Diesel exhaust fluid injector control

Abstract

A control map and method for controlling a diesel exhaust fluid (DEF) injector 160 of an engine exhaust system. A volume of DEF injected by the injector is controllable by controlling the timing of opening and closing of the injector, and the injector is controllable to open and close at a range of different frequencies. The control map comprises a mapping between a plurality of possible flow rate requests and a corresponding plurality of injection schemes, each injection scheme extending over a time period and involving a number of injections. For flow rate requests that are less than a first threshold flow rate the number of injections is more than for flow rates that exceed the first threshold flow rate. A length of each injection within each injection scheme may be the same for all injections within the respective injection scheme. The injections in each injection scheme may take place at consistent intervals, or at least one of the injection schemes may comprise a first period in which injections are bunched together and a second period in which injections are more spaced apart.

Description

Technical Field The disclosure relates to the control of diesel exhaust fluid injectors for injecting diesel exhaust fluid into a diesel engine aftertreatment apparatus. Background Diesel exhaust fluid, which comprises NH3, is used in diesel aftertreatment systems to react with NOx formed during diesel combustion to produce nitrogen and water for release to atmosphere. The volume of diesel exhaust fluid injected into the diesel aftertreatment system needs to be carefully controlled relative to the volume of NOX so that the reaction is balanced. It is also important that the diesel exhaust fluid mixes well with the NOX to facilitate the reaction. An amount of diesel exhaust fluid injected by a diesel exhaust fluid injector is generally controlled by opening and closing a control pintle of the injector. The control pintle has two stable states: fully open and fully closed. In order that the spray characteristics of the injector are kept as constant as possible, there is no ability to control the pintle to come to rest at any point that is intermediate the fully open position and the fully closed position. However, movement from fully open to fully closed may not be instantaneous. Similarly, movement from fully closed to fully open may not be instantaneous. Therefore, while spray characteristics are constant when the pintle is fully open, the spray characteristics may vary during the non-instantaneous transitions between open and closed and between closed and open. Diesel exhaust fluid injectors are routinely configured to be actuatable only at a certain frequency, f. This means that in each period, p (where p = 1 / f), the injector must undertake one (and only one) cycle of opening and closing. A maximum injection rate over a time period, t, may be achieved by arranging for the injector pintle to be open throughout the time period t. To achieve an injection rate that is less than the maximum injection rate, it is necessary to control opening and closing of the pintle such that an average injection rate across the time period, t, matches a desired injection rate. If there is a desire for injection at 50 % of the maximum injection rate during period, t, this may be achieved by arranging for the injector pintle to be open for t / 2 and closed for t / 2. If there is a desire for injection at 25 % of the maximum injection rate during period, t, this may be achieved by arranging for the injector pintle to be open for t / 4 and closed for 3t / 4. For example, an injector may have a frequency, f = 0.5 Hz. This means that the injector must open and close once (and only once) during each 2 second period. Where the maximum injection rate is required, the injector will open at the start of the 2 second period and close at the end of the 2 second period. Where a 50 % injection rate is required, the injector will open once for a duration of 1 second during the 2 second period. (For example, the injector may remain closed for 0.5 seconds at the start of the 2 second period, before opening for 1 second, and then being closed for the final 0.5 seconds of the 2 second period.) Where a 25 % injection rate is required, the injector will open once for a duration of 0.5 seconds during the 2 second period. (For example, the injector may remain closed for 0.75 seconds at the start of the 2 second period, before opening for 0.5 seconds, and then being closed for the final 0.75 seconds of the 2 second period.) Quality of the spray of the injected diesel exhaust fluid may be impacted during the non-instantaneous transition of opening and closing the pintle. Effectiveness of the mixing of the diesel exhaust fluid with the NOX is impacted with low frequency injection because there is an extended duration in which diesel exhaust fluid is injected at an effective rate that is greater than required followed by an extended period in which no diesel exhaust fluid is injected. Summary of the disclosure Against this background, there is provided a control map for a controller for a diesel exhaust fluid injector, wherein the diesel exhaust fluid injector has two stable states, comprising an open state and a closed state, and a transition state which occurs during transition between the open state and the closed state and between the closed state and the open state, wherein a volume of diesel exhaust fluid injected by the injector is controllable by controlling timing of opening and closing the diesel exhaust fluid injector, and wherein the injector is controllable to open and close at a range of different frequencies, wherein the control map comprises a mapping between: (a) a plurality of possible flow rate requests; and (b) a corresponding plurality of injection schemes, each injection scheme extending over a time period, t, and each injection scheme involving a number, n, of injections wherein n is a variable; wherein for flow rate requests that are less than a first threshold flow rate and more than a second threshold flow rate that is lower than the first threshold flow rate, the number of injections, n, is more than for flow rates that exceed the first threshold flow rate. In this way, for each possible flow rate request, by adopting different frequencies across the range of possible flow rate requests (where a flow rate request equates to a requested volume of diesel exhaust fluid injected during a period, t), an appropriate balance can be provided between injection spray quality and mixing. This recognises that the balance between spray quality and mixing is not necessarily consistent across all possible flow rates. In a further aspect of the disclosure, there is provided a method of controlling injections of a diesel exhaust fluid injector, wherein the diesel exhaust fluid injector has two stable states, comprising an open state and a closed state, and a transition state which occurs during transition between the open state and the closed state and between the closed state and the open state, wherein a volume of diesel exhaust fluid injected by the injector is controllable by controlling timing of opening and closing the diesel exhaust fluid injector, and wherein the injector is controllable to open and close at a range of different frequencies; the method comprising: receiving a flow rate request; consulting a control map to determine an injection scheme associated with the flow rate request; and controlling the injector in accordance with the injection scheme, wherein the control map comprises a mapping between: (a) a plurality of possible flow rate requests; and (b) a corresponding plurality of injection schemes, each injection scheme extending over a time period, t, and each injection scheme involving a number, n, of injections wherein n is a variable; wherein for flow rate requests that are less than a first threshold flow rate and more than a second threshold flow rate that is lower than the first threshold flow rate, the number of injections, n, is higher than for flow rates that exceed the first threshold flow rate. Drawings Embodiments of the disclosure are illustrated in the following drawings in which: Figure 1 shows a schematic representation of an engine assembly including aftertreatment system and showing coolant and DEF fluid lines; Figure 2 shows a technique, in accordance with the prior art, for varying the volume of diesel exhaust fluid injected between 10% and 50% injector capacity by varying a proportion of time over which the injector pintle is open, wherein the injector pintle is open only once per time period, t; Figure 3 shows an approach, in accordance with the present disclosure, whereby multiple options for achieving 50% injector capacity are available by varying the number of times the injector pintle is open per time period, t; Figure 4 shows, for two different options for achieving 50% injector capacity, how the proportion of time during which the injector pintle is in a transition state, varies considerably; Figure 5 shows a range of options available for achieving 100% injector capacity together with data associated with those options; Figure 6 shows a range of options available for achieving 90% injector capacity together with data associated with those options; Figure 7 shows a range of options available for achieving 75% injector capacity together with data associated with those options; Figure 8 shows a range of options available for achieving 50% injector capacity together with data associated with those options; Figure 9 shows a range of options available for achieving 30% injector capacity together with data associated with those options; Figure 10 shows a range of options available for achieving 10% injector capacity together with data associated with those options; and Figure 11 shows two options for achieving 30% injector capacity. Detailed description Figure 1 shows a highly schematic representation of an engine assembly 100. The engine assembly 100 may comprise an engine block 110, an aftertreatment system 120, an engine management system 130, a diesel exhaust fluid pump 140, a diesel exhaust fluid tank 150, a diesel exhaust fluid injector 160, a coolant circuit 170, a diesel exhaust fluid circuit 180 and an electronic signal path 190. The engine assembly 100 may further comprise an exhaust conduit 115 configured to transfer exhaust gas from the engine block 110 to the aftertreatment system 120. The coolant circuit 170 may comprise a first branch 172 configured to provide thermal control to the diesel exhaust fluid injector 160 and a second branch 174 configured to provide thermal control to the diesel exhaust fluid tank 150. The second branch 174 may comprise a valve 176. The diesel exhaust fluid tank 150 may comprise a header 152 and a level sensor 154. The header 152 may accommodate part of the second branch 174 of the coolant circuit 170. The diesel exhaust fluid circuit 180 may comprise: the diesel exhaust fluid pump 140; a first path between the header 152 of the diesel exhaust fluid tank 150 and the diesel exhaust fluid pump 140; a second (return) path between the diesel exhaust fluid pump 140 and the diesel exhaust fluid tank 150; and the third path between the diesel exhaust fluid pump 140 and the diesel exhaust fluid injector 160. The engine management system 130 may control the diesel exhaust fluid pump 140, via the electronic signal path 190. The engine management system 130 may also control the diesel exhaust fluid injector 160. In this way, an appropriate quantity of diesel exhaust fluid may be injected into the aftertreatment system 120 at an appropriate rate. The engine management system 130 may change the flow rate requested at regular intervals based on a wide range of factors including sensor values. Such sensor values may include values for NOxand for temperature, among many other potential parameters. The aftertreatment system 120 may be configured to facilitate mixing of the injected diesel exhaust fluid from the diesel exhaust fluid injector 160 with the exhaust gas from the exhaust conduit 115. The aftertreatment system 120 may comprise a mixer (not shown) for this purpose. In some cases, the injector and the mixer may be a part of the exhaust conduit 115. Effective mixing may be important for an efficient reaction of the diesel exhaust fluid with the exhaust. It may be desirable not only to seek to match the quantities of diesel exhaust fluid and the exhaust to achieve a balanced reaction but also to seek optimal mixing of the two in order to avoid an unbalanced reaction notwithstanding that the constituent reactants are balanced. Atomization of the injected diesel exhaust fluid assists with effective mixing of the diesel exhaust fluid with exhaust gas, which increases the effectiveness of the aftertreatment system 120. As stated above, an amount of diesel exhaust fluid injected by a diesel exhaust fluid injector 160 is generally controlled by opening and closing a control pintle of the diesel exhaust fluid injector 160. The control pintle has two stable states: fully open and fully closed. Therefore, volume of injection is controlled by a proportion of time for which the control pintle is open. Thus, 50% maximum injection is achieved by opening the control pintle for 50% of the time. Figure 2 shows a range of curves for how to achieve different injection volumes, equivalent to 50%, 25%, 20% and 10% of the maximum injection volume. In each case, the control pintle opens only once in a period t (where t = 1 / f and f is the frequency of the injector). The present disclosure involves adoption of a variable frequency such that the injector pintle may open more than once in the period, t, such that there are more options for how to achieve the different injection volumes of less than 100%. Figure 3 shows various different options, in accordance with the present disclosure, for opening and closing the control pintle at different frequencies in order to achieve the same 50% injection volume. Figure 4 illustrates two options, in accordance with the disclosure, for opening and closing the control pintle at different frequencies in order to achieve 50% injection volume. In the first example (the top curve) the control pintle opens only once during the period. In the second example (the bottom curve), the control pintle opens six times during the period. In both cases, the control pintle is open for approximately 50% of the period and closed for approximately 50% of the period, resulting in injection at approximately 50% of the injector maximum. However, in the first example there are only two transitions 10 (one transition to open and one to close) while in the second example there are six transitions 10 (six transitions to open and six to close). Thus, in the second example, the proportion of the total time during which the control pintle is transitioning between its two stable states (open and closed) is six times that of the first example. Comparative representative data is shown in the table below the two curves. Note that these data are for a representative injector rated to inject at a maximum of 10 kg / hr though the principles of the disclosure would apply to injectors capable of injecting at any flow rate. The transition times (and hence the transition time percentages) may be different for different injectors. The gradients in the curves in Figure 4 have been exaggerated to make the illustration clearer. Since the quality of the injection spray is impacted during any transition time, in the second example a larger proportion of the overall period experiences less predictable and / or reduced injection spray quality. While, in the first example, the period of lower quality of injection is minimised, at the same time the total period of injection is longest. This may have the consequence of undesirable in flow mixing / evaporation and wetting of the walls of the injection mixer (so called wall-wetting). Wetting reduces the temperature of the aftertreatment apparatus, results in accumulation of diesel exhaust fluid on the walls and reduces the likelihood of atomization of the injected diesel exhaust fluid. This in turn may reduce availability of the diesel exhaust fluid for reaction with the exhaust, which may also be undesirable. Thus, for any particular hardware combination, there is a balance to be struck between the period of time for which injection spay quality is impacted (more likely in the second curve) and the likelihood of poor in flow mixing / evaporation and high wallwetting (more likely in the first curve). Moreover, for any particular aftertreatment architecture and mixer design, an optimal balance between a smaller number of longer injections and a larger number of shorter injections may differ depending upon the flow rate requested (which might be expressed as a requested percentage flow rate of the maximum flow rate). For high requested flow rate, fewer injections of longer duration may generally be preferable since fewer transitions minimises the proportion of time during which injection is poor whilst in flow mixing / evaporation and wall wetting is less likely for higher requested flow rates. However, for medium and lower requested flow rates it may be more complicated to balance the benefits of a lower number of (longer) injections (thus reducing the number of transitions during which injection quality is impacted) with the benefits of (a higher number of) shorter length injections (thus reducing the likelihood of poor in flow mixing / evaporation and wallwetting). Therefore, for any particular aftertreatment architecture and mixer design it may be beneficial to provide a control map (or a data library) linking each potential requested flow rate (kg / hr) with an optimal injection pattern. Figures 5 to 10 show different injection options for achieving requested flow rates of 100%, 90%, 75%, 50%, 30% and 10% of the maximum flow rate providable by the injector. For one particular aftertreatment architecture and mixer design, the optimal pattern (and hence the pattern set out in the control map) may be that shown with a star beside it. Thus, for the particular aftertreatment architecture and mixer design, the optimum number of injections to achieve 100% may be 1, to achieve 90% may be 4, to achieve 75% may be 2, to achieve 50% may be 10, to achieve 30% may be 10 and to achieve 10 % may be 4. It will thus be noted that the optimum number of injections may not very linearly relative to the required flow rate (although for other aftertreatment architectures and mixer designs it may be linear). For a different aftertreatment architecture and mixer design, the optimal pattern for achieving the various different requested percentage flow rates may be different. Thus, the starred options in Figures 5 to 10 are not limiting. Figure 11 shows that in addition to varying the number and duration of injections per period, it may also be that for some aftertreatment architectures and mixer designs it may be preferable to space the injections unevenly through the period, as shown in the second row of Figure 11, rather than to space them evenly (per the first row). Uneven spacing of injections may facilitate more sophisticated thermal control of the aftertreatment system. For example, a longer gap between injections at some point during the period that at another point during the period may enable a greater increase in temperature to be achieved prior to the impact of a cooling effect which may result from an injection. This longer gap might be preceded (or followed) by multiple (e.g. two) injections in quicker than normal succession, thus reducing poor in flow mixing / evaporation and wetting (by not having one long injection) but whilst concentrating the cooling impact of the injections in a shorter period. This may then facilitate a longer gap before the subsequent injections in order to facilitate thermal recovery. Thus, overall, the thermal impact of the injections on the aftertreatment system may be modified for the same number and length of injections. In the arrangement of the second row of Figure 11 it should be noted that although there are periods of bunched injections and periods of more spaced inventions, it remains the case that one of the four injections occurs in each of the four equal divisions of the period, t. More generally, it may be that for a period requiring n injections, the period is divided into n equal sub-periods, and each sub-period comprises exactly one injection. Industrial Applicability The control map and method of the present disclosure may be applicable for use in any number of engine assemblies that comprise an aftertreatment system involving injection of diesel exhaust fluid. For each possible flow rate request of diesel exhaust fluid, by adopting different frequencies across the range of possible flow rate requests (where a flow rate request equates to a requested volume of diesel exhaust fluid injected during a period, t), an appropriate balance can be provided between injection spray quality and mixing. This 5 recognises that the balance between spray quality and mixing is not necessarily consistent across all possible flow rates.

Claims

1. A control map for a controller for a diesel exhaust fluid injector, wherein the diesel exhaust fluid injector has two stable states, comprising an open state and a closed state, and a transition state which occurs during transition between the open state and the closed state and between the closed state and the open state, wherein a volume of diesel exhaust fluid injected by the injector is controllable by controlling timing of opening and closing the diesel exhaust fluid injector, and wherein the injector is controllable to open and close at a range of different frequencies, wherein the control map comprises a mapping between: (a) a plurality of possible flow rate requests; and(b) a corresponding plurality of injection schemes, each injection scheme extending over a time period, t, and each injection scheme involving a number, n, of injections wherein n is a variable;wherein for flow rate requests that are less than a first threshold flow rate and more than a second threshold flow rate that is lower than the first threshold flow rate, the number of injections, n, is more than for flow rates that exceed the first threshold flow rate.

2. The control map of claim 1 wherein the number of injections, n, in each injection scheme falls within a range nmax to nmin where nmax is determined in accordance with a maximum frequency of the diesel exhaust fluid injector and a nmin is determined in accordance with a minimum frequency of injector, wherein the minimum frequency of injector is 1 / t.

3. The control map of any preceding claim, wherein t is a constant.

4. The control map of any preceding claim wherein a length of each injection withineach injection scheme is the same for all injections within the respective injection scheme.

5. The control map of any preceding claim wherein injections in each injection scheme take place at consistent intervals.

6. The control map of any of claims 1 to 4 wherein at least one of the injection schemes of the plurality of injection schemes comprises a first period in which injections are bunched together and a second period in which injections are more spaced apart.

7. The control map of any preceding claim wherein the corresponding plurality of injection schemes is dependent upon characteristics of a mixer configured to facilitate mixing of diesel exhaust fluid with exhaust gas.

8. The control map of any preceding claim wherein for flow rate requests that are less than the second threshold flow rate, the number of injections, n, is less than for flow rates that are between the first threshold and the second threshold.

9. The control map of any preceding claim wherein the number of injections, n is dependent upon a duration of each transition state.

10. An engine assembly controller comprising:an engine controller;an aftertreatment controller; and the control map of any preceding claim.

11. An engine assembly comprising:an engine;an aftertreatment system configured to receive exhaust from the engine;an injector configured to inject diesel exhaust fluid into the aftertreatment system; andan engine assembly controller of claim 10.

12. A method of controlling injections of a diesel exhaust fluid injector, wherein the diesel exhaust fluid injector has two stable states, comprising an open state and a closed state, and a transition state which occurs during transition between the open state and the closed state and between the closed state and the open state, wherein a volume of diesel exhaust fluid injected by the injector is controllable by controlling timing of opening and closing the diesel exhaust fluid injector, andwherein the injector is controllable to open and close at a range of different frequencies;the method comprising:receiving a flow rate request;consulting a control map to determine an injection scheme associated with the flow rate request; and5 controlling the injector in accordance with the injection scheme,wherein the control map comprises a mapping between:(a) a plurality of possible flow rate requests; and(b) a corresponding plurality of injection schemes, each injection scheme extending over a time period, t, and each injection scheme involving a number, n, of injections10 wherein n is a variable;wherein for flow rate requests that are less than a first threshold flow rate and more than a second threshold flow rate that is lower than the first threshold flow rate, the number of injections, n, is higher than for flow rates that exceed the first threshold flow rate.15

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