Engine spin-up control with natural torque smoothing

A hybrid vehicle includes an engine having at least one cylinder and an electric machine that drives the engine during a start-up period. A control module monitors a rotational speed of the engine and regulates torque generated by the electric machine based on the rotational speed during the start-up period and a maximum discharge power available to power the electric machine.

FIELD OF THE INVENTION

The present invention relates to hybrid vehicles, and more particularly to a torque smoothing control system of a hybrid vehicle.

BACKGROUND OF THE INVENTION

Hybrid vehicles are driven by multiple powerplants including, but not limited to an internal combustion engine and an electric machine. The electric machine functions as a motor/generator. In a generator mode, the electric machine is driven by the engine to generate electrical energy used to power electrical loads or charge batteries. In a motor mode, the electric machine supplements the engine, providing drive torque to drive the vehicle drivetrain.

When the hybrid vehicle is at rest and no drive torque is required, the engine is deactivated. Vehicle launch is initiated by the operator in one of several manners including, but not limited to, depressing an accelerator pedal and releasing pressure from a brake pedal. Prior to activating the engine, the electric machine spins up or drives the engine to a desired idle speed. As the engine spins up to idle speed, compression torque induces speed sag. This causes resonance within the powertrain, which can be sensed by the operator.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a hybrid vehicle that includes an engine having at least one cylinder and an electric machine that drives the engine during a start-up period. A control module monitors a rotational speed of the engine and regulates torque generated by the electric machine based on the rotational speed during the start-up period and a maximum discharge power from an energy storage device to power the electric machine.

In other features, the hybrid vehicle further includes a driver input device. The control module determines the maximum discharge power based on the driver input device. The control module determines a start-up schedule based on the driver input device and determines the maximum discharge power based on the start-up schedule. The start-up schedule includes a torque command and the maximum discharge power.

In other features, the driver input device includes an accelerator pedal. The control module determines a start-up schedule based on a position of the accelerator pedal.

In still other features, the driver input device includes a brake pedal. The control module determines a start-up schedule based on a position of the brake pedal and/or a pressure within the brake system.

In yet another feature, the control module regulates the rotational speed based upon a desired idle speed when the rotational speed achieves a speed threshold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now toFIG. 1, an exemplary hybrid vehicle10includes an engine12and an electric machine14, which drive a transmission16. More specifically, the electric machine14supplements the engine12to produce drive torque to drive the transmission16. In this manner, fuel efficiency is increased and emissions are reduced. The engine12and electric machine14are coupled via a belt-alternator-starter (BAS) system18. More specifically, the electric machine14operates as a starter (i.e., motor) and an alternator (i.e., generator) and is coupled to the engine12through a belt and pulley system. The engine12and the electric machine14include pulleys20,22, respectively, that are coupled for rotation by a belt24. The pulley20is coupled for rotation with a crankshaft26of the engine12. In one mode, the engine12drives the electric machine14to generate power used to recharge an energy storage device (ESD)28. In another mode, the electric machine14drives the engine12using energy from the ESD28. The ESD28can include, but is not limited to, a battery or a super-capacitor. Alternatively, the BAS system18can be replaced with a flywheel-alternator-starter (FAS) system (not shown), which includes an electric machine operably disposed between the engine and the transmission or a chain or gear system that is implemented between the electric machine14and the crankshaft26.

The transmission16can include, but is not limited to, a manual transmission, an automatic transmission, a continuously variable transmission (CVT) and an automated manual transmission (AMT). Drive torque is transferred from the engine crankshaft26to the transmission16through a coupling device30. The coupling device30can include, but is not limited to, a friction clutch or a torque converter depending upon the type of transmission implemented. The transmission16multiplies the drive torque through one of a plurality of gear ratios to drive a driveshaft32.

A control module34regulates operation of the vehicle10based on the launch control system of the present invention. The control module34controls fuel injection and spark to selectively activate and deactivate cylinders of the engine12. More specifically, when the vehicle10is at rest, none of the cylinders of the engine12are firing (i.e., are deactivated) and the engine12is stopped. During vehicle launch (i.e., acceleration from rest), the electric machine14drives the crankshaft to spin-up the engine12to an idle RPM and to initiate vehicle acceleration. During periods where low drive torque is needed to drive the vehicle, the engine cylinders do not fire and the valves can be deactivated. Drive torque is provided by the electric machine14. When deactivated, fuel and spark are cut-off to the cylinders of the engine. Further, opening and closing cycles of the intake and exhaust valves can be prevented to inhibit air flow processing with the cylinders.

An accelerator pedal36is provided. A pedal position sensor36is sensitive to a position of the accelerator pedal36and generates a pedal position signal based thereon. A brake pedal40is provided. A brake pedal position sensor42is sensitive to a position of the brake pedal40and generates a pedal position signal based thereon. The control module34operates a brake system43based on the brake pedal position signal to adjust a pressure within the brake system, which in turn regulates a braking force of brakes (not shown). A speed sensor44is responsive to the rotational speed (RPMEM) of the electric machine44. The speed sensor44generates a speed signal. The control module34operates the vehicle10based on the pedal position signals generated by the pedal position sensors38,42and the speed signal generated by the speed sensor44, as described in further detail below. The engine speed (RPMENG) can be determined based on the speed signal. More specifically, RPMEMcan be multiplied by the known pulley ratio to provide RPMENG.

When deactivated, fuel and spark (i.e., ignition) are cut-off to cylinders of the engine12. Vehicle launch is initiated based on operator input. For example, vehicle launch can be initiated by an operator depressing the accelerator pedal36or relieving pressure from the brake pedal40. Each of these actions indicate an operator's desire to initiate vehicle movement. The vehicle launch control system of the present invention implements the electric machine14to spin-up the engine12to a desired idle speed, prior to firing (i.e., activating) the engine12.

The vehicle launch control system determines a start-up schedule based upon the manner in which vehicle launch is initiated by the operator. The start-up schedule defines the parameters, at which the electric motor14is operated to spin-up the engine12to the desired idle speed. For example, in the case where the operator initiates vehicle launch by depressing the accelerator pedal, an aggressive launch is indicated. Therefore, the start-up schedule is chosen to enable a rapid engine spin-up. In the case where the operator initiates vehicle launch by relieving pressure from the brake pedal, a relaxed launch is indicated. Therefore, the start-up schedule is chosen to enable a less rapid engine spin-up. It is also anticipated that the start-up schedule can be determined based on the pressure within the brake system43. A plurality of alternative start-up schedules can be implemented to handle other cases, such as an operator depressing the accelerator pedal while maintaining pressure on the brake pedal.

In one routine, the launch control system provides passive torque smoothing. More specifically, the launch control system determines a desired start-up schedule based on the driver input. The start-up schedules each define operating parameters including, but not limited to, operation mode, torque command, torque slew rate, maximum discharge power (PMAX) and minimum battery voltage (VMIN). The operation mode includes a torque control mode, which indicates that the control module34is to regulate operation of the electric machine14based on torque, as opposed to speed. The torque command is set to a torque value that is higher than that which the system can deliver or some high value dictated by the physical constraints of the system (e.g., bearing loads). The torque slew rate defines the rate at which torque change occurs (e.g., Nm/sec). PMAXis the maximum power that is to be discharged from the ESD28to drive the electric machine14and VMINis the minimum voltage that the ESD28is allowed to achieve.

The torque command is intentionally set higher than that which is achievable in order to limit the torque output of the electric machine14based on VMINand PMAX. The parameters of the start-up schedules vary based on the driver input. More specifically, PMAXvaries based on the desired aggressiveness of the launch. For example, in the case of a more aggressive vehicle launch, PMAXis higher than that of a less aggressive launch.

Once the start-up schedule has been selected, the control module34drives the electric machine14, which drives the engine12. As the rotational speeds of the engine12and electric machine14increase, a point is achieved where air compression within the cylinders slows the rotational speed. Upon detecting a decrease in rotational speed, the launch control system increases the torque generated by the electric machine14to inhibit undesired speed sagging and powertrain resonance. Upon achieving a desired rotational speed, the launch control system switches from the torque control mode to a speed control mode, whereby the speed of the electric machine14is profiled to gently raise the engine speed to blend with the slight speed increase of the following fuel delivery.

As graphically illustrated inFIG. 2, the electric machine14is capable of producing more torque for a given PMAXand torque command, which is set arbitrarily high. More specifically, for a given PMAX, the torque capability decreases as RPMEMincreases. Also, the torque envelope is greater as PMAXis increased, but not for all RPMEM's. For example, based on the exemplary values ofFIG. 2, there is no advantage in drawing more battery power than 6 kW when RPMEMis less than approximately 400 RPM since the system is already saturated. However, above 400 RPM, increasing PMAXyields more mechanical output.

Referring now toFIG. 3, exemplary values of electric machine speed and torque are provided for an exemplary launch. The mechanical resonance of the exemplary vehicle occurs at approximately 300 RPM. Therefore, the launch control system functions to pass through the 200–400 RPM region as quickly as possible. During this region, the electric machine torque is naturally boosted as the RPM sags due to compression torque. This is because more electric machine torque is becomes available as the RPM sags for a given discharge power. Assuming the target engine idle speed is 700 RPM, once the engine has reached approximately 600 RPM, the operation mode is set to the speed control mode. The speed to which the electric motor controls the rotational speed of the engine can be profiled from 600 to 670 RPM. In this manner, stitching the electric creep and the refueling or cylinder activation (e.g., fueled creep or accelerator tip-in) modes together more seamlessly.

The initial PMAXis not necessarily set to max power output (e.g., >6 kW). In this manner, vehicle jerk is suppressed when the engine12is accelerated from 0 to a finite RPM. The torque must also transition smoothly from the compression torque smoothing phase to the constant-speed phase (e.g., approximately 250 ms inFIG. 3). In this manner, vehicle jerk is suppressed when the engine12stops accelerating. In addition, the engine crankshaft26is pre-positioned before the engine spin-up to minimize the torque required to accelerate the engine12.

The power command calibrations can be tabulated as a function of engine coolant temperature. The launch control system can also boost PMAXif a certain engine RPM is not observed after a predicted time (e.g., a “crank stuck” scenario). The calibrations are also affected by the tuning of the powertrain mounts. It is desirable to ensure that the remaining compression beating frequencies (150–250 ms inFIG. 3) are as far as practical from the natural pitch mode frequency of the powertrain mounts. The same idea applies to the accessory drive tuning as well.

In an alternative routine, the launch control system regulates the electric machine14based on differential gains for negative electric machine RPM derivatives. In this case, each start-up schedule defines operating parameters including, but not limited to, operation mode, torque slew rate, a maximum discharge power for a first positive range (PMAXPOS1), a maximum discharge power for a second positive range (PMAXPOS2), a maximum discharge power for a negative range (PMAXNEG) and minimum battery voltage (VMIN). For example, PMAXPOS1can be set to 5.7 kW during positive acceleration for speeds under 500 RPM. PMAXPOS2can be set to 6.5 kW during positive acceleration for speed greater than or equal to 500 RPM. PMAXNEGcan be set to 7.5 kW during zero or negative acceleration for speeds greater than 500 RPM or any speed for greater than or equal to 100 ms after spin-up start. It is appreciated that these values are merely exemplary in nature.

The control module34monitors the electric machine14speed and acceleration and has the capability to set separate PMAXvalues for positive and negative motor accelerations. Similar to the first routine described above, the electric machine power is selected separately as a function of RPM bands. The initial power is calibrated to swiftly accelerate the crankshaft26from a stop without rocking the powertrain. The higher speed power is calibrated to quickly accelerate the crankshaft26through the powertrain resonance speeds. The advantage of the alternative routine is that the motor boost during the compression speed-sag is counteracted with a custom power level. The remaining functions of the launch control system are similar to that of the first routine, described in detail above.

Referring now toFIG. 4, the general steps executed by the launch control system of the present invention will be discussed in detail. In step100, control determines whether to initiate launch. Launch is initiated based on the operator input, as described in detail above. If control determines not to initiate launch, control loops back. If control determines to initiate launch, control determines the start-up schedule (SS) based on the operator input in step102. In step104, control issues launch commands (i.e., operation mode, torque slew rate, PMAX, etc.) based on the start-up schedule. Control operates the electric machine based on the launch commands in step106and regulates the electric machine torque, torque slew rate and ESD discharge power based on RPM in step108. In step110, control determines whether the RPM is greater than or equal to an RPM threshold (RPMTHRESH). If the RPM is not greater than or equal to RPMTHRESH, control loops back to step106. If the RPM is greater than or equal to RPMTHRESH, control initiates idle speed control in step112and control ends.

The launch control system of the present invention controls electric machine torque by using torque slew rate and PMAXas control variables for an arbitrarily high torque request. PMAXcan vary as based on electric machine RPM and/or based on RPM derivative. The launch control system transitions from the torque control mode to the speed control based on an electric motor RPM threshold.