Powertrain for a motor vehicle and control method thereof

A powertrain has an internal combustion engine provided with an accessory transmission that includes a first pulley connected to a crankshaft of the engine, a second pulley connected to a shaft of an electric machine, and a belt connecting the first and the second pulleys to each other to rotate in the same direction of rotation (R). The electric machine is operated to supply an active torque to the second pulley during start-up of the internal combustion engine by means of a starter motor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is filed under 35 U.S.C. § 371 as the U.S. National Phase of International Patent Application No. PCT/IB2020/061848, filed Dec. 11, 2020, which designated the U.S. and which claims the benefit of Italian patent application no. 102019000023805, filed on Dec. 12, 2019, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention concerns a powertrain for a motor vehicle and a method for the control of said powertrain.

The present invention is preferably but not exclusively applied in a hybrid powertrain, which will be referred to hereinafter without any loss of generality.

BACKGROUND ART

As is known, hybrid traction vehicles comprise an internal combustion engine and at least one electric machine which can be used as a generator or as a motor to deliver torque in combination with (or alternatively to) the internal combustion engine, according to the vehicle operating conditions.

A configuration in which an electric machine is used connected between the internal combustion engine and the vehicle transmission is conventionally called “P2”. Modular units are known designed to be interposed between the internal combustion engine and the transmission of a vehicle (and therefore currently called “P2 modules”), which comprise, in addition to the electric machine, one or more clutches to selectively connect to the transmission the internal combustion engine and/or the electric machine, as well as the relative actuators and transmission elements.

Powertrains for motor vehicles provided with an accessory drive arranged on one end of the internal combustion engine opposite the transmission (normally indicated by “front end”, or “P0”) are also known. The accessory drive, generally a belt drive, connects one or more accessories to the crankshaft. Said accessories typically comprise an electric machine operating as an alternator and/or motor and a compressor for the air conditioning system, and the accessory drive comprises respective pulleys connected to the crankshaft and to each of the accessories, a belt cooperating with the above-mentioned pulleys and a tensioner designed to ensure a minimum belt tension.

If the electric machine is used exclusively as an alternator (generator), it is common practice to interpose between the same and the relative pulley a one-way joint or free wheel, having the object of transmitting the torque from the pulley to the electric machine when the latter is driven by the engine, but allowing the electric machine to override the pulley in the transitory operating conditions in which this can occur (for example in the case of a sudden deceleration of the crankshaft. This prevents the high inertia of the electric machine subjecting the belt to undesired tension peaks.

If on the other hand, as occurs increasingly frequently, the electric machine is used also as a motor, the use of a free wheel as described above is not possible, since the electric machine must be able to receive or transmit torque to the relative pulley according to the operating conditions.

In this case, to ensure a minimum belt tension in all operating conditions, a dual tensioner is used having a first tensioner pulley acting on the belt span upstream of the electric machine and a second tensioner pulley acting on the belt span downstream of the electric machine, since both spans can be the slack span according to the operating conditions.

The known solutions described above can be combined so as to produce a hybrid powertrain having an electric machine in position P0 (below: the electric machine P0) and an electric machine in position P2 (below: the electric machine P2); the two machines can be used with different control strategies (for example one only as motor, the other in a reversible manner).

Furthermore, the internal combustion engine can be started by means of the electric machine P0, the electric machine P2 or by means of a third electric machine (a traditional starter motor) associated with the engine flywheel.

For example, if the electric machine P0 is used only as a motor and the internal combustion engine is started by means of the electric machine P2 or the starter motor, the tension of the accessory drive belt can reach unacceptable minimum values, as can be seen inFIG.3, which can determine noisiness and premature wear of the belt.

DISCLOSURE OF INVENTION

The object of the present invention is to produce a powertrain that solves the above-mentioned problem.

The above-mentioned object is achieved by an accessory drive, such as a powertrain for a vehicle that has an internal combustion engine, a first electric machine operating at least as a starter motor and located on a first side of the internal combustion engine facing in use towards the transmission, a control unit, and an accessory drive that includes a first pulley connected to one end the engine's crankshaft on a second side of the engine opposite in use to the transmission, a second electric machine located on the second side of the internal combustion engine, a second pulley connected to the second electric machine, and a belt connecting the first and at least the second pulley together to rotate in the same direction of rotation as the crankshaft. The control unit is programmed to control the second electric machine to transmit an active torque to the second pulley during start-up of the internal combustion engine by means of the first electric machine, thereby maintaining a predetermined minimum tension level in a span of the belt between the first and the second pulleys.

The present invention also concerns a control method of the powertrain. The method includes driving the second electric machine to transmit an active torque to the second pulley during start-up of the internal combustion engine by means of the first electric machine, thereby maintaining a predetermined minimum tension level in a span of the belt between the first and the second pulleys in the direction of rotation.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference toFIG.1, the number1indicates overall a powertrain for a motor vehicle.

The powertrain1comprises an internal combustion engine2(below, for the sake of brevity, “heat engine2”) having a crankshaft3and a hybrid module4.

The hybrid module4is interposed, in use, between the heat engine2and a transmission T of the vehicle, and is connected to one end5of the crankshaft3facing the transmission T.

The hybrid module4comprises an electric machine6and a decoupling clutch7designed to selectively connect the electric machine6to the crankshaft3and to the transmission T.

The clutch7comprises at least a clutch plate8axially fixed and rotationally integral with a flywheel9connected at the end5of the crankshaft3, and a plate10axially movable and rotationally integral with an intermediate shaft11of the module4. The clutch7lastly comprises a hydraulic actuator12acting on the plate10for control of the clutch.

The intermediate shaft11is connected to a pulley14by means of a torsional vibration damper of known type and not illustrated, housed inside the pulley.

The pulley14is connected by a belt15to a pulley16connected to the electric machine6.

A flex plate17is fixed to the pulley14, said plate constitutes an output member of the module4designed to be connected to a torque converter18constituting an input member of the transmission T.

The powertrain1lastly comprises an accessory transmission19connected to one end20of the crankshaft3opposite the end5.

The accessory transmission19(FIG.2) comprises a first pulley21fixed to the end20of the crankshaft3, a second pulley22associated with a second electric machine23and a third pulley24to drive a compressor25of a conditioning system. The three pulleys21,22and24are connected to one another by a belt26. A conventional tensioner28acts on a span26aof the belt26comprised between the second pulley22and the third pulley24. The pulleys21,22and24rotate in the same direction of rotation R indicated by an arrow inFIG.3and coinciding with the direction of rotation of the crankshaft3.

The powertrain1comprises a third electric machine30operating as a starter motor, provided with an output pinion31that engages with a toothing of the flywheel9. Expediently, the third electric machine30is a brushless motor able to rapidly start the heat engine2.

The powertrain1lastly comprises a programmable control unit34, which controls the electric machines in response to input signals representing the operating conditions of the powertrain1and of the driver controls.

The operation of the powertrain1is the following.

When the clutch7is closed, the intermediate shaft11is connected both to the heat engine2and to the electric machine6and to the vehicle transmission T.

In this condition, the control unit34can operate the electric machine6both as generator (to recharge the battery during the thermal traction, or as a regenerative brake), and as a motor for the delivery of an additional torque (boosting) in combination with the electric machine23, according to the operating conditions. The specific control logics of the electric machines are not described here since they do not form part of the invention.

When the clutch7is open, the electric machine6can be used for the electric traction, electric braking and coasting with the heat engine off.

The heat engine2is started (both cold and starting from a condition of electric traction) by means of the third electric machine30. During this phase, the clutch7is open.

During starting of the heat engine2, the electric machine30drives the crankshaft3and the first pulley21connected to it.

According to the present invention, to prevent the tension dropping below predetermined levels in the span26cof the belt26, immediately downstream of the first pulley21in the direction of movement of the belt, the electric machine23is operated by the control unit34so as to supply an active torque to the pulley22. This additional torque causes an increase in the tension in span26cof the belt26, and therefore eliminates the technical problem highlighted.

For a better understanding of the effect of the additional torque according to the present invention,FIGS.3and4illustrate the trend of the tension of the belt26in an accessory transmission of a conventional powertrain, and in the accessory transmission of the powertrain of the invention.

In particular, the tensions are measured in the three spans26a,26band26carranged respectively between the pulleys22and24, between the pulleys24and21, and between the pulleys21and22, and shown in the graphs as a function of the time. In the time interval between 0 and 2 s the trend of the tensions is observed with the heat engine off (driven by the electric machine6); as from 2 s the trend of the tensions is observed in the starting phase of the heat engine2by means of the electric motor30.

As can be easily seen inFIG.3, with a mean belt tension value in the order of approximately 350 N when the motor is driven, significant tension oscillations occur at start-up in the belt span26c, in the order of approximately 400 N peak-peak, with minimum peak values in the order of 20-30 N. Such low values, essentially due to the inertia of the electric machine23in the presence of the torsional vibrations induced by the combustion at start-up, can determine slipping and noise.

FIG.4illustrates the trend of said tensions in the presence of the additional torque transmitted by the electric machine23to the pulley22simultaneously with start-up by means of the electric motor30.

The value of the active torque to be transmitted to the pulley22can be easily determined experimentally. For example, for a three-cylinder engine with 1.0 l displacement, the active torque necessary is in the order of only 5 Nm. The active torque must be maintained only for the duration of the first ignition cycles of the engine; once a stable combustion regime has been reached, the active torque is no longer necessary and can be interrupted.

FromFIG.4the effect of increase in the tension values in span26cof the belt26is evident, with minimum peak values in the order of 210 N.

Lastly, it is clear that modifications and variations can be made to the powertrain1described that do not depart from the scope defined by the claims.