GB2404220A

GB2404220A - variable speed mechanically-driven vehicular water pump with supplementary electrical drive

Info

Publication number: GB2404220A
Application number: GB0317157A
Authority: GB
Inventors: Martin Green; Iain William Gouldson; Christopher David Bush; James Richard Vanschagen; Stephen John Joyce; Robin Ashby
Original assignee: Visteon Global Technologies Inc
Current assignee: Visteon Global Technologies Inc
Priority date: 2003-07-23
Filing date: 2003-07-23
Publication date: 2005-01-26
Also published as: GB0317157D0

Abstract

The mechanical drive shaft 3 is engageable with the pump impeller 4 by a clutch 8,12,20,22. The electrical drive comprises a rotor, in the form of magnetic elements 12 attached to the impeller 4, a stator 24,15 including coils 28 arranged around the impeller 4 in proximity to the coils 28, and a power supply to the stator. The clutch 8,12,20,22 is arranged to rotationally disengage the rotary driving member 3 and the impeller 4 when the electrical drive is energised to rotate the rotor 12 at a rotational speed above that of the rotary driving member 3. Other embodiments include the use of an over-run clutch, and a combined electrical drive and magnetic clutch, the latter is not implicitly claimed for.

Description

Variable Speed Water Pump The present invention relates to a variable

speed water pump, and particularly a mechanically-driven water pump for a motor vehicle that has a supplementary electrical-drive.

Pumps for pumping liquids are often driven directly by a motor, either at a constant speed, or at a variable speed, for example by a pulley arrangement in which the pump speed, and hence the mass flow of a pumped liquid, is directly tied to the motor speed. Sometimes it is desirable to have a control of the pump speed that is independent of the motor Peed This is particularly true in automotive applications, where the pump is used to pump a coolant fluid such as an ethylene glycol/ water mixture around an internal combustion (IC) engine cooling circuit.

When an engine is started, the engine block, metal surfaces within combustion chambers and the catalytic converter are all cold. The engine will not run at maximum efficiency or at low emissions until the engine and converter have warmed up.

In addition, the internal engine wear rate is higher when the engine is cold than when it is at its normal operating temperature. It is therefore generally desirable to get an engine up to its operating temperature as rapidly as possible.

IC engines therefore normally have a thermostatic valve in the coolant circuit to prevent coolant from flowing through a radiator when the engine is cold. The coolant does, however, still flow around the engine and usually also through a bypass circuit encompassing a heat transfer matrix for 2 - passenger compartment heating. Heat is therefore still lost from areas that could benefit from a rapid warm up rate, particularly the internal metal surfaces of the combustion chambers.

Another problem with a coolant pump driven directly at the engine speed, is that the required cooling capacity does not generally depend in a direct linear relationship with the engine speed. The pump must therefore be sized so that it can provide adequate cooling at a low engine speed, for example 2000 rpm and at a high load, for example when towing a trailer uphill.

A further problem with a coolant pump driven directly at the engine speed, is that at high engine speeds a significant amount of engine power can be lost by driving the pump at an unnecessarily high speed. This is so mainly because the power needed to drive a pump normally increases with the cube of pump speed. For a typical automotive coolant pump, the power loss at an engine speed of 6000 rpm can be between about 1.0 kW and 2.5 kW, depending mainly on engine size.

All of these factors lessen engine efficiency.

A further problem may arise when the engine is turned off, as residual heat in the engine may cause stationary water in the engine block to boil. This issue becomes more important in a motor vehicle having a hybrid electric motor/Is engine, in which the IC engine may undergo frequently stop/start cycles during normal driving.

It has therefore been proposed to use an electric motor - 3 powered from the motor vehicle battery to drive the pump.

However, an electrically-driven pump requires an increase in the capacity of the electrical generation and battery system, also adding to cost and complexity. In addition, electrically driven pumps suffer from the inevitable electrical losses in converting mechanical power to electrical power, and then back to mechanical power, which can reduce any gain in engine efficiency owing to a quicker warm up from cold, or from driving the pump at a lower speed.

It is an object of the present invention to provide a more convenient motor vehicle coolant pump.

According to the invention, there is provided a motor vehicle coolant pump for pumping liquid coolant, comprising a rotary impeller for pumping the coolant, a mechanically driven rotary driving member for supplying mechanical power to rotate the impeller, an engaging means for rotationally engaging together the rotary driving member and the impeller, and an electrical drive for rotating the impeller, wherein: - the electrical drive comprises a rotor, a stator, and an electrical connection for supplying electrical power to the stator, the rotor comprising a plurality of magnetic elements affixed to the impeller and the stator including a plurality of electrical coils arranged around the impeller in proximity with said magnetic elements, the arrangement being such that when electrical power is supplied to the stator through said electrical connection, the electrical drive is energized to rotate the rotor; and the engaging means is arranged to rotationally disengage À 4 the rotary driving member and the impeller when the electrical drive is energized to rotate the rotor at a rotational speed above that of the rotary driving member.

Also according to the invention, there is provided a motor vehicle, comprising an internal combustion engine, a motor vehicle coolant pump, a cooling circuit, a source of electrical power, an engine control unit (ECU) and a number of engine sensors including an engine temperature sensor, wherein the coolant pump is as described above, the engine being arranged in use to drive mechanically the coolant pump via rotary driving member in order to pump coolant through the cooling circuit to cool the engine, and the ECU being arranged in use to monitor the sensed engine temperature and to energise the coolant pump from the source of electrical power to rotate the rotor at a rotational speed above that of the rotary driving member in order to reduce the sensed engine temperature when the engine temperature exceeds a predetermined limit.

The rotary driving member may then rotate the impeller during normal engine operation. When however, the engine is stopped, the engaging means is arranged to rotationally disengage the rotary driving member form the impeller when the electrical drive is energised, thereby allowing the electrical drive to rotate the impeller and pump coolant fluid, for example through an engine cooling circuit of a motor vehicle. The invention therefore provides an electrical run-on facility in a coolant pump when a motor vehicle engine is stopped.

Additionally or alternatively, when the engine speed is running, but not driving mechanically the rotary driving - 5 - member at a high enough speed for the pump to provide sufficient cooling, then the electrical drive may be energised to rotate the impeller at a higher speed than would otherwise be the case with a purely mechanical drive. The invention therefore provides an electrical boost facility when a motor vehicle engine is running.

In one embodiment of the invention, the engaging means is automatically arranged to rotationally disengage the rotary driving member and the impeller when the electrical drive is energized to rotate the rotor at a rotational speed above that of the rotary driving member. One way of doing this is if the engaging means is an over-run clutch arranged between the driving member and the impeller. The term "over-run clutch" as used herein means a clutch which transfers rotational energy only when the impeller is rotating more slowly than the rotary driving member.

In an alternative embodiment, the engaging means is a selective engaging means arranged to selectively disengage the rotary driving member from the impeller when the electrical drive is to be energized to rotate the rotor. One way of doing this is if the selective engaging means is a magnetic clutch arrangement between the impeller and the rotary driving member. A magnetic clutch provides the advantage of being electrically controllable, for example by an engine control unit.

The magnetic clutch arrangement may include a stationary electromagnet arranged in proximity with the magnetic elements, the electromagnet moving the magnetic elements and hence the impeller towards or away from the electromagnet - 6 when the electromagnet is energised in order to engage the impeller with a clutch surface fixed rotationally with the rotary driving member.

The stationary electromagnet may then be formed by the coils.

In one embodiment of the invention, the magnetic elements are then preferably arranged along a circumference around an axis through the impeller and. The coils may then be arranged around the circumference at an axial spacing from the magnetic elements.

Alternatively, the stationary electromagnet may be separate from the coils. In an alternative embodiment of the invention, the magnetic elements are then arranged along a circumference around an axis through the impeller and the coils are arranged radially outside the circumference.

In either case, the clutch surface may engage with a surface of the magnetic elements when the electromagnet is energised.

The magnetic elements may be formed from a soft magnetic material. The electrical drive may then be a switched reluctance type electric motor.

Alternatively, the magnetic elements may be formed from a magnetised magnetic material. The electrical drive may then be a stepped type electric motor.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: - 7 Figure 1 shows a cross section view through a motor vehicle coolant pump according to a first embodiment of the invention, having a rotary drive shaft, an impeller, a magnetic clutch for controllably engaging the drive shaft and the impeller, and a separate electrical drive for rotating the impeller when disengaged from the drive shaft; Figure 2A shows a cross-section in a mid-plane through a stator ring of the electrical drive of Figure 1; Figures 2B and 2C show, respectively, a cross section view in a central axially extending plane through the stator ring, and a side view of the stator ring of Figure 2A; Figure 3A shows a plan view of the impeller of Figure 1, showing impeller vanes that project from a circular back planes Figures 3B and 3C show, respectively, a cross section view through a central axially extending plane through the impeller in the direction of line II-II of Figure 3A, and a partial cross section and plan view of the opposite side of the back plane of Figure 3A showing an annular arrangement of stepped magnetic elements; Figure 4 shows a cross section through a radial plane of the impeller and stator ring of Figures 2A and 3A; Figure 5 is a cross section view through a motor vehicle coolant pump according to a second embodiment of the invention, having a rotary drive shaft, an impeller, and combined a magnetic clutch and electrical drive for, respectively, controllably engaging the drive shaft to the impeller, and a for rotating the impeller when disengaged from the drive shaft; Figures 6A-C, 7A-C and 8A-C show various side, plan and cross-section views through disassembled components that form a stator of the combined a magnetic clutch and electrical drive of Figure 5i Figure 9A shows a side view of the assembled stator of Figures 6-8; Figures 9B, 9C and 9D show, respectively, a central cross section in an axial plane, and a pair of opposite end views, of the assembled stator of Figure 9A; and Figure 10 shows a cross section view through a motor vehicle coolant pump according to a third embodiment of the invention, having a rotary drive shaft, an impeller, an over-run clutch for controllably engaging the drive shaft and the impeller, and an electrical drive for rotating the impeller when disengaged from the drive shaft.

Figure 1 shows a cross section view through a motor vehicle coolant pump 1 according to a first embodiment of the invention, having a rotary driven member 2 that comprises a drive shaft 3, and an impeller 4 for pumping coolant 6 inside a pumping chamber 7 between an inlet 9 and an outlet 11. The pump 1 also comprises a magnetic clutch for controllably À 9 engaging the drive shaft and the impeller. The clutch comprises an annular electromagnet 8 mounted within a pump housing 10, in proximity with an annular arrangement of magnetic elements 12 which are affixed to a circular back plate 13 of the impeller 4. Impeller vanes 15 project from the back plate 13 into the pumping chamber 7.

The drive shaft 3 has a stepped cylindrical shape that extends along an axis 14 from an external pulley wheel 16 towards the impeller 4. The drive shaft 3 is rotationally mounted on bearings 18 within the housing 10 and is sealed by seal 19. A disc 20 is rotationally coupled to and projects radially from the drive shaft 2. The disc 20 extends inside the pumping chamber 7. The disc 20 is a clutch plate with a clutch surface 22 facing towards the ring of magnetic elements 12 on the impeller 4.

In use, the magnetic elements 12 may be either attracted to or repelled by the electromagnet 8. In the former case, the magnetic elements 12 will be permanent magnets. In the latter case the magnetic elements 12 may be permanent magnets, but preferably are of a soft magnetic material that will be attracted to the electromagnet when this is energised by a supply of electrical current (not shown).

Optionally, the impeller 4 is spring biased either towards or away from the clutch plate 20, depending on whether the magnetic elements are attracted or repelled by the electromagnet 8. In either case, the it is preferred that the clutch surface 22 comes into friction contact with the magnetic elements 12 when the electromagnet is energised or de-energised to cause the magnetic elements 12 and hence - 10 impeller to move axially along the drive axis 14 towards the electromagnet and clutch plate 20. Such contact is illustrated in Figure 1.

When the clutch plate 20 is not in contact with the magnetic elements 12, the impeller 4 is freely rotatable on the drive shaft 3 about a central sleeve bearing 21. Rotation of the impeller 4 may therefore be inhibited when an engine has been started in order to inhibit movement of coolant 6 through the engine, thus ensuring a quicker warm up of the engine. The clutch plate 20 may also be disengaged or partially disengaged from the impeller 4 when an engine is running at high speed, beyond that necessary to drive the pump 1 at a rotational speed which would provide the optimum flow of coolant 6 to the engine.

A separate electrical drive is provided for rotating the impeller, if necessary, when the impeller 4 is disengaged from the drive shaft 3. The electrical drive comprises a stator ring 24 mounted within the pump housing 10, and the series of stepped magnetic elements 12 having external slots 23 around the periphery 15 of the impeller back plate 13.

Although the magnetic elements 12 may be formed from a unitary material such as a ring of iron, features such as the slots 23 make the magnetic elements distinct.

The stator ring 24 and impeller 4 are shown in more detail in the various views of Figure 2A-C, 3A-C and 4. The stator ring 24 includes a series of stepped magnetic poles 26 between which are coils 28, which may be individually energised by the application of electrical current (not shown) in order to rotationally drive the stepped magnetic elements 12 affixed to the back plate 13 of the impeller 4.

The impeller vanes 15 may therefore be turned by the separate electrical drive 12,24 when additional cooling of the engine is needed, for example when an engine is switched off.

Another time when additional cooling is needed is during heavy load conditions in hot weather. In this circumstance, the engine will be turning the driven member 2 and impeller, but the engine and vehicle speed may insufficient to cool the engine. In this case, the magnetic clutch 8,20 may be disengaged, and the separate electrical drive 12,24 energised in order to drive the pump 1 at a higher speed than may otherwise be the case by driving the impeller 4 at a higher speed than shaft 2.

A motor vehicle coolant pump 101 according to a second embodiment of the invention is shown in Figure 5, in which components similar to those of the first embodiment are indicated by corresponding reference numeral incremented by 100.

The second embodiment 101 has a rotary driven member 102 that comprises a drive shaft 103, and an impeller 104 for pumping coolant 106 inside a pumping chamber 107 between an inlet 109 and an outlet 111. The pump 101 also comprises a magnetic clutch for controllably engaging the drive shaft 103 and the impeller 104. The clutch comprises an annular electromagnet 108 mounted within a pump housing 110, in proximity with an annular arrangement of magnetic elements 112 which are affixed to a circular back plate 113 of the impeller 104.

Impeller vanes 115 project from the back plate 113 into the - 12 pumping chamber 107.

The drive shaft 103 has a stepped cylindrical shape that extends along an axis 114 from an external pulley wheel 116 towards the impeller 104. The drive shaft 103 is rotationally mounted on bearings 118 within the housing 110 and is sealed by seal 119. A disc 120 is rotationally coupled to and projects radially from the drive shaft 102. The disc 120 extends inside the pumping chamber 107. The disc 120 is a clutch plate with a clutch surface 122 facing towards the ring of magnetic elements 112 on the impeller 104.

In use, the magnetic elements 112 may be either attracted to or repelled by the electromagnet 108. In the former case, the magnetic elements 112 will be individual permanent magnets.

In the latter case the magnetic elements 112 may be permanent magnets, but preferably are of a soft magnetic material that will be attracted to the electromagnet when this is energized by a supply of electrical current (not shown).

Optionally, the impeller 104 is spring biased either towards or away form the clutch plate 120, depending on whether the magnetic elements are attracted or repelled by the electromagnet 108. In either case, it is preferred that the clutch surface 122 comes into friction contact with the magnetic elements 112 when the electromagnet is energised or de- energised to cause the magnetic elements 112 and hence impeller to move axially along the drive axis 114 towards the electromagnet 108 and clutch plate 120. Such contact is illustrated in Figure 5.

When the clutch plate 120 is not in contact with the magnetic - 13 elements 112, the impeller 104 is freely rotatable on the drive shaft 103 about a central sleeve bearing 121. Rotation of the impeller 104 may therefore be inhibited when an engine has been started in order to inhibit movement of coolant 106 through the engine, thus ensuring a quicker warm up of the engine. The clutch plate 120 may also be disengaged or partially disengaged from the impeller 104 when an engine is running at high speed, beyond that necessary to drive the pump 101 at a rotational speed which would provide the optimum flow of coolant 106 to the engine.

The second embodiment 101 differs from the first embodiment 1 in that the electrical drive and magnetic clutch are not separate, but are formed from the same coils and magnetic element components 108,112. The coils and components are illustrated in more detail in Figures 6A-C, 7A-C and 8A-C, which show various side, plan and cross-section views through disassembled components that form the stator 108 of the combined a magnetic clutch and electrical drive 108,112 of Figure 5.

The combined stator 108 is formed from three distinct components 40,41,42, each of which has a cylindrical former 44,45,46. Each former has a central annular wall 48,49,50 and an outer array of twelve fingers 52,53,54 that extend axially from a back plate 60, 61,62 and which are spaced equidistantly around the circumference of each former 44,45,46.

Reference is now made also to the various views of the assembled stator 108 shown in Figures 9A, 9B, 9C and 9D. The annular space between each central wall 48,49,50 and the corresponding array of fingers 52,53,54 holds a plurality of - 14 coils 56,57,58. Each of the farmers 40,41,42 has twelve individual coils 56,57,58, each coil corresponding with one of the fingers 52,53,54.

The farmers 40,41,42 with coils 56,57,58 are stacked one inside the other to form the completed stator 108, which is then mounted within the pump housing 110. Each one of the coils 56,57,58 has electrical connections (not shown) through the housing, so that the coils may be individually energized.

The corresponding fingers 52,53,54 and the inner annular walls 48,49,50 form a magnetic circuit together with those magnetic elements 112 in proximity with the fingers 52,53,54.

By energising the coils 56,57,58 in the correct sequence at the correct frequency, the stator can rotationally drive the magnetic elements 112 and so turn the impeller 104. In this mode of operation, the coils 56,57, 58 therefore act as an electric drive 124, in an analogous manner to the electric drive 24 of the first embodiment 1.

Furthermore, by energising all or a circumferential set of coils 56,57,58 with a constant current, the coils can attract or repel the magnetic elements 112, thereby acting in a similar manner to the electromagnet 8 of the first embodiment.

Alternating and direct currents to the coils 56,57,58 can advantageously be combined in order to provide independent control of both the magnetic clutch effect and rotary electric drive of the impeller 104.

The second embodiment 101 therefore can provide the same functionality and benefits as the first embodiment 1 - 15 described above.

A motor vehicle coolant pump 201 according to a third embodiment of the invention is shown in Figure 10, in which components similar to those of the first embodiment are indicated by corresponding reference numeral incremented by 200.

The third embodiment 201 has a rotary driven member 202 that comprises a drive shaft 203, and an impeller 204 for pumping coolant 206 inside a pumping chamber 207 between an inlet 209 and an outlet 211. The pump 201 also comprises an over-run clutch 70 for controllably engaging the drive shaft 203 and the impeller 204. The over-run clutch 70 automatically engages the impeller 204 with the drive shaft 203 when the drive shaft is turned to transmit mechanical power into the impeller 204. The resistance of the pumped coolant 206 keeps the impeller 204 engaged with the drive shaft 203. The over run clutch does, however, automatically disengage with the impeller 204 if the impeller is turned by an external force at a rate faster than that of the drive shaft 203.

As with the previous embodiments 1,101, the impeller 204 has an annular arrangement of magnetic elements 212 with a stepped outer profile. These magnetic elements 212 are affixed to a back plate 213 of the impeller 204. Here, the back plate 213 has a stepped annular upper surface 72. The magnetic elements 212 are seated in an intermediate step 71 in the upper surface 72 at a circumference 217 mid way between the drive axis 214 and an outer circumference 74.

Unlike the first two embodiments 1,101, the plurality of magnetic elements 212 are not affixed to the outer periphery - 16 74 of the impeller back plate, but to an intermediate periphery 217 formed by the step 71 in the impeller back plate 213. This arrangement provides the benefit of being more compact in the radial direction.

As with the second embodiment, the third embodiment 201 has a series of coils 75 spaced annularly around the drive shaft 203 inside the housing 210. These coils are spaced radially outside the ring-shaped arrangement of magnetic elements affixed to the impeller back plate 213. The coils 75 may be energised in sequence to rotate the impeller 204 with this is disengaged from the drive shaft 203.

As with the other embodiments 1,101, impeller vanes 215 project from the back plate 213 into the pumping chamber 207.

The drive shaft 203 has a stepped cylindrical shape that extends along an axis 214 from an external pulley wheel 216 towards the impeller 204. The drive shaft 203 is rotationally mounted on bearings 218 within the housing 210 and is sealed by seal 219.

The main difference between the third embodiment 201 and the second embodiment 101 is that the over-run clutch 70 is an automatically operating mechanical clutch that disengages whenever a force is applied to the impeller 204 to rotate the impeller are a greater rate than the rotational rate of the drive shaft 203.

In use, the magnetic elements 212 may be rotationally driven by the electric drive 224 in the manner of a stepper motor or a switched reluctance motor. In the former case, the magnetic - 17 elements 212 will be permanent magnets. In the latter case the magnetic elements 212 may be of a soft magnetic material that will be attracted to the electromagnet when this is energised by a supply of electrical current (not shown).

Unlike the first and second embodiments, 1,101, the third embodiment 201 is not capable of inhibiting the rotation of the impeller 204 when an engine has been started in order to inhibit movement of coolant 106 through the engine. The design does, however, benefit from a simpler electronic arrangement, and still provides the benefits of being capable to provide run-on cooling when an engine has been stopped.

The electric motors 12,112,212;24,124,224 described above may be either a permanent magnet type or a variable reluctance type. ! The invention providers a number of benefits for a motor vehicle. The first and second embodiments 1,101 provide a lower pump speed when the engine is cold, resulting in a quicker warm up, and also can be used to limit pump speed when this is necessary. All three embodiment 1,101,201 provide electrical run-on capability when the engine is hot at idle or after engine shut off, in which the pump is run electrically to circulate coolant and so cool the engine more effectively. In additional, all embodiments permit use of the electrical drive to get improved cabin heating when engine is off, and all embodiments avoid the problem of excessive electrical power consumption. This is because in normal operation, the electrical drive does not need to be used. The invention therefore makes use of mechanical power consumption by the pump during normal engine operation, which provides an ! - 18 improvement of up to 1.5% in fuel economy in steady state operation, as compared with an all-electrical motor vehicle water pump.

The first and second embodiments 1,101 provide even greater improvements in fuel economy during engine warm up. For example, in the urban part of the New European Drive Cycle (NEDC) test, the improvement in fuel economy can be up to 4%.

In general, the improvement will be greater for diesel engines as compared with gasoline engines.

The invention therefore provides a convenient and effective solution to the problems of controlling engine temperature in a motor vehicle. - 19

Claims

Claims: 1. A motor vehicle coolant pump for pumping liquid coolant,

comprising a rotary impeller for pumping the coolant, a mechanically driven rotary driving member for supplying mechanical power to rotate the impeller, an engaging means for rotationally engaging together the rotary driving member and the impeller, and an electrical drive for rotating the impeller, wherein: - the electrical drive comprises a rotor, a stator, and an electrical connection for supplying electrical power to the stator, the rotor comprising a plurality of magnetic elements affixed to the impeller and the stator including a plurality of electrical coils arranged around the impeller in proximity with said magnetic elements, the arrangement being such that when electrical power is supplied to the stator through said electrical connection, the electrical drive is energized to rotate the rotor; and - the engaging means is arranged to rotationally disengage the rotary driving member and the impeller when the electrical drive is energized to rotate the rotor at a rotational speed above that of the rotary driving member.
2. A motor vehicle coolant pump as claimed in Claim 1, in which the engaging means is automatically arranged to rotationally disengage the rotary driving member and the impeller when the electrical drive is energized to rotate the rotor at a rotational speed above that of the rotary driving member.
3. A motor vehicle coolant pump as claimed in Claim 2, in which the engaging means is an over-run clutch.
4. A motor vehicle coolant pump as claimed in Claim 1, in which the engaging means is a selective engaging means arranged to selectively disengage the rotary driving member from the impeller when the electrical drive is to be energized to rotate the rotor.
5. A motor vehicle coolant pump as claimed in Claim 4, in which the selective engaging means is a magnetic clutch arrangement between the impeller and the rotary driving member.
6. A motor vehicle coolant pump as claimed in Claim 5, in which the magnetic clutch arrangement includes a stationary electromagnet arranged in proximity with said magnetic elements, the electromagnet moving the magnetic elements and hence the impeller towards or away from the electromagnet when the electromagnet is energised in order to engage the impeller with a clutch surface fixed rotationally with the rotary driving member.
7. A motor vehicle coolant pump as claimed in Claim 6, in which the stationary electromagnet is formed by said coils.
8. A motor vehicle coolant pump as claimed in Claim 7, in which the magnetic elements are arranged along a circumference around an axis through the impeller and the coils are arranged around said circumference at an axial spacing from said magnetic elements.
9. A motor vehicle coolant pump as claimed in Claim 6, in which the stationary electromagnet is separate from said coils.
10. A motor vehicle coolant pump as claimed in Claim 9, in which the magnetic elements are arranged along a circumference around an axis through the impeller and the coils are arranged radially outside said circumference.
11. A motor vehicle coolant pump as claimed in any of Claims 6 to 10, in which the clutch surface engages with a surface of said magnetic elements when the electromagnet is energised.
12. A motor vehicle coolant pump as claimed in any preceding claim, in which the magnetic elements are formed from a soft magnetic material.
13. A motor vehicle coolant pump as claimed in any preceding claim, in which the magnetic elements are formed from a magnetised magnetic material.
14. A motor vehicle coolant pump as claimed in any preceding claim, in which the impeller has a number of vanes for pumping coolant, the vanes projecting from a back plate, and the plurality of magnetic elements being affixed to the impeller back plate. ;
15. A motor vehicle coolant pump as claimed in Claim 14, in which the back plate has a circular periphery, and the plurality of magnetic elements is affixed to said periphery of the impeller back plate.
16. A motor vehicle coolant pump as claimed in Claim 15, in which the vanes project from one side of the back plate, and the plurality of magnetic elements are affixed to an opposite side of the impeller back plate.
17. A motor vehicle, comprising an internal combustion engine, a motor vehicle coolant pump, a cooling circuit, a source of electrical power, an engine control unit (ECU) and a number of engine sensors including an engine temperature sensor, wherein the coolant pump is as claimed in any preceding claim, the engine being arranged in use to drive mechanically the coolant pump via rotary driving member in order to pump coolant through the cooling circuit to cool the engine, and the ECU being arranged in use to monitor the sensed engine temperature and to energise the coolant pump from the source of electrical power to rotate the rotor at a rotational speed above that of the rotary driving member in order to reduce the sensed engine temperature when the engine temperature exceed a predetermined limit.
18. A motor vehicle coolant pump, substantially as herein described, with reference to or as shown in the accompanying drawings.
19. A motor vehicle with a motor vehicle coolant pump, substantially as herein described, with reference to or as shown in the accompanying drawings.