GM Introduces the Precept

Automotive Manufacturing & Production, Jan, 2000 by JOHN McCORMICK

By their nature, long-term projects take a long time to materialize. The PNGV (Partnership for a New Generation of Vehicle) program was started in 1993 with the idea of developing a production-ready, mid-size sedan by 2004 that can achieve 80mpg, while still meeting all of the other constraints provided by federal regulations, consumer preferences, performance and cost.

General Motors, Ford and DaimlerChrysler are all approaching this challenge in their own ways and elements of their work have been emerging slowly. However GM has just opened a sizeable window on its own PNGV concept

Dubbed Precept, this demonstration vehicle debuts at the 2000 North American International Auto Show. Precept is a remarkable effort, starting with an exterior design that has taken the coefficient of drag (Cd) to a new low for a five-passenger vehicle. Engineers rolled out the clay models in advance of the other, internal technologies apparently because they just couldn't wait anymore. The Cd on the Precept is only 0.163: half of the average mid-size sedan and even lower than GM's two-passenger EV1 electric car, which has a Cd of 0.190.

To hit this record-breaking level, engineers started with a rear-engine design. Engineers say the front-engine layout went against the grain of the ultimate aerodynamic shape, the teardrop design. Now, with the engine in the trunk, all of the thermal management could also be sent aft.

Instead of a drag-inducing front radiator air intake, intakes for airflow sit just behind the rear wheels and exhaust runs out through a vent spanning the back-end of the vehicle. Whatengineers found is that this intake/exhaust system not only eliminates the drag coming in, but actually reduces the drag caused by the wake moving out. A thermal map of the exhaust shows that, in effect, the airflow completes that coveted teardrop design.

The efficiency theme continues under the largely aluminum skin, where engineers crafted an aluminum spaceframe structure that weighs in at 334.4 lb., a 45% reduction from a traditional steel model. Engineers could have gone with exotic materials to really shake out weight, but remember that cost will eventually be one of the parameters PNGV designs need to own up to in 2004.

Rightnow, the Precept only meets crash regulations for the year 2000. Tom Lobkovich, Body Engineering Manager at GM and his team will be back at the design board when the second phase of the project kicks in for 2004. "We would, in the course of executing 2004 prototype vehicles, address (the safety) issue. But I can tell you our experience in developing the EV1 which meets all of the FMVSS safety standards as well as NCAP, basically indicates that you can certainly do aluminum structures to meet all of the applicable safety standards."

Under the hood (actually the trunk), GM is using a hybrid diesel-electric powerplant hooked up to a complex dual-axle regenerative system using both heat and electric traction to boost efficiency. Engineers say right now, their battery specification is nickel-metal hydride (NiMH) packs provided by Ovonic Battery Co., a joint venture with GM. The 28 battery modules weigh 197lb. and produce more than 43 kW of power. A liquid cooling system circulates between the battery cells, maintaining an optimum operating temperature.

But there is another contender in the battery market: lithium-polymer batteries (LPB). The technology, developed under the auspices of another government program, the United States Advanced Battery Consortium (USABC), may give the NiMH technology a run for its money. The LPB battery is a solid cell made of a metallic lithium anode, a dry conductive solid polymer electrolyte and a vanadium oxide cathode backed with an aluminum current collector. As such it will be a lot safer in a crash because there is no volatile solvent. Also, the LPB works much better with packaging constraints.

On the Precept, engineers designed the LPB pack to sit under the vehicle floor pan with a total height of just 179 mm, a length of 1295mm and a width of 516 mm.

A decent battery isn't the end of the equation. Engineers are tapping every possible resource throughout the vehicle to optimize the entire system. Front wheels use an electric traction system: a combination of an electric motor, a singlestaged planetary-gear reduction unit and a differential, all in one single assembly. The three-phase alternating current motor puts out a peak 35kW to conventional steel half-shafts hooking up with each wheel. In return, the motor also sends power back to the batteries during regenerative braking.

At the rear, the wheels are driven by three major elements: a compression-ignition, direct-injection (CIDI) diesel engine; a multi-purpose unit with a power-inverter module and an automatically-shifted manual transaxle. The 1.3-liter dual-overhead cam diesel, developed by Isuzu, is a four valve per cylinder design with a variable nozzle turbocharger. Output is 40 kW (54 hp). Other innovations include the direct-injection common rail system chosen for its low emissions, and a servo-controlled air inlet valve that sits downstream of the compressor's outlet. This valve shuts down the engine quickly when it is not in operation and also reduces airflow during startup, reducing the energy demand during cranking.