EVs have provided multiple technology advances which are now being leveraged for use in small aircraft; diesel-electric locomotives may be the next beneficiary.
There’s no question that all aspects of EVs have improved dramatically in the past few years, from basic components to system-level architectures. Now that all-electric vehicles (EVs) are available if not in wide use as standard consumer choices, a logical consideration is how some of their technology can be adapted to other modes of transportation. Planes, Trains and Automobiles, anyone?
(Note that “pure” EVs account for about 2% of vehicles sales in the U.S. in 2020 and are slightly higher in other regions, but not by much. As for what that percentage might be in two, five, and 10 years, the experts with their crystal balls and automakers are all over the place on this, with some saying it will be half of vehicles by 2030, others much lower. Your prediction is as valid as theirs, seems to me.)
The EV-driven advances are opening the way to re-think other forms of transportation, as these advances can be leveraged and adapted. That’s the normal ripple effect of technology progress, which opens new opportunities in related and sometimes unrelated areas. The R&D, evaluation, mass production, and road testing of EVs in areas such as batteries as well as battery management, motors, and motor control are a technology boost for electrification of other transportation modes.
For example, there is renewed interest in electric-powered air taxis, which are vertical takeoff and landing (VTOL) aircraft for short-distance trips. Unlike conventional helicopters that generally have a single main rotor plus a tail rotor to counter the main-rotor torque, it’s now practical to use an arrangement that is similar to a small drone with a distributed propulsion system. In this scheme, there are four, six, or more small individual motors and propellers mounted on extended arms. Electric power allows a freedom in motor arrangement, design, and control that conventional helicopters cannot offer.
Of course, it’s never that easy. Conventional fixed-wing aircraft can glide to some extent if the engines fail, while conventional helicopters can autorotate their blades (for a while) and generate enough lift to make an acceptable landing in many situations. However, these air taxis have no such lifting surfaces and glide “like a rock” if power is lost. So extra reliability and redundancy are especially critical because loss of power is quickly catastrophic.
Some of these air taxis use the tiltrotor approach to blend the benefits of both fixed and rotary-wing attributes. The six-motor eVTOL from Joby Aviation uses such an arrangement, Figure 1, which is also used in the military’s Boeing Bell V-22 Osprey, a non-electric VTOL. While the Osprey had a difficult birth largely due to fluid leakage through the rotating joints; in contrast, it’s much easier to “twist” the cables carrying electric power to the rotor motors, especially as they only have to rotate 90 degrees.
But why limit electric aircraft to air taxis? There’s also work being done on small aircraft, such as the Velis Electro from Pipistrel Aircraft, Figure 2, and Alice from Eviation, Figure 3. Both are designed for relatively short distances and modest capacity.
NASA and others are thinking bigger, of course. If you can have an all-electric small plane, maybe you can work toward a larger one? That’s where things get very difficult. While some technology advances scale up nicely and beneficially, aircraft do not. The amount of power you need for a larger aircraft — think Boeing 787 Dreamliner — far outweighs what batteries can provide.
There are some basic numbers to acknowledge: the large kerosene-fueled turbofan engines of the 787 deliver about 12,000 watt-hours per kilogram, while top-tier Li-ion batteries come in at about 300 Wh/kg, a 40× difference. Even if the electric motors of the e-aircraft are somewhat more efficient than the jet engines, there’s still a very long way to go. Plus, liquid-fueled aircraft have a secondary advantage, as their weight decreases as they burn fuel, unlike the situation for battery-powered vehicles.
What About Railroads?
Air taxis and small aircraft may get attention due to their “cool” factor, but there’s a more mundane place where electric power may actually make sense and do so sooner. Railroads in the United States carry a significant amount of the freight — how much and what percentage depends on the metrics you look at — and they are more efficient than trucks. Due to the distances and other factors in the U.S., these freight trains are not catenary-powered all-electric engines as they are in some countries. Instead, they powered by diesel-electric locomotives where an onboard diesel motor (typically 5K to 12K horsepower) turns an alternator, and the alternator output drives axle-mounted traction motors.
Compared to aircraft, the technical issues associated with railroads are more bounded, standardized, and defined, which is a good thing. A recent paper “Economic, environmental and grid-resilience benefits of converting diesel trains to battery-electric” in Nature Energy, by a team headed by researchers at Lawrence Berkeley National Laboratory (LBNL/California), offered a detailed proposal and analysis for fully electrifying these diesel-electric locomotives.
The arrangement would have a power car right behind the locomotive loaded with batteries and connected to the traction motor power subsystem in place of the diesel engine and alternator (which could remain in place as backup). This power car could be a custom-designed tender or perhaps a flatcar with large battery-laden modules which could be lifted on/off as needed, or used elsewhere as battery-backup units, Figure 4.
The paper looks at the proposal from multiple technology, cost, and practicality perspectives, and makes a strong case for cost savings as well as environmental benefits. I’m certainly not in a position to find areas where their assumptions — and there are many – are perhaps overly optimistic. I do like that they even had a chart showing the sensitivity of their conclusions to deltas in their original assumptions, Figure 5.
There are several ironies in their proposal. The first locomotives were steam powered, of course, and almost always pulled a tender loaded with wood or coal for fuel. So, their proposed arrangement has some “back-to-the-future” aspects to it.
Further, the problem of providing electric power to the locomotive is not just one for full-size, real trains — even model railroaders have the problem. Power for these models, regardless of scale, comes through the tracks (rails) to the wheels, and is picked up by tiny brushes that deliver the current to the electric motor in the model.
While this use of rails for dual purposes seems like a great scheme and something for almost nothing, it has some practical issues related to the pick-up brushes, of intermittent connections, gaps in the rails, and “reverse loops” where a track loops back onto itself and so creates a short circuit. To avoid this, the model’s conductor must manually switch track polarity when a train is in the loop or add an automatic polarity-reversal module.
Now, some model-railroad product vendors such as the AirWire900 Battery Powered Wireless DCC Control System from CVP USA offer systems with on-board batteries to be carried in a tender behind the locomotive, giving a run time of 20-30 minutes. Providing motor power via an on-board pack eliminates all of the issues associated using the tracks for power rails in addition to their original role as railroad-vehicle tracks. In some ways, the proposal and analysis in the LBNL paper are large-scale renditions of what modelers are trying to do.
What’s your view on the viability of all-electric air taxi, small aircraft, and even mid-to-large aircraft? Desirable but impractical? Maybe in a decade or so? Or, are the battery energy- density numbers and associated technical issues simply too hard to overcome in the foreseeable future? And what about battery-powered freight trains: will they be adopted before aircraft?
This article was originally published on EE Times.
Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.