There were several electric vehicles (EVs) on display at the New York International Auto Show, but one of them is 38 ft (11.6 m) long, and weighs 8000 lb (3630 kg). It has a cockpit for just the driver, and will be trying to push the land speed record for an EV to over 400 mph at the Bonneville, UT, Salt Flats this August.
It's a college student project, but there's no reason to sell the engineering team short. They are graduate students in Ohio State University’s Center for Automotive Research (CAR), which holds the existing world EV record—307 mph.
The team is working with a premier engineering company, Venturi Monaco, a racing EV team with a solid record of success, and is using a new generation of 2000 lithium-ion-phosphate pouch cells from A123 Systems. The total pack capacity is 100 kW·h and the weight is 3400 lb (1542 kg). Although the cells are similar to other A123 pouch cells, the ones in the “Bullet” EV’s pack are not yet commercially available, according to student team leader David Cooke.
Latest in “Bullet” series
This EV is the fourth in OSU’s Bullet series, although designated Bullet 3. No. 1 was an EV with nickel metal-hydride batteries, No. 2 was a hydrogen fuel-cell vehicle, and No. 2.5 was a first effort with Li-ion batteries (32,113 cylindrical cells from A123) and is the holder of the existing world record.
The Bullet body is carbon fiber, mounted on a steel space frame. The coefficient of drag is just 0.127, thanks to an extreme effort at tapering the body sides and a driver “seating” position that is nearly horizontal.
“We had to justify everything that might increase the width, even screw heads,” said Evan Maley, a member of the student team. The body was designed at OSU by masters degree students, with CFD assistance from TotalSim USA.
To reduce width, the Bullet 3 uses one centrally located electric motor for each (front and rear) axle of the four-wheel-drive system, instead of a motor at each wheel. The battery pack has been tested for full power. However, the electrical system, including its two 10,500-rpm peak-rated permanent magnet synchronous motors, has only been tested to 2.1 MW (2816 hp), although design-rated for a total of 3000 hp (2238 kW).
With continued tweaking of the inverters and motor control, the Bullet 3 team expects to reach the full design power, Cooke said.
Powertrain by Venturi Monaco
Basic development of the powertrain, including the motors, was done by Venturi Monaco, with the student team performing the integration work. Venturi’s area of expertise is engineering for environmentally oriented racing, and it has developed EVs, hybrids, and biofuel vehicles.
The 24.5-in (622-mm) wheels are of forged bullet aluminum, No. 7075 alloy, a high-strength aircraft grade aluminum that gets both a pre- and post-treatment. The tires are a special ultra-lightweight design by Mickey Thompson Performance Tires & Wheels, and have only 0.0625-in (1.6-mm) of tread rubber.
In a track run, there is up to 1.5 in (38 mm) of growth in the diameter of the wheel-tire assembly, Maley said. This change creates a significant challenge for the electronic control system for the drivetrain, which needs precise information on the speed of the vehicle.
The carbon-fiber body and the high power outputs (which create interference) limit the use of wireless to a GPS signal, which in conjunction with a six-axis accelerometer detects all wheel motion to correct the GPS signal. In addition, there is an airspeed signal as a backup, using a bank of pitot tubes—a variant of the airspeed indicators in planes.
The Bullet 3 has a two-speed transmission, so when the driver requests a shift, the control system slows each motor and then commands the shift. The computer next gradually raises motor speed at a rate of 1000 rpm per second, and at a specific rpm an overrunning clutch engages—a mechanical operation.
The motor is cooled by automatic transmission fluid, which is chilled immediately before the run to reduce its temperature to 25-30°C (77-86°F). During the run, the oil in the motor will rise to about 130°C (266°F).
The clutch and gearbox are not subject to high temperatures, so light racing gear oil is circulated, primarily to lubricate the bearings and reduce friction between gear teeth.
Motor electronics “ice” cooling
The motor electronics require active cooling. The circuit is filled with water, then is partly drained, leaving just enough to keep the pump primed. The system then is filled with ice chips just before the run, and by the time the run is over, all the ice will have melted.
Vehicle weight “is not your friend when you’re trying to accelerate,” noted Cooke. But he pointed out that the coefficient of friction of the Salt Flats course is just 0.3 to 0.6, which provides half the grip of asphalt. So if the vehicle is too light, there’s a traction issue, Cooke added. However, to get the overall performance needed, the Bullet 3 is heavier than what the team would consider ideal from a standpoint of what is required for traction.
Because acceleration is so important, in the drawing-board phase the OSU team did look at capacitors, which discharge more quickly. However, the batteries chosen maintain a relatively flat voltage curve as they discharge, going from 20% to 90% depletion with little voltage drop, he added. So on balance, they were the best choice, Cooke said.
The braking system uses a combination of aircraft disc brakes and two parachutes to stop the car.
FIA rules for record
The rules for the world land speed record in miles per hour (there also can be runs for a record in kilometers) are set by FIA (Federation Internationale de l'Automobile). Speed is measured over a mile—between mile 5.5 and 6.5 on the 12-mi Bonneville course. The car is run both ways on the same course within an hour, and the two numbers are averaged.
When the vehicle completes its first run, the team has a brief period to prepare for the second run, including recharging the battery pack. There isn't enough time for a full recharge, Maley said, but it takes only about 20 min for a 20-90+%. So the overall sizing of the battery pack factors this into the design. The charging operation uses a diesel generator (rated at 150 kVA) feeding through a dc battery supply, with the charge rate regulated by a control system from a laptop.
The driver will be Roger Schoer, the performance driver and instructor at the track of the East Liberty, OH, Transportation Research Center, where the OSU team does some of its testing.
Ohio State’s CAR began its EV program by participating in the open wheel Formula Lightning college series that ran through the 1990s. It also has been a participant in Formula SAE, Supermileage SAE, and Baja SAE programs.Continue reading »