Cutaway view of
GM Precept hybrid-eletric concept
Designing a full-sized family sedan that achieves PNGV's goal of 80 mpg is a daunting engineering challenge, one which GM accomplished in the form of the landmark Precept hybrid-electric car.

Building The Precept

An Interview with Bill Shepard, Program Manager, GM Precept Concept Vehicle

By Bill Moore

07 June 2000 -- Developing the technology to build a full-sized passenger car that gets 80 miles per gallon (2.62 liters/100 km) doesn't come easy, but having built the EV1 battery electric car helped the folks at GM's Advanced Technology Vehicle (ATV) center in Troy, Michigan get a definite "leg up" on their Dearborn and Auburn Hills competitors, according to Bill Shepard, the program manager for the GM Precept.

The Precept is General Motor's response to Vice President Al Gore's 1993 challenge to develop a new type of passenger vehicle for the 21st Century, one that could deliver up to 100 mpg fuel efficiency and reduced emissions. Called the Partnership for a New Generation Vehicle (PNGV), the joint private industry, public agency partnership has led to the development of technologies that will eventually find their way into the cars and trucks of the new century.

ATV Enters The Fray

Bill Shepard told EVWorld.Com that it wasn't until 1996 that ATV got involved in the development of the Precept concept vehicle. Prior to that, he said a lot of groundwork had to be done to develop the necessary underlying technologies that would go into the Precept and its competitors, the Ford Prodigy and DaimlerChrysler ESX-series of hybrid-electric cars.

There was, "a lot of work with the government agencies lining up research and development that was needed at the various federal laboratories," Shepard stated. " There was a lot of component and system level development that took place early on that enabled us to have some features and some systems all geared toward generation of the highest fuel economy at our disposal when, the Big Three, sat down design these cars beginning about '96."

Taping Its EV1 Expertise

With the EV1 design and development program nearing completion in 1996, Shepard said GM "rounded up" many members of that team to begin work on designing the PNGV car that would eventually become known as the Precept.

"Fortunately, we had a cadre of people here that had just gone through that program that had many of the same priorities in terms of efficiency, trying to get the last mile out of every little bit of energy possible, which obviously was the same strategy that we needed to use in going into the PNGV program."

While the goal of the EV1 program was to wring out the maximum mileage out of a battery charge, the goal of the PNGV was to extract the maximum amount of energy from a given unit of fuel. What that fuel would be, wasn't yet known though all three carmakers would eventually settle on advanced diesel engines called CIDI (compression ignition, direct injection) because they offered a 15% improvement in efficiency over the best gasoline engines.

"We had some core expertise, as we call it, in electric drive and controls that we were able to immediately apply to the parallel hybrid task. We also selected an aluminum body structure strategy that also carried over from the EV1 and various elements of the efficient chassis design; going for lowest rolling resistance from the tires, for example."

From this point forward, GM's PNGV team proceeded "very deliberately," according to Shepard.

"We tried to make sure that we got every idea possible up on the table that could further fuel economy and heard from everyone in the process."

Going The Extra Mile... Plus Nine

By admission of everyone involved in the PNGV program, achieving a consistent 70 mpg performance in a full-sized, five passenger vehicle is a challenge. The Prius only get's its 66 mpg in simulated Tokyo urban driving conditions of an average 12 mph. More typically it delivers combined averages in the low to mid 40 mpg. The Honda Insight appears to be regularly giving its new owners averages in the 50-55 mpg range, though on the highway, it can deliver its 70 mpg numbers. But it is only a small, two-seater, not a full-sized sedan.

So, for Ford and DaimlerChrysler to create 5-passenger vehicles with 70+ mpg fuel efficiency is, in itself, no mean feat. But the folks at GM's ATV center were convinced they could do better, though there would be some obvious tradeoffs, like unconventional engine placement in the rear of the car and very little storage space in the front of the car. (Original VW Bettle owners will appreciate this situation).

"We thought that we could learn more as a corporation in exploring the very highest levels of propulsion architecture required to get that extra 10 miles per gallon. I believe we could have come up with a vehicle that was more similar to those of our partners as Ford and DaimlerChrysler, for example a front mounted heat engine and less hybridized configuration... a smaller electric drive, driving the front wheels and could have achieved levels of performance in the 70's (mpg) range, but again, we thought we could learn more by going all the way to 80 (mpg)."

"We knew we would have to do things differently. We knew we would have to make compromises. We would have to do something that would be more costly..." Shepard stressed that all of the PNGV cars are concept vehicles only. They are not intended for production, that is the next phase of the ten year-long PNGV program. Instead, these vehicles are test beds for exploring the technology, not their manufacturability or marketability. GM's strategy was that once they had achieved the absolute maximum performance based on current technology, it would be easier to "back off" on the design for commercialization purposes rather than having to push the technology even future. The result would be an affordable, high mileage vehicle that could still out perform the competition.

Check Your Paradigms At The Door

Apart from starting with a basic foundation of knowledge gleaned from the EV1 program, Shepard told EVWorld that everyone was told to "check their paradigms at the door."

"We wanted to make sure that no one was bound by their experiences, even on the EV1... We had to have absolutely an open mind of where we might go from here," Shepard commented. GM's ATV engineers used what he called the "boundary study method," which explores one particular vehicle variable at a time, irrespective of its influence on other variables.

"Focusing on just one attribute at a time allowed us to push the envelope in each direction further out," he said. This process went on for about seven months at the end of which the engineers had a lot of hypothetical vehicles that excelled in one particular area such as cabin comfort or high mileage. Now it was time to find the ideal compromise that would lead to a more balanced vehicle, one that admittedly didn't do as well in any particular category, but offered the best blend of all around performance, comfort and safety.

Pushing the MPG Stratosphere

Shepard explained that there has to be very careful, close integration between all parts of the vehicle design process. He said that you can't tell the body designer to go come up with a design and the power plant guys an drive system and then mate them together as you might in a more conventional vehicle. Instead, there had to be very close collaboration between all the development teams.

As a result, the Precept incorporates something on the order of 130 innovations, each of which contributes to the overall efficiency of the design.

One of the biggest percentage contributors is the Precept's four-wheel regenerative braking system which Shepard estimates adds "several" miles per gallon in the urban driving cycle. One of the payoffs for this approach is not just improved fuel economy, but reduced brake wear. According to Shepard in a typical urban environment where drivers have to do a lot of braking, virtually all of it can be handled by the car's regenerative system which recaptures the vehicles inertia and converts it to electricity, as well as slowing the car. This will greatly prolong the service like of the friction brakes on the car.

The Precept's rear-mounted CIDI "heat" engine is coupled to a second electric motor which has six separate functions including starting the heat engine, synchronizing the manual transmission, providing regenerative braking and additional horsepower for accelerating.

Digital Displays

Another interesting innovation is the Precept's all digital instrument panel which resembles the display on a laptop computer. Not only does it display your usual set of instruments like speedometer and fuel gauge, but it also incorporates photo "stitching" technology which take the images from three video cameras and blends them into a single panoramic image of what's behind and beside the vehicle. The tiny cameras are mounted in the place of the side and rearview mirrors of the vehicle, saving one to two miles per gallon, too boot.


07 June 2000 -- Despite having two electric motors onboard the Precept, the car was not designed to operate as a ZEV (zero emission vehicle), Shepard explained. While the electric drive is powerful enough to "launch" the car from a stop up to about 8 miles per hour, the battery pack is not large enough to sustain the vehicle for any distance. Instead, once the car begins to move forward under electric power, the CIDI engine kicks in to provide the main motive power for the vehicle.

However, while the car may not be a true ZEV in the strictest sense of the word, it does make the most of its hybrid-electric architecture. Shepard pointed out that with the exception of the Precept, all other EVs and HEVs -- including GM's own EV1, as well as the competing PNGV cars - only have regenerative braking on one axle, while the Precept has it on both axles. He said that this allows the car to recapture more of its own kinetic energy, further boosting its efficiency.

Conventionally Unconventional

While both Ford and DaimlerChrysler chose to put their power plants in the front of their CIDI-electric hybrids, GM took the more controversial approach of mounting their "heat" engine in the rear of the car. Shepard explained that early on there was debate within the development team over this choice. It would compromise an important selling point for many consumers, ample trunk space. But the team felt it was more important to push the proverbial design envelope, wringing out the highest mileage performance possible. Shepherd stated that since the Precept was only a concept vehicle, one no destined for production, preserving trunk space wasn't as critical as achieving maximum fuel efficiency.

In the miles-per-gallon race, at least, the decision paid off. Placing the engine in the rear allowed GM to minimize the vehicle's drag to the point that it is the most aerodynamically clean 5-passenger car ever built with a drag coefficient of 0.16. This is not only cleaner than the smaller, two-passenger EV1 (0.19), but it's even better than an F-16 jet fighter plane.

From the perspective of the driver, the car's interior is a little more conventional, but even here GM's engineers exercised their weight-saving, energy-minimizing creativity. The front seats of the Precept resemble more an expensive lawn chair than they do the typical, deeply padded automobile seat. The back and bottom is made of a synthetic mesh material you can see through. Not only does this save weight, but it also reduces the cooling load on the car, allowing cabin ventilation to reach the driver and his passenger's backs and backsides.

A Hypothetical Fuel Cell Precept

Not only did GM construct a CIDI-based hybrid-electric version of the Precept; they went a step further and constructed a second prototype, this one powered by a fuel cell. However, Shepard was quick to point out that this vehicle is a "hypothetical" engineering exercise on GM's part and is not meant to imply that the company is ready to build a fuel cell-powered vehicle. Instead, the company wanted to explore the various packaging issues associated with installing such cutting edge technology.

One benefit of the rear-engine design is the large radiator surface area it affords on the rear-quarter panels of the vehicle. Resembling honey-combs at the rear of the car, these air intakes not only provide the Precept's CIDI engine with amble radiator surface area while minimizing performance-robbing drag, it also smoothes out the air flow behind the vehicle. And when you install a fuel cell, it pays an added benefit. Because the fuel cells contemplated for cars like the Precept will be in the 50 to 70 kW ranges, they will require more heat dispersion surface area than a conventional internal combustion engine of comparable power. Having large, rear-mounted radiators would let GM's fuel cell disperse its heat with little added aerodynamic drag.

The fuel cell Precept also boosts hydrogen metal hydride storage. Using metal hydride powders as the storage medium for pure hydrogen gas could solve one of the most niggling problems vexing engineers, how to store enough hydrogen, the lightest of all gases, to provide decent range for the vehicle. Both Ford and DaimlerChrysler's fuel cell prototypes have had to rely on either compressed hydrogen stored in bulky cylinders in the trunk of the car or in super-cold, liquid hydrogen. Neither approach would give a fuel cell car much range. In the case of compressed hydrogen, a few dozen miles at best and liquid hydrogen perhaps a 100 miles or so. Storing volatile hydrogen gas in these inert metal powders eliminates any change of an explosion or fire. In addition, Shepard said that depending on the size of the hydride storage tank, a car similar to the Precept could travel up to 500 miles between refueling stops with a pair of tanks small enough to fit under the rear seats of the car. Best of all, given the improved efficiency of the fuel cell over the CIDI engine, a "hypothetical" fuel cell Precept could get the equivalent of 100 miles per gallon of fuel.

However, before we get too excited, Shepard pointed out that there is currently no publicly accessible hydrogen-fueling infrastructure anywhere in the world. So, not only do fuel cells and hydride storage systems have to mature apace, so does the necessary fueling infrastructure.

In the Blink of an Eye

Driving the Precept will initially feel a little different for owners used to the subtle but reassuring vibration of an internal combustion engine. Like the Prius and Insight, the Precept automatically shuts down its heat engine when the vehicle is at a stop, saving fuel and reducing emissions. Shepard remarked that the car feels a lot like the EV1 when it starts out, but at 8-10 mph, the CIDI engine comes alive. He said drivers might notice a slight transition between the two propulsion systems, but he believes it is less noticeable than in other hybrids like the Prius. And it is fleeting, indeed, on the order of two tenths of a second or the blink of an eye.

According to Shepard, the Precept meets PNGV's acceleration goal of 0-60 mph in 12 seconds. While he admitted this isn't particularly blistering performance, it is typical of similarly sized vehicles with baseline engines.

He added that the car also meets all federal crash safety standards, something some observers have expressed concern over because of the car's rear engine placement.

The Emissions Conundrum

While the CIDI engine offers a not-to-be-ignored 15% improvement in fuel efficiency over a comparable gasoline engine, it also has some serious emissions hurdles that will have to be overcome, Shepard explained. The new generation diesel engines which have found their way into all three carmakers PNGV vehicles are vastly improved over their predecessors, he stated. New compression ignition, direct injection engines are less noisy, product less smoke and are more drive-able. However, microscopic particulate matter and NOx (nitrogen oxides) are a major concern, specially since the EPA's new Tier II emissions standards have lowered the bar even further, a bar under which none of the current PNGV vehicle can pass.

"It (CIDI emissions) remains an issue," he admitted. "We acknowledge that we cannot meet the very strictest emissions standards that have been proposed as part of the federal Tier II emissions strategy. It is something that is going to have to be worked on. There are after-treatment strategies that are being worked on: particulate traps, things of that sort that could, conceivably make compression ignition technology even cleaner than it is. But it is tough to say that it will ever be equal to the spark-ignition engine."

Money Well Spent?

Has the money spent by both the car companies and America taxpayers been well spent? Shepard is unequivocal in his response. "Absolutely! There is so much learning coming into the industry. And when I say industry, I mean not only the Big Three, but the supplier community, the federal research labs. That money isn't just focused at the Big Three. The entire industry is learning from this experience, academia included. The universities have been funded out of the PNGV program."

Shepard said that the end objective of the PNGV program is to take the knowledge and breakthroughs and apply them to commercial products. "We know we have to get this into the hands of the consumer." He also added that the funds to build the Precept did not come from federal taxpayer dollars, but was funded entirely by General Motors. The same applies to Ford and DaimlerChrysler, all of whom paid for all of the development costs of their respective hybrids.

Precept's Progeny

Putting the Precept on the road is only the end of the beginning in terms of the ultimate goal of PNGV. GM, Ford and DaimlerChrysler have committed to the final phase of the 10-year-long effort by developing serious, consumer-ready, pre-production prototypes. Here the focus shifts to manufacture-ability and cost containment, goals just as daunting as reaching 80 miles per gallon, if not more so.

"We want to get to making a business out of advanced-class vehicles," he stated. "It's a matter of taking the very focused, singular vehicle objective and pushing fuel economy out into family passenger cars but it's also spreading that learning into the core products incrementally as we're able to come maturity of development for some of these systems we've worked on, as well."

Times Article Viewed: 9015
Published: 01-Jan-2000


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