MANAGEMENT FOR HYBRID ELECTRIC VEHICLES
Unique Mobility, Inc.
A hybrid electric vehicle comprising an internal combustion engine and a battery bank as sources of power is being investigated.
In this study, the conversion of mechanical work from the engine to electrical power using an alternator and regenerative phase converter is investigated. The unique characteristics of the three components above are analyzed to identify the corresponding limitations. More importantly, the favorable range of operation of these units operating together is identified.
An experimental van using the hybrid concept is constructed and its starting and control modes are described. An analysis of engine, alternator and converter is performed to formulate a control strategy. Special attention is given to the converter and its algorithm due to its important interface role between the alternator and battery bank.
Electric vehicles have the potential to greatly reduce the local pollution levels within urban and suburban environments. The technologies exist today to provide acceptable commuting vehicles for a sizable group of the commuting public that would operate on existing battery technologies. It has been determined that such a vehicle is impractical to develop because of the limited usage and market acceptability. Thus, without severe regulations or incentives promoting their use, electric vehicles are still a remote possibility until the development of a cost effective energy storage means that offers range and performance that is presently available in the vehicle market.
PURPOSE OF HYBRIDS
Advances in technology have brought about electric drive train components that offer low weight and high efficiency and performance capable of competing with internal combustion vehicles. Electric vehicles offer the potential of a highly sophisticated drive system that could include features such as anti-lock braking, anti-slip differentials, four wheel drive operation, as well as regenerative braking and motor-in-wheel drive. Even without the ultimate battery, there are compelling reasons to develop an electric drive system.
The hybrid vehicle offers the bridge to carry the electric drive train development into the next decade. Vehicles of this type could have similar performance and superior range and emissions characteristics. The implementation of a series hybrid, as described in this paper, offers the opportunity to operate a clean burning natural gas engine through a continuously variable electronic transmission that recovers both regenerative braking energy and available idling energy. The development of vehicles of this type will also allow other potential clean burning fuel technologies and/or fuel cell technologies to be introduced to the market through the availability of a flexible vehicle platform.
DESCRIPTION OF SYSTEM
The system discussed in this paper is a series hybrid system consisting of a natural gas engine and an electric drive train. The term "series" refers to the fact that the mechanical power output of the engine is converted first to electricity, and then back to mechanical drive power. The engine speed in this type of system is totally decoupled from the vehicle wheel speed. The only relation of engine speed to vehicle speed is determined by the average power required to maintain vehicle performance. The engine is also downsized from what one would normally consider for a vehicle of a particular size and weight. This is due to the use of a small, high power density battery pack that serves as a transient energy source during acceleration as well as an energy sink during braking. Thus the battery acts like a flywheel or load leveler to remove the load transients from the engine and thus increase its operating efficiency. The ability of the system to control both engine speed and torque independently allows the engine to be operated at the most efficient point possible for any driving condition.
The proper operation of the engine is paramount to the increased efficiency of the system. This paper will illustrate the analysis of the control algorithm for the engine and the resulting system performance. The implementation of the system into a natural gas powered Chrysler Caravan will be illustrated.
The three components of the system being investigated in this study are represented by blocks in Figure 1. The command signal to the engine will control its throttle position while the command to the converter will determine the operating point of both alternator and converter.
The following are some of the parameters of each of the above components:
The performance characteristics of each component of the system are determined by either test or calculation. This information forms the basis for the optimization process which is the subject of this study.
In this section we will look at the characteristics of the three components of the system shown in Figure 1.
1) I.C. Engine
Figure 2 shows the results of tests done on the I.C. Engine for available torque vs. speed. The fact that torque values vary by a small amount for each of the four tests has no more significance than making the test practical to perform. While testing for torque, the amount of fuel consumption (BSFC) by the engine was monitored. These values of BSFC are plotted vs. torque in Figure 3 showing that speed has a small effect on the distribution of these points. The cluster of points tends to indicate a range of torque at which the BSFC is minimum. This appears to be between 38 and 48 lb ft and is labeled "Zone A".
Alternatively, Figure 4 shows the variation in BSFC as a result of changing demands on the BHP of the engine. The "Hook" curves resulting from constant speed operation show that there is a minimum value of BSFC over a wide range of BHP if the engine speed is allowed to change. This confirms the finding in Figure 3 of minimum BSFC at a particular range of engine torque.
The losses of the alternator are determined at various values of current and alternator speed n. These losses consist of the copper, iron, bearing and windage losses. Following are variables affecting each of these losses:
The copper losses are affected by the change in temperature of the alternator which in turn affects the winding resistance R. The two terms in Equation (2) for the iron losses refer to the losses in the stator teeth and in the flux return-path, respectively. The flux-density in each of these two parts is and . The bearing loss in Equation (3) is also a function of the rotor weight which can be calculated from the physical parameters of the alternator. The windage loss in Equation (4) is additionally a function of the physical parameters of the rotor.
The converter performance characteristics are greatly influenced by the chosen value of the duty-cycle of the commutation. Figure 5 shows the input power vs. output power at two values of duty-cycle.
Following are the equations used in calculating the curves in Figure 5.
The converter characteristics are then described in mathematical form for particular values of (1-D).
The system represented in Figure 1 is then analyzed with its three components operating together. The criteria for this condition are the specific values of output power at the converter terminals. The values chosen in this analysis are 5, 10, 15, 20 and 25 kW.
It was determined from the converter characteristics that its efficiency is highest when operating at the lowest possible duty cycle D. The efficiency is also highest when operating at the lowest current possible.
Other equations are used in calculating the system performance. These are:
Values of alternator current are calculated over the full range of alternator speed and at various values of converter output power. The results are plotted in Figure 6 up to a maximum current of 150 A.
These curves show the inverse non-linear relationship between alternator speed and current at constant values of converter output power. They, therefore, show the minimum speed needed for the alternator to achieve a certain amount of output power at the converter while staying below 150 A of alternator current. These curves also identify the usable range of the efficiency curves of Figure 7 for each value of output power.
Figure 7 shows the values of efficiency for the combination of alternator and converter over the full speed range. They were calculated from the following equation:
Where is the input power to the alternator, calculated from the following expression:
Figure 7 also shows that the efficiency drops significantly at low speeds. The minimum operating speed is also dictated by the limit of 150 A set upon the alternator current which was demonstrated by superimposing Figure 6 and Figure 7. For a particular value of output power, the speed corresponding to 150 A is the minimum speed allowed on the efficiency curve. However, it is advantageous to operate at a speed higher than this minimum value to achieve a higher efficiency.
Other efficiency curves are plotted in Figures 8 and 9 for the alternator and the converter, respectively, using the following formulae:
Figure 10 shows the temperature rise in the alternator over the speed range while certain amounts of output power are obtained from the converter. As expected, high speeds are required as the output power demand from the converter is increased. Figure 6 can also be superimposed on Figure 9 to identify the minimum speeds permissible for particular values of output power. However, we can also use the results of Figures 11 and 12 to establish these boundaries. Figures 11 and 12 give values of the temperature rise in the alternator over the full speed range when the current is limited to 150 A and 50 A, respectively. When these limits are superimposed on the curves of Figure 10, the permissible range of operation is identified.
Next, we can tie the engine performance with the alternator requirement via the following relationships:
Figure 13 shows the curves of output torque needed from the engine vs. engine speed to deliver the output power values at the converter. The safe operating limits on the engine torque and speed are the maximum values of the x and y axes.
Zone A appearing on the curve of Figure 13 is defined in Figure 3 by the range of engine torque corresponding to the minimum value of BSFC. The minimum and maximum values of speed defined by Zone A are then determined from Figure 13. These values are plotted in Figure 10 to show the range of temperature rise corresponding to Zone A.
Figure 14 shows the range of engine speed vs. converter output power within the limits defined by Zone A. Operating the engine at those speeds corresponds to the lowest values of BSFC.
Figure 15 is a plot of engine BHP vs. engine speed at various converter output power. Zone A, which identifies the lowest possible BSFC is superimposed on the curves of Figure 15. Even though this zone may appear narrow, the engine speed can vary by several hundred rpm without a substantial change in the BHP (for a particular value of converter output power). These values of BHP are plotted against the converter output power in Figure 16.
The relationship over this range appears to be almost linear and can be described with the following expression:
Expressed differently, the efficiency of the alternator and converter combined is:
Looking at the engine performance in more detail, Figure 17 shows how fuel consumption is affected by the output BHP required of the engine. The y axis is BSFC * BHP i.e. cu ft/hour and the relationship with BHP can be expressed as follows:
APPLICATION OF HYBRID SYSTEM IN CHRYSLER CARAVAN
The complete hybrid drive system has been implemented in a mini-van platform. The system is illustrated in Figure 18. The drive system components consist of a natural gas engine, a UNIQ motor and controller operating in a starter/alternator configuration, two UNIQ drive motor and controllers and planetary gear reduction sets for independent front wheel drive, a vehicle control module, an engine management system, and a battery pack. The entire system operates around nominal voltage of 180 volts dc. The 12 volt systems are supplied from the a 60 amp dc-dc converter. Heat and air conditioning are supplied from the engine. Power steering and brakes are supplied from independent electric systems.
The operation of the van was meant to be as familiar as possible. Turning the ignition on without starting, allows the vehicle to be driven from the battery only. Turning the ignition to start, activates the engine and may be done while the vehicle is in motion. Once the engine is started, the entire vehicle must be switched off to kill the engine. Since the transaxle was replaced with fixed ratio planetary gear sets there is no need for the shift lever. The vehicle is reversed by pressing a single switch which illuminates while in reverse.
The main power system in the van is supplied from a distribution box underneath the van. This allows any one module to be disconnected for servicing without disturbing the others, and also allows for a single cable pair from the battery box for easy removal. The power electronics are always connected to the power bus and are activated electronically with only 2 ma of standby current draw. This removes the expensive and unreliable contactors from the power path, which also reduces the inrush currents associated with capacitory charging. The electronics is also designed to stay active while the vehicle is in motion even if the switch is turned off.
The driver instrumentation includes the speedometer, which is now electronic, including the odometer, a tachometer which shows engine speed, battery voltage and current, natural gas fuel level, and drive controller status.
The vehicle controller module is the main junction for the driver controls such as ignition and throttle. All of the power electronics associated with the drive receive their control inputs from this box. Some of the functions performed by this device are listed below:
Vehicle Speed Control
Battery Under and Over Voltage Limit
Data Acquisition Interface
Brake and Reverse Light Interface
Limited Slip Control
Engine Management Interface
The engine management unit is the device that controls the natural gas engine's speed and torque. These parameters are determined by the inputs from the vehicle controller and are dependent on battery current, incremental battery state of charge, and vehicle speed. Ideally, the batteries should end the trip in the same state of charge as begun. The engine speed is controlled by modulating a vacuum powered cruise control unit on the engine carburetor. The engine torque is controlled by supplying the desired regeneration current level to the alternator controller. Because the regeneration is done in a boost fashion, power may be derived from the engine at any speed. The engine management system also controls the engine speed in transient situations such as braking and waiting at stop signs or lights. The control function is set up to slowly vary the engine speed to prevent drastic changes in operating point for better economy and emissions.
The use of hybrid systems in vehicles can help in the introduction of alternatively fueled vehicles as well as electric vehicles to build the necessary infrastructure to support and manufacture cost effective clean vehicles.