Machinedesign 1298 Bgsu Formula Lightning Car 0 0

The race to the electric car

May 1, 2000
Automotive racing has been the birthplace of most performance and safety innovations we now take for granted in our passenger cars

The idea was to challenge undergraduate university students to develop highperformance, electric powered race cars. Under the auspices of Centerior Energy Co., Independence, Ohio, in cooperation with the Solar and Electric Racing Association, (SERA) Formula Electric Racing was born. Now, in the second of a fouryear racing schedule, 11 universities across the country compete in the Formula Lightning program (see box, “Who’s racing”).

To focus attention on the electrical powerplant and its performance, all Formula Lightning racers have a standard chassis, disc brakes, coil-over-shock suspension, and rack and pinion steering. The open-wheel, Indy-style racers are all 163-in. long, 77-in. wide, and weigh 975 lb — not including drivetrain and batteries. These racing cars are designed for speeds up to 160 mph.

Although the sleek Formula Lightning racers may look like Indy cars, they certainly don’t sound the same. The deafening high-pitched whine of an internalcombustion engine at 10,000 rpm is replaced by a nearly inaudible swoosh as the electric racers streak past at speeds approaching 100 mph.

The Electric Falcon

At Bowling Green State University (BGSU), Bowling Green, Ohio, the starting point for the Electric Falcon design team was a review of a SERA prototype Formula Lightning car initially powered by a dc motor rated at 29.84 kW and drawing more than 400 A. Although this prototype reached almost 90 mph, it could not sustain this speed for long without depleting its batteries.

Motor and control design. The BGSU design team knew that the existing power-drive combination needed improvement. To develop and integrate components into a winning system, students, faculty, and marketing partners used concurrent engineering and rapid prototyping.

The BGSU design team set out to develop an ac induction motor rated at 60 kW at 8,000 rpm with a peak output torque of 80 lb-ft. The final motor design evolved from a C-TAC 3-phase, ac motor rated at 3.7 kW at 1,750 rpm manufactured by Lincoln Electric Inc., Cleveland.

The motor design team determined that, by operating the motor at 10,000 rpm, the basic motor produced approximately 30 kW. By rewinding the stator, the final power doubled to achieve the targeted output of 60 kW.

To achieve the 60 kW rating, the students, using reverse engineering, modified the mechanical design of the motor through lighter material selection and increased structural integrity. The bell housing design was entered into a MasterCAM software program using a coordinate measuring machine (CMM). A finite- element analysis of the design indicated that the new motor housings needed reinforcement to handle the higher torque and speed ratings. The final bell-housing design was then cast and machined.

Cooling system. Upgrading motor performance produced more heat than the original air cooling system could dissipate. So, the design team devised a system of spraying an oil mist on the rotor and the motor bearings. This system cools the rotor to acceptable levels and, in the process, recovers much of the generated heat. By cooling the oil in radiators placed in front of the battery packs, the batteries are warmed, boosting their efficiency on cool days. The students calculate that by cooling and lubricating the rotor and bearings in this manner, bearing life will increase up to 50%.

The new oil-spray cooling system was simulated and modeled using AutoCAD and became part of the concurrent engineering process.

Controller configuration. The final controller went through several phases. Originally, an ac vector drive unit designed exclusively for use in electric vehicles was used. Although the racer could achieve speeds of 74.6 mph, it only delivered half of the desired 300 A to the motor, thus limiting acceleration and maximum power.

The design team then adapted a 350-A, peak, EMS Model 2055 high-speed inverter by bypassing its rectifier section and accessing the inverter section. Bench testing showed this new design capable of much greater power than the first drive, but it was too large to fit in the Electric Falcon. Again, the design team was innovative and turned this problem into an advantage.

The controller, originally packaged in a 9.8 in. x 18.9 in. x 31.9 in. steel housing, was disassembled into five units. Each of these was repackaged in aluminum and polycarbonate containers and strategically placed in the car so the weight distribution would enhance vehicle performance. In addition, elimination of the steel housing, diode-bridge rectifier and cooling fans reduced the weight by more than 70 lb. This system has been updated to a vector drive to achieve the desired results.

Braking system. Braking involves standard hydraulic disk brakes and regenerative torque through reversing circuits. Both systems are controlled by brake pedal movement. Above 10 mph, gradual depression of the brake pedal actuates a rheostat to control regenerative energy to slow the vehicle and recover nearly 80% of the braking energy. Heavy pedal pressure adds hydraulic braking and slows the car quickly. Below 10 mph, braking is all hydraulic.

Batteries. After investigating various commercially available designs, the Falcon team found that lead acid Optima batteries provided superior power-toweight and dollar ratios, and they could be mounted in any position (facing up, down, or sideways) — a decided advantage for adjusting car ballast, Figure 1. The battery chemistry uses starved electrolyte so, in the case of an accident, the risk of a sulfuric-acid hazard is reduced.

The Electric Falcon is powered by 312 Vdc and requires 26, 12-V batteries. A total of 104 batteries — four complete sets — were purchased, tested, and grouped according to their ability to hold power. Each of the four sets was grouped into eight packs — six with three batteries each, and two packs with four.

The battery packs are modular and fit into any available space within the car. Ballast is adjustable by swapping a fourpack with a three-pack, and vice versa, in any appropriate area of the car.

The safety committee requires that the batteries be completely enclosed so, in case of an accident, all components stay within the packs. The design team chose to use a polycarbonate insulator for the battery pack enclosures rather than the aluminum the safety committee recommended. Because of aluminum’s high conductivity, the team felt there may be a problem if, in case of an accident, the batteries break open inside the pack and energize the case with live voltage.

The team paid special attention to the connections inside the battery packs, because every joint and standard batterypost connector has some resistance causing I2R (power) losses. One-piece couplers are used on about half of the internal pack connections. These couplers allow the positive and negative terminals to butt together to minimize these power losses.

Electric-car battery technology is advancing rapidly with ideas promising both lighter weight and longer run-time between charges. The BGSU team is evaluating several advanced battery designs such as lithium-metal hydride, zinc-bromide, lithium-metal sulfide, and metal air.

Power train. The drivetrain concept evolved from a direct drive through hypoid gearing to a differential, Figure 2. This meant, with no transmission, the car had only one gear. Several different power-train approaches, including transmissions and torque converters, were investigated. A Supercharger Systems Inc. cog-belt system provides the highest efficiency and lightest weight. It also enables overall ratios at the track to match racing conditions.

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The present direct- drive system with a vector drive provides good accelerating torque in the 20-70 mph range. Road course results indicate that electric vehicles will need two speeds to optimize low-end performance and high-speed efficiency. Work is now underway on the development of a twospeed gearbox.

Safety. The Electric Falcon team faced a major problem concerning the design of a non-arcing, high-voltage, manual disconnect. Such a device is required so the drive can be disconnected from the motor under load, which could either weld conventional contacts shut or explode.

The team devised a safety shield that draws shut between separating connectors within a sealed box. Both the box and shield are made of insulating plastic. The male-female connectors, manufactured by Radsok, contain rifling in the barrel of the female connector to ensure a reliable, clean connection.

The quick disconnect mechanism contains preloaded springs with a clevis-pin, detent assembly that holds the contacts in a normally-closed position. Through flexible cables from either the cockpit, Figure 3, or from the center of the rear cowling, Figure 4, the pin can be pulled to activate the disconnect mechanism.

Efficiency. The Electric Falcon proved to be energy efficient when tested over an 0.8 mile circuit consuming 5.55 kW-h during a 16.7 mile run. Total battery energy available is about 8 kW-h. Racing strategy is to finish the race with a minimal power reserve. Long races require a pit stop for fresh batteries.

Efficiency of the Electric Falcon motor and inverter is about 90% and near 80% for the present belt and hypoid-gearing system. It is hoped the new gearing design will boost its efficiency to a level greater than that of the motor.

BGSU goes racing
The Formula Lightning racing program, beside the commercial benefits, offers university students an exciting opportunity to wrestle with real-world engineering and technical challenges. The faculty of the College of Technology, Bowling Green State University quickly recognized the practical benefits of the electric racing program and became a charter member in 1994.

“We can never lose sight of the fact that one of the functions of a university is to create new knowledge,” explains Dr. Thomas Erekson, dean of the College of Technology at BGSU. “Considering the economy of this region, automobiles are a perfect match for us. The ideas that we will develop and test will be available to companies that can use them to build a better product. Moreover, electric vehicle technology is environmentally sound and a technology with a future.”

Almost every student in the College of Technology has been affected by the Electric Falcon program. In addition to classroom assignments, 30 students volunteered their free time to work on the program. They formed two teams. The Power Team, under the direction of Associate Professor Anthony Palumbo, designed, developed, and tested the battery system, motor, gear system, and the computer-driven controller system. A second team, under the direction of Barry Piersol, assistant to the dean, developed and implemented marketing, promotions, and fund-raising campaigns.

The undergraduate student teams were assisted by graduate students, faculty, and community volunteers.

“It’s more like coaching a sport than teaching a class,” says Prof. Palumbo. The program involves many student groups working on various projects — from breadboarding, testing, modifying, and evaluating results to working the pits. For example, when investigating the choice of wiring material —copper or aluminum — a class determined the required performance criteria, developed and carried out testing programs, called suppliers, and made final recommendations that were incorporated in the car.

All this has to be done under constant deadlines, just like in industry. As Prof. Palumbo puts it, “There is none of this, ‘Well, if we don’t finish this now, we’ll get to it next semester.’ It’s got to be done now, if you want to run in the next race. All these demands have produced a remarkable esprit de corps among everyone involved.”

How are they doing?

In 1994, the first year of Electric Formula racing, Bowling Green’s Electric Falcon entered two races. On July 9 during the Cleveland Grand Prix — a 2.37 mile road course — the Electric Falcon finished fifth with a best lap speed of 78.148 mph. Twice the Electric Falcon showed impressive performance by going from sixth place after the first turn to third by the end of the second lap. After an improperly connected battery cable during a pit stop dropped the Falcon to ninth (last) place, it regained fifth place by the end of the race.

On August 18, the Electric Falcon raced at Indianapolis Raceway Park. After qualifying at 80.548 mph on the 0.686-mile oval, the Electric Falcon ran in third place for the entire race.

This year’s first race was the Arizona Public Service Electrics, a weekend of racing where more than 60 electric vehicles competed in five classes. The Electric Falcon finished second with an average speed of 57.813 mph. Arizona State University won with a speed of 58.319 mph.

Also on this year’s schedule are the Electric Vehicle Grand Prix, May 5-6 at Richmond International Raceway, Richmond, Va.; the Cleveland Electric Formula Classic, July 21-23, Burke Lakefront Airport, Cleveland; and the Electricore Formula Lightning Race, Indianapolis Raceway Park.

If your company is interested in becoming a marketing partner with

BGSU, contact Barry D. Piersol, assistant to the dean, at 419-372-7580. His address is College of Technology, Bowling Green State University, Bowling Green, Ohio 43403-0300.

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