GB2418873A

GB2418873A - Soot filter control system

Info

Publication number: GB2418873A
Application number: GB0422142A
Authority: GB
Inventors: Ian Pegg; Keith William Barter; Michael James Watts; Michael Willcock
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2004-10-06
Filing date: 2004-10-06
Publication date: 2006-04-12
Anticipated expiration: 2024-10-06
Also published as: JP2006105149A; CN1789676A; GB2418873B; GB0422142D0; CN1789676B; DE102005046709A1

Abstract

A system is disclosed for controlling activation of a filter 16 for removing soot from oil used to lubricate an internal combustion engine 5. The system is operable to determine when soot needs to be removed from the oil and when operating conditions are suitable for removing the soot and uses a variable flow rate pump 15 to allow the flow rate or pressure to be set to a level most suitable for soot removal.

Description

A SOOT FILTER CONTROL SYSTEM

This invention relates to internal combustion engines and in particular to the removal of soot from the oil used to lubricate such an engine.

One of the limits to longer service intervals for an internal combustion engine and particularly for a diesel engine is excessive soot build up in the lubricating oil during certain operating conditions. Secondary filtration is often deployed to remove this and enable longer service intervals but in order to allow more effective filtration the oil pump needs to be larger than required without this service Interval requirement.

This is because with a conventional arrangement the oil pump is a gear type unit driven by the engine and the pump Is fitted with a pressure relief valve, either across the pump unit as an assembly or from the main gallery back to the pump. These pump units are not very efficient as their flow rate is engine speed dependent and the relief valve only ensures that no adverse pressures are generated. One known arrangement for secondary filtration Is to connect a secondary filter to a bypass flow passage which returns oil from the pump back to an oil reservoir without passing through the bearings of the engine when it is necessary to prevent adverse pressure being supplied to the engine. The secondary filter Is often in the form of a centrifugal filter and with such an arrangement oil will only pass through the secondary filter when the bypass passage is open Because of the effect of viscosity on the pressure produced by the pump, the oil pump bypass opens at low engine speeds when the oil is cold but only at higher engine speeds when the oil is hot. When the oil Is cold a centrifugal filter does not work very effectively due to the high viscosity, leading to low exit pressure from the jets giving low filter speed. The lower filter speed produces a low centripetal force thereby reducing the filtration efficiency. The high viscosity also reduces the migration of the particles preventing the majority from reaching the wall and being separated. When the oil Is hot the pump only has flow in the relief circuit at higher speeds because the pump has to be sized for hot idle conditions to minmse the parasitic - 2 losses. This means that a user of the engine running the engine at low speeds In hot conditions for the majority of the time is not able to effectively filter the oil through the secondary filter.

In addition, because of the need to generate more flow in order to operate the secondary filtration, this Will lead to Increased parasitic losses and will reduce the fuel economy thereby reducing the economic gains obtained by increasing the service intervals.

It is an object of this invention to provide a system that can more efficiently remove soot from the oil used to lubricate an internal combustion engine.

According to a first aspect of the invention there Is provided a system for removing soot from oil used to lubricate an Internal combustion engine having a lubrication system through which the oil is circulated wherein the system comprises a variable flow rate pump to circulate the oil through the lubrication system, a filter to remove soot from the oil circulated through the lubrication system, a controller to determine when soot needs to be removed from the oil and, when it is determined that soot needs to be removed, to control the pump so as to provide sufficient flow to efficiently remove the soot from the oil.

The controller may be operable to control the pump to provide a minimum flow rate of oil from the pump.

The controller may be operable to control the pump to provide a predetermined flow rate of oil from the pump.

The system further comprise at least one oil temperature sensor to provide a signal to the controller indicative of the temperature of the oil and the controller may be operable to control the removal of soot from the oil by only operating the pump to remove soot from the oil when the temperature of the oil is above a predetermined minimum temperature The filter may be a bypass filter and a valve may be used to control the flow of oil to the filter.

The valve may be a pressure controlled valve arranged such that oil Is only able to flow to the filter when the pressure of the oil is above a predetermined pressure.

The system may further include at least one oil pressure sensor to provide a signal to the controller indicative of the pressure of the oil near to an outlet from the pump and the valve may be controlled by the controller to only open when the measured pressure is above a minimum predetermined pressure.

Alternatively, the filter may be a full flow filter positioned between the pump and an oil cooler for the engine and a valve controlled return passage is provided to return oil from a position between the filter and the oil cooler to an Inlet to the pump when it has been determined that soot needs to be removed from the oil.

The valve may be a pressure dump valve arranged such that oil is only able to flow back to the pump inlet when the pressure of the oil leaving the filter is above a predetermined pressure. Alternatively, the system may furthemnclude at least one oil pressure sensor to provide a signal to the controller indicative of the pressure of the oil from the pump and the valve may be controlled by the controller to only open to allow oil to flow back to the pump inlet when the measured pressure is above a minimum predetermined pressure.

In either case the filter may be a vortex filter.

According to a second aspect of the invention there is provided a method for removing soot from oil used to lubricate an internal engine having a lubrication system through which oil is circulated by a variable flow rate pump and a filter to remove the soot from the oil circulated through the lubrication system, the method comprising determining when soot needs to be removed from the oil and when it is determined that soot needs to be removed from the oil operating the pump so as to pass the oil through a soot removal filter with sufficient flow to effectively remove the soot therefrom.

The method may further comprise operating the pump so as to pass the oil through a soot removal filter with sufficient flow to effectively remove the soot therefrom only when the temperature of the oil Is above a predetermined minimum temperature.

According to a third aspect of the invention there is provided a filtration system for a fluid circuit having a device requiring a supply of pressursed fluid wherein the filtration system comprises a variable flow rate pump to supply pressurised fluid to the device, a filter to remove debris from the fluid and a controller to determine when debris needs to be removed from the fluid and to control the variable flow rate pump so as to provide sufficient flow to the filter to efficiently remove debris from the fluid when it is determined that debris needs to be removed.

The filter may be one of a centrifugal filter and a vortex filter.

The filter may be connected to a main flow circuit from the pump by a valve. The valve may be one of a pressure relief valve and a control valve operated by the controller.

The invention will now be described by way of example with reference to the accompanying drawing of which: Fig. 1 is a block diagram of a first embodiment of an engine and soot reduction system according to a first aspect of the invention; Fig 2 is a block diagram of a second embodiment of an engine and soot reduction system according to a first aspect of the Invention; Fig 3 is a block diagram of a third embodiment of an engine and soot reduction system according to a first aspect of the invention; and Fig.4 is a schematic flowchart of a method according to a second aspect of the invention With particular reference to Fig.1 there is shown a diesel engine 5 having a lubrication system to circulate oil from a reservoir or sump 6 to the engine 5 for lubrication purposes and return the oil to the sump 6. An oil cooler 7 and a full flow filter 8 are included as part of the lubrication system to cool and remove debris from the oil A system is provided to remove soot from the oil circulating through the lubrication system, the system comprises a controller 10, a temperature, quality and level sensor 12, a pump outlet pressure sensor 20 an engine oil pressure sensor 21, a variable flow rate pump 15, a bypass filter 16 in the form of a centrifugal filter used to selectively remove soot from the oil and a solenoid valve 17 which controls the flow of oil to the bypass filter 16.

The controller 10 is arranged to receive input signals from the temperature, quality and level sensor 12, the engine oil pressure sensor 21, the pump outlet pressure sensor 20 and from various other sensors (not shown) associated with the engine 5 which can be used to determine the speed and load on the engine 5, to process the received signals and to control the operation of the variable flow rate pump 15 and the solenoid valve 17 based upon the received signals.

In the example shown the bypass filter 16 is a centrifugal filter but other filter types capable of removing soot from the oil could be used. 6

In this case, the variable flow rate pump 15 is a rotary vane pump having a number of predetermined flow rates. However, it will be appreciated that a pump In which the flow rate can be continuously varied between lower and upper limits could also be used.

The controller 10 receives a signal used to determine when soot needs to be removed from the oil. Soot may require removal when a specific level of soot has accumulated in the oil or when the engine 5 Is operating in conditions In which large quantities of soot are likely to be transferred to the oil. The actual process used to determine that the soot needs to be removed is the subject of our co-pending application entitled "A soot management system for an engine", and is not described herein in detail. This determination can be performed by programming embodied within the controller 10 or be made using a separate controller and be provided to the controller 10 as an input signal. In either case the controller 10 receives a signal which it can use to determine whether soot removal is required.

In this example this signal Is in the form of a flag set in a memory of the controller 10 If the flag is set to 1 then soot needs to be removed and if the flag is set to 0 then soot does not need to be removed.

Dunng normal operation of the engine 5 when soot does not have to be removed from the oil, the controller 10 controls the pump 15 to provide the desired pressure required by the engine 5 and selects the most suitable flow rate to provide this pressure. That is to say the operating conditions of the engine 5 including the oil pressure in the engine 5 as sensed by the pressure sensor 21 are received by the controller 10 and the pump 15 Is controlled accordingly to provide the required oil pressure in the engine 5 The required oil pressure can be obtained from data stored in a look up table or can be calculated using predetermined algorithms relating oil pressure to engine speed and load. - 7

When the controller 10 determines that soot needs to be removed from the oil it controls the pump 15 in a soot removal mode to optimise the removal of soot from the oil.

The first step in this process is to confirm that the oil is above a predetermined temperature. This is particularly important when a centrifugal filter is used because this type of filter is inefficient when the oil is very viscous. If it is determined from the temperature, quality and level sensor 12 that the oil is warm enough and hence of a sufficiently low viscosity for efficient filtration to occur the next step is to control the pump to produce a predetermined flow rate. This flow rate is chosen to provide substantially the most efficient use of the bypass filter 16 that is to say, the flow rate through the bypass filter 16 that will cause the centrifugal filter to rotate at a sufficiently high speed for efficient soot removal to take place. In the example shown in which a pump having a number of discrete flow rate settings is used, the flow rate from the pump 15 is set to the flow rate setting that will produce the predetermined flow rate.

Because the engine 5 also requires a supply of oil to maintain efficient lubrication the flow rate from the pump 15 will be higher than that needed solely for the filter 16.

When the outlet pressure sensor 20 indicates that a predetermined minimum pressure has been reached the valve 17 is opened by the controller 10 allowing oil to pass through the bypass filter 16 this ensures that flow to the bypass filter 16 will not occur until sufficient pump pressure and flow are present to rapidly bring the bypass filter 16 up to speed.

The valve 17 can be of a type that is able to regulate the flow of oil through both the bypass filter 16 and the main oil flow passage through the full flow filter 8 and the oil cooler 7 to the engine 5 or be of a type where, when opened to provide flow to the bypass filter 16, a predetermined leakage of oil is permitted to flow to the engine 5. In either case it is important to maintain a flow of oil to the engine 5 to prevent damage to the engine 5 from occurring.

As the flow rate of oil required by the bypass filter 16 requires the generation by the pump 15 of a flow rate and pressure much higher than that required for normal engine lubrication purposes then, even when the oil is flowing freely through the bypass filter 16, there is still sufficient pressure to maintain effective engine lubrication When the controller 10 determines that soot no longer needs to be removed from the oil it is operable to close the valve 17 shutting off the supply of oil to the bypass filter 16 and reduces the flow rate setting of the pump 15 to a normal operating setting as required by the engine 5.

It will be appreciated that the output from the pump 15 is not governed by the speed of the engine 5 and so the pump 15 can be operated to provide a high flow rate of oil even when the engine 5 is idling. This ensures that, irrespective of the duty cycle under which the engine 5 Is operating, soot can be removed from the oil provided the oil is not too VISCOUS.

By using the bypass filter 16 only when it has been determined that soot needs to be removed from the oil, less energy is wasted than with a conventional system in which oil is passed through the bypass filter irrespective of whether soot filtration is required and irrespective of whether the oil is sufficiently warm to permit efficient filtration. In addition, by controlling the pump to provide an optimised flow rate for the bypass filter 16 when it is being used and only allowing filtration to occur when the oil is of a sufficiently low viscosity, the most efficient use of the bypass filter 16 Is achieved. It will be appreciated that because a much higher flow rate is required for filtration than for normal lubrication the less time spent operating at this elevated flow rate the less energy will be consumed.

It will be appreciated that the system could use an oil temperature sensor of any type capable of measuring the oil temperature and that although the system is particularly useful for a diesel engine it can be applied to any engine where soot needs to be removed from the oil.

With reference to Fig.2 there is shown a system which is In many respects the same as that previously described with respect to Fig.1. The only significant differences are that the solenoid valve 17 is replaced by a spring controlled pressure dump valve 117 and the pump 115 in this case Is of a continuously variable output type.

The system operates in a similar manner to that described above in that the pump 115 is used to provide oil at normal pressures and flow rates to the engine 5 until the controller determines that soot needs to be removed and it has been confirmed that the temperature of the oil is sufficiently high to permit efficient filtration to occur. However, in this case there is no actual control of the valve 117 it merely opens to allow flow through the bypass filter 16 when the pressure from the pump 115 rises above a predetermined level. In this embodiment this is achieved by controlling the pump 115 to provide a minimum flow rate by Increasing the output from the pump 115 to a level where it is known that the flow rate from the pump will equal or slightly exceed the minimum required flow rate for the bypass filter 16. This minimum flow rate will provide a pressure at the pump outlet greater than the required pressure to open the valve 117 and so as the flow from the pump 115 is increased a point will be reached where the valve 117 will open to allow oil to flow through the bypass filter 16. As before this flow rate or pressure is higher than that required to lubricate the engine 5 and so the oil will only flow through the bypass filter 16 when soot has to be removed and at a rate which will use the bypass filter 16 in an efficient manner.

Although the Invention has been described with reference to a centrifugal filter where it Is known that efficient filtration will only occur above a predetermined oil flow rate and when the oiHs above a predetermined temperature it is equally applicable to other types of filters - 10 such as for example vortex filters where the soot is removed by causing the oil to rotate rapidly as it passes through the filter. In such a filter the viscosity of the oil also needs to be low for efficient filtration and a high flow rate Is required. It is also applicable to a membrane type of filter because with such a filter the pressure loss across such a filter with a sufficiently fine structure to capture the small soot particles requires the use of a lot of energy and even with such a filter it is easy to filter the oil when it Is less viscous.

With reference to Fig. 3 there is shown a third embodiment of a system according to the invention which in many respects is the same as those previously described but differs from those previously described in that the filter used to remove the soot from the oil is arranged in a full flow position and not in a bypass line.

As before a diesel engine 5 has a lubrication system to circulate oil from a reservoir or sump 6 to the engine 5 for lubrication purposes and return the oil to the sump 6. An oil cooler 7 and a conventional full flow filter 8 are included as part of the lubrication system to cool and remove debris from the oil.

The system to remove soot from the oil circulating through the lubrication system comprises of a controller 10, a temperature, quality and level sensor 12, a pump outlet pressure sensor 20 an engine oil pressure sensor 21, a variable flow rate pump 215, an inline soot filter 9 in the form of a vortex filter and a valve 217 to control the flow of oil through a bypass line back to the pump 15 The valve 217 is opened when soot removal is taking place and can either be a pressure dump valve controlled by the pressure in the lubrication system at that position or can be electronically controlled by the controller 10. In either case, the purpose of the valve 217 is to return a large percentage of the oil to an inlet of the pump 215 without it passing through the oil cooler 7 so as to maintain the oil at a relatively high temperature during filtering of the oil and so improve filtration efficiency.

During the time when the valve 217 Is open, so as to allow a high flow rate of oil to return to - 11 the pump 215, a sufficient flow rate of oil will continue to be supplied to the engine 5 in order to ensure that sufficient lubrication is maintained at all times A vortex filter as meant herein is a filter in which the fluid passing through the filter is caused to rotate to form strong vortices which through centripetal acceleration cause the soot particles to be deposited on the outer wall of the filter. One of the advantages of such a filter is that unlike a conventional membrane filter it is able to remove small particles from the oil without producing a significant restriction to flow.

The controller 10 is as before arranged to receive input signals from the temperature, quality and level sensor 12, the engine oil pressure sensor 21, the pump outlet pressure sensor 20 and from various other sensors (not shown) associated with the engine 5 which can be used to determine the speed and load on the engine 5, to process the received signals and to control the operation of the variable flow rate pump 215 based upon the received signals.

In this case the variable flow rate pump 215 Is a rotary pump having a constant speed of rotation and the flow rate Is varied by varying the displacement per rotation of the pump 215. The pump 215 has a number of predetermined flow rates chosen to provide the required flow rates for the engine 5 and for filtration.

The controller 10 receives a signal used to determine when soot needs to be removed from the oil. This signal is in the form of a flag set in the memory of the controller 10. If the flag Is set to 1 then soot needs to be removed and if the flag is set to 0 then soot does not need to be removed.

Dunng normal operation of the engine 5, when soot does not have to be removed from the oil, the controller 10 controls or sets the pump 15 to provide the desired pressure required by the engine 5. That is to say, the operating conditions of the engine 5 Including - 12 the oil pressure in the engine 5 as sensed by the pressure sensor 21 are received by the controller 10 and the pump 215 is set to a flow rate that will provide the required oil pressure in the engine 5. The required oil pressure can be stored in a look up table or can be calculated using predetermined algorithms relating oil pressure to engine speed and load. During this mode of operation the flow through the vortex filter 9 is relatively low and so virtually no soot Is removed from the oil during this mode of operation. Because of the construction of the vortex filter 9, the restriction to flow is minimal and its Inclusion within the oil lubrication system has virtually no effect on the energy required to provide normal lubrication to the engine 5.

When the controller 10 determines that soot needs to be removed from the oil it sets the pump 215 in a soot removal mode to optimise the removal of soot from the oil.

The first step in this process is to confirm that the oil is above a predetermined temperature and, if it is determined from the temperature, quality and level sensor 12 that the oil is warm enough for efficient filtration to occur, the next step is to control the pump 215 to produce a predetermined flow rate. This flow rate is chosen to provide substantially the most efficient use of the vortex filter 9 that is to say the flow rate through the vortex filter 9 that will cause the oil passing therethrough to rotate at a sufficiently high speed for efficient soot removal to take place It will be appreciated that If the flow rate from the pump 215 can be varied in a continuous manner then this flow rate can be an exact minimum flow rate to produce the desired optimum filtering but If the flow rate from the pump 215 can only be varied in a stepped manner, as in this case, then the pump 215 will be set to use the output flow rate which provides a flow rate the same as or slightly above the predetermined minimum flow rate required to produce efficient filtration with minimum energy usage. - 13

The flow rate from the pump 215 can be measured Indirectly by using the pump outlet pressure sensor 20 if a continuously variable output type of pump Is used and the pump 215 can then be controlled to produced the desired flow rate based upon the measured pressure but if a stepped output type of pump is used such as In this embodiment then the pump merely has to be set to the output setting required to produce the desired flow rate.

As the flow rate of oil required by the bypass filter 16 requires the generation by the pump 215 of a flow rate and pressure higher than that required for normal engine lubrication purposes then, even when the oil is flowing freely through the bypass line back to the pump 215, there is still sufficient pressure to maintain effective engine lubrication.

When the controller 10 determines that soot no longer needs to be removed from the oil it is operable to reduce the flow rate of the pump 215, this will automatically close the valve 217 because the pressure will fall below the level required to maintain the valve open.

It will be appreciated that as before the output from the pump 215 is not governed by the speed of the engine 5 and so the pump 215 can be operated to produce a high flow rate even when the engine 5 is idling. This ensures that, irrespective of the duty cycle under which the engine 5 is operating, soot can be removed from the oil provided the oil is of a sufficiently low viscosity.

As with the previous embodiments the pump is only operated at a high flow rate when soot must be removed and when the oil is at a suitable temperature for filtration thereby saving energy and improving the fuel economy of the engine.

With reference to Fig. 4 there is shown the method for controlling a variable flow rate pump in order to remove soot from the oil used to lubricate and engine. - 14

The method starts at step 100 which is when the engine is activated or started. The method advances to step 110 where it is determined whether soot needs to be removed from the oil and this is achieved by Interrogating a flag using the test: ls Flag >0 If Yes then go to step 120 else return to step 110 At step 120 the method determines whether the oil temperature is above a predetermined minimum temperature using the test: ls T > Tmin where T is the measured temperature of the oil and Tmin is an oil temperature chosen to represent the minimum temperature at which the viscosity of the oil is sufficiently low to permit efficient filtering of the soot from the oil If the measured temperature is below this minimum temperature then the method returns to step 110 but if it is above the minimum temperature the method advances to step where the pump is controlled to provide the required flow rate for efficient filtration of the soot from the oil.

The method then returns to step 110 to recheck that soot removal is still required. This may be after a predetermined time has elapsed or may be part of a continuous process in which the soot content of the oil is continuously monitored and the status of the flag is continuously updated.

Therefore in summary it will be appreciated that this invention provides several advantages including the ability to filter out soot at any time irrespective of the speed of the engine, prevents filtration when the oil viscosity is to high so as to obtain efficient filtration at all times, permits the pressure used for filtration to be controlled to achieve the optimum - 15 removal of soot, allows the speed of a centrifugal filter to be controlled to provide good filtration or the flow velocity through a vortex filter to be adjusted to maximise filtration and only uses the high flow rates required for filtration when soot actually has to be removed thereby reducing energy usage.

It will be appreciated that the Invention Is not limited to the removal of soot from oil and could be applied to other filtration systems for fluid circuits having a device requiring a supply of pressurised fluid. In which case, the filtration system would comprise a variable flow rate pump to supply pressurized fluid to the device, a filter to remove debris from the fluid and a controller to determine when debris needs to be removed from the fluid and to control the variable flow rate pump so as to provide sufficient flow to the filter to efficiently remove debris from the fluid when it is determined that debris needs to be removed. The filter could be a centrifugal filter or be a vortex filter or any other type of filter requiring a high flow rate to provide efficient filtration.

The filter could be connected to a main flow circuit from the pump by a valve in which case, the valve could be a pressure relief valve or couldbe a control valve operated by the controller.

The device could be for example and without limitation, a transmission, differential or axle requiring a supply of lubricating oil or a hydraulic motor requiring a supply of pressurised fluid.

It will be appreciated by those skilled in the art that although the Invention has been described by way of example with reference to a number of specific embodiments it is not limited to these embodiments and that various alternative embodiments or modifications to the disclosed embodiments could be made without departing from the scope of the Invention. - 16

Claims

1. A system for removing soot from oil used to lubricate an internal combustion engine having a lubrication system through which the oil is circulated wherein the system comprises a variable flow rate pump to circulate the oil through the lubrication system, a filter to remove soot from the oil circulated through the lubrication system, a controller to determine when soot needs to be removed from the oil and, when it is determined that soot needs to be removed, to control the pump so as to provide sufficient flow to efficiently remove the soot from the oil.

2. A system as claimed in claim 1 wherein the controller is operable to control the pump to provide a minimum flow rate of oil from the pump.

3. A system as claimed in claim 1 wherein the controller Is operable to control the pump to provide a predetermined flow rate of oil from the pump

4 A system as claimed in any of claims 1 to 3 in which the system further comprises at least one oil temperature sensor to provide a signal to the controller indicative of the temperature of the oil and the controller is operable to control the removal of soot from the oil by only operating the pump to remove soot from the oil when the temperature of the oil is above a predetermined minimum temperature

5. A system as claimed in any of claims 1 to 4 in which the filtems a bypass filter and a valve is used to control the flow of oil to the filter.

6. A system as claimed in claim 5 wherein the valve is a pressure controlled valve arranged such that oil Is only able to flow to the filter when the pressure of the oil Is above a predetermined pressure. - 17

7. A system as claimed in claim 5 wherein the system further includes at least one oil pressure sensor to provide a signal to the controller indicative of the pressure of the oil from the pump and the valve is controlled by the controller to only open when the measured pressure is above a minimum predetermined pressure.

8. A system as claimed in any of claims 1 to 4 wherein the filter is a full flow filter positioned between the pump and an oil cooler for the engine and a valve controlled return passage is provided to return oil from a position between the filter and the oil cooler to an inlet to the pump when it has been determined that soot needs to be removed from the oil.

9. A system as claimed in claim 8 wherein the valve is a pressure dump valve arranged such that oil Is only able to flow back to the pump inlet when the pressure of the oil leaving the filter is above a predetermined pressure.

10. A system as claimed in claim 8 wherein the system further includes at least one oil pressure sensor to provide a signal to the controller indicative of the pressure of the oil from the pump and the valve is controlled by the controller to only open to allow oil to flow back to the pump inlet when the measured pressure is above a minimum predetermined pressure.

11. A system as claimed in any of claims 8 to 10 wherein the filter is a vortex filter.

12. A method for removing soot from oil used to lubricate an internal combustion engine having a lubrication system through which oil is circulated by a variable flow rate pump and a filter to remove the soot from the oil circulated through the lubrication system, the method comprising determining when soot needs to be removed from the oil and when it is determined that soot needs to be removed from the oil - 18 operating the pump so as to pass the oil through a soot removal filter with sufficient flow to effectively remove the soot therefrom.

13. A method as claimed in claim 8 wherein the method further comprises operating the pump so as to pass the oil through a soot removal filter with sufficient flow to effectively remove the soot therefrom only when the temperature of the oil is above a predetermined minimum temperature.

14. A filtration system for a fluid circuit having a device requiring a supply of pressurised fluid wherein the filtration system comprises a variable flow rate pump to supply pressurised fluid to the device, a filter to remove debris from the fluid and a controller to determine when debris needs to be removed from the fluid and to control the variable flow rate pump so as to provide sufficient flow to the filter to efficiently remove debris from the fluid when it is determined that debris needs to be removed.

15. A filtration system as claimed in claim 14 wherein the filter is one of a centrifugal filter and a vortex filter.

16.A filtration system as claimed in claim 14 or in claim 15 wherein the filter is connected to a main flow circuit from the pump by a valve.

17. A filtration system as claimed in claim 16 wherein the valve is one of a pressure relief valve and a control valve operated by the controller

18. A system substantially as described herein with reference to the accompanying drawing. - 1 9

19. A method substantially as described herein with reference to the accompanying drawing.

20. A filtration system substantially as described herein.