To make a change from the ongoing TV fantasy drama The Fall of the American Empire, aka The Game of the Throneless, let me introduce you to the Siemens SP260D.
This is an electrical aircraft engine. More details here.
This is only the second of Siemens’ efforts in the line, though they have been making electric motors since the 1890s. (AEG beat them to it, in 1889.) The striking datum is the power-to-weight ratio: 260 kW (footnote) from 50 kg, making 5.2 kW/kg. What should we compare this to?
A table of power-to-weight ratios for a sample of engines on the market today.
Car and aircraft engines cannot be compared like-for-like. Cars need their peak power only for minutes at a time, plane engines are expected to deliver it for hours. What is striking is that in both categories, electric motors beat ICEs very handily. Since electric motors for vehicles are young, they have more potential for improvement. Siemens plan to replace the aluminium end-shield with a lighter carbon-fibre composite. More ambitiously, Magnix are working on a superconducting motor.
The one ICE that comes close to electrics is the exotic Formula 1 engine made lovingly by hand by an arm’s-length subsidiary of Mercedes, in England not Germany. The ratio is what results when a crew of fanatic Brummie dwarfs are given a free hand to throw considerations of cost, durability, reliability, fuel efficiency and noise out of the window in the single-minded pursuit of raw power. Formula 1 races only last about 90 minutes; the engines are not used in endurance races like Le Mans. You can’t see this weird machinery (600 kW from 1.6 litres? My car is happy with 75) as a possible future for general-use ICEs, though some innovations are pioneered in the sport.
ICE defenders will hasten to say that the motor is only part of the story. Siemens and Magnix have very few customers, because their motors are well ahead of even the best batteries for storing energy. As a stopgap until the superbatteries arrive, several aircraft companies such as Zunum are working on hybrids. In these the electric motor still handles the propulsion, but it it is partnered by a range extender ICE, which charges the batteries. This is designed to run all the time at the optimum revs, and can be as efficient as an ICE allows.
Electric motors are superior to ICEs in other ways. They are very efficient – over 90% - and therefore cooler; the simple design slashes the number of moving parts to go wrong, making them highly reliable; they are more flexible, for instance delivering full torque at low speed in a car; they are much quieter; and of course don’t emit CO2 and other pollutants.
The 140-year reign of the ICE is drawing rapidly to a close. Both the electric and internal combustion motors were invented in the first third of the 19th century, but it both cases it took till the 1880s for the inventions to reach commercial form. They quickly divided the world between them, like Diocletian. Electric motors took over all the fixed uses, in factories, offices, and houses; the ICE took the mobile ones, in cars, trucks, ships and planes.
The deal has broken down, for two reasons. In the 1960s, electronic controls were developed by ABB allowing AC motors to run at variable speeds and outputs, which they could not do well before. Now the efficiency of electric motors applies across variable loads. Then in 1980 John Goodenough and others invented the lithium-ion battery, that made batteries a serious option for powering vehicles.
Good riddance to the ICE. There is a sad side to it, like all major changes. Even a non-techie like me knows in principle what a camshaft, a fuel injector, or a gearbox does. I even know how to double-declutch. The language of switched reluctance and cathode battery degradation is as alien to our generation as terrets and martingales. But that’s what the young will learn now.
Footnote
No, I am not going to convert normal SI units to BTUs per cubic groat or what half-brained units are still current in the shrinking handful of countries that stick to the Imperial “system”. It’s not a system, just a collection of dubious historical anecdotes.
Some miscellaneous comments:
(1) “Electric and ICE quickly divided the world between them.” Of course they did. If you could get your electricity at the end of a wire, with the other end connected (at quite a distance) to a highly efficient generating station, why would you want to burn your own fuel to create motive power?
(2) Side note on the above-I miss the streetcars of my youth. They were quiet, they didn’t stink up the city, and they had those cute little bells to warn the pedestrians, instead of the obnoxious horns of the buses we hear so often now.
(3) I think I don’t like the comparison of power-to-weight shown in the table. The weight carried by an airplane for its propulsion is the weight of the engine plus the weight of the fuel and its container. For all I know, that comparison may still be very favorable to the newest electric propulsion options. But either way, I think the weight of the fuel should be shown as part of the picture.
I thought I made your point 3 in the paragraph beginning “ICE defenders will hasten to say…”. Adding fuel to the indicator creates boundary problems, and IMHO obscures rather than clarifies the comparison, but I Am Not An Engineer. I used the conventional definition, pointing out its limitations. Another issue is gearboxes, usually unnecessary in VSD electric motors.
It was curiously hard to find weights for car ICE engines. I picked the Honda only because I finally located its weight, not on Wikipedia or the manufacturer’s website but a fan forum. The F1 engine weight is not a design choice but a minimum imposed by the race organisers. Either typical car ICE designers don’t care much about weight, within a range, or they think that their customers don’t, or both. For aircraft engines, it’s always been a key datum.
I apologize if I came across as an “ICE defender.” That was certainly not my intention. I just think it is a truer comparison if the total weight attributable to one or the other system is portrayed, rather than just the engine weight.
And by the way, yes, I recognize that getting good data for the comparison is no simple feat. We know for sure that the companies in the business have engineering staffs who have all that information at their fingertips. They do, in fact, care quite a bit about engine weight, as well as fuel weight, since they affect both fuel economy and handling. Sadly, I’m sure those data are closely held.
How does your indicator work? The powertrain includes, as a minimum, an energy store and a propulsion engine. Let’s say the store is either gasoline (12.7 kWh/kg) or a lithium-ion battery (0.25 kWh/kg). Gasoline has about 50 times the energy density of the battery, down from 100 in 2012. The electric motor+battery system is 5 times more efficient, so overall gasoline’s effective density advantage shrinks to 10:1. But storage has a time dimension. To compare the systems, you have to specify a use case: and these are multiple.
For electric planes, current battery storage limits endurance to one hour, which is only acceptable for niche uses like trainers, aerobatics and air taxis. The technology is not yet ready for standard uses in general, commercial or military aviation. In cars, there is a heated and long-running argument, with great commercial importance, over the acceptable single-charge range of a BEV. A mid-range Tesla 3 can go 260 miles, a 2018 Nissan Leaf 151 miles (both EPA). If your benchmark is 250 miles, the Leaf fails; if it’s 150 miles, the Tesla is over-engineered. In any case, the metric of interest is usable range, not power density.
More than good enough, I’d say.
Ba-dump-bump.
Of course it is necessary, but … I will miss the sounds. I love the sound of small plane engines overhead. The bigger ones are just okay. I was not lucky enough to experience a good streetcar. I do like a nice bell.
cmon, RBC… I have you at the top of my RSS feed… and lately that’s only meant that I keep getting prompted to go to bogus posts.
In a past life, I’ve fixed stuff like this.
In my current life, I’m distressed about needing to disconnect from RBC.
I’d be sorry about this if I had any idea what you are complaining about. Nobody makes you read any post. To me, sustainable aviation is a goal worth some attention.
Besides being *intensely* fascinating. I for one thank you for this post, regardless of whether it’s on-topic (for this blog) or not (it -is- on-topic, but even if it weren’t!)
Hi, I apologize for all the spam recently, my computer was hacked. The problem should be fixed for good now. Sorry for the inconvenience!
A few random comments from a former USAF aircraft maintenance officer:
(1) Comparing only to reciprocating engines is foolish. Although there’s certainly a long history in small-craft aviation of using piston engines (because the size efficiency has historically — that is, with essentially common-alloy metal parts — been lower for reciprocating engines, and the heat-exchange-on-the-ground problem is pretty darned severe for various forms of turbines), post-1960s materials and designs certainly make a turbine part of a potential hybrid engine. The problem THAT creates is one of aircraft conformation, since any turbine demands a straight-through air path from front to back that is, umm, difficult to accomodate as the primary propulsion source in something the size of a Piper Cub… but not necessarily if one or two a battery-charging, small, constant-speed engines are slung away from the cabin. (Not as easy as isolating the entire propulsion system to either in front of or behind the cabin, but achievable.)
(2) One of the huge advantages of turbines is that they are far less sensitive to fuel quality and composition than are reciprocating engines, and much easier to “tune” to alternate blends. Hypothetically (well, not so hypothetically, I’ve seen it done in the field), one can get sufficient performance as a charger instead of direct source of thrust from some pretty wonky fuel combinations… meaning that as fossil fuels become less common, it will actually take less effort to use a turbine than a reciprocating engine.
(3) One of the biggest barriers is and will be instrumentation. The RF interference thrown off by one of these “small” electrical engines will not play nice with civilian-grade navigation and control technologies. That’s going to mean not just hardening, but use of higher-grade internal electronics, which are neither cheap nor easily available. (Not necessarily milspec, but not off-the-shelf either.) The less said about what “proprietary technology” will do with getting usable results from a pitot tube, the better (trust me, I’ve been there).
(4) That points to the next big barrier. Bluntly, civilian maintenance techs do not and will not have the training, experience, equipment, or even tech manuals to handle electrical engines for at least a decade after military adoption of the technology (the source of most aircraft maintenance techs and almost all of their instructors).
(5) As a nonpilot, I don’t know how serious the different performance characteristics are going to be in pilot training; I just refuse to discount them as insignificant. And given that for small aircraft “pilot error” (including “bad weather” when the aircraft shouldn’t have entered that weather pattern in flight) is at least a significant factor in over 75% of mishaps, that’s A Problem. (Just like it was with turboprop and jet engine adoption, both of which took A LOT longer to percolate down than their advocates thought.) See point 4, too, because inadequate/incorrect maintenance is more of a problem than even official mishap investigations admit to.
Always a pleasure when a post attracts comment from somebody with real expertise to contribute!
Ad (3): Surely Siemens have thought of the RF interference issue? They are flying small planes already with their motors. From the problems you mention, perhaps it’s just as well that large-scale introduction will have to wait a bit for really good batteries, giving time to sort out the issues in small planes.
Ad (4): Electric motors have been built, and sold by the million, and maintained (when they need it, which isn’t much) by civilians, since 1880. They are 50-60 years older than the gas turbine, though of course designs keep improving. It’s not a rocket science problem, still less brain surgery.
It does not look as if the military are driving the current wave of interest in electric aircraft propulsion, though no doubt there are secret programmes for stealthy UAVs and so on. If the “military first” paradigm continues to hold, electric flying will stall. But why must it? Land electric vehicles are doing fine without much military boost – IIRC the Pentagon’s main contribution has been to support self-driving and GPS. They should probably be doing more; a large proportion of US casualties in Iraq and Afghanistan were connected to transporting liquid fuel.
If LiquidPiston can create reliable, duriable engines using its rotary engine design, it would be very competitive in many applications using triditional ICEs and some using electric motors.
The aim is to stop burning fossil fuels. Better ICEs don’t help much, though they will slightly reduce the volume of liquid biofuels needed for applications where battery-electric just does not work. The Wankel rotary went into a few cars, but SFIK no major manufacturer uses it now.
The LiquidPiston design is sort of an inverted Wankel with, supposedly, some improvements on the Wankel without some of its problems.
LiquidPiston will show up in military applications before it’s seen in commercial ones. For one, the military wants quieter drone engines than the ones it’s currently using. While not as quiet as an electric motor, LiquidPiston would be much quieter without the RF interference that Jaws thinks could be a problem.
The military is also looking at it for hybrid vehicles and motor/generators.