High Voltage

Hybrid cars provide a glimpse into the future of green aircraft.

Volt

Volt

** The Volt packs 700 pounds of lithium-ion
batteries under the floor. These batteries will
take you 40 miles.**

(December 2011) For the past week I have been driving a Chevy Volt. The Volt, as you are undoubtedly aware, is a plug-in hybrid — GM's, and I believe America's, first mass-produced car of this type. I was staying at Truro, Massachusetts, on Cape Cod, and it was fun to make a 25-mile round trip to Provincetown for breakfast every morning without ever using a drop of gas. No doubt the euphoria of independence from the gas pumps to which I have been tethered all my life would eventually have worn off, but during my brief affair with the Volt, I loved it dearly. We took our host out to dinner to make up for the blip in her electric bill.

Why am I telling you this? Because we are entering the age of the plug-in hybrid airplane.

A hybrid power plant is generally defined as one that makes use of two (or more) energy sources. A diesel-electric locomotive or ship is not, in the current sense, a hybrid vehicle (unless it also has batteries), because the electric motors that drive the wheels or screws get their power directly from the diesel-fueled motor-generator, or “genset.” Electricity is never an independent source of power, as in the Toyota Prius, which starts up and begins to accelerate on battery-supplied electric power alone before its gasoline engine cuts in, or the Volt, which can go 40 miles or so on an overnight charge taken from a household wall outlet. In the current understanding of the term, a hybrid vehicle is one that has both an internal-combustion (IC) engine and an electric motor and can be propelled by either one or, optionally, both at once. (To clarify the terminology, an engine, strictly speaking, consumes fuel; a motor gets power from an external source, such as electricity or hydraulic pressure — and turns it into motion.)

Hybrid IC-electric power systems are not new; indeed they are nearly a century old. They were used in submarines before World War II: Diesel engines would propel the boat on the surface and charge its batteries at the same time; submerged, it ran on battery power alone. The motive in that case was obvious; the diesel engine needed air to operate. The main purpose of the hybrid power system in the nonsubmersible Prius is different: to enhance fuel economy both by recovering energy that would normally be lost as heat in braking and by allowing use of a smaller, more efficient gasoline engine for cruising. It is because of the energy recovered during braking that hybrid cars get superior fuel economy in stop-and-go driving but offer a smaller advantage on the open road.

The Volt has the same energy-recovery features as the Prius but goes a step further. It can connect to the grid, obtaining power at about a third to a quarter of the per-mile cost of gasoline. In an effort to emphasize the Volt’s great virtue, which is that it possesses, thanks to its gasoline engine, the same range as a conventional car, GM refers to it not as a plug-in hybrid but as an “extended range electric vehicle.” Whether the Volt is an electric car whose range is extended by a gasoline engine or an IC-engined car whose efficiency is enhanced by an electric subsystem is a matter of perspective. In any case, the Volt is truly a hybrid.

The Volt’s 700-pound subfloor pack of lithium-ion batteries stores 16 kilowatt-hours of electrical power. A kilowatt is one and a third horsepower, so this equates to an output of about 21 horsepower for an hour. To optimize the battery life, however, only about 65 percent of the full charge, or 13 horsepower-hours, is available.

At 50 mph the Volt has, I estimate, about 48 pounds of aerodynamic drag and another 35 to 40 pounds of rolling resistance to overcome, which requires an output of 11 to 12 horsepower, or around 10 horsepower-hours to go 40 miles. The difference between 13 and 10 is due mostly to drivetrain losses.

The Volt has four seats. An equivalent airplane would be much cleaner, aerodynamically, than the car, but even at only twice the speed (a measly 87 knots) it would still require considerably more horsepower because the power required by an airplane increases nearly in proportion to the cube of its speed. The Volt’s 700-pound battery pack might keep it aloft for an anxious half-hour. So with current battery technology, purely electric propulsion does not seem practical for airplanes except in certain limited applications, such as self-launching sailplanes.

The Volt’s “extended range” scheme, on the other hand, might work for an airplane, if we assume that, with battery research going on at a frenzied pace all over the world, significant advances in energy density — kilowatt-hours per pound of battery weight — will be made. Suppose that an airplane had sufficient battery capacity to climb rapidly to altitude. It could then cruise on a relatively small motor-generator (a “genset”), perhaps a turbodiesel or a turbo-compound rotary operating at its most efficient speed and mixture, with fuel consumption 25 percent lower than that of avgas-burning reciprocating engines. Some of the genset power would feed back into the battery — which would never be fully drained anyway — providing a power reserve for climbs and go-arounds. The battery could also be recharged during descent — the aerial equivalent of a hybrid car’s regenerative braking — though recovery of more than a fraction of the energy used to climb would be impossible for several reasons: A propeller makes a poor windmill, some potential energy is used just to keep moving forward, and nobody would descend at the same aerodynamically efficient speed at which he climbed. On the ground, the airplane would be refueled with diesel fuel, cooking oil or Chanel No. 5 — whatever it took — and the battery pack would be pulled out and replaced with a freshly charged one, or perhaps rapidly recharged while the aeronauts had lunch.

Such a propulsion system flies — pardon the pun — in the face of the dictum attributed to Ford Trimotor designer William Stout, “Simplicate and add more lightness,” a phrase which, parenthetically, would have been lightened, and improved, by the removal of more. (The author-pilot Antoine de Saint-Exupéry similarly said that a designer knew that he had succeeded when there was nothing left to remove — a principle that he unfortunately failed to apply to his own books. The Little Prince should have been four pages long. But I digress.)

What justifies replacing a direct-drive IC engine with a system requiring both an engine and a motor, both batteries and fuel tanks, and complex control systems to keep the whole thing transparent to the pilot? It has to be a significant gain in efficiency.

This is where the calculations get tricky. The Volt, which like all hybrids bombards its driver with displays intended to let him know how green he is being, would end each fuel-free outing with the announcement that we had achieved 250-plus miles per gallon. Actually, this was too modest; we had used no fuel at all and had therefore achieved infinite miles per gallon. The EPA had quite a struggle to come up with a miles per gallon equivalent for partially electric cars. But it would have been a different story if we had been going 300 miles at 100 mph. It is claimed that a hybrid aircraft power plant would be 25 percent more efficient than a simple recip, but in the absence of practical experience I don’t think such estimates mean much.

Perhaps practical experience will soon be available. AeroVironment has built a high-altitude, long-duration unmanned aircraft using multiple electric motors and a Toyota engine, and at the Paris Air Show this year Siemens, EADS (the European aerospace conglomerate that includes Airbus) and Diamond Aircraft unveiled an experimental hybrid Dimona with a 100 hp electric motor, batteries and a rotary-driven genset. It’s always a bit mystifying to see an outfit the size of EADS dabbling with two-seat motorgliders, and one suspects some ulterior motive, like a hybrid UAV that travels to its target under IC engine power and then switches to a silent electric motor to perform surveillance. You can dream up all sorts of uses for hybrid power plants, so long as you don’t compel them to meet the current standard of practical personal transportation.

The standards of performance we have become accustomed to are very hard for a battery-based system to meet. Current electric airplanes are either small, or slow, or limited in range and duration, or cumbersome because of gliderlike proportions, or all of the above. There are no battery-powered, 900-mile-range, 170-knot four-seaters, and there will be none for the foreseeable future. Hybrid systems escape the constraints of batteries; they can do anything a recip can do and anything a pure-electric airplane can do. No doubt they can deliver improvements in efficiency, but whether those improvements warrant the costs of the extra complexity is doubtful. Look at the Volt. It’s a sweet piece of engineering, no doubt, but it’s a $41,000 car. How many trips to Provincetown is that?