Controlling torque in a flywheel powertrain

A method for controlling a powertrain for an automotive vehicle includes determining a desired flywheel torque, determining, with reference to the desired flywheel torque, a desired torque capacity torque of a clutch through which torque is transmitted between the flywheel and wheels of the vehicle, operating the clutch to produce the desired clutch torque capacity, determining a slip error across the clutch, and changing a gear ratio of a continuously variable transmission located in a drive path between the clutch and said wheels to a gear ratio that reduces the slip error.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a powertrain for a motor vehicle and, more particularly, to controlling torque transmitted between a flywheel and vehicle wheels.

2. Description of the Prior Art

A hybrid vehicle powertrain combines a conventional propulsion system, which includes an internal combustion engine and a step-change automatic transmission, with an energy storage system to improve fuel economy over a conventional vehicle powertrain. Various techniques for regenerating and storing kinetic energy of the vehicle include electric systems, which include an electric motor and electric storage battery; hydraulic systems, which store energy in a pressurized hydraulic tank and flywheel systems, which store energy in a rotating flywheel or disc.

A major challenge present by flywheel systems is the need to provide robustly and smoothly the flywheel torque to the wheels. One way that has been proposed is to determine the wheel torque through the measurement of wheel slip as an input into the control of the flywheel torque delivery to the wheels. This measurement can be difficult since it is very stiff and can change with environmental conditions, creating the need to have very accurate ratio control. Wheel slip is a difficult measurement to make in a vehicle particularly under low loads, and it has a high gain. Small errors in this measurement can cause large control perturbations.

SUMMARY OF THE INVENTION

A method for controlling a powertrain for a motor vehicle includes determining a desired flywheel torque, determining, with reference to the desired flywheel torque, a desired torque capacity torque of a clutch through which torque is transmitted between the flywheel and wheels of the vehicle, operating the clutch to produce the desired clutch torque capacity, determining a slip error across the clutch, and changing a gear ratio of a continuously variable transmission located in a drive path between the clutch and said wheels to a gear ratio that reduces the slip error.

The invention further contemplates a system for transmitting power in a powertrain including a flywheel, vehicle wheels, a clutch through which torque is transmitted between the flywheel and said wheels, a continuously variable transmission located in a drive path between the clutch and said wheels, and a final drive mechanism located in series relation with said transmission in a drive path between said transmission and said wheels.

During vehicle acceleration conditions, the flywheel torque provides added positive torque to supplement powertrain torque transmitted to wheels. During vehicle deceleration conditions, the flywheel provides negative torque to wheels thereby replacing friction wheel brake torque so that kinetic energy of the vehicle is recovered and stored for further vehicle propulsion rather than being wasted through friction in the brake pads.

The output torque of the flywheel is controlled to provide accurate torque to the wheels consistently to meet driver demanded wheel torque. The torque demand of the driver is the combination of the output of the base powertrain plus the torque output of the flywheel.

The method employs easy to measure input speed signals rather than wheel slip signals. Open loop control of the flywheel output torque reduces fluctuation of torque at the wheels. Closed loop control of clutch slip causes only minor perturbations to the output torque.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings,FIG. 1illustrates schematically a hybrid powertrain10for a motor vehicle. The front wheels12,13are driveable connected to a power source14, such as an internal combustion engine (ICE) through a transmission16, preferably a multiple-speed step change transmission, and a front differential mechanism18. The rear wheels20,21are driveable connected to a hybrid power source, such as a flywheel assembly22, through a transmission24, preferably a continuously variable transmission (CVT), driveshaft26and rear differential mechanism28. The CVT24, which produces a stepless, continuously variable ratio of the speed of its input to the speed of its output. Belt drive mechanisms and traction drive mechanisms are used to transmit power through the CVT24.

In this hybrid powertrain10, wheel braking energy is regenerated and stored mechanically in flywheel22. The speed ratio of the CVT24is varied to allow rotating energy to be either stored in the flywheel22or released from the flywheel and transmitted to the wheels20,21within the operating range of the CVT.

FIG. 2shows an example of possible speed ratios for a given application. The flywheel assembly22includes a flywheel29connected to the sun gear30of a simple planetary gearset32, which further includes a ring gear34, fixed against rotation, a carrier36, and planet pinions36, supported on the carrier and meshing with the sun gear and ring gear. The speed ratio produced by gearset32is preferably about 5.07. Planetary gearset32is referred to as Gear3.

Carrier36is connected to a gear38, which meshes with a pinion40. Preferably, gear38rotates about 2.6 times faster than pinion40. Pinion40and gear38are collectively referred to as Gear2. Flywheel torque is the torque transmitted on shaft41.

Pulleys42,44are driveably connected by a drive belt46, whose radial position on the pulleys varies. The CVT ratio varies in the preferred range between 2.54 and 0.42. Clutch48driveably connects pulley42and pinion40.

Pulley44is connected to a gear50, which meshes with a pinion52. Preferably, gear50rotates about 1.59.6 times faster than pinion52. Pinion52and gear50are collectively referred to as Gear1.

Driveshaft26, which includes two constant velocity (CV) joints54,56, is driveably connected to the ring gear58of rear differential mechanism28through a pinion60. The rear differential mechanism28produces a final drive ratio of about 3.58

Clutch torque capacity is the variable magnitude of torque transmitted the clutch in any of its operating conditions i.e., slipping, partially engaged, or fully engaged.

Assuming uniform pressure across the friction surfaces of the clutch,
Torque=Nsurfaces*Fclutch*mu*(Douter^3−Dinner^3)/3*(Douter^2−Dinner^2))  (1)
wherein Torque is clutch torque capacity, Fclutchis the magnitude of force applied normal to each friction surface of the clutch, mu is the coefficient of friction, N is the number of clutch friction surfaces, Douteris the outer diameter of the friction surfaces, and Dinneris the inner diameter of the friction surfaces.

The force on the servo piston that activates the clutch is a function of the hydraulic pressure applied to the piston,
Fpiston=Pressure*Pi*(Douter piston^2−Dinner piston^2)/4  (2)
wherein Fpistonis force on the servo piston, Douter pistonis the outer diameter of the piston surface, and Dinner pistonis the inner diameter of the piston surface. Therefore, the torque capacity of clutch48is a function of the variable pressure on the actuating servo piston. Pressure refers to the net pressure on the servo, i.e. the actual pressure minus the pressure needed to overcome the servo return springs.

FromFIG. 2, the output torque transmitted to wheels20,21is a function of clutch torque
Toutput=Tclutch*CVTratio*Gear 1 Ratio*FDratio  (3)

Based on the piston and clutch geometry, the clutch pressure-clutch torque relationship can be established. To control the flywheel torque, the clutch pressure is controlled to provide the desired output torque.

In addition to the output torque, the slip across clutch48is controlled. By controlling clutch slip, the flywheel29will be either collecting energy or releasing energy. The direction of the energy flow depends on the direction of the clutch slip. Positive clutch slip occurs when the Gear2side of clutch28rotates faster than the CVT side of the clutch. Positive slip will result in energy being transmitted from flywheel29to the wheels20,21as the speed of the flywheel decreases. Negative clutch slip occurs when the CVT side of clutch28rotates faster than the Gear2side of the clutch. Negative slip will result in rotating energy being stored in flywheel29as its speed increases.

FIG. 3illustrates a block diagram of the steps of the control method, which eliminates need to use wheel slip as the input variable to control the flywheel torque to the wheels. Instead, to maintain clutch slip, the CVT ratio is adjusted through closed loop control of clutch slip.

Driver demand64, represented by the degree of displacement of the accelerator pedal66or displacement of the engine throttle68, is used as a reference to determine from a torque allocation variable70, the desired flywheel output torque72.

At step74, the desired flywheel output torque72is divided by the CVT ratio and the gear ratio of the powertrain components in the power path between the wheels and the CVT pulley44, i.e., the product Gear1Ratio*FD ratio, to determine the desired torque capacity78of clutch48.

At step80, the desired torque capacity78of clutch48is divided by the gain of the clutch (clutch torque per pressure unit) to determine the magnitude of pressure82with which to actuate the clutch and produce the desired clutch torque capacity78.

A function84stored in electronic memory produces as output a signal representing the desired clutch slip86that corresponds to the current operation conditions. Desired clutch slip86preferably has a low magnitude because slip represents energy loss that heats the friction surfaces of clutch48.

The difference between measured or actual clutch slip88and desired clutch slip86is determined at summing junction90, whose output is the slip error92, which is multiplied by the gain94of the CVT24(CVT gear ratio per unit of clutch slip) to determine the CVT ratio error96.

A closed loop controller98, preferably a PID controller, receives as input the CVT ratio error96and produces as its output a command signal100representing a change in CVT ratio that will minimize the slip error90. In response to command signal100, the radii at which drive belt46engages pulleys42and44of the CVT24are changed, such that the CVT ratio is changed by the magnitude of the commanded change in CVT ratio100.