timing window. Timing line "D" indicates the end of logic and thus the end of the drive current and the return of the pumping plunger 32 to its start position under the influence of spring 44.

0071The time interval A-C is typical for a normal valid pumping stroke in which a full charge of reagent is pressurized and discharged through its corresponding nozzle. Glitch detections which fall within the C1-C2 window are also considered valid, and the variance may be due to friction, temperature, system voltage, wear or other factors. Indeed, the window position and/or width may be made to vary as a function of any of these parameters during use if active feedback is provided.

0072Time interval A-B illustrates an invalid pumping event in which the actuator 36a faced low resistance due, for example, to lack of reagent to pump. Timing point B comes from the detection of the glitch in the current trace (broken line) which occurs when the solenoid armature and associated pumping plunger reach the full travel stop, as shown by the broken line.

0073Time interval A-D is the duration of the applied logic pulse and thus drive current.

0074Although the present invention has particular benefits when employed in a reagent dosing system for an internal combustion engine, it will be appreciated that the invention is also applicable to other systems in which total fluid delivery is to be measured and/or the level of a fluid tank is to be monitored without the need for an additional fluid level sensor. For example, the invention also has application in a fuel injection system for delivering an accurate volume of fuel to an injection nozzle of the engine for combustion purposes. Further, the disclosed technique for keeping track of totalized fluid dispensed and computed fluid remaining on board may be used to establish mission duration for other applications such as remotely piloted or robotized vehicles.