Variable cam phaser apparatus

Abstract

 An electrically actuated cam phaser apparatus for transferring a driving torque from a drive member (1) to a driven member (2) and for selectively adjusting the angular relationship between the drive and driven members, wherein the cam phaser includes an actuator (4) comprising a single phase brushless electric motor having a rotor and a stator, whereby the rotor is rotatably driven by the drive member.

Description

[0001] The present invention relates to a cam phaser apparatus, particularly, but not exclusively a variable cam phaser apparatus used in valve trains of automobile engines.

[0002] It is desirable to be able to adjust the cam phase (i.e. the angular relationship between the crankshaft and the camshaft) of engines during engine operation in order to vary the timing of the opening and closing of the inlet and/or exhaust valves to improve engine performance, emissions and/or fuel consumption. Various types of arrangements exist that are capable of achieving this adjustment. Such systems are required to efficiently convert rotational movement from the crankshaft into rotational movement of the camshaft whilst allowing dislocation between these rotational movements in order to allow variation of the cam phase.

[0003] Current systems typically utilise hydraulic actuators using high pressure oil to enable relative angular displacement between drive and driven members (e.g. cam sprocket or pulley and camshaft) of the valve train. Such systems have difficulty operating at extremes of temperature, or low engine speed, in particular during engine start up when the oil pressure is too low to allow hydraulic actuation.

[0004] Whilst attempts have been made to design electrically actuated variable cam phase arrangement, such usually require a complex triple shaft arrangement such as planetary gears or a harmonic drive arrangement.

[0005] Examples of triple shaft gear systems suitable for use with a cam phaser comprise planetary gear systems, with a sun gear, planetary gears mounted on a planet carrier and a ring gear, or harmonic drive systems with a wave generator, flex-spline and circular spline.

[0006] Another form of triple member system equivalent to a planetary gear system comprises a magnetic gear having an outer ring comprising a plurality of magnets (equivalent to the ring gear), a concentric inner ring comprising a plurality of circumferentially spaced magnets (equivalent to a sun gear) and an intermediate ring or carrier member mounted in an annular space between said outer and inner rings and comprising a plurality of circumferentially spaced pole pieces (equivalent to planetary carrier upon which a plurality of planetary gears are mounted).

[0007] Typically, where such triple shaft gear systems are applied to a cam phaser of an engine, one shaft is connected to the cam sprocket or pulley to comprise a drive member (typically the planet carrier or circular spline) and a second shaft is connected to the camshaft (typically the ring gear or the flex-spline) to comprise a driven member, the remaining third shaft (typically the sun gear or wave generator) being connected to an actuator for varying the cam phase (i.e. the angular relationship between the drive and driven members).

[0008] The actuator normally comprises an electric motor having a stator fixed to the engine and a rotor mounted on the adjusting part of the cam phaser. Whilst direct current (DC) motors can be used, such require brushes and slip ring which are prone to wear and thus require frequent maintenance. Therefore a preferred alternative is to use a brushless alternating current (AC) motor.

[0009] A problem with simple single phase brushless AC motors is that the conventional single-phase motor has certain rotor positions for which it is not self-starting, even when the stator windings are energized full wave. Such rotor positions are generally referred to as null positions. The performance range of such arrangements of synchronous motors is also limited, in particular by the irregular torque profile in the start-up range.

[0010] Two approaches have been used to overcome the starting difficulty of the single-phase motor. In one approach, the motor is constructed with an asymmetrical stator and/or rotor air gap geometry which serves to preposition the rotor so that starting is not attempted from a null position. In the other approach, the motor is effectively operated as a two-phase machine by providing a separate starting winding, and energizing it with phase displaced current. The phase displaced current is typically generated with an impedance element such as a capacitor.

[0011] For the fine control required for a cam phaser, three phase motors are therefore generally used in preference to single phase motors. However, such three phase motors require a full electronic bridge, three winding phases on the stator and three sensors, increasing the cost and complexity of the cam phaser.

[0012] According to a first aspect of the present invention there is provided an electrically actuated cam phaser apparatus for transferring a driving torque from a drive member to a driven member and for selectively adjusting the angular relationship between the drive and driven members, wherein the cam phaser includes an actuator comprising a single phase brushless electric motor having a rotor and a stator, whereby the rotor is rotatably driven by the drive member.

[0013] Preferably the cam phaser apparatus comprising a triple member gear system said triple member gear system comprising a first member connectable to said driven member, a second member connectable to said drive member, and a third member comprising an adjusting means of said triple member gear system, said third member being connected to said actuator for applying a braking torque and/or a motoring torque to said third member to vary the angular relationship between the first and second members, the rotor being directly connected to or mounted on the third member. The triple member gear system may be one of a planetary gear system, a harmonic drive or a magnetic gear,

[0014] By drivingly connecting the rotor to the drive member, via the triple gear system, the actuator will never need to supply or absorb torque at 0 rpm, and therefore there is no need for a 0rpm starting torque capability for the actuator.

[0015] According to a second aspect of the present invention there is provided an internal combustion engine having a cam phaser apparatus in accordance with the abovementioned first aspect of the invention.

[0016] Preferably the stator is mounted on the cylinder head of the engine and the rotor is mounted on the third member of the cam phaser apparatus.

[0017] An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing, in which:

Fig. 1 is a schematic view of a cam phaser apparatus according to an embodiment of the present invention;

[0018] As illustrated in Fig 1, the cam phaser apparatus includes a first member 1 of a triple member gear system, said first member 1 being connected to a crankshaft of the engine via a pulley or cam sprocket and an endless belt or chain. In the case of a planetary gear system, such first member 1 could be the planet carrier and planetary gears. In the case of a harmonic drive, the first member 1 could be the circular spline. In the case of a magnetic gear, the first member 1 could be the intermediate ring or pole pieces.

[0019] The camshaft of the engine is connected to a second member 2 of the triple member gear system. In the case of a planetary gear system the second member 2 could be the ring gear. In the case of a harmonic drive, the second member 2 could be the flex-spline. In the case of a magnetic gear, the second member 2 could be the outer ring.

[0020] A third member 3 of the triple member gear system is connected to a single phase brushless electric motor 4 for altering the cam phase of the engine. In the case of a planetary gear system, the third member 3 could be the sun gear. In the case of a harmonic drive, the third member 3 could be the wave generator. In the case of a magnetic gear, the third member 3 could be the inner ring. The housing of the motor is fixed with respect to the cylinder head while the rotor of the motor rotates with the camshaft.

[0021] When it is desired to advance the cam phase of the camshaft, a braking torque is applied to the third member 3 by the electric motor 4 to decelerate the third member 3, thus advancing the camshaft angle with respect to the crankshaft.

[0022] When it is desired to retard the cam phase of the cam shaft, a motoring torque is applied to the third member 3 by the electric motor to accelerate the third member 3, thus retarding the camshaft angle with respect to the crankshaft.

[0023] When no advance is required it is necessary to maintain power to the motor 4 to ensure that the motor 4 compensates for the natural tendency of the motor to decelerate due to frictional losses.

[0024] Because the rotor of the motor 4 is effectively rotatably driven by the engine, via the camshaft pulley, when the engine is operating, the motor will never need to supply or absorb torque at 0 rpm, and therefore there is no need for a 0 rpm starting torque capability for the motor and a simple single phase AC electric motor can be readily used.

[0025] Various modifications and variations to the described embodiment of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with a specific preferred embodiment, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.

Claims

1. An electrically actuated cam phaser apparatus for transferring a driving torque from a drive member to a driven member and for selectively adjusting the angular relationship between the drive and driven members, wherein the cam phaser includes an actuator comprising a single phase brushless electric motor having a rotor and a stator, whereby the rotor is rotatably driven by the drive member.

2. A cam phaser apparatus as claimed in claim 1, wherein the cam phaser apparatus comprising a triple member gear system said triple member gear system comprising a first member connectable to said driven member, a second member connectable to said drive member, and a third member comprising an adjusting means of said triple member gear system, said third member being connected to said actuator for applying a braking torque and/or a motoring torque to said third member to vary the angular relationship between the first and second members, the rotor being directly connected to or mounted on the third member.

3. A cam phaser apparatus as claimed in claim 2, wherein the triple member gear system is one of a planetary gear system, a harmonic drive or a magnetic gear,

4. An internal combustion engine having a cam phaser apparatus as claimed in any preceding claim.

5. An internal combustion engine as claimed in claim 4, wherein the stator is mounted on the cylinder head of the engine and the rotor is mounted on or formed integrally with the cam phaser apparatus.

Drawing