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.
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.