Hybrid is the word we use now to define a vehicle that combines different propulsion technologies. But we wouldn’t think of calling the most successful hybrid of all time a hybrid. We call it the Diesel engine, or more formally the diesel-electric locomotive.
It was clear more than a century ago that the internal combustion engine had many advantages over steam power. But in a railroad locomotive it was very difficult to transfer the output of a piston engine to the drive wheels. The necessary gears, transmissions, clutches and other hardware just didn’t hold up. And the piston engine had to spin at comparatively high rpm to produce the necessary torque so that made it very difficult to get the heavy train moving.
The solution was to connect the piston engine to a generator. The piston engine turns the generator without gears and clutches and can operate at a constant and efficient rpm. The generator produces electricity that is wired to drive motors on the locomotive axles. Electric motors can generate gobs of torque from zero rpm making them perfect for getting a train rolling and then operating over a range of speeds.
Nobody would think of replacing the Diesel engine in a train locomotive with a battery. But that’s what most people are thinking when it comes to electrically powered airplanes. Just as in a locomotive, an electric motor has many advantages in powering an airplane. But batteries just don’t cut it as an energy source. And I don’t believe battery technology will advance far enough in my lifetime to power an airplane with speed, payload and range similar to our current piston fleet. And who wants to go backward on those qualities?
But if we install an electrical power generating source in the airplane we are freed from the low energy density of a battery but can still gain the advantages of electric motor propulsion.
Electric motors are much more compact than a piston engine of similar output so a nacelle could be designed for low drag, not simply to accommodate the engine.
Electric motors produce torque at low rpm and propeller efficiency benefits greatly from low rpm, particularly when operating at high airspeeds.
A battery could also be used to supply a brief burst of power for takeoff so the actual generating engine would need be sized for cruise, not takeoff and initial climb or go-around. The battery would act like the water injection systems that upped the power in some pistons and turbojets years ago. A couple minutes worth of extra energy would be enough.
With hybrid power the weight and mass of the propulsion system could be distributed ideally within the airframe. Many have tried to locate piston engines midship but the compromise of drive shafts has not worked out well. Even turbine engine efficiency suffers when long ducts are used to feed induction air.
With hybrid power the power producing engine could be positioned in the best spot for weight and balance, and to minimize drag of the necessary structure around it.
Tesla has made lots of headlines with its all-electric battery fueled car. But Porsche has blown Tesla into the weeds with its new 918 Spyder that is a true hybrid with both battery and gasoline engine power. The electric motors that drive the 918 wheels are extremely efficient and powerful and the car has beat all of Porsche’s conventional cars in lap time around the famous Nurburgring track in Germany. The piston engine in the 918 is connected to the wheels along with electric motors, but it seems certain a Porsche with electric motor propulsion only can’t be far in the future.
I think battery-only power is going to be restricted to vehicles–including airplanes–with very limited applications. But a true hybrid holds great promise. The power generating source can be a gasoline or diesel piston engine, a turbine, or perhaps a hydrogen fuel cell, or even some technology not yet invented.
An electric airplane could be great, as long as it’s making its own electrical power. I don’t want to watch my range go out the window just because I turned the heater up or the air conditioning down. However, if you’re goal is to avoid burning fossil fuel entirely and you are willing to sacrifice speed, range and payload there is already an airplane that suits that mission–a glider.