Hybrid transmission having electro-magnetically actuated pawl clutch

An electro-magnetically actuated pawl clutch is adapted to establish a fixed overdrive ratio in a powersplit type hybrid gearing arrangement. The electro-magnetically actuated clutch includes an inner race splined to the transmission input shaft and an outer race fixed to a first gear that is supported for rotation about the input shaft. The inner race includes two magnetically separated toothed rings. Electrical current in a non-rotating coil establishes a magnetic field in the inner race. The magnetic field causes a set of pawls to pivot with respect to the outer race and to engage at least one of the toothed rings. The pawls and teeth are designed such that, when engaged, the outer race can rotate faster than the inner race but cannot rotate slower. The first gear meshes with a second gear fixed to the transmission output shaft.

TECHNICAL FIELD

This disclosure relates to the field of vehicle clutches. More particularly, the disclosure pertains to an electro-magnetically actuated pawl clutch used within a hybrid electric powertrain.

BACKGROUND

Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.

Some transmissions, called discrete ratio transmissions, are configured to establish a finite number of speed ratios between an input shaft and an output shaft. When the currently selected ratio is no longer appropriate, a discrete ratio transmission must shift to a different one of the available speed ratios. Other transmissions, called continuously variable transmissions (CVTs), are capable of establishing any speed ratio between lower and upper limits. CVTs are capable of making frequent fine speed ratio adjustments which are not perceivable by vehicle occupants.

Many transmissions use hydraulically actuated friction clutches to establish various power flow paths. Hydraulic actuation is suited for clutches that selectively couple rotating elements to one another because pressurized hydraulic fluid can be routed from a stationary housing to rotating components between seals. Therefore, the hydraulic actuator can rotate with one of the rotating elements. When there are multiple hydraulically actuated clutches, the clutches often share an engine drive pump and share many of the valve body components used to regulate the pressure.

Hybrid vehicle transmissions improve fuel economy by providing energy storage. In a hybrid electric vehicle, for example, energy may be stored in a battery. The battery may be charged by operating the engine to produce more power than instantaneously required for propulsion. Additionally, energy that would otherwise be dissipated during braking can be captured and stored in the battery. The stored energy may be used later, allowing the engine to produce less power than instantaneously required for propulsion and thereby consuming less fuel.

SUMMARY OF THE DISCLOSURE

An electro-magnetically actuated clutch includes a non-rotating electromagnet coil, a toothed inner race, an outer race, and a magnetically conductive pawl. The toothed inner race, which is supported for rotation about the coil, has left and right magnetically conductive rings which are magnetically separated from one another. The outer race, which may be magnetically non-conductive, is supported for rotation about the inner race. The pawl is supported for rotation with the outer race and is pivotable into engagement with the left and right rings in response to current in the coil. Both rings may have teeth. The teeth of one of the rings may be offset from the teeth of the other ring such that a majority of the engagement force is distributed to one ring.

A clutch includes an electromagnetic coil, left and right magnetically conductive rings, a race supported for rotation with respect to the rings, and a magnetically conductive pawl. The electromagnetic coil may be non-rotating while the rings and the race are supported for rotation. The left and right rings each have a cylindrical surface adjacent to the coil and a toothed surface opposing the cylindrical surface. The electromagnetic coil may be radially inside the rings. The left and right rings are magnetically separated from each other but may be fixedly coupled to one another. The race is supported for rotation with respect to the rings and may be radially outside the rings. The pawl is pivotable with respect to the race into engagement with the left and right rings in response to current in the coil.

A transmission includes an electromagnetic coil, left and right magnetically conductive rings, an outer race, and a magnetically conductive pawl. The coil may be fixed to a transmission case. The rings are both fixedly coupled to an input shaft and are magnetically separated from one another. The outer race is supported for rotation with respect to the input shaft. The pawl is supported for rotation with the outer race and is pivotable into engagement with the left and right rings in response to current in the coil.

DETAILED DESCRIPTION

A group of rotating elements are fixedly coupled to one another if they are constrained to rotate as a unit in all operating conditions. Rotating elements can be fixedly coupled by spline connections, welding, press fitting, machining from a common solid, or other means. Slight variations in rotational displacement between fixedly coupled elements can occur such as displacement due to lash or shaft compliance. In contrast, two rotating elements are selectively coupled by a shift element when the shift element constrains them to rotate as a unit whenever it is fully engaged and they are free to rotate at distinct speeds in at least some other operating condition. Two rotating elements are coupled if they are either fixedly coupled or selectively coupled. Two rotating elements are driveably connected if a series of gears and shafts is capable of transmitting power from one to the other and establishes a fixed speed ratio between the two elements.

FIG. 1schematically illustrates a kinematic arrangement for a power-split type hybrid electric vehicle. Power is provided by engine10which is fixedly coupled to planet carrier12via transmission input shaft11. A set of planet gears14are supported for rotation with respect to carrier12. Sun gear16and ring gear18are each supported for rotation about the same axis as carrier12and each mesh with the planet gears14. Generator20is fixedly coupled to sun gear16. Layshaft gear22is fixedly coupled to ring gear18and meshes with layshaft gear24. Layshaft gear24is fixedly coupled to layshaft gears26and28via shaft30. Layshaft gear32meshes with layshaft gear28and is fixedly couple to motor34. Layshaft gear26meshes with layshaft gear36which is the input to differential38. Differential38drives wheels40and42allowing slight speed differences as the vehicle turns a corner.

Generator20and motor34are both reversible electric machines. The terms generator and motor are used merely as labels. Both machines are capable of converting electrical power to mechanical power or converting mechanical power to electrical power. For example, each machine may be a synchronous motor in combination with a three phase inverter. Both machines are electrically connected to battery44. In some circumstances, engine10may generate more power than is delivered to the vehicle wheels40and42with the excess power stored in battery44. In other circumstances, power may flow from battery44permitting engine10to produce less power than the instantaneous demand of the vehicle. For example, the engine10may be off while power to propel the vehicles comes from battery44.

The powertrain ofFIG. 1can be operated in a continuously variable mode with battery44neither providing nor absorbing power. The torque applied to generator20and the torque applied to layshaft gear22are both related to the torque generated by engine10based on the number of teeth on sun gear16and the number of teeth on ring gear18. Specifically,

Tgen=NsunNsun+Nring⁢TengTgear⁢⁢22=NringNsun+Nring⁢Teng
where Tengis the torque generated by engine10, Tgenis the torque absorbed by the generator20, Tgear22is the torque absorbed by gear22, Nsunis the number of teeth on sun gear16, and Nringis the number of teeth on ring gear18. The engine speed is a weighted average of the generator speed and the speed of gear22.

When the vehicle is moving slowly, gear22rotates slowly and generator20rotates faster than engine10. Power generated by the engine is split by the planetary gear set. A portion of the power is transmitted mechanically to shaft30from carrier12to ring gear18to gear22to gear24. The remaining power is transmitted from sun16to generator20which converts the power to electrical power. Motor34converts the electrical power to mechanical power which is transmitted to shaft30by gear32and28. Although both power transfer paths are subject to some parasitic losses, conversions between electrical power and mechanical power typically involve more power loss than purely mechanical transfer. As the ratio of the speed of shaft30to the speed of engine10increases, a point is reached at which generator10is stationary. At this ratio, all of the power is transferred mechanically. At higher overdrive ratios, generator20rotates in the opposite direction as engine10and acts as a motor. Power circulates from generator20through the mechanical power flow path to shaft30, through gears28and32to motor34which acts as a generator. The parasitic losses associated with the circulation of power tend to make operation at overdrive ratios inefficient.

The powertrain ofFIG. 1includes an additional power flow path to provide efficient power transfer at overdrive speed ratios. Specifically, layshaft gear46is supported for rotation about transmission input shaft11. Layshaft gear48is fixedly coupled to shaft30and meshes with layshaft gear46. Clutch50selectively couples layshaft gear46to shaft11. When clutch50is engaged, power is transferred mechanically from engine10to shaft30via gears46and48. In this fixed ratio mode of operation, battery44can provide additional power via either generator20or motor34or can be charged via either electrical machine. Use of the fixed ratio mode for steady state cruising significantly reduces fuel consumption because both the engine and the transmission operate efficiently.

Since clutch50is the only clutch in the powertrain ofFIG. 1, use of a hydraulically actuated clutch would require addition of a pump and valve body. Therefore, a different method of actuating clutch50is desired.FIGS. 2 through 4illustrate an electro-magnetically actuated pawl clutch suitable for selectively coupling gear46to shaft11.

FIG. 2is a pictorial view of an electromagnetic clutch suitable for use in the hybrid powertrain ofFIG. 1. An inner race includes two rings52and54. As applied to the hybrid powertrain ofFIG. 1, each of these rings is fixedly coupled to input shaft11. An outer race54is fixedly coupled to gear46. A plurality of pawls58are retained in the outer race and rotate with the outer race. In the disengaged state illustrated inFIG. 2, the pawls are tucked into the outer race so as not to contact the inner race. In this state, relative rotation between the inner race and the outer race in either direction may occur. Springs may bias the pawls toward this disengaged position. The outer surfaces of the rings of the inner race have teeth60. When the clutch is in an engaged state, the pawls58pivot into engagement with this these teeth. The tooth profile is ramped on one side such that relative rotation is permitted in one direction but not in the other. In the orientation shown inFIG. 2, when the inner race rotates clockwise relative to the outer race, the ramped profile of the teeth push the pawl back toward outer race. However, the teeth preclude counter-clockwise rotation of the inner race relative to the outer race. (A few degrees of rotation may occur before a pawl catches.)

FIG. 3shows a cut-away pictorial view of clutch50. The left and right inner race rings are joined by a plurality of posts62. The pawls are retained axially in the outer race56by a retainer64. An electro-magnetic coil is radially inside and concentric with the inner and outer races. The coil includes a magnetically conductive coil housing66having a U-shaped cross section. Electrical windings68are wrapped circumferentially in the gap of the coil housing. When the windings are energized with current, a magnetic field is established in the coil housing. One side of the U is axially aligned with the left inner race ring while the other side of the U is axially aligned with the right inner race ring. The radial clearances between the coil housing66and the left and right rings54and52are set as small as practical consistent with free rotation. Rings52and54are made of magnetically conductive material while the posts62separating them are made of magnetically non-conductive material. Thus, when the coil is energized, one ring becomes a magnetic North pole and the other ring becomes a magnetic South pole. The pawls are made of a magnetically conductive material such that they are attracted to the left and right rings when the coil is energized, engaging the clutch. Once the pawls come into contact with the left and right rings, they complete the magnetic circuit. With the magnetic circuit thus complete, other than the two small air gaps, little power is required to maintain the pawls in this state.FIG. 4is an exploded view showing the assembly of clutch50.

FIG. 5is a cross section of the clutch as installed in the kinematic arrangement ofFIG. 1. The coil66is fixedly coupled to a transmission housing70. A stationary coil is advantageous because no slip rings or other measures are required to convey electrical power to the windings. The inner race is fixedly coupled to input shaft11. Notice that left ring54is wider than right ring52. The two rings may be circumferentially offset slightly such that the torque is reacted entirely or almost entirely through the left ring54. In this way, the non-magnetic posts62do not need to transmit appreciable torque. Right ring52serves a magnetic function but does not carry significant mechanical load. Outer race56is fixedly coupled to gear46which is supported for rotation about input shaft11. In alternative embodiments, the toothed rings may form an outer race and the pawls may be retained in an inner race. In that case, the coil would be located radially outside the outer race. The tooth profile would be on the radially inward surface of the outer race while a cylindrical surface of each ring faces outward adjacent to the coil.FIGS. 6 and 7show the pawl in the disengaged and engaged positions respectively.

Friction clutches are capable of transmitting torque between elements that are rotating at different speeds. The transmitted torque tends to bring the components to the same speed. A pawl clutch, on the other hand, selectively couples elements by establishing a positive engagement as opposed to frictional engagement. As a result, a pawl clutch can only transmit torque between elements that are rotating at the same speed. Engaging a pawl clutch when the elements are at different speeds would produce a sudden change in speeds which is likely to be unpleasant to vehicle occupants and may even cause transmission components to fail. Therefore, control of element speeds at the time of clutch engagement is important.

When the vehicle is at low speed, the transmission ofFIG. 1is operated in the continuously variable mode. No current is supplied to coil68so clutch50is disengaged. When the controller determines that operation in the fixed ratio overdrive mode is preferable, the controller first transitions to a more overdrive speed ratio than the fixed ratio. Then, the controller commands current to coil68causing the pawls to pivot. The clutch does not engage immediately because gear46is rotating faster than shaft11in this condition. The controller, still controlling the speed ratio in the continuously variable mode, gradually permits the engine speed to increase. Once the fixed ratio is reached, clutch50will engage.