Plugging in the Feds: The New Hybrids
By Chris Ellis
'President-elect' Barack Obama has promised that 'half of all cars purchased by the federal government will be plug-in hybrids or all-electric by 2012.' Some of this requirement can be met by Chevrolet Volts and plug-in versions of the Toyota Prius, but there is a need for plug-ins which can offer more, in terms of both size and performance. This article describes a generic powertrain for sedans the size of a Chevrolet Malibu, Ford Taurus or Dodge Charger and larger, and for all sizes of SUV. The article starts with the specification and then offers explanations for the various choices.
The plug-in battery pack needs to be modular, to provide users with a choice of AER (All-Electric Range), from some 20 miles at city speeds to over 60 miles on freeways at the legal limit. Assuming an average electricity consumption of 250 Wh per mile in the city, the available capacity of the smallest pack needs to be 5 kWh. Because the state-of-charge swing needs to be less than 75% to improve battery life, the nominal battery capacity will be around 7 kWh. The battery pack will be located under the trunk floor, in what was the spare wheel well. Consequently, run-flat tires will be fitted as standard.
The base engine could be an E85/gasoline version of a small I-4, similar to the 100 bhp unit in the Volt. The optional more powerful engine might be a turbocharged version of the same engine, producing some 150 bhp on gasoline and 180 bhp on E85. Perhaps surprisingly, the more powerful engine should use less E85 than the base engine when cruising because it will have properly implemented flexible fuelling (see Saab BioPower), unlike most other systems claimed to be optimal. Most systems adjust ignition and injection timing, but Saab uses variable boost turbocharging to modify the effective compression ratio to take full advantage of the higher octane rating of most biofuels. The result is that the 'mpg gap' between gasoline and E85 can close to the point where it costs less per mile to use E85.
The engine will be connected to the front wheels via a dual-clutch transmission (DCT), which combines the efficiency of a manual transmission with the ease of use of an automatic. The engine and transmission will be mounted in the conventional transverse position, under the hood. Because the DCT operates here as part of a parallel hybrid configuration, engine power can normally be delivered even more smoothly than a DCT or conventional automatic operating on its own.
The 'foundation technology' for the powertrain is a surge power unit, mounted inside the transmission tunnel down the centerline of the vehicle. The Malibu, Taurus and Charger are already available with a conventional 4WD system requiring a transmission tunnel which is large enough, with some minor fettling, to take a surge power unit. The surge power unit will, in its basic form, drive (and be driven, during regenerative braking) by the rear wheels, via the rear final drive. The sketch indicates where the key components will be located in the vehicle. The surge power unit will have an available energy capacity of some 500 Wh, with a peak output power rating of over 260 bhp (~200 kW), and be kept topped up by the plug-in battery, and the engine when the battery is flat. Most importantly, the surge power unit should have the ability to return over 60% of the kinetic energy of the vehicle during a full brake/accelerate cycle. This contrasts with the less than 35% returned by most current hybrid drives. For the FBI (and police?), a more powerful version should be available which the front as well as rear wheels, via a 4WD version of the DCT. Cars with V-8 engines will become so 'last millennium'.
It will be clear from the above that an electric-only version (BEV) could be achieved by replacing the engine and DCT with an additional battery pack in the engine bay. Another option will be to omit the plug-in battery, leaving the surge power unit to support a very effective 'fuel-only' hybrid version. Imagine how attractive this will be in countries like Brazil, where ethanol is cheap and produces little net CO2.
Because the base plug-in battery pack doesn't have to absorb surges of regenerative power or support peak acceleration, it needs to deliver or absorb no more than 25 kW continuously, allowing a choice of battery chemistry optimized for energy density and least-cost-per-mile, rather than peak power. The low power requirements will also help ensure long battery life.
In principle, the surge power unit could be based on a high-power motor/generator and battery (or ultra-capacitor), or a hydraulic system or one based on kinetic energy storage. Kinetic energy storage (i.e. using high-speed flywheels) is theoretically the most promising in terms of efficiency, power and, ultimately, production costs, and is expected to be used in Formula 1 racing next year. It may well be that the American Le Mans Series, with it new Green Challenge, will become the crucible for the kit that the likes of Scully and Mulder may be driving by 2012.
To those who love the argument 'series versus parallel', please note that the foundation technology advocated here supports both. For example, the IC engine could be replaced with a small gas turbine generator or a fuel cell stack with only minor changes to the surge power unit. However, in an extended-range electric vehicle (more commonly known as a plug-in hybrid) with a genuine 20-mile-plus all-electric range, almost all the city driving is going to be with the engine off. Consequently, most of the fuel used each year is going to be burnt at highway speeds, where most engineers agree that a parallel connection is more efficient. While there is a sound argument for a series hybrid configuration in a fuel-only city runabout or transit bus, the addition of a substantial AER to something like a (modified) new Honda Insight should confirm that most plug-in hybrids will probably be parallel.
All of which suggests what a 'Mk II' Volt might be like. Let Saab/Opel develop a direct injection BioPower version of their little 1.0 liter I-3 already in production (they probably have done it already), delivering up to 120 bhp. Let it drive the front wheels only, via a 4WD version of a 'dry-clutch' DCT, and connect the 100 kW surge power unit to the DCT via the DCT's rear drive shaft. This combination can then provide up to 250 bhp in 'Sports Mode', because the power of the surge power unit and the engine are additive in this parallel configuration, unlike a series hybrid. Arguably, there could be a cheaper entry model with a basic version of the same engine, delivering only some 70 bhp. Combined with the surge power unit, this could still provide up to 200 bhp for acceleration. Now add your choice of plug-in battery module, to provide 20, 40 or 60 miles of all-electric running.
Several years ago, in an article in HybridCars, I suggested there is, despite all the doubters, a 'silver bullet' which could give the United States the ability to overcome the problem of its dependence on imported oil. It's called a Plug-in Biofuel Hybrid (PBH), preferably running on renewable electricity and 2nd-generation biofuels. Just like a rifle cartridge, it has several key components - the engine, the plug-in battery and the surge power unit, and they are all going to need further work before we have a fully effective solution. However, it seems that the next President already understands what is needed, and is determined to see that we get it. Let's all hope so.
For more on this topic, take a look at 'Riding to the Rescue in an Opel X' on EV World.
For the big picture, as painted three years ago, you might like to read (with your sense of humor fully engaged), part 3 of 'The Cars of 2015'.
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