Patent Description:
As is well known, it is important for vehicles to be provided with one or more filters in the air intake to prevent dust and other damaging particulate matter from being drawn into the engine. In automobiles, the air filters used do not tend to become quickly clogged and may only need replacing during the regular service intervals of the vehicle. For working vehicles such as tractors or combine harvesters, which tend to operate in very dusty environments, filter clogging or blocking can very quickly lead to reduced performance, and frequent stops to clean or replace filters is economically damaging.

To address the issue, a number of self-cleaning filter systems have been developed which act to remove accumulated dust and dirt from filter surfaces whilst the vehicle continues to operate, thereby reducing downtime for the vehicle and increasing the intervals between the replacement of filters. Many self-cleaning filters use some kind vacuum cleaner to clean the filter. Sometimes the filter rotates but it is also known to design the cleaning device in a rotating manner.

European patent application <CIT> describes a self-cleaning filter for a harvesting machine comprising a rotating vacuum device on top of the filter to remove the dirt. It is disclosed that it is not necessary to clean always but to clean in certain periods or phases which can be initiated based on certain parameters based on the data measured by sensors. These parameters can be the air pressure at the inlet of the fan or the temperature of the cooling water.

European patent application <CIT> discloses a method to unblock a perforated rotary filter screen by applying a stream of high pressure air on the filter screen and moving it in a transversal direction. Together with the rotation of the filter element it is cleaned in a helical or spiral path. The stream can be either directed from the upstream or the downstream side of the filter element, but it is better according to the patent to apply it from the upstream side.

International patent application <CIT> discloses a filter element to which a pressure pipe of a mobile cleaning device can be attached. With this pipe a cleaning fluid can be injected into the filter element in form of multiple pulses of compressed air. The connection piece at which the pressure pipe is attached is closed if the pipe is not attached.

United States patent application <CIT> discloses a self-cleaning air filter system with two phases, a working phase and a cleaning phase. The filter element comprises a casing, a filter exterior and a filter interior. A vent is located inside of the filter. In the working phase a negative pressure exists inside of the filter to pull air inside. A positive pressure is applied during the cleaning phase by a valve inside of the duct, which applies an air blast into the vent and onto a deflector. The deflector redirects the air radial to the filter. The pressure created by the blast is kept alive with the duct, because it is not compensated so fast. The blast of air can be applied automatically, periodically or manually.

Chinese patent application <CIT> discloses an air filter system with a self-cleaning function. A first air source and a first air valve is used to dislodge dirt located on the filter element. A second air source and a second valve is used to create an air stream to the outside. The filter exterior with the dislodged dirt is connected to this stream with an outlet pipe. With the Venturi effect the dusty air is pulled to the air stream and out of the vehicle.

<CIT> discloses a method of operating a cleaning process on a self-cleaning air filter according to the preamble of claim <NUM>.

<CIT> discloses a self-cleaning air filter system and method in which during a pulse cleaning period pulses of air pressure are applied to the clean air side of an air filter to dislodge contaminants. A scavenging apparatus is used to draw the dislodge contaminates away from the filter. The flow rate of the scavenging apparatus can be increased during pulsed cleaning period.

It is an object of the invention to provide an improved method of operating a self-cleaning filter system.

In accordance with a first aspect of the present invention there is provided a method of operating a cleaning process on a filter in a vehicle air intake, comprising:.

the method further comprising monitoring a filter load of the air filter, comparing the filter load with a filter load threshold, and initiating performance of the cleaning subroutine when the filter load threshold is reached or exceeded, characterized by increasing the filter load threshold after each performance of the cleaning subroutine up to a maximum filter load threshold level.

Increasing the flow of air through the exhaust duct to increase suction at the same time as applying one or more pulses of pressurised fluid (preferably gas or air) to the downstream side of the filter in order to dislodge accumulated dust and debris on the upstream side of the filter notably improves the efficiency of the dust and debris removal process. The advantage of using pressurised fluid pulsation is that, unlike some known systems, it is not necessary to stop the airflow to the engine whilst the flow is reversed to actively blow backwards (from downstream to upstream) through the filter.

Whilst efficiency of the dust and debris removal process in improved by the cleaning subroutine, it will not remove <NUM>% of the accumulated dust and debris. Accordingly, without periodic increase of the initial (lowest) threshold level to initiate the cleaning subroutine, the time taken to reach the threshold level would shorten with each cycle, leading to inefficient operation as the cleaning subroutine (including increased fan speed and pulsation) is performed with increasing frequency. Suitably, an indication to change the filter is generated, such as in the form of a warning light or message in the vehicle cab, when the maximum filter load threshold is reached or exceeded.

Note that the terms "upstream" and "downstream" are used herein in the context of the usual direct of airflow as the vehicle operates. Hence, the air intake opening to the atmosphere is upstream of the filter, and the engine receiving filtered air is downstream of the filter.

The cleaning subroutine steps of applying a pulse and carrying away material may be repeated at least once prior to the step of decreasing the flow of air through the exhaust duct. In an embodiment, the cleaning subroutine steps of applying a pulse and carrying away material are repeated more than once prior to the step of decreasing the flow of air through the exhaust duct.

In an embodiment, the extraction device for generating a flow of air through the exhaust duct comprises a fan located adjacent the outlet end of the exhaust duct and wherein the step of increasing the flow of air through the exhaust duct from the inlet end to the outlet end comprises increasing the rotational speed of the fan and the step of the decreasing the flow of air through the exhaust duct from the inlet end to the outlet end comprises decreasing the rotational speed of the fan. The speed of the fan may be increased from a first speed to a second speed to increase the flow of air through the exhaust duct. The speed of the fan may be decreased from the second speed to the first speed to decrease the flow of air through the exhaust duct.

In a preferred arrangement, the method further comprises using as the fan a cooling system fan of the vehicle, which cooling system fan is arranged to cool one or more components of the vehicle. Using an existing fan of the vehicle reduces component count and enables a relatively more compact arrangement in the vehicle engine bay.

In alternative embodiments, the extraction device arranged to draw air through the exhaust duct from the inlet end to the outlet end may be any one of the following: a venturi system, a flutter valve, or an exhaust ejector.

The method may comprise recording when (or otherwise noting how often) the cleaning subroutine is performed and generating an indication to change the filter after a predetermined number of iterations of the cleaning subroutine.

The triggering of the cleaning subroutine may be performed automatically at predetermined intervals or it may be triggered semi-automatically (e.g. the user may be presented with an indication that cleaning is now due - which indication they can follow or ignore). These intervals can differ in their time lengths, and an interval may begin with a previous running of the subroutine triggered by the reaching of a filter load threshold.

In an embodiment, the method comprises using a weight sensor to measure the weight of the filter in order to determine the amount of contaminant held in the filter. The method may comprise triggering the cleaning subroutine when the weight of the filter reaches a stored threshold value.

In an embodiment, the method comprises using a particle sensor to determine the degree of soiling in air upstream of the filter and/or a vacuum sensor to detect the level of vacuum downstream of the filter. The method may comprise using data from the particle sensor and/or the vacuum sensor to determine the degree of contamination (i.e. clogging) of the filter. The method may comprise triggering the cleaning subroutine when the degree of contamination of the filter reaches a stored threshold value. The method may comprise generating an indication to change the filter when the degree of contamination of the filter reaches a further stored threshold value. The method may comprise triggering the cleaning subroutine when the degree of vacuum measured by the vacuum sensor reaches a stored threshold value. The method may comprise generating an indication to change the filter when the degree of vacuum measured by the vacuum sensor reaches a stored threshold value.

To reduce inefficiency, the method preferably further comprises blocking of the performance of the cleaning subroutine until a predetermined time (for example <NUM>, <NUM> or <NUM> minutes) has elapsed since the preceding performance of the cleaning subroutine. The predetermined time may be a system variable that is determined by the ambient conditions: for example, in wet weather where there is less atmospheric dust being drawn in by the filter element, the minimum permitted period between cycles of the cleaning subroutine may be extended.

Preferably, the method further comprises determining one or more vehicle operating parameters (such as vehicle velocity, engine load, rotational speed or engine temperature, or pressure in a source of the pulse of pressurised fluid), comparing the same with respective stored threshold values, and blocking performance of the cleaning subroutine if a respective stored threshold value is not reached. This identifies circumstances in which it is inappropriate to trigger the cleaning subroutine, for example when there is insufficient pressure in the source of pressurised fluid to generate pulsation to a desired level. Blocking, by a user of the vehicle, of automatic initiation of the cleaning subroutine is suitably also enabled, for example to avoid blowing out clouds of dust as the vehicle passes through a populated area.

In an embodiment the method comprises receiving environmental data from an external source relating to one or more of the following for the region in which the vehicle is operating: weather forecast, the temperature, the air quality, the humidity. The method may also comprise reviewing recorded data from previous actions (for example: same cleaning procedure for the same or similar task). The method may comprise using the environmental and/or recorded data from previous uses to adjust the parameters of the cleaning subroutine such as the speed/force of the extraction device, increasing the force of pressure pulsations. The environmental data may be received from a server or other data source wirelessly.

In a further aspect, the invention provides a self-cleaning air filter system for a vehicle, comprising:.

For improved efficiency in manufacturing, the filter and filter housing and the source of pressurised fluid and at least one conduit are preferably constructed as a subassembly on a common supporting frame.

The extraction device may be a fan located adjacent the outlet end of the exhaust duct.

The invention further provides a vehicle including the above-described self-cleaning air filter system, with the said fan operable to draw air through the exhaust duct being a cooling fan in a cooling system of the vehicle, which cooling fan is arranged to direct cooling air towards at least one radiator or heat exchanger on a downstream side of the cooling fan, with the outlet end of the exhaust duct being positioned on the upstream side of the cooling fan.

The invention will now be described, by way of example only, in which.

Referring to <FIG>, a utility vehicle in the form of a tractor <NUM> is shown having a cab <NUM> and an engine compartment <NUM>. A chassis <NUM> which is partly visible connects front wheel suspension and steering assembly <NUM> and rear axle assembly <NUM>. A forward direction of the tractor is indicated by the arrow F.

<FIG> schematically represents components within the engine compartment <NUM>, with an engine <NUM> mounted on the chassis <NUM>. Forward of the engine <NUM> is a cooling package <NUM> comprising one or more radiators in cooling circuits for fluids (water, engine oil) of the vehicle. Forward of the cooling package <NUM> is a cooling fan <NUM> which draws air through a vent or grille arrangement in the front of a hood assembly (not shown) which surrounds the engine compartment <NUM>. Air is blown from the fan <NUM> through the cooling package <NUM> to lower the temperature of the liquids.

Above the engine <NUM> is mounted a self-cleaning air filter subassembly, indicated generally at <NUM>. The subassembly <NUM> comprises a filter element <NUM> within a filter housing <NUM>, and number of other components (described below) mounted to a common mounting frame <NUM>.

<FIG> schematically represents the air filter <NUM> (in the form of a generally cylindrical annular filter element) within the correspondingly shaped filter housing <NUM>. The filter housing <NUM> has an air inlet <NUM> and an air outlet <NUM>. The air inlet <NUM> is provided with an extended inlet body portion <NUM> (<FIG>) and is configured to receive atmospheric air and deliver the same to an external (upstream) surface of the filter <NUM>. The air outlet <NUM> is configured to deliver air that has passed through the filter <NUM> to the engine <NUM>.

Referring additionally to <FIG> and <FIG>, a source of pressurised fluid <NUM> (gas or air) and at least one conduit <NUM> coupled to deliver the pressurised fluid from the source to the air outlet (downstream) side of the filter <NUM> is attached to the mounting frame <NUM>. The source of pressurised fluid <NUM> may comprise an accumulator or reservoir, or may comprise a compressor. Whilst <FIG> shows a single conduit <NUM>, the embodiment of <FIG> and <FIG> has two conduits 42a, 42b which latter arrangement allows delivery of pressurised fluid to more than one location within the filter element <NUM>.

Rather than reversing the airflow direction through the filter element <NUM> to blow accumulated dust and debris out of the filter element <NUM>, the pressurised fluid source <NUM> is used to deliver a pulse of pressurised air into the airspace at the core of the annular filter element <NUM> which generates a shock wave which, in turn, serves to loosen dust and debris embedded on the outer surface of the filter.

To generate the pulse of pressurised air, the or each of the conduits <NUM>, 42a, 42b includes a valve <NUM> operated by a controller <NUM> to briefly connect the source <NUM> to the filter <NUM>. A pulse tube <NUM> may be used to connect the conduit <NUM> to the interior of the filter <NUM>, and a pulse distributor <NUM> mounted within the interior of the filter housing <NUM> may be provided to guide the shock wave resulting from the pulse to optimise the loosening effect.

The controller <NUM> is suitably a programmable device programmable to carry out an operating method of the system as will be described below with reference to <FIG> and <FIG>. The controller <NUM> may be a stand-alone device, or it may form a subsystem of a general vehicle or engine control system of the host vehicle <NUM>. In order to control some aspects of the method, the controller <NUM> is connected to a number of sensors <NUM> (<FIG>) which detect a variety of vehicle operating parameters, such as engine <NUM> temperature and pressure output from the source <NUM>.

An exhaust duct <NUM> has an inlet end and an outlet end. The exhaust duct <NUM> extends through the inlet extended body portion <NUM>, and the inlet end of the exhaust duct <NUM> is positioned within the filter housing air inlet <NUM>. The outlet end of the exhaust duct <NUM> is positioned on the upstream side of the vehicle cooling fan <NUM>, so that operation of the fan <NUM> to direct cooling air towards the cooling package <NUM> will also draw air along the exhaust duct <NUM> from the inlet to the outlet including any loose dust or dirt on the surface of the filter element <NUM>.

In order to enhance cleaning efficiency, the controller <NUM> is programmed or otherwise configured to cause the system to periodically perform a cleaning subroutine in which, whilst one or more pulsations are introduced to the downstream side of the filter element <NUM>, the operating speed of the cooling fan <NUM> is increased to increase the suction at the inlet end of the exhaust duct <NUM> as the pulsation shock-wave dislodges dust and dirt from the upstream side of the filter element <NUM>.

<FIG> is a flow chart representation of a cleaning process operation for the apparatus of <FIG>, starting at step <NUM> with determination that the vehicle engine is running. The chart then splits into two branches, in dependence on whether the process is being triggered manually, by a user of the vehicle, or automatically.

In the right-hand branch, it is determined at step <NUM> that a filter cleaning operation has been triggered manually. Following this, at step <NUM>, a parameter control step determines a number of vehicle operating parameters (for example by sensors <NUM>; <FIG>) that may determine whether the cleaning process can proceed. Such parameters may include:.

Following completion of the parameter control check at step <NUM>, to process moves to initiating the performance of a cleaning subroutine at step <NUM>. The initiating is prevented if a cancel parameter is identified at step <NUM>, which cancel parameter may the existence of a negative determination from any of the parameter control checks at step <NUM>, or an input indicating that the user has activated a manual cancellation of the process.

When initiated, the cleaning subroutine comprises firstly increasing the rotational speed of the cleaning fan <NUM> at step <NUM>. The increase may be <NUM>-<NUM>% or may be substantially higher.

Next, a first pressure pulsation is applied to the filter element <NUM> at step <NUM>, and the dust and debris dislodged by the pulsation is carried away by the exhaust duct <NUM> at step <NUM>. Whilst only a single pulsation may be used, it is generally preferable to repeat the steps of pulsation and carrying away at least once, as indicated at steps <NUM> and <NUM>. The cleaning subroutine concludes at step <NUM> with the rotational speed of the cooling fan <NUM> being reduced to its original level - that is to say the level prior to step <NUM>.

Following completion of the cleaning subroutine of steps <NUM> - <NUM>, a check with a vacuum sensor is made at step <NUM> of the filter load.

As will be recognised, the cleaning subroutine cannot be <NUM>% effective at removing dust and debris from the filter element and, over time, the extent to which the filter element becomes permanently blocked to airflow increases. <FIG> is a graph showing air filter loading as a percentage value (vertical axis) over a period of vehicle operation punctuated by periodic performances of the cleaning subroutine (cleaning counter - horizontal axis).

In <FIG>, the uppermost trace <NUM> represents <NUM>% filter loading (i.e. a "full" filter). Below this, a second trace <NUM> at <NUM>% filter loading indicates a maximum level used when in automated operation (described below) to provide an operational safety margin. A third trace <NUM> represents instantaneous filter loading: as can be seen, after each performance of the cleaning subroutine, the filter loading drops back towards zero. However, as can also be seen, for each successive period of operation (between cleaning subroutines), the filter loading rises more sharply than the preceding period as the extent to which the filter is permanently blocked increases.

A fourth trace <NUM> in <FIG> represents a filter load threshold. As will be described below, in an automated mode according to the invention, initiating performance of the cleaning subroutine occurs when the filter load <NUM> reaches or exceeds this threshold <NUM>. Furthermore, recognising how the extent of permanent filter blocking increases over time, the method of operation according to the invention includes increasing the filter load threshold <NUM> after each performance of the cleaning subroutine up to a maximum filter load threshold level, suitably trace <NUM> in automatic operation, with trace <NUM> (<NUM>% loading) permitted only for a manually-initiated (override) operation. Reverting to <FIG>, following the completion of a cleaning subroutine, if step <NUM> determines that the filter load has reached <NUM>% (or trace <NUM> for automatic operation), an indication is generated to the user at step <NUM> that the filter element needs to be replaced.

In an addition to the filter load check at step <NUM>, a check of the number of iterations of the cleaning subroutine may be performed at step <NUM> and that number compared to a stored threshold value. In the loading monitoring example of <FIG>, it can be seen that maximum loading has been reached after <NUM> cycles of the cleaning subroutine (although in practise the number of cycles will be much higher), so a maximum number of iterations (e.g. <NUM>, <NUM>, <NUM> or <NUM>) may be specified, following which the indication is generated to the user at step <NUM> that the filter element needs to be replaced, regardless of filter load level.

As indicated above, performance of the cleaning subroutine may be triggered automatically, and this is represented in <FIG> by step <NUM> (following the engine on check <NUM>) which determines whether a sufficient interval (e.g. <NUM> minutes) has elapsed since the last performance. Following a positive determination at step <NUM>, one or both of a pair of precondition checks are made at steps <NUM> and <NUM>.

Assuming a positive outcome, a parameter control check (equivalent to that at step <NUM>) is performed at step <NUM>, following which automatic operation is confirmed at step <NUM>.

A further parameter check as to the vehicle velocity (optionally via sensors <NUM>) is performed at step <NUM> before initiation of the cleaning process at step <NUM> and the check of cancellation parameters at step <NUM> so a cleaning subroutine doesn't happen when the vehicle is in the garage or is standing in a city at a traffic light. The reason behind this is so an unlucky pedestrian isn't blasted with a dust cloud or the dust is distributed in the garage.

In the foregoing, the applicants have described a method of operating a cleaning process on a filter in a vehicle air intake, comprises passing air through the filter from an upstream side to a downstream side, and providing an exhaust duct having an inlet end adjacent the upstream side of the filter and an outlet end adjacent a cooling fan of the vehicle arranged to draw air through the exhaust duct from the inlet end to the outlet end. In a periodic cleaning subroutine, the rotational speed of the cooling fan is increased <NUM> from a first speed to a second speed, whilst a pulse of pressurised fluid is applied <NUM>, <NUM> to a downstream side of the filter, and the exhaust duct carries away <NUM>, <NUM> material dislodged from the upstream side of the filter by the applied pulse, before the rotational speed of the fan is reduced <NUM> to its original level. A filter load of the air filter is monitored and compared with a filter load threshold, and the cleaning subroutine performed when the filter load threshold is reached or exceeded. The filter load threshold is increased after each performance of the cleaning subroutine up to a maximum filter load threshold level.

<FIG> illustrates an alternative embodiment of the self-cleaning filter arrangement in accordance with the invention. The embodiment as illustrated in <FIG> is similar to the previous embodiment described above, to which the reader should refer for details. Only significant differences between the self-cleaning filter arrangement according to this further embodiment over that of the previous embodiment will be described in detail.

Whereas in the previous embodiment a cooling fan <NUM> of the vehicle is used as an extraction device to draw air through the exhaust duct <NUM>, <FIG> illustrates how alternative forms of extraction device <NUM>' for generating an air flow through the exhaust duct <NUM> to extract dust and debris can be used. Accordingly, in this further embodiment, the extraction device <NUM>' can take any suitable form for drawing air through the exhaust duct and could be a venturi system, a flutter valve, or an exhaust ejector, for example.

<FIG> also illustrates a number of additional sensors which can be used to provide data input to the controller <NUM>. These include a weight sensor <NUM> for measuring the weight of the filter <NUM>, a vacuum sensor <NUM> to measure the vacuum downstream of the filter and/or a particle filter <NUM> to determine the degree of soiling (dust and debris contamination) in the air entering the filter.

In use, as the filter clogs its weight will increase and the controller <NUM> can be configured to use weight data from the data sensor <NUM> to monitor changes in the weight of the filter in order to determine when a cleaning subroutine should be triggered and/or the filter replaced. In an embodiment, the controller <NUM> is configured to trigger the cleaning subroutine when the weight of the filter reaches a stored threshold limit. In an embodiment, the controller <NUM> is configured to trigger a warning that filter should be changed when the weight of the filter reaches a stored maximum threshold limit for the weight of the filter.

The level of vacuum downstream of the filter will also increase as the filter clogs and in an embodiment the controller <NUM> monitors vacuum data from the vacuum sensor <NUM> to determine how clogged the filter is. In an embodiment, the controller <NUM> is configured to trigger the cleaning subroutine when the measured vacuum reaches a stored threshold limit. In an embodiment, the controller <NUM> is configured to trigger a warning that filter should be changed when the measured vacuum reaches a maximum threshold limit.

In an embodiment, the controller <NUM> monitors data from the particle sensor <NUM> and uses this data to predict how clogged the filter is and/or to vary the frequency with which the cleaning subroutine is carried. Thus when vehicle is being operated in very dusty conditions as determined from data received from the particle sensor, the controller may increase the frequency of the cleaning subroutine compared to when the vehicle is being operated in clean air.

The controller <NUM> may also use data from any of the weight, vacuum, and/or particle sensors to predict when a filter change may be required to provide predictive maintenance. The data from the sensors may be combined with data relating to vehicle usage, e.g. engine operating hours. For example, the controller <NUM> can be configured to record data regarding the degree of contamination of the air entering the filter from the particle sensor over time and/or in relation to time of use of the vehicle (e.g. engine operating hours) to predict when the filter is likely to be blocked based on data obtained from previous use of the filter in similar conditions. Such data may be acquired by the controller <NUM> itself during runtime of the system or acquired externally, say though controlled testing of the air filter system, and downloaded or otherwise made available to the controller <NUM>. The controller <NUM> can be configured to generate a warning signal to change the filter when it determines this to be necessary and may even be configured to automatically order a replacement filter, say from a dealer, using a predictive maintenance protocol.

It will be appreciated that a self-cleaning air filter system in accordance with the invention include any one or more of the weight, vacuum, and/or particle sensors in any combination and that such sensors can be incorporated into the previous embodiment. It will also be appreciated that the controller <NUM> may be configured to use data obtained from any two or more of the sensors in combination in order to determine when a cleaning subroutine should be triggered and/or the filter replaced.

In addition to using data provided by sensors on-board the vehicle, additional data may be provided to the controller <NUM> from external sources, such as a databank server <NUM> in communication with the controller. Such additional data may include environmental data (indicated schematically at <NUM>) relating to the area or region in which the vehicle is operating at any given time. Such environmental data may include any one or more of the following: weather forecast, the temperature, the air quality, the humidity. The additional data may also include measured data from previous use of the self-cleaning filter system under similar environmental conditions. This may be acquired by the controller <NUM> itself during runtime of the system or acquired externally, say though controlled testing of the air filter system, and downloaded or otherwise made available to the controller <NUM>.

Claim 1:
A method of operating a cleaning process on a filter (<NUM>) in a vehicle air intake, comprising:
- passing air through the filter from an upstream side to a downstream side;
- providing an exhaust duct (<NUM>) having an inlet end adjacent the upstream side of the filter, an outlet end, and an extraction device (<NUM>; <NUM>') to generate an air flow through the exhaust duct from the inlet end to the outlet end; and
- repeatedly performing a cleaning subroutine, which cleaning subroutine comprises, in sequence:
∘ increasing the flow of air through the exhaust duct (<NUM>) from the inlet end to the outlet end;
∘ applying a pulse of pressurised fluid to a downstream side of the filter (<NUM>);
∘ carrying away by the exhaust duct (<NUM>) material dislodged from the upstream side of the filter as a result of the applied pulse; and
∘ decreasing the flow of air through the exhaust duct (<NUM>) from the inlet end to the outlet end;
- the method further comprising monitoring a filter load of the air filter (<NUM>), comparing the filter load with a filter load threshold, and initiating performance of the cleaning subroutine when the filter load threshold is reached or exceeded; characterized by increasing the filter load threshold after each performance of the cleaning subroutine up to a maximum filter load threshold level.