A highly computerized car manages energy consumption like a dieter counts calories. Precept, General Motors' Partnership for a New Generation of Vehicles (PNGV) entry, contains 47 microprocessor-based controllers packaged in an electrical system supporting 12-, 28-, and 42-volt power supplies. Smart staging—used for the 12-volt electrical supply that runs most of the vehicle's accessories—operates the power sources on an efficiency curve that trips one device during light loads and multiple devices for heavy loads.
"We wanted to lower the watts used because a power draw equates to a drop in fuel efficiency," said Joe LoGrasso, electrical engineer on Precept. Performance models for Precept show that 50 W (0.07 hp) of electrical accessory consumption equate to a 0.4 km/L (1 mpg) impact.
"We went after everything relating to energy consumption, even the dome light," said Ron York, GM's PNGV Program Manager. The dome light consists of 20 LEDs mounted in a circuit board embedded in the headliner.
While the exterior incandescent lights and halogen headlamps on today's typical vehicle consume 170 W (0.23 hp), Precept uses 94 W (0.13 hp) via high-intensity-discharge (HID) forward lighting and LED signal lighting. "The reduction in energy consumed by exterior lighting was equivalent to about 1.5 mpg," LoGrasso said. All of Precept's interior and exterior lights (with the exception of HID headlamps) are LEDs.
The sedan's command center consists of four controllers to handle overall energy management, hybrid propulsion systems, body electronics, as well as interior climate and thermal system requirements. Central controllers communicate digitally with remote nodes (representing 25 smaller controllers) via a Class II databus that handles up to 10.4 kilobits of information per second. Main controllers also communicate with each other and the primary vehicle system via a Controller Area Network (CAN) bus at 500 kilobits per second.
In explaining how Precept defines and compartmentalizes its functions, LoGrasso said, "The key to the success of the vehicle's control systems was use of a functional decomposition method. This was the first time that the Hatley-Pirbhai method was used to engineer an entire automobile. We also created software using an auto-code technique developed from system models for the four primary controllers."
In addition to electrical/electronics, the five-passenger sedan demonstrates advancements in vehicle architecture, thermal systems, energy storage, chassis, body exterior/interior, and aerodynamics. "Much of what you see, especially at the component level, represents the harbinger of what you'll see in the future from GM," said Robert Purcell, Executive Director of GM Advanced Technology Vehicles.
Precept's propulsion system has an electric traction system in the front, and a heat-engine traction system in the rear. The electric traction system combines an electric motor, single-stage planetary gear reduction unit, and differential in one assembly. The vehicle's liquid-cooled permanent-magnet three-phase ac motor is rated at a peak power output of 35 kW (47 hp) and a continuous output of 18 kW (24 hp).
"We put the electric motor up front to allow for narrower front rail spacing and a greater maximum steer angle," explained Bill Shepard, Total Vehicle Integration Engineer. The hybrid power modules use generation III Advanced Insulated Gate Bipolar Transistor technology. The first generation 600-V/750-A power module required six units—each about 30 cm2 (5 in2). Generation III is a single 600-V/400-A unit.
Three main elements compose the heat-engine traction system: a compression-ignition direct-injection (CIDI) engine; a multi-purpose unit with PIM that serves as electric motor and generator; and an automatically shifted manual transaxle.
Created by Isuzu, the CIDI engine is a three-cylinder, 1.3-L, 12-valve unit delivering a maximum of 40 kW (54 hp). The engine never runs below 1500 rpm. In 0.2 s, the multi-purpose unit reaches engine spin-up, and in 0.7 s the engine starts. The CIDI engine starts in any gear, and clutchless shifting is possible. The multi-purpose unit mates to the CIDI engine via the transaxle. It provides 10 kW (13 hp) of power to help propel the car, crank the engine to its 1500-rpm starting speed, drive the HVAC compressor when the engine is switched off, and synchronize gear shifts. In generator mode, the engine can drive the multi-purpose unit to charge the battery, or the multi-purpose unit can produce power by regenerative braking.
Electrical power feeds many of the accessories features. "Some of the accessory features were designed as efficiency enablers, including the ABS with 4-wheel regenerative braking; panoramic rear-vision system; slot antenna; LED signal and interior lighting; halogen HID headlamps; 42-volt heated windshield, blower motor, and cooling fans; and lightweight full range speakers," said LoGrasso.
The base design for the automatically shifted manual was the five-speed transaxle used in the Opel Astra. "The transaxle is a brand new development. There is no reverse gear; we use the front motor (electric traction system) to put the car in reverse," Advanced Traction Manager Mark Kosowski said, adding there is also no fifth gear.
A power PC processor-based controller handles the entire hybrid propulsion system. The 32-bit, 266-MHz device channels two-way communication with the accelerator and brake pedals, shifter, energy management system, electric traction system, multi-purpose unit, and the controllers for the heat engine, transmission, thermal system, and brakes.
The demonstration Precept will test two battery types: advanced nickel/metal hydride (NiMH), and the world's first application of a fully solid-state lithium polymer battery (LPB) in a hybrid vehicle. In the NiMH test application, 28 modules with 10 cells each comprise the 89-kg (197-lb) battery pack located under the front seats. "We wanted to put the batteries in a space that was otherwise poorly utilized," said Shepard.
The thermally insulated stainless steel encased LPB operates with an internal temperature of 50-85°C (122-185°F). "Each of the seven modules has its own control and monitoring system. It's a very integrated architectural design," said GM's Dennis Davis, energy storage engineer. LPB development work involved 3M, Hydro-Quebec, Argo-Tech, and the United States Advanced Battery Consortium.
Since Precept carries only 17 L (4.5 gal) of fuel, efficiency means the car must operate a system only when needed at the minimum amount required as well as bypass energy disposal by reusing or redirecting it. When possible, shortcuts to energy routing result in fuel dividends.
"It's important that fuel economy not take a big hit on hot days, so condensate removed inside the cabin while the air conditioner operates is put back to the primary air inlet," Shepard explained. The amount of energy needed to condition the ambient air coming to the interior is reduced up to 200 W (0.27 hp) compared to a traditional system. "We're just taking all the little bits of waste and utilizing it," Shepard said. Precept uses 16 heat exchangers for thermal functions, compared to six heat exchangers for a typical vehicle.
Another atypical aspect of the Precept is its 195-kg (430-lb) chassis hardware. A typical family sedan's chassis hardware (suspension, steering, brakes, fuel, and related equipment) adds up to as much as 450 kg (990 lb). Nearly every component in the short- and long-arm front/rear strut suspension is aluminum. Suspension componentry mass totals 34 kg (75 lb). Steering components, having a mass of 8.3 kg (18 lb), include carbon fiber composites for the steering column, rack housing, and intermediate shaft. The chassis corner assemblies, including friction brake components, have a mass of 46.7 kg (103 lb)—less than half the mass of corner assemblies used on a typical five passenger sedan. Precept mixes friction (brake-by-wire) and regenerative braking. The demonstration sedan is the first hybrid with dual axle regenerative braking to recover energy at all four wheels. Each of the 16 x 4.5-in aluminum wheels has a mass of 3.8 kg (8.4 lb), with the set of four wheels and tires totaling 41.4 kg (91.2 lb). The exhaust system (past the catalytic converter and including the muffler) is titanium.
Precept's 152-kg (335-lb) aluminum spaceframe is composed of 64 kg (144 lb) of stampings (44%), 32 kg (71 lb) of castings (22%), and 49 kg (108 lb) of extrusions (34%). Multiple joining processes resulted in 600 rivets, 1600 spot welds, and 15 m (49 ft) of MIG welds. Aluminum panels (doors, decklid, hood, front fenders, and quarter panels) account for 74 kg (163 lb).
Front and rear fascia and rocker covers are reinforced reaction injection molded composite parts. The roof is a structural composite panel with Kevlar reinforcement. Carbon-fiber composites total 20 kg (44 lb) and include front and rear bumper beams, headlamp and taillamp mounting panels, front storage tub, and select belly pans.
In the interior, a textured vinyl material covers the floor areas. The front seats—each having a mass of 9.7 kg (21 lb)—use powder-coated steel and aluminum frames to support mesh material on seat cushion and back. "That's 30% lighter than today's seats," said Tom Lobkovich, a body and interior engineer. The rear package shelf and headliner are made of expanded polypropylene, as is the substrate for the instrument panel and door trim pads. Total mass for the expanded polypropylene is 35 kg (77 lb).
The Precept's parallel hybrid traction system.
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Precept development involved GM's Advanced Technology Vehicles, Design Center, Research and Development, Aerodynamics lab, Opel, Isuzu, and several key suppliers. The concept showcases more than 130 technical innovations—44 records of invention. The technology demonstrator to be unveiled at the 2000 North American International Auto Show in Detroit is the forerunner of the 2004 production prototype.
Kami Buchholz
