# Building a powerful coiled spring engine

I trust Brandon Sanderson to have done his research - in his book "The Rithmatist" he writes about coiled spring batteries that power a passenger train. I do wonder what could be achieved with a spring engine, in a world where the internal combustion engine hasn't been invented?

Specifically:

1. What would be required (tech and material wise - metalurgy, math etc.) to build a powerful engine, such that would power a full passenger train at practical-for-serious-transportation speeds, say 50 miles per hour? For how long would one battery last (Note that Sanderson wisely put in the ability to switch engines/batteries. A train/ship is allowed to stop for a couple o' minutes, then to keep going)?

2. How powerful can we get? Could we go 100 mph? more? Could we get ships and tanks and other power-hungry machines going on spring engines?

Think of how much gas can be saved, cost and green environment, etc...

EDIT: I found an article where something similair is discussed: why not a wind up car . ?In my question, however, I'm talking about a world where combustion/electric engines don't exist.

According to Wikipedia, a modern Lithium-Ion battery has a specific energy of 0.36-0.95 MJ/kg or 100–265 W·h/kg.

According to this rather recent research paper, a battery made from carbon nanotube springs has a recorded specific energy of 6.7 kJ/kg, with a theoretical limit of ca. 0.67 MJ/kg, so very similar to modern Li-Ion batteries. Carbon nanotubes are among the most optimal materials for mechanical energy storage, so that value is near the high end of mechanical energy storage.

A typical modern locomotive weighs around 100 tons and its engine has a power output of 2-3 MW. Therefore, in order to power a 3MW locomotive at full power for one hour, you will need 3 MW-hours of energy, or a fully loaded battery with a weight of at least 11.3 tons. For 2 hours, you need 22.6 tons of battery, and so on. Modern high-speed locomotive engines can have an even higher output and energy requirement.

In conclusion: While it seems possible to have mechanical battery-powered trains, even for several hours long trips, the weight of a battery will always reduce its efficiency, and therefore such trains would overall use more power than an equivalent present-day electric train. Since trains go on fixed tracks anyway, adding static power lines and getting rid of the batteries is an obvious improvement.

Batteries for cars and other vehicles that do not have to follow fixed paths cannot be powered by static power lines and always have to carry their own fuel or batteries. As current development shows, battery-powered cars etc. are still possible and economical compared to combustion-powered equivalents.

• Cheers, Hackworth. That's that then. I'll leave it to Sandeson to figure out the rest. Thanks for the paper link, I wonder about appliances though. I guess my spring+gears+crank crossbow can always keep me safe if some cumbustion-engine-epidemic were to occur. – Nahshon paz Aug 26 '15 at 11:17
• Note that, in real life, the majority of the world's railways aren't electrified. Most often, it's that they don't have the traffic density to justify the expense. – Tristan Klassen Aug 27 '15 at 14:58
• note that some trains weight less: (pedestrianobservations.wordpress.com/2012/03/13/…) – Nahshon paz Sep 2 '15 at 9:08

Think of how much gas can be saved, cost and green environment, etc...

Did Sanderson say how the springs were to be wound? This would take as much effort as the springs would release in use. Let's say horses were used to wind the springs, why not simply use horses to draw the passenger train instead?

I suppose some forms of natural energy could be used, e.g. hydro-spring power instead of hydro-electric. Again though you have to transport the springs to point of use. Springs tend to be heavy. Transporting springs by spring-power seems self-defeating.

EDIT

Power out equals less than power in. Spring power was thoroughly developed when clocks were spring powered and clockwork toys were popular. A clockwork toy could go very fast but only for a minute or two. Undoubtedly it would be possible to have a large spring-powered drag racer. It would be under power for a few seconds. You could have a longer distance form of transport that ran as slowly as a clock. To get both speed and duration together would be impossible.

• That's easy. You use magic to wind the springs :-) But seriously - what we want is a lot of concentrated power, so pulling the train by horses to 50mph isn't practical. I realize that the winding will take a long time (we'll cook up some technologies for that, later) – Nahshon paz Aug 26 '15 at 8:38
• If you use magic to wind the springs then use magic to drive the train - or use magic springs that are infinitely strong and work without unwinding. I'll add to my answer. – chasly - reinstate Monica Aug 26 '15 at 8:50
• sigh... Never mind the pun about the magic. I'm thinking along the lines of gear tooth wheels and a crank to hold the spring from unwinding when no power is exerted on it. This way, the horses can have a rest and wind the spring indefinately (well, until something breaks). I actually made a crossbow based on this as a kid, works like a charm. – Nahshon paz Aug 26 '15 at 9:02
• All of this was do-able on a small scale from the invention of clockwork. No-one used it for real-life transport because it wasn't efficient. en.wikipedia.org/wiki/Clockwork – chasly - reinstate Monica Aug 26 '15 at 9:14