(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.