Tuesday, January 12, 2010

More Electric

In aviation circles, the term "More Electric" points to redesigning aircraft systems to run off electric power, as opposed to hydraulic or pneumatic. The idea is that wires and electric motors can be lighter, more efficient, and more reliable than the piping, valves, and various other moving parts associated with fluid power systems. And since weight and downtime are two very large enemies of aircraft designers and operators, we're starting to see a trend in the more electric direction.

For example, the Airbus A380 was the first commercial aircraft to utilize pure electric actuation to deploy engine thrust reversers and to hold open engine cowlings for servicing. The Boeing 787 goes a few steps further in doing away with bleed air systems from its engines. In its place, large starter-generators provide the electrical power for de-icing, cabin pressurization, and climate control. In addition, wheel braking is a pure electric application in the 787. And both these aircraft utilize electrohydrostatic actuation, where electrical power is used to create localized hydraulic pressure in what is essentially a hybrid of the two methods.

With the 2010 North American International Auto Show kicking off this week in Detroit, it's easy to see that the world's automobile manufacturers are moving in a more electric direction as well. In the realm of the car there are many good reasons for this shift, and also the potential to take the movement even further than in aviation.

For decades the simplest (cheapest) means of powering systems and accessories in an internal combustion-powered car was to bolt on a unit to the engine and spin its shaft with a belt connected to the crankshaft. Power steering, braking, air-conditioning, engine cooling, alternators, and superchargers were all "tacked-on" in this fashion, and in the century of cheap gas this worked just fine. Energy was converted into shaft power from a chemical source (gasoline), then converted again into electrical or hydraulic power, and then finally dissipated through work. It was easy to understand and repair, and it got the job done.

That said, much of the gasoline's chemical energy was wasted in this process. In essence, each of these individual systems was designed for peak power demands, and hence oversized for the majority of daily driving conditions. Having a shaft, pulley, and belt for each component added weight and moving parts and increased the overall number of conversion stages, which in turn increased overall losses. And having to idle or dump excess energy when demand slacked introduced another source of inefficiency.

Moving in a more electric direction mitigates a number of these issues by replacing fluid and shaft power with larger alternators or starter/generators. With increased electrical generating capacity, more systems can be run using electrical power. As shaft-to-electric power conversion is extremely efficient, there are fewer losses at the conversion stage. Since all systems are now running off the same electric "currency", load management is made easier, and the sizing of the generating device can be optimized for a smaller power requirement. And since electric motors do not have to spin while  idling, parasitic power requirements are essentially eliminated.

Additionally, more electric architecture provides the capability for cars to convert kinetic energy into electrical energy with only a few tweaks. We've already added a larger generating device to the engine, so if we can couple that to the driveshaft in some fashion we can get a regenerative braking system almost for free. It would require a slightly larger battery to collect the energy, but that's a pretty easy weight-vs.-efficiency trade to complete. And hey, generators are essentially motors run in reverse, so with a larger one attached to the engine and connected to a bigger battery, idle start-stop is only a short step away.

This is the basic idea behind the GM "mild" hybrid system, and while gains achieved weren't dramatic, they were notable and at minimum additional cost. Honda's Integrated Motor Assist (IMA) hybrid system operates on the same basic principles, but with an electric motor large enough to provide motive torque. IMA was also developed as an optimized system, with engine, motor/generator, and battery sizing all interrelated to create the most efficient combinations possible. Toyota's Hybrid Synergy Drive decouples the concept further, adding a complex transmission to connect engine, motor/generator, and drive wheels, allowing driving and driven components to be optimized in real time to maximize overall efficiency.

As cars don't have to fight gravity nearly as much as aircraft, the lower energy density of batteries and hydrogen vs. hydrocarbon fuels isn't a limiting factor in choosing a powerplant. Many auto manufacturers are looking to a future where primary propulsion is provided by one or more electric motors. Advantages in noise (no combustion = quiet), maintenance (bearings are the only contacting moving part), performance (maximum torque available at 0 RPM, and linear torque curve with speed), simplicity (higher RPM range can eliminate the need for multi-gear transmissions), weight (higher power densities), size (higher power densities), emissions (there aren't any), and point efficiency (electric motors are upwards of 95% efficient) are compelling. Electric motors don't care where their voltage came from, so primary generation can come from solar, wind, geothermal, hydro, nuclear, coal, gas, oil, treadmill, whatever. Local energy storage is the big question that still remains, and I'll save that for another discussion.

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