The Hybrid Vehicles of the Vehicle Research Institute of Western Washington University

Dr. Michael R. Seal, Director
Vehicle Research Institute
Western Washington University
(360) 650-3045

ABSTRACT

Western Washington University has built two ground-up hybrid electric vehicles and converted a four door Neon to an electric hybrid vehicle. The first vehicle, Viking 21, was utilized to test various systems before incorporating them into Viking 23 which is the vehicle discussed in this paper along with the Neon conversion. Viking 23's front wheels are driven by two brushless D.C. motors through a purpose built 4 speed transaxle incorporating a limited slip differential. The rear wheels are driven with a natural gas, fuel injected, four cylinder, 16 valve engine through a six speed gearbox and mechanical diodes at the rear wheel hubs.

A Chrysler Neon has been was converted to a methane electric hybrid vehicle complete with a fully functioning HVAC system. This car competed in the DOE sponsored hybrid electric vehicle competition in June 1995.

INTRODUCTION

It is becoming increasingly evident that the current battery technology available for electric cars is such that true inter-city capability is not possible in the foreseeable future. Therefore, the design team for Viking 23 decided that a solar electric, parallel hybrid vehicle would be a realistic replacement for today's car. A range of up to 100 Km at an average urban speed of 50 km/h in zero emission mode was set as a goal. An attempt will be made to do this with no power drawn from the existing electric power grid by utilizing solar cells to charge the battery. For inter-city use, a target range of 500 Km at an average speed of 100 km/h was set. The parallel configuration was chosen because parallel mechanical driveline efficiency is greater than that possible for a series hybrid. One disadvantage of a parallel hybrid is that a larger, more powerful internal combustion (IC) engine is required for hill climbing and maximum performance than would be needed for the series configuration. On the other hand, the parallel hybrid has the advantage that no large generator is needed to collect the IC engine power to charge the battery and run the electric motors.

The focus for the Neon was slightly different in that it was to be a good methane car with a relatively short all electric range to qualify for the competition. It is also a parallel hybrid.

POWER TRAIN CONFIGURATION

Viking 21/23 utilizes a parallel configuration with an electric drive to the front axle and internal combustion engine (ICE) drive to the rear axle has been chosen to take advantage of the high inherent efficiency of a gear and chain drive transmission system when in ICE mode. The electric drive mode is used for nearly all urban use and accounts for about 90% of all trips.

The Neon project uses a Morse silent link chain to electrically drive the input shaft of the Neon five speed transmission. Thus it is front wheel drive in both the electric and ICE modes.

ELECTRIC DRIVE SYSTEM - Since electrical energy is always limited, and especially so if the power is to be generated with solar cells mounted on the Viking 23, every effort must be made to conserve energy. Because their efficiency is more than 95%, Unique Mobility rare-earth permanent magnet brushless D.C. motors and controllers are used for much of the operating envelope. The two 10 kW motors use a roller chain to drive into the four speed front transaxle that has a small limited slip differential built into it (Fig. 2).

The two front motors provide regenerative braking when the brake pedal is depressed part way. A linear potentiometer controlling the level of regeneration is linked to the brake pedal. When a more severe level of braking is required than can be provided with regeneration, further pedal travel actuates the front and rear master cylinders. Four aluminum calipers act on the 255mm cast iron brake discs at each wheel. The calipers have a retraction system which ensure that the brakes don't drag when not applied.

Although the brushless D.C. motors are very efficient, there is very little overload capability and the torque curve is such that starting and climbing on a steep grade (30%) would not be possible without the high torque multiplication available in first gear. Reverse gear must be handled with the front motors only, as there is no reverse gear in the I.C. motorcycle gearbox. Changing the direction of rotation of the motors to CCW provides reverse movement through the front transmission first gear.

The Neon uses a single water cooled 30 kW brushless D.C. motor driving the front transmission input shaft at the outboard end. Regenerative braking is handled in the same way as for Viking 23.

SOLAR ARRAY - To maximize the power output from the BP solar cells on Viking 23, they were cut into rectangles 76mm wide and 101mm long. This cell configuration allows for maximum packing to increase the potential power from the area covered by silicon cell material. A number of the cells were split in two so that a better fit could be obtained to the aerodynamic body shape. The cells were soldered together in series strings of 152 cells that were then paralleled to each other through maximum peak power trackers. The cells were encapsulated with Dupont Tefzel ZMC 0.6 mm tensilized fluoropolymer film. This material was bonded with .71 mm EVA film in a vacuum bag heated to 85 C, held for one half hour, cooled, and then demolded.

SOLAR CELL BATTERY CHARGING - To meet the range goals on solar power, it is necessary to minimize power consumption and maximize the solar collection area. It was decided an area of 8sq meters for the planform of the car was as much as a two seat urban car should have. Using 17% efficient solar cells, the total power possible would be 1360 Watts. Because of the need to provide windows and an aerodynamic body form, the actual maximum solar input was only 720 Watts. Although cells along the sides of the car would provide more power at low sun angles, it was decided that cell breakage due to impact from the doors of other cars opening into the side made this impractical. The car is equipped to be recharged from a 110v 15 amp circuit as well. As the various panels on the car don't all face exactly the same direction, and because of partial shading of panels, the car is equipped with four maximum peak power trackers to improve solar cell performance.

The Neon hybrid is intended to be a battery depleting hybrid whose sole means of charging is from the grid.

BATTERY - Saft nickel-cadmium 24 amp/hour cells make up the 11.4 kW hour battery on Viking 23 primarily because of the extremely high power density of this cell. (150 W/Kg at 80% depth of discharge (DOD) and 400 W/Kg at 50% DOD). Four parallel strings of 78 cells each are combined to provide an 108 volt output suitable for powering the two electric motors with a total weight of 296 Kg. The Neon uses 6 kW hours of Saft NiCd battery to give 144 volts at the motor.

AUXILIARY POWER UNIT - Viking 23 provides full range performance with the ICE with an high performance 900cc motorcycle engine driving through its standard six speed transmission on Viking 23. The final drive is by roller chain to a jack shaft that has a CV jointed axle at each end. These axles are connected to mechanical diodes at the wheel hubs. The diodes allow complete automatic disconnection of the rear drive train when the ICE is not running. An additional advantage of the one-way clutches in the rear hubs is that the ICE low inertia dog clutch gear box is disconnected at output during shifts simply by reducing the throttle pedal position, interrupting the fuel injection signals for a moment. The shifter has dual linear shift gates. The shift knob is twisted 90 degrees to change gates. The linear shift pattern is front to rear with 4 speeds for the electric drive and 6 speeds for the ICE. The driver can shift gears on the electric drive system or the ICE at will.

The Neon hybrid drives normally as an ICE on methane and the clutch pedal is depressed and latched down when the car is driven in electric mode.

Compressed Natural Gas - Compressed natural gas (CNG) is used to fuel the 16 valve, 4 cycle, 4 cylinder, water cooled motorcycle engine in Viking 23. CNG provides a very clean cold start. As no liquid fuel can adhere to the intake manifold, no enrichment is required. A multipoint electronic fuel injection (E.F.I.) is used for natural gas. As Viking 23 has exhibited the ability to travel 21.1 Km/l consumed at constant 80 km/h, it is possible to travel more than 500Km on the interstate freeway with one tankful of CNG. At present, the principal emission control is a three way catalyst strategy that provides very low NMHC and CO emissions and reduces NOx under heavy load. The new system, utilizing a zirconium dioxide oxygen sensor feedback loop fuel control allows use of a 3 way catalyst system that will meet the California ULEV standards in IC engine mode, and of course qualify as a zero emission vehicle (ZEV) in electric only mode. For most urban driving cycles, the solar panel will provide enough power that this car will not have a significant effect on electrical power requirement from the grid. A new feature is the use of barrel throttle valves adjacent to the intake ports which promote high swirl at low throttle settings to give improved low speed torque, fuel economy and emissions. The addition of proportional E.G.R. reduces NOx and an electrically heated catalyst helps reduce cold start emissions. Extra air is introduced into the oxygen catalyst for further CO and HC reduction.

The Neon hybrid uses the 2 liter engine supplied in the standard car with compression raised to 14:1 and a modified factory fuel injection system utilizing methane (CNG) injectors in place of gasoline units.

CONTROL STRATEGY

The Viking 23 is designed to run at speeds up to 66 Km/h as an electric vehicle and to operate as an ICE vehicle at higher speed. The car can be driven at 90 km/h in electric mode, but the battery would be depleted quickly. In the natural gas ICE mode the car can be driven at any speed but efficiency and exhaust emissions are poorer than in the electric mode. The car drives in reverse in the electric mode only. When the roads are very slippery, both drive systems can be activated to give all wheel drive. The driver can select either drive system by activating switches on the dashboard. The driver can select either or both drive systems depending on the type of driving contemplated, battery state of charge, or CNG fuel pressure remaining in the system. In general, ICE's have best BSFC at wide open throttle and work best at highway speeds where aerodynamic loads require substantially more power than at lower speeds. As ICE power units have extremely low efficiency at low speeds, urban stop and go driving is ideal for electric mode driving. Regenerative braking is used in ICE mode as well as electric mode so that energy recovery is possible whenever the battery can absorb it.

The Neon can be driven in ICE mode in the same fashion as a standard vehicle. It can also be driven as a normal electric car with a limited range.

EFFICIENCY

This type of hybrid vehicle can achieve substantially better efficiency when operating in electric mode than when in ICE mode because the brushless DC motor operates at better than 90% efficiency through a wide operating range, whereas the ICE might achieve one third this efficiency for a relatively narrow operating window. By using multispeed transmissions for each drive mode, the power units can be kept in their most efficient range through virtually all of the driving cycle.

DRAG REDUCTION - Viking 23 has been designed to reduce its drag through the following strategies:

Weight reduction - Viking 23 has a monocoque chassis weighing 45kg made from carbon fiber honeycomb panel. The body is also carbon fiber with a foam core that also weighs 45kg without the solar cells. The use of carbon fiber provides an overall weight saving of 124kg over the aluminum paneled, steel tube chassis and fiber glass body of the previous Viking 21. Another 34kg was saved by using a Honda CBR 900 IC engine in place of the Yamaha 1200cc in Viking 21.

Another 34 kg was saved by using a Honda CBR 900 IC engine in place of the Yamaha 1200cc in Viking 21. Smaller savings were accrued wherever possible. The Viking 21 milled aluminum front suspension uprights were replaced with carbon fiber skinned urethane foam units that weighed only 682 grams instead of the 1.6 Kg of the aluminum parts.

Aerodynamic drag reduction - The aerodynamic shape of the Viking 23 body was optimized at 1/10th scale in the Vehicle Research Institute (VRI) wind tunnel. The long tail necessary to provide increased solar cell area gave an excellent opportunity to reduce eddies at the rear of the car to almost zero. The smooth underbody of the car slopes upward toward the rear at 4 degrees from a low point at the front wheels in an upward curve to its horizontal tail edge. The outside mirrors are mounted on long streamlined stalks so that the turbulence they create will not disturb airflow down the flanks of the car. The mirrors are electrically controlled from inside the car. The cabin doors use air inflated seals to provide an aerodynamic hermetic seal to prevent the wall of air thrown up by cracks around the doors from interrupting the flow of air down the car. The inflatable tube around the edge of the door is 24mm in diameter at its cross section inflated, and only 1.5mm thick when collapsed so that the door can open. When the tube is inflated to 2.4 Bar absolute, the door is locked in position which also increases the torsional stiffness of the body. As the door gap is completely sealed, there can be no air leaking out of the cab at this point causing turbulent flow along the body. All cabin air is brought through ducts under the nose of the car. Air can be heated by passing through the electric motors and be released through the demister vents at the base of the windshield or through the fresh air ducts leading to the dash board vents. Exit ducts from the cabin lead to a low pressure outlet zone. The hood of the car is so low that the only way to have legal height headlights on the hood would be to have pop up headlamps with the increased aerodynamic drag they induce. Instead, the lights are mounted behind the bottom edge of the windshield where they don't disturb the air stream and the windshield wipers clean them. Drag is also reduced by fully skirting the rear wheels, and partially skirting front wheels. The coast down data provided a Cd of .275 and the frontal area = 1.4177 square meters. Therefore, the Cd x A = .399 square meters. The power required to overcome aerodynamic drag is 3607 Watts at 80 km/h. As the power required to overcome the rolling drag of tires is 3902 Watts, the total power required is just over 7.5 kW. If we allow 96% efficiency for the electric motors, 98% efficiency for the controllers and a final drive efficiency of 98%, the overall power requirement is 8.16 kW. Therefore, as the battery has a 11.4 kW/hr rating, the car should be able to travel 112Km at a steady speed of 80 km/h. Although stop and go driving and hilly terrain would reduce this range, the use of regenerative braking can recover 50% of the energy normally lost as heat during normal braking and braking when descending hills. Normally, in electric mode the car only drives the front wheels, and the rear axles freewheel at the overrunning clutches, so there is minimum power transmission loss.

SAFETY

To provide frontal crash safety in Viking 23, the motor drive controllers are mounted in the nose where they can serve as 18" of deformable structure. The carbon fiber chassis provides a very stiff reaction structure to protect the occupants. The doors contain carbon fiber tube side beams to withstand side intrusion in a crash from this direction. The carbon fiber side beams are also immensely stiff and provide excellent side protection. The rollover structure consists of a carbon fiber epoxy hoop structure reinforced around the door aperture through the "B" pillars with two supports rearward to the rear bulkhead.

MANUFACTURABILITY & MATERIALS

Although certain features of Viking 23 do not lead to ease of manufacturability such as the use of solar cells on the entire top surface of the vehicle, the actual carbon fiber structure would not be as expensive as is commonly thought if new methods of mass production of carbon fiber parts are adopted. Reaction injection molding of large composite parts is currently in use for the low production Lotus Esprit model and could easily be expanded to higher quantities with further cost savings. As parts count is significantly reduced with this production method, fewer fasteners are needed and no rust protection or paint is required which can account for one half of the cost of a steel body in white. The use of a lightweight structure makes the use of power brakes and power steering unnecessary. Material cost for carbon fiber composites has dropped dramatically in recent years from $100/lb to about $10.00/lb.

CONCLUSIONS

The Viking 23 car has fulfilled most of the goals set for it. The final weight of 900 kg was 24% greater than planned which reduced the expected performance to a certain extent. Doubling the battery size to give increased range in ZEV mode was responsible for most of the difference in weight. The additional battery size unfortunately added 145 Kg to the extreme rear of the chassis. The battery size increase was a result of a desire to provide better fuel efficiency numbers when compared with large battery capacity series hybrid cars on the SFUDS cycle. As Viking 23 is now configured, 90% of typical automobile trips can be managed in ZEV mode which provides electric car efficiency without significant performance loss on the highway in ICE mode.

As a consumer acceptable hybrid vehicle the Viking 23 appears to be a success. The control strategy of the electric/ICE modes is easily grasped and the performance in each mode is more than adequate for city and highway use. The stiff composite chassis provides a solid feel, enhanced handling, and less interior noise over the previous Viking 21. Increased electric range, and very low emissions in CNG ICE mode combine to make the Viking 23 hybrid car a model vehicle for the 21st century.

The Neon has met its goals of providing dramatically improved economy and zero emissions in electric mode. It meets ULEV standards in ICE mode while maintaining driveability and passenger carrying capability.