WO2014031099A1 - Vehicle powertrain and method - Google Patents

Abstract

A vehicle comprises a powertrain with at least a first configuration of an electrical motor for available for propulsion and an internal combustion engine available for propulsion, a source of generated electricity and a rechargeable energy storage system. A control system selects a powertrain state based on a modeled target state, an evaluation of objective and subjective costs, as well as the present powertrain state.

Description

VEHICLE POWERTRAIN AND METHOD

BACKGROUND

[001 ] Technical Field:

[002] The technical field relates generally to powertrain control and, more particularly, to solution of the operational space for torque/power producing systems in hybrid powertrains.

[003] Description of the Technical Field:

[004] Hybrid powertrains may be defined as those powertrains having two or more subsystems for the conversion of different types of potential energy to torque or mechanical power. A common characteristic of hybrid powertrains is that the power conversion subsystems differ from one another in terms of efficiency, sometimes greatly. Hybrid-electric and hybrid-hydraulic powertrains, especially those which pair an internal combustion (IC) engine with the electric or hydraulic motor, illustrate this characteristic well. In a hybrid-electric or hybrid-hydraulic powertrain the electric or hydraulic motor is often the primary source of torque while the IC engine operates as a secondary source of torque due to the differences in efficiency. For applications in motor vehicles the electric (or hydraulic) motor is preferred for propulsion while an IC engine is sometimes not even directly available for propulsion.

[005] The stores of potential energy available to the power conversion elements of a hybrid powertrain differ as well. Stores of potential energy include storage batteries, fuel cells and capacitors for electric motors, hydraulic accumulators (a type of pressure tank) for hydraulic motors and fossil fuels for an IC engine. The store of potential energy for the primary torque source in a hybrid powertrain can be referred to as a Rechargeable Energy Storage System (RESS). Electric and hydraulic motors quite often operate to recapture vehicle kinetic energy (regenerative braking) and convert it for storage as potential energy in the RESS. [006] The weight cost of storing a unit of potential energy in an RESS has been much greater than the weight cost incurred for a like unit of energy provided by fossil fuels. An RESS unit of manageable size for a motor vehicle will, as a consequence, support a smaller range of vehicle operation than the fossil fuel will support. An RESS can be built in a variety of ways, including from fuel cells, batteries and hydraulic accumulators. An RESS built from batteries for a hybrid-electric powertrain will have limitations in terms of its ability to deliver power. For example, internal losses increase as the square of electric current flow. A comparable penalty does not apply to operation of an IC engine.

[007] An IC engine in a motor vehicle hybrid powertrain can be used to exploit the high energy to weight advantage of a fossil fuel to extend vehicle range. In addition, the fact that alternative torque sources and sinks exist in a hybrid powertrain, and the availability of an RESS which the IC engine can be used to recharge, may allow the IC engine to operated at closer to its peak potential efficiency than IC engines achieve in conventional powertrains. An IC engine can also provide power to overcome power delivery limitations of some types of RESS units, for example batteries.

[008] Powertrain control systems are conventionally programmed to exploit the comparative efficiency of the electric or hydraulic motors for propulsion and their capacity for regenerative braking to recharge the RESS. The systems are subject to constraints or operational limitations which relate, among other things, to operating characteristics of the RESS, power electronics, the converters and inverters as well as the IC engine.

[009] Open and closed loop control systems have been proposed for hybrid (and electric) powertrains. Problems exist with each approach. Multiple open loop systems do not take into account changes to the system which inhibit efficiency. Multiple closed loops produce conflicts between control pathways and can reduce system stability and compromise overall effectiveness. [0010] Open loop control systems are implemented based on a mathematical model of how a powertrain is anticipated to perform under given conditions. Since feedback relating to actual system response is not used system response to changing conditions is especially quick, but model error is not accounted for. Nor is corrective response taken to disturbances. Output is not compared to a reference and does not depend on any past system state. Open loop control is applicable to stationary systems or systems where the behavior of the system is well known. It is less applicable to event such as an IC engine start, including IC engine auto start, because the combustion rate can be unpredictable.

[001 1 ] To compensate for model error, feedback can be added to produce closed loop control. System output is monitored and fed back to a controller which reacts to changes in the system's state to maintain a target "set point." Model error can be identified from the feedback signals, if consistent, but system "hunting" for torque/power demand allocation to the IC engine and motors can result. Where the system is well behaved in stability terms (robust control) then short response times to target may be gained. However, closed loop systems can be prone to over shoot and with poor tuning can exhibit steady state errors. They are prone to over correction. They exhibit higher costs in part due to demand for accurate feedback. If feedback is unstable the system may become unstable.

[0012] Systems which have combined open and closed loop principals have been attempted. These have tended to become large programs and to produce a high level of system hysteresis. They can be prone to learning errors. Multiple closed loops used with various hybrid, electric, fuel cell, or advance engine technologies management systems can produce conflict which reduces overall effectiveness and stability.

SUMMARY

[0013] A method for operating a vehicle power train having primary and secondary sources of power/torque and utilizing a rechargeable energy storage system to supply energy to the primary source of power/torque is taught. A control system is provided which, responsive to an operator or other exogenously supplied power demand, determines a powertrain state in which power or torque demand is allocated to the primary power source, the secondary power source, or both. An envelope of motor output torques and internal combustion output torques available is generated to support this determination. From the envelope the powertrain states which can meet output torque demand are identified. The projected effects of the available powertrain states on the state of energy (SOE) of the RESS are estimated. Hardware constraints on primary and secondary torque sources are evaluated. The direction of primary source torques which increase and decrease the output torque are determined.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a side elevation of a vehicle with a power take-off application vocation which may be equipped with a hybrid powertrain.

[0015] FIG. 2 is a high level block diagram of a control system for a hybrid-electric powertrain for a motor vehicle such as that of FIG. 1 .

[0016] FIG. 3 is a control flow diagram.

[0017] FIG. 4 is a state machine illustrating operation of the hybrid-electric powertrain of FIG. 2.

DETAILED DESCRIPTION

[0018] In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. The terms power and torque are used to characterize output demand on electrical motors 30, 32 and from an internal combustion (IC) engine 28. Control systems can and are designed to generate demand in terms of target power or target torque and the techniques elaborated herein are applicable to either approach. Unless otherwise specified, or excluded by context, references to torque include (mechanical) power.

[0019] A vehicle may have access to multiple sources of torque. These can include: IC engines; motors (which may be electric or hydraulic); flywheels; a source of inertial torque such as motion of the vehicle itself; motion of a power take-off (PTO) vocation mounted on the vehicle; or a rotating device such as a diesel engine, an exhaust turbine or electrical machine which is spinning down. While the powertrain control methods described here are primarily applicable to hybrid powertrains some aspects of the methods can be applied to vehicles with electric powertrains in which electric motors are the exclusive source of propulsion and to vehicles incorporating mild hybrid modifications. Some, but usually not all, of these devices may be tapped to maintain charge on an RESS. Usually only an IC engine, an exhaust turbine (turbo-compounding applications), or the motors can be directly utilized for propulsion. Some PTO vocations offer possibilities for energy recapture. Clutches may be treated as sources of torque and serve as a proxy for systems such as PTO vocations.

[0020] Referring now to the figures and in particular to FIG. 1 , a hybrid aerial lift truck 1 is illustrated. Hybrid aerial lift truck 1 serves as an example of a medium duty vehicle which incorporates a hybrid powertrain and which supports a PTO vocation, here an aerial lift unit 2 mounted on a truck bed 12. The aerial lift unit 2 can be either a sink or a source of torque to be managed in order to gain operational efficiencies. Specifically hybrid aerial lift truck 1 incorporates a hybrid-electric powertrain described below which supplies torque to extend the aerial lift unit 2 and to propel the vehicle and which carries an RESS for supplying energy to one of the prime movers of the powertrain.

[0021 ] The aerial lift unit 2 includes a lower boom 3 and an upper boom 4 pivotally interconnected to each other. The lower boom 3 is in turn mounted to rotate on the truck bed 12 on a support 6 and rotatable support bracket 7. The rotatable support bracket 7 includes a pivoting mount 8 for one end of lower boom 3. A bucket 5 is secured to the free end of upper boom 4. Bucket 5 is pivotally attached to the free end of boom 4 to maintain a horizontal orientation at all times. A hydraulic lifting unit 9 is interconnected between bracket 7 and the lower boom 3 by pivot connection 10 to the bracket 7 pivot 13 on the lower boom 3.

[0022] Hydraulic lifting unit 9 is connected to a supply of a suitable hydraulic fluid under pressure by which the unit is lifted. The supply may be an automatic transmission or an hydraulic accumulator. Where an automatic transmission is used as a pump it may be powered by the prime movers for hybrid mobile aerial lift truck 1 . Typically an IC engine or an electric/hydraulic motor serves as the prime mover.

[0023] The outer end of the lower boom 3 is interconnected to the lower and pivot end of the upper boom 4. A pivot 16 interconnects the outer end of the lower boom 3 to the pivot end of the upper boom 4. An upper boom compensating assembly 17 is connected between the lower boom 3 and the upper boom 4 for moving the upper boom about pivot 16 to position the upper boom relative to the lower boom 3. The upper-boom compensating assembly 17 allows independent movement of the upper boom 4 relative to lower boom 3 and provides compensating motion between the booms to raise the upper boom with the lower boom. Upper boom compensating assembly 17 is usually supplied with pressurized hydraulic fluid from the same sources as hydraulic lifting unit 9. Outriggers (not shown) may be used installed at the corners of the truck bed 12. Pressurized hydraulic fluid for these operations may be supplied by an hydraulic pump or an accumulator.

[0024] Referring to FIG. 2, a hybrid-electric powertrain 20 and an electronic control system 22 suitable for use with vehicle 1 are illustrated. Electronic control system 22 includes controller area network (CAN) datalinks 18, 44 which allow communication of data among the various controllers. A third datalink connects the electronic system controller (ESC) 24 to a slaved remote power module (RPM) 140. Public datalink 18 and hybrid-datalink 44 allow the exchange of data between numerous controllers for coordinated operation of powertrain 20 components in response to monitored vehicle operating variables. The control strategy, whatever architecture is chosen, should be versatile and flexible. The architecture therefor should provide for standardized interfaces where information is provided by the same ring via the same interface in all environments and content collecting interfaces are enabled to be reused in different contexts.

[0025] Control system 22 here is adapted to a hybrid-electric powertrain 20 having an IC engine 28 and two electrical motors 30, 32 which support operation of a PTO vocation 2 and which can provide traction power to drive wheels 26. Control system 22 can readily be programmed to operate other types of powertrains. Control system 22 is not limited to a hybrid-electric powertrain and hydraulic systems are a possible alternative.

[0026] Other alternative hybrid configurations can substitute a compressed natural gas (CNG) engine for the IC engine as a secondary power source. A fly wheel is an example of a system which can serve as a prime mover and an RESS concurrently. A hybrid and even an all electric powertrain can have multiple energy sources which can support propulsion or RESS recharging. Possibilities include: an IC engine (for a hybrid); motors; exhaust turbines (turbo-compounding systems for a hybrid); power-take off operations; and vehicle inertia.

[0027] A control architecture can use variants of rings/frames in order to provide different implementations of functionality within different products. While the implementation behind an interface will differ in the individual variants of a ring, the information provided by an interface is intended to be the same in all variants of the ring.

[0028] The IC engine and motors of a hybrid system normally run in either a torque or a power control mode. Electric motor power/torque can be positive (motor) or negative (generator). Similarly an IC engine power/torque can be positive (engine) or negative (engine brake). Hybrid Supervisory Control (HSC) 48 coordinates all torque commands to each device. Speed control is typically performed with motors only, but recent IC engines also provide speed control and clutch to clutch transmissions use speed control. HSC 48 can work in the torque domain only while a transmission controller/torque converter module (TCM) converts torque to pressure and a motor control processor (MCP) 27 operates motors 30, 32 to convert torque to current and an engine control module (ECM) 46 operates IC engine 28 converts torque for fuel/spark/air (ECM 46 may functionally include an engine brake controller). HSC 48 determines which devices produce what torque to respond to driver demand while staying within system constraints, such as battery power limits. The ECM 46 determines engine response including, when possible, shutting down an IC engine 28.

[0029] Consumers of an IC engine torque interface should be insensitive to whether it is estimated with the model for a gas engine or a diesel engine. Consumers of an engine coolant temperature interface should be insensitive to whether it is estimated locally or estimated remotely and received over serial communication. In particular, solution of the timing, selected power or torque output level and, depending upon the transmission 38, the operational speed of an IC engine 28 may be selected by coordination of operation of the IC engine with other components of the hybrid-electric powertrain 20.

[0030] Torque control architecture has been favored over speed control architecture for several reasons for hybrid-electric power trains. In an IC engine speed control the ECM regulates torque (power) to maintain engine speed. It cannot effectively control battery power/energy. IC engine speed regulation is less effective due to closed loop only control. Regarding motor speed control, motor power not easily regulated and a coordinated motor speed profile difficult to do.

[0031 ] The process of solution for the operational space for the IC engine 28 is carried out by execution of a program. Program control is usually located in the ESC 24 or the HSC 48, which are nodes of both the public datalink 18 and the hybrid datalink 44. Program elements though may be distributed among other controllers such as the engine controller 46. The program operates on, and provides criteria which may be quantized in terms of, a number of vehicle operating variables. Examples of such operating variables include: RESS state of charge (SOC) or state of energization (SOE), which is supplied by a battery management system (BMS) 64 for the traction batteries 34 which function as the RESS for powertrain 20; traction batteries 34 pack voltage from BMS 64; traction batteries 34 power limits from BMS 64; traction batteries 34 current limits from BMS 64; traction batteries 34 temperature from BMS 64; fuel tank 62 fuel level from a sensor (not shown) and reported over the public datalink 18 through the engine controller 46; IC engine 28 coolant temperature reported by the engine controller 46; transmission temperature from a torque converter/transmission controller (TCM) 42; electrical motors (motor/generators) 30, 32 temperature reported by HSC 48; and hybrid inverter 36 temperature reported by the HSC. In addition various validity checks may be provided, and data relating vehicle configuration 65 may be used such as whether the vehicle is in a tow haul mode (engine off may be restricted); transmission range (engine off may be restricted; transmission reverse grade mode (engine off may be restricted); four wheel drive mode (engine off may be restricted); or a forced remote vehicle start has been requested. The foregoing list is neither all inclusive nor exhaustive and additional variables may be reported by sensors package 70. A value for ambient temperature may be provided by the sensors package 70 or from the engine controller 46 which sometimes has access to readings from a temperature sensor in an engine air intake.

[0032] BMS 64 monitors the state of traction batteries 34 in terms of useable energy, power capability, health, voltage and temperature. Energy efficiency for traction batteries 34 is usually expressed as a percentage of the electrical energy stored in the traction batteries that is estimated as recoverable during discharging. For an electrolytic cell this is the theoretically required energy divided by the energy actually consumed in the process. Inefficiencies arise from a number of factors including current inefficiencies and heat losses due to polarization. The rate at which current is discharged affects the final result. The commonly reported RESS variable of current limits may be reported both in terms of a root mean square (rms) and peak limits.

[0033] The effectiveness of a control system for a hybrid-electric (or an electric) vehicle increases as various constraints are incorporated into the model. These can be characterized as objective and subjective (including economic) costs. Objective costs are related to losses determined by physical law, and thus, for a given powertrain, are fixed for a given mode of operation. These include: RESS power loss (i²r); electric machine costs (P Electrical Power - P Mechanical Power); IC engine power loss (chemical energy of fuel - mechanical output); brake power loss (energy loss due to friction). Subjective or economic costs can include: IC engine cost for low temperatures and due to control emissions (combined with objective cost of engine power loss); RESS usage cost; large discharge/charge events (battery usage cost is high); small discharge/charge (battery usage cost is low); penalties [kW] (within limits = 0, otherwise high cost); motor torque, voltage and current limits; inverter voltage and current limits; RESS power, current and voltage limits; and output torque limits (based on driver requested torques).

[0034] The significance of the various variables differ. The condition of traction batteries 34 may deteriorate more quickly at high temperatures than otherwise if current in flow is not limited. Current outflow from the battery may be aggravated by air conditioning and other refrigeration demands because cooling systems on hybrid-electric vehicles often rely on accessory electric motors to run compressors and coolant circulation pumps. Under conditions of extreme cold traction batteries 34 may be unable to support high current outflows, which can relate directly to IC engine 28 starting. High current levels equate with high energy consumption and accelerated wear or "aging" of powertrain 20 components. BMS 64 operational strategy may provide stable and repeatable operation from the traction batteries 34 and this strategy may have consequences in the determination of IC engine 28 operational space.

[0035] Data quantifying such adverse system responses allow, under some conditions, a cost comparison to be made between the additional costs incurred by operation of the vehicle using its IC engine 28, mixing propulsion demand between electric motors 32 and the IC engine 28 or propelling the vehicle only from electric motor(s) 32, 30. They can also be used to determine how much regenerative braking to use to meet braking demand. One comparative disadvantage of electric and hybrid-electric vehicles with respect to an IC engine based powertrain is the high cost of electric powertrain components. Limiting the exercise of electrical components in a hybrid-electric powertrain 20 under conditions characterized as "adverse" can extend the service lives of the components and reduce maintenance costs and vehicle down time by enough to pay for immediate savings in operating costs lost due to increased hydro-carbon fuel consumption. In any event the values reported for the traction batteries 34 related batteries by BMS 64 play a role in the timing of starting and stopping of IC engine 28 and it output levels.

[0036] Optimization occurs at two levels for hybrid power trains - at a strategic level and at a tactical level. Strategic optimization defines targets, i.e. a target hybrid state or a target input speed. How optimization is pursued in terms of the specified engine mode, input torque commands, etc., while taking into account present constraints is the system's tactical response.

[0037] Tactical optimization may be expressed in equation form as Objective Costs - Subjective Costs - Penalties (within limits = 0, otherwise one has a high cost)." As applied to engine costs it is known that IC engine power losses equal the theoretical chemical energy of fuel less mechanical output (this is objective). The IC engine 28 cost for low temperatures, catalyst warm-up and high altitude is subjective. We account for a cost bias for deceleration fuel cut-off (DFCO) Request by the ECM 46 as being subjective. There is also an IC engine 28 torque changes cost to maintain stability which would be considered subjective were it used. Output torque costs are modeled in terms of brake power loss (energy loss due to friction (objective)); output torque limits based on driver requested torques (characterized as a penalty). These include: RESS power loss (i²r); electric machine costs (P Electrical Power - P Mechanical Power); IC engine power loss (chemical energy of fuel - mechanical output). Subjective or economic costs can include: IC engine cost for low temperatures and due to control emissions (combined with objective cost of engine power loss); RESS usage cost; large discharge/charge events (battery usage cost is high); small discharge/charge (battery usage cost is low); penalties [kW] (within limits = 0, otherwise high cost); motor torque, voltage and current limits; inverter voltage and current limits; RESS power, current and voltage limits; and output torque limits (based on driver requested torques).

[0038] RESS costs include objective power losses stemming from current flow as described above. SOC/SOE control costs include RESS usage cost and IC engine fuel costs which are characterized as subjective (for high SOC discharging is encouraged, for low SOC charging is encouraged). Violation of RESS power limits as arbitrated from voltage and current limits with a resistance factor is equated to a subjective cost and penalty. Long-term RESS usage cost is handled as a subjective cost as is RESS state of life (subjective, planned).

[0039] Electrical machine or "motor" costs are expressed in terms of PEIectrical Power - PMechanical Power, an objective consideration. Motor torque limits are treated as a penalty.

[0040] Ideally, cost values from the optimization algorithm then add hysteresis values to the cost values for transitions from one mode to another mode based on: the evaluated mode; and, the previous optimum mode OR the actual mode. Then up to 4-6 costs can be compared and an optimum mode with the minimum cost can be estimated. Modes must include all operating strategies for the vehicle - such as regenerative coast down, turbo- compounding and regenerative energy capture through braking, etc.

[0041] Hybrid-electric powertrain 20 illustrates the many possible examples of powertrains where rules of operation may be varied to meet propulsion and braking demand. Hybrid- electric powertrain 20 is configurable for series, parallel and mixed series/parallel operation. It can be applied to a PHEV for all electric operation and, if necessary, be operated in a strictly IC engine 28 mode under some conditions.

[0042] Hybrid-electric powertrains for vehicles have generally been of one of two types, parallel and series. In parallel hybrid-electric powertrains propulsion torque can be supplied to drive wheels by an electrical motor, by an IC engine, or a combination of both. In series type hybrid systems drive propulsion is directly provided only by the electrical motor. The IC engine is used to run a generator which supplies electricity to power the electric traction motor and to charge storage batteries. In a series type system the control system may operate under a rule under which the internal combustion engine is started at a minimum threshold battery SOC, run at its most efficient brake specific fuel consumption output level until the RESS battery reaches a maximum allowed SOC whereupon the IC engine is turned off. [0043] Hybrid-electric powertrain 20 includes the IC engine 28, two electrical motors 30, 32 which can be operated either as generators or traction motors and a series of clutches 52, 54 56 and (optionally) 58, which allow great flexibility in configuring powertrain 20 as for series, parallel or blended operation, or for pure electric or pure IC based propulsion modes, electrical motors 30, 32, when operating as generators, can be either back driven from drive wheels 26 (regenerative braking) or driven directly by the IC engine 28. In hybrid-electric powertrain 20 the IC engine 28 can provide direct propulsion torque or can be operated in a series type hybrid-electric powertrain configuration where it is usually used to drive electrical machine 30 for the purpose generating electricity. Hybrid-electric powertrain 20 also includes a planetary gear 60 for combining power output from the IC engine 28 with power output from the two electrical motors 30, 32. A transmission 38 couples the planetary gear 60 with the drive wheels 26. Power can be transmitted in either direction through transmission 38 and planetary gear 60 between the propulsion sources and drive wheels 26. During braking planetary gear 60 can deliver torque from the drive wheels 26 to the motor/generators 30, 32 or, if the vehicle is equipped for engine braking, to engine 28, distribute torque between the motor/generators 30, 32 and IC engine 28.

[0044] The plurality of clutches 52, 54, 56 and 58 provide various options for configuring the electrical motors 30, 32 and the IC engine 28 to propel the vehicle through application of torque to the drive wheels 26, to generate electricity by driving the electrical motors 30, 32 from the engine, and to generate electricity from the electrical motors 30, 32 by back driving them from the drive wheels 26. Electrical motors 30, 32 may be run in traction motor mode to power drive wheels 26 or they may be back driven from drive wheels 26 to function as electrical generators when clutches 56 and 58 are engaged. Electrical motor 32 may be run in traction motor mode or generator mode while coupled to drive wheels 26 by clutch 58, planetary gear 60 and transmission 38 while at the same time clutch 56 is disengaged allowing electrical motor 30 to be back driven through clutch 54 from engine 28 to operate as a generator. Conversely clutch 56 may be disengaged and clutch 58 engaged and both electrical motors 30, 32 run in motor mode. In this configuration electrical motor 32 can propel the vehicle while electrical motor 30 is used to crank IC engine 28 for starting. Clutch 52 may be engaged to allow the use of IC engine 28 to propel the vehicle or to allow use of a diesel engine, if equipped with a "Jake brake," to supplement the capacity of vehicle regenerative and service brake operation to stop the vehicle by engine braking. When clutches 52 and 54 are engaged and clutch 56 disengaged engine 28 can concurrently propel the vehicle and drive electrical motor 30 to generate electricity. Still further operational configurations are possible although not all are used. Elimination of some configurations allow clutch 58 to be considered as "optional" and for it to be replaced with a permanent coupling.

[0045] The selective engagement or disengagement of clutches 52, 54, 56 and (if used) 58 allows hybrid-electric powertrain 20 to be configured to operate in a "parallel" mode, in a "series" mode, or in a blended "series/parallel" mode. To configure powertrain 20 for series mode operation clutches 54 and 58 could be engaged and clutches 52 and 56 disengaged. Propulsion power is then provided by electrical machine 32 and electrical motor 30 operates as a generator. To implement powertrain 20 for parallel mode operation at least clutches 52 and 58 are engaged. Clutch 54 is disengaged, electrical machine 32 and IC engine 28 are available to provide direct propulsion. Electrical motor 30 may be used for propulsion. A configuration of powertrain 20 providing a mixed parallel/series mode has clutches 52, 54 and 58 engaged and clutch 56 disengaged. Electrical machine 32 operates as a motor to provide propulsion or in a regenerative mode to supplement braking. IC engine 28 operates to provide propulsion and to drive electrical motor 30 as a generator.

[0046] Hybrid-electric powertrain 20 can draw on at least two reserves or stores of potential energy, one for the electrical motor 30, 32 and one for the IC engine 28. Capacity for producing electrical energy for the electrical motor 30, 32 is stored in an RESS such as traction batteries 34. Batteries 34 also exhibit rates of charging and discharging which may be limited in comparison to energy flow into or from a fuel tank 62 or capacitors. The availability of power from the electrical power reserve may be referred to as its state of energization (SOE) or, more usually with batteries, as its state of charge (SOC). In either case the value is indicated as a percentage. Combustible fuel for engine 28 is typically a hydro-carbon and, if liquid or gaseous, maybe stored in a fuel tank 62. The fuel tank 62 is resupplied from external sources and unlike the batteries 34 (which function as the vehicle's RESS) cannot be regenerated by operation of the vehicle. Typically a fuel tank 62 can be replenished in a far shorter time period than can the traction batteries 34 can be recharged and the energy density per unit of mass is far greater for most combustible fuels than can be achieved by charging traction batteries 34. Both storage systems are subject to a maximum energy storage limit.

[0047] Other energy stores may be considered under some circumstances. Vehicle moving inertia is chief among these. This energy is tapped by using the drive wheels to back drive one or both electrical motors 32, 30 to generate electricity which may be used to charge the traction batteries 34. Similarly an extended or elevated PTO vocation/aerial lift unit 2 can be considered a store of energy.

[0048] Electronic control system 22 also provides control over the PTO vocation/aerial lift unit 2 and PTO motor 122 via RPM 140. PTO motor is illustrated as powered by hydraulic fluid pumped from transmission 38. Private datalink 174 links the ESC 24 with a slaved remote power module (RPM) 140. RPM 140 provides control over the PTO vocation. As illustrated this occurs in cooperation with other controllers including particularly a transmission controller 42. Were the powertrain an hydraulic powertrain PTO motor 122 could be supplied with hydraulic fluid from an accumulator serving as the RESS. Where the PTO vocation/aerial lift unit 2 is an aerial lift (which when extended can have attributes of an hydraulic accumulator), or involves a rotating element with substantial rotational momentum, it is conceivable that such a source could be tapped to back drive electrical motors 32, 30. Such resources could not usually be tapped "on demand" and the opportunities to exploit such transient resources would be subject to narrow and unpredictable timing constraints.

[0049] Traction batteries 34 may also be charged from external sources (for example plug-in hybrid electric vehicles (PHEV)) or by operation of the powertrain 20. As already described, electrical motors 30 and 32 may operate as generators to supply current to recharge traction batteries 34 over a high voltage energy bus 17 from the high voltage energy distribution subsystem. Hybrid inverter 36 provides voltage step down or step up and, if electrical motors 30, 32 are alternating current devices, current rectification and de-rectification between the electrical motors and batteries 34. Fuel, a form of stored energy, may be converted first to mechanical power and then to electrical energy and thereby "moved" from the fuel tank 62 to the traction batteries 34. Traction batteries 34 may also be recharged through regenerative energy capture techniques such as regenerative braking, turbo compounding, regenerative energy capture through coast or spin down.

[0050] Control system 22 also coordinates operation of the elements of the powertrain 20 and the service brakes 40 in response to operator/driver commands to move (accelerator or throttle position "ACC/TP") and stop (BRAKE) received by ESC 24. Energy reserves in terms of the SOC of traction batteries 34 are managed taking into account the operator commands. The control system 22 selects how to respond to the operator commands to meet programmed objectives including efficiently maintaining the SOC of traction batteries 34 as well as protecting powertrain 20 components.

[0051] In addition to the datalinks 18, 44, control system 22 includes the controllers which broadcast and receive data and instructions over the data links. Among these controllers is the ESC 24. ESC 24 is a type of body computer and is not assigned to a particular vehicle system. ESC 24 has various supervisory roles and is connected to receive directly or indirectly various operator/driver inputs/commands including brake pedal position (BRAKE), ignition switch position (IGN) and accelerator pedal/throttle position (ACC/TP). Sensor package 70, or the engine controller 46, can be used to collect other data such as ambient air temperature (TEMP). In response to these and other signals ESC 24 generates messages/commands which may be broadcast over datalink 18 or datalink 44 to an anti-lock brake system (ABS) controller 50, the transmission controller 42, the ECM 46, HSC 48 and an accessory motor controller 23 and the MCP 27. [0052] Accessory motor controller 23 controls high voltage accessory motor 25 in response to directions from other CAN nodes. High voltage accessory motor 25 represents several direct current motors used to support the operation of components such as an air conditioning compressor (not shown), a battery cooling loop pump (not shown) or a power steering pump (not shown). On many hybrid-electric vehicles there is no option to power such components by drive belts from the IC engine 28 and the motors driving these components are parasitic loads on a electrical machine operating in generator mode or they draw power over a high voltage power distribution sub-system 19 from the traction batteries 34. Under conditions of high heat and humidity greater demands are likely to be placed on air conditioning and for battery cooling. When these circumstances coincide the greatest stress due to heat is likely to be placed on the powertrain 20 components particularly motor internal resistances rise and batteries 34 temperatures with increased frequency and depth and (dis)charge cycles.

[0053] Operator demand for power or torque on powertrain 20 is a function in part of ACC/TP. ACC/TP is an input to the ESC 24 which passes the signal to the hybrid supervisory controller 48. Where engine 28 is supplying power or torque both for propulsion and for charging of the traction batteries 34 an allocation of the available power from engine 28 is made by the HSC 48. The BMS 64 may also be a source of demand for power or torque.

[0054] Maintaining batteries 34 SOC is subject to various constraints including the present SOC of the traction batteries 34 and a dynamic limit on the rate at which the traction batteries 34 can accept charge. The traction batteries 34 and engine 28 can be selected so that the engine can be run at its most efficient brake specific fuel consumption during pure charging operation up to a nominal SOC, usually 80% of a full charge. Thus the dynamic limit on the rate of charge can be disregarded during periods when both charging and propulsion are demanded from the powertrain 20. The HSC 48 monitors batteries 34 SOC and when charging of batteries 34 is indicated allocates available torque from the engine 28 or from the drive wheels 26 during dynamic regenerative braking of electrical motors 30 and/or 32 as controlled by MCP 37 to generate electricity for charging traction batteries 34. [0055] A vehicle equipped with a hybrid powertrain designed to recapture kinetic energy has sources of torque other than an IC engine and traction motors. Regenerative braking exploits inertia torque of the vehicle. Motors and clutches which have been spun up may be used as sources of torque under some circumstances. A power take-off vocation, such as a utility truck "cherry picker," can be used as a source of torque under some circumstances. Control over such a powertrain can be implemented in different ways.

[0056] The ability to start and stop an IC engine on vehicles, particularly non-hybrid or mild hybrid vehicles, has long been recognized as a mechanism to improve fuel economy and is related to methods described here. Examples of such vehicles can be found among contemporary passenger cars, with implementation based on a spark ignition (SI) engine coupled with either a belted starter/alternator (BSA), an integrated starter/alternator (ISA), or in some cases timing shut down of the IC engine to trap a fuel/air charge in one cylinder at or just after movement of a cylinder piston to top dead center of the compression stroke. Among issues to be addressed in developing engine start and stop operations in a hybrid-electric powertrain are minimizing customer perceived discomfort (or lack of performance) while providing a mechanism that does not increase warranty issues. Heavy and medium duty compression ignition IC engine applications present different sets of issues to address than spark ignition IC engines present.

[0057] Referring now to FIG. 3. operation of the control system over IC engine 28 is described with reference to a data flow diagram incorporating a state determination block 80, a tractive effort optimization block 83, a state optimizer block 81 and an active filter block 82. State determination machine 80 determines a state for powertrain 20. The possible states which form the space of acceptable solutions (solution space) to torque or power demands made on powertrain 20 are supplied as feedback (target states) from a state optimizer block

81 which operates on the state determined by the state determination machine, the target tractive effort and the tractive effort commanded. State transitions occur through active filter

82 and a feedback signal indicating completion of a transition (past state) is provided by the active filter to the state determination block 80 as an input. The system is driven by exogenous requests for vehicle tractive effort, which results in generation of a target tractive effort value (supplied to the state determination block 80 and to the state optimization block 81 ) by the tractive effort optimization block 83 and an optimal effort value supplied by the same source to the state determination bock 80.

[0058] The exogenous system constraints supplied to the state determination block 80 are derived from operating variables which include:

RESS state of charge (SOC) or state of energization (SOE) for the traction batteries 34;

traction batteries 34 pack voltage;

traction batteries 34 power limits;

traction batteries 34 current limits;

traction batteries 34 temperature;

fuel tank 62 fuel level;

IC engine 28 coolant temperature;

transmission temperature;

electrical motors (motor/generators) 30, 32 temperature; and

inverter 36 temperature.

From these variables the optimum torque available from each prime mover, IC engine 28, electrical motor 30 and electrical motor 32 is generated. As an extreme example, traction batteries 34 current limits or temperature may indicate that battery power is unavailable and that, as a result, optimum torque from electrical motors 30, 32 operating in motor mode is zero. Torque demand from the vehicle operator, possibly combined with torque demand from other sources such as a PTO controller (RPM 140) or BMS 64 may be combined with desired recharging of the traction batteries 34 to produce a target powertrain state. Other constraints may be applied to the state determination block 80 such an IC engine only mode on account of the vehicle being in a towing mode.

[0059] For state determination block 80 there exists an envelope of motor torques which satisfy hardware constraints. There is a combination of electrical machine torque combinations with can produce the driver's torque output request. Motor torque generated effects available battery power. There are hardware constraints on output torque capacity. The direction of change in motor torque will result in an increase or a decrease motor torque capacity.

[0060] State machine 80 supplies two outputs to the state optimizer 81 and active filter 82, output torque Te commanded and a system state determination. The active filter 82 takes into account time and frequency constraints for a given engine and takes into account and limits engine torque Te to the constraints, determines transition requirements for fueling or compression, considers engine stop/start overrides and constraints and finally sends torque requests with response time limitations for IC engine 28 and electrical motors 30, 32 to the engine controller 46 and HSC 48.

[0061] The active filter 82 provides an opportunity for second layer optimization to develop limits or hysteresis values based on the current system state from the state machine 80, the previous optimum state and the future target state for the powertrain.

[0062] Referring to FIG. 4, an exemplary state machine for a range extended electric vehicle (REEV) is described. The hybrid aerial lift truck 1 of FIG. 1 may be realized as a REEV. A REEV is a vehicle that is a Plug-In Hybrid Vehicle (PHEV) with All Electric Range (AER) capability. On a PHEV the RESS is rechargeable by connection to external mains. It may be rechargeable by operation of an IC engine or, in part, by regenerative braking. Though a REEV is equipped with an IC engine it would be theoretically possible that the IC engine would never be run during the service life of the vehicle, though constraints built into the system should prevent such an eventuality. A REEV operates in one of two main propulsion modes or states, a charge depleting (CD) mode 90 and a charge sustaining (CS) mode 95. For purposes of simplification, it is assumed here that on start that the vehicle RESS is fully charged (SOC/SOE at its upper limit) and the state machine assumes the main CD state/mode 90. [0063] In CD state 90 the powertrain is only allowed to use RESS energy to propel the vehicle. Without reaching an SOC/SOE trigger point for transitioning to charge sustaining mode/state 95 certain conditions may arise which allow, or require, the IC engine to be started. From CD mode 90 two sub-states 91 , 92 may be reached in which IC engine operation is possible. The first of these is an Engine On CD state 91 in which the IC engine is running. As long as the control system state remains in the Engine ON CD state 91 the IC engine is not allowed to cycle off. Another sub-state is Engine Auto Stop/Start CD state 92 in which engine operation is possible but not mandated. Here engine auto stop/start is engaged. A variety of circumstances may produce transitions between these sub-states and CD mode 90. For example, state 91 may be invoked due to low ambient temperatures. This is termed a defrost condition and exploits the capacity of an IC engine to rapidly generate what in other circumstances would be waste heat. This allows rapid defrosting and heating for passenger comfort. The system transitions back to state 90 based on engine or transmission coolant temperature, with an appropriate hysteresis band to avoid cycling between state 90 and sub-state 91 . Another possible cause of a transition to sub-state 91 can be termed fuel maintenance. Hydro-carbon fuels are subject to deterioration over time. This is dealt with by periodically burning the fuel to allow for its replacement. Another possible cause for transition to sub-state 91 might be termed a "grade protection mode" in which the IC engine is held on to assure minimum vehicle grade climbing capability. Other conditions can be allowed for such as hood position, diesel engine special operating conditions (for example diesel particulate filter regeneration), conditions other than a low state of SOC/SOE which limit RESS output power, etc.

[0064] A CD engine auto stop/start possible sub-state 92 is provided which allows IC engine operation. This may occur on account of a constraint of available RESS power, for example high battery temperature and can occur to maintain vehicle performance. Sub-states 93 and 94 are analogous to sub-states 91 and 92 and occur for similar reasons but to and from CD mode 95. [0065] A transition to charge sustaining (CS) mode 95 occurs upon RESS SOC/SOE falling a below a minimum threshold for maintaining CD mode. Transition from CS mode 95 back to CD mode 90 can occur as a result of an intervening plug-in charging event, if the IC engine is run to return charge on the RESS above a minimum return threshold (which is higher than the threshold for maintaining CD mode) or if regenerative braking happens to return sufficient charge to the RESS to meet the return threshold.

[0066] An engine on/no auto stop/start function state 96 may be provided for a vehicle where the IC engine can be directly applied to a function other than back driving an electrical motor in order to recharge an RESS. State 96 affords continued vehicle operation under circumstances where the electric traction motor is "inoperable," which may occur for a number of reasons. The engine on/no auto stop/start function state 96 may be reached from either CD mode 90 or CS mode 95. Renewed availability of the traction motor results in the state of the machine returning whence it came, respectively CD mode 90 or CS mode 95.

[0067] It is clear that the electric motors 32, 30 chosen for any application should be as efficient as possible. Predicting what the efficiency of any electric machine however present certain issues to be resolved. An electric machine operating as a motor is, in energy terms, fairly simple. Electrical power is the input, and mechanical work is the primary output, with, however, some of the energy being converted into heat. The input power and mechanical output power are straightforward to measure. Input power is the product of voltage and current. Mechanical output power equals the torque and angular speed on the output shaft. However, the efficiency of an electric motor is not so simple to measure and describe as might be supposed. The problem is that it can change markedly with different conditions, and there is no single internationally agreed method of stating the efficiency of a motor. This is perhaps one of the largest issues in developing an EV or HEV application and why it is so important to understand the performance expectations and potential gearing options from a transmission such as transmission 38 for a particular application. In the exemplary embodiment this extends to which electrical motors 30, 32 to operate if there is a choice. [0068] There are four main factors that affect electric machine efficiency. In general, electric motors become more efficient as their size increases. This is often very difficult to accommodate within vehicle packaging confines and often results in a higher cost machine. In addition, higher speed electric motors are more efficient than lower speed ones. This factor has more influence on overall efficiency than motor type. The reason for this is that one of the most important losses in a motor is proportional to torque, rather than power. A lower speed motor will have a higher torque, for the same power, and hence higher losses. The third factor is the cooling method. Motors that are liquid cooled run at lower temperatures, which reduces the resistance of the windings, and as a result improves efficiency. While cooling might appear to only give a 1 % efficiency, this efficiency is relatively large when the entire powertrain 20 is considered. The last factor has to do with how the electric motor is operated. The efficiency of an electric motor might well be very different from any figure given in the specification, if it operates well away from optimum speeds and torque. In some cases an efficiency map may be provided. Where two electrical motors such as electrical motors 30, 32 are provided, the types of machine applied may be different to expand the solution space

[0069] For a given machine the maximum efficiency may be as high as -94%, but this efficiency is only obtained for a fairly narrow range of conditions and a single temperature point, say about 70°C. It is quite possible for a machine to operate at well below 90% efficiency, for example during periods of a light regenerative energy capture event. This efficiency factor would be further affected by inverter operating efficiency and RESS efficiency. As long as these factors are taken into account, efficiency maps are a convenient way to represent motor drive subsystem of a large, complex, system like a vehicle. Efficiency maps guide the torque-speed combinations at which a specific electrical motor drive is efficient and how its output can be combined or displaced by an IC engine, thus allowing for a more efficient design. As with any efficiency map, it must be understood with respect to the system it will be operating in.

Claims

What is claimed is:

1 . A method for operating a hybrid powertrain having at least first and second machines for converting energy from first and second sources to torque, the first source being a rechargeable energy storage system, the method comprising the steps of: in response to an exogenous output torque request, generating a target torque effort level and an optimal torque effort level; responsive to the target torque effort and optimal torque effort levels generated and further responsive to a set of exogenous system constraints, determining a state for the hybrid powertrain allocating torque demand to the first and second sources and generating a tractive effort commanded level; and responsive to the determined state and to the tractive effort commanded, determining a set of target states.

2. The method of claim 1 , further comprising the step of: filtering transition to the determined system state.

3. The method of claim 2, wherein the first source is an electric motor and the second source is an internal combustion engine.

4. The method of claim 2, further comprising the step of: responsive to determination of the state for the hybrid power train and allocation of tractive effort, determining a solution space for the first and second sources.

5. The method of claim 4, the step of determining the system state and allocating tractive effort being further responsive to feedback of the solution space.

6. A vehicle power train comprising: primary and secondary sources of torque; a rechargeable energy storage system to supply energy to and to receive energy from the primary source of torque; a control system including means responsive to exogenous torque demand for generating a target tractive effort and an optimized tractive effort; the control system including means responsive to target tractive effort and optimized tractive effort for establishing a powertrain system state and tractive effort command allocating torque demand to the primary and secondary sources of torque; the control system including means responsive to the powertrain system state and tractive effort command for developing a set of target states; and feedback of the powertrain system state and tractive effort command as a restraint on the establishing of the powertrain system state.

7. The vehicle powertrain of claim 6, further comprising: the control system wherein the means for establishing are subject to exogenous constraints on the powertrain system state.

8. The vehicle powertrain of claim 7, further comprising: the control system including an active filter responsive to establishing a system state for filtering transition to the system state from a prior system state; and feedback connecting prior system states as a restrain on the means for establishing.

9. The vehicle powertrain of claim 8, further comprising: means communicating the target tractive effort to the means for developing target system states.

10. The vehicle powertrain of claim 9, the primary and secondary sources of torque being an electrical motor and an internal combustion engine, respectively.

11 . The vehicle powertrain of claim 9, further comprising: the means for developing a set of target states including means for estimating a state of energy (SOE) of the rechargeable energy storage system.

12. The vehicle powertrain of claim 9, further comprising: the means for establishing the system state including means for determining the direction of primary source torques which increase and decrease the output torque.