The Cars of 2015 – the 'Prequel'
By Chris Ellis
The 'Sequel', the latest prototype in GM's fuel cell program, has recently been delayed because of "the cost of financing", according to Larry Burns, GM's VP, R&D. This brief paper suggests that GM might care to consider a 'Prequel'.
The Prequel would be aimed at the same Sport Activity Vehicle (SAV) market as the Sequel, but would take advantage of all that we now know about aerodynamics, biofuels, hybrid technology and suspension systems. In other words, the Prequel is a cross between a Prius and an Escape hybrid, but with a flexible fuel engine capable of running on any mix of gasoline and E85, and with the option of a plug-in battery. The paper also suggests why, if the Prequel can be this affordable, environmentally friendly, economical to run and just plain desirable, there may never need to be a sequel.
The 'skateboard' which contains the fuel cell stack, hydrogen cylinders, controllers, etc, of the Sequel is 11 inches thick. Imagine now a Prequel with identical cabin dimensions, but with the cabin, and therefore the roof, ten inches nearer the ground. Compared with a Prius, it would be both roomier and 2 inches lower.
Assume that the coefficient of drag of the Sequel and the Prequel are the same, at 0.28, as is their weight (1500 kg, 3307 lbs), and they are running on identical tires. Assume that the 15% reduction in height reduces the frontal area of the Prequel relative to the Sequel by 15%. Then, if the power required at the wheels by the Sequel at a steady 70 mph is 18.2 kW, the Prequel is likely to require only 15.6 kW, some 14% less.
Chrysler Akino interior includes 'club seating' in the rear and swivel-out seats in the front.
The Prequel will be all-wheel-drive, like the Sequel. The Sequel drives the front wheels with a single inboard electric motor. The most significant difference between the GM EV1 and its prototype (the Impact) was that the Impact had two inboard electric motors, each driving a front wheel, while EV1s (and the Sequel) have a single large inboard motor driving both front wheels though a conventional differential. The main concern that caused the change was the possibility of sudden motor failure throwing the vehicle off the road or into the path of oncoming traffic. This is one reason the Prequel won't adopt the rear 'wheel motors' of the Sequel.
The second reason is that wheel motors inherently increase unsprung weight, which most suspension designers strive hard to reduce, and will pay extra to achieve. The nearest example is the Prius, which is fitted with expensive magnesium alloy wheels as standard to shave a few pounds off the unsprung weight at each corner. The principal reason why the Prius has magnesium wheels is to improve the ride, not to reduce fuel consumption by less than 1%.
Of course, the ride could be improved further by bringing the disk brakes inboard. This is far from novel, either on the track or on the road. One reason why the Jaguar XJS had such a good ride for a sports car was its inboard rear brakes. However, the Prequel will make do with outboard disks all round. The front disks will be conventional items, but, given the light braking loads at the rear, reduced further by powerful regenerative braking, relatively thin unventilated aluminum brake rotors will be used at the rear, plus aluminum calipers, to keep unsprung weight to a practical minimum.
The engine will be almost identical to the 180 bhp BioPower unit already offered with the latest Saab 9-5 in certain markets, except that it will be mounted at the rear and almost flat, under the floor of the trunk. The transmission will be a five-speed AMT. The Surge Power Unit will be mounted under the hood and have an available energy capacity of over 300 Wh and a peak power rating of over 150 kW (200 bhp). The optional plug-in battery will have a usable capacity of 7 kWh, sufficient to support engine-off running for up to 30 miles at up to 65 mph, and the associated electric motor will have a continuous power rating of 15 kW. In this configuration, the Prequel qualifies as a PBH (Plug-in Biofuel Hybrid capable of over 300 mpg of gasoline), with all the associated tax credits, access rights, etc.
The Sequel uses A arms all round, but they need to be relatively short to save space. In the Prequel, the A arms can be much longer, almost as large as those fitted to an IRL or Formula 1 race car, to give superb handling and ride. They also facilitate the variable ride height system (VRHS), which allows the Prequel to cut its ground clearance to just over four inches above 60 mph yet raise it to over 12 inches if the driver requests it and the speed is less than 5 mph. Between 5 and 60 mph, the ride height is between 8 and 6 inches. The maximum ground clearance of the H1 Hummer is 16 inches. All other Hummers have less than 11 inches of ground clearance.
|0 – 60 mph||<7 secs||<10 secs||<11 secs|
|CO2 g/km||<30?||0 to 150?||104|
GM had better get going, because Ford have already shown their SAV concept. www.americancarfans.com/news.cfm/NewsID/2050301.013/ford/1.html
Imagine Ford's SAV with a powertrain and VRHS similar to the Prequel. How could the fuel cell Sequel compete with that? More affordable? Not this side of 2020. Better performance? Clearly not, unless the Sequel gets the same PowerBeam surge power unit. Better looking? If you prefer 'plump', maybe. More environmentally friendly? As, but not more. Cheaper to run? The price of hydrogen will need to be low, very low.
GM could have something like the Prequel in customers' hands by 2009. But that would result in so many bruised egos that it probably won't happen. Not GM then, but Ford haven't invested anywhere near as much emotional capital in fuel cells. Nor have Toyota. ("What are we going to do for an encore, Chief?") Meanwhile, over in Germany......
When GM eventually decides to build something like the Prequel, consider how easy it would be to offer an 'EV2'. Out comes the engine and AMT. Assuming the EV2 is required to be able to cruise at no more than 80 mph, then the electric motor needs no more than a 25 kW continuous rating, which implies a peak power of about 40 kW. Assuming that anyone who frequently travels more than 100 miles a day will buy a Prequel, with or without a plug-in battery, then the EV2 doesn't need to offer a range of more than 100 miles. This indicates a usable capacity of around 25 kWh. When the EV1 was on the drawing board, lead acid was the only realistic battery option. By 2009, there will be several more. One that already has the toughness and durability is the Zebra.
With the same PowerBeam SPU as the Prequel, the EV2 will be able to muster a total of 200 kW, adequate for most SAV users. Because the SPU insulates the battery from the shock loads of regenerative braking and acceleration, the battery may well last eight years or more, despite being subjected to thousands of deep discharges. The SPU also allows the electric motor to avoid operating at high power at low rpm, when it's efficiency may be less than 80%, thus further improving system efficiency and reducing the energy required per city mile.
The shadow of the EV1
From GM's perspective, the investment required to be able to offer EV2s in addition to Prequels is very low. However, it would be a great shame if 'once bitten, twice shy' was proved true yet again. Let's hope Bob Lutz, who wasn't around to be bitten last time, will see off the dogs this time!
For background on the Prequel go to www.thepowerbeam.com/CARS2015.pdf
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