Any suggestions on ways to power large-scale clockwork vehicles? I'm building a world with highly advanced mechanical technology in a D&D-esque fantasy setting, but I haven't decided how to fuel them. I've considered just big hairsprings and flywheels, but I'd like some outside opinion. Oh yes, steam is not developed until later, so let's avoid combustion for now.
Leonardo da Vinci designed and reputedly built a spring powered car in the late 1400's (as a spectacular prop for Duke Ludovico Sforza). Even with rather gigantic (for the time) springs, modern reproductions of the vehicle could run for about 40m before the springs unwound. This should give you the idea of how much energy density that mechanical power storage has.
Reproduction of Leonardo's car
With modern (read 20th century) technology, this can be amplified quite a bit, but generally by using flywheels, springs etc. in conjunction with a high power density device like an internal combustion engine. The best example is the KERS (Kinetic Energy Recovery System) used in formula one race cars. Since the engines of F1 cars can reach power outputs of over 1000hp, the KERS need to be able to rapidly "absorb" some of that energy and just as quickly return it to the drive axle so the car can rapidly accelerate out of the corners. KERS is limited to 60Kw (@ 80Hp), which shows that even with modern technology and materials science, this isn't an easy thing to do).
So in a setting where technology is late middle ages, I suspect that a "car" will have a performance much closer to Leonardo's device than an KERS assisted F1 race car. Still, if you use a spring driven device to assist a horse drawn carriage up a hill, then you might be onto something.
If we assume it to be a magical setting where we don't have to follow laws of physics very adequately...
Let's build a gigantic windmill, a mile high and with wide arms so it can harness tremendous windpower. Then, let's make equally gigantic springs from special (magical?) kind of iron. They would be mounted on the windmill and wound up. After that they be used as engines in trains and ironclads. A spring should store enough windpower that it would be sufficient for a journey from the windmill to far away places, with passengers and cargo, and then back. After the return the spring would have to be mounted on the windmill and wound up again.
I have been running a steampunk-esque campaign for a few friends for just over two years now, and some of my players have developed some very unique ideas on how to power very large mechanical devices. Even without steam power, clockwork mechanisms can be "powered" mechanically in many different ways. One of my very favorites is a real life wind-powered machine called a Strandbeest. They are probably the closest real life equivalent to a giant "clockwork" vehicle, that is powered only by the wind. In my D&D campaign, the impossible scale of many of the airships and vehicles has been overcome by the creation of a couple new elements. If you are okay with implementing some "fantasy" type elements in your world, you can create some very unique mechanical power sources. The notable reference would be an element that has near frictionless properties, so that It can be used to power some (physics-breaking) perpetual motion machines (only if you are willing to overlook their improbability). Some other notable methods of acquiring mechanical power could be tidal generators, wind, water wheels, or even human power.
Since it is a fantasy setting you could capture small elementals or demons and have them running on treadmills inside the workings. Those treadmills then provide the power to everything else.
While springs or flywheels of the time would probably have a much too low energy density to be useful here, you could use compressed air energy storage.
Compressed air is, to our standards, an awful energy storage :
Advanced fiber-reinforced bottles are comparable to the rechargeable lead-acid battery in terms of energy density
And the lower pressure and heavier bottles of the time would be worse. Still, probably the best option.
Charging them would be energy- and infrastructure-intensive: windmills, watermills or big horse-powered mills are also, to our standards, ridiculously inefficient, but they could power a crude compressor and slowly fill your carbon steel bottles. Be careful though, even well-made, expensive bottles may explode if you fill them too much or hit them too hard.
Also note that due to gas expansion, the exhaust will be cold. Refrigerators are literally based on this principle. Enough that you will want heaters on some of your mechanical parts to avoid them freezing. Charcoal, alcohol, oil-based mixtures, that stuff Greek fire was made of... Also note that hot gas is more efficient, so if you do have a good heat source, it may somewhat help.
At least refuelling a vehicle may be fast, probably by swapping bottles - assuming you have standard adaptors..
I visited a clock museum, I saw a couple of mechanisms there. One of the mechanisms, I saw there was a weight driven system. Another mechanism that you could use is the pendulum. Other than those the main system I saw was the hair spring, so that would probably be the best bet. Here is a link to the mechanism and winding of Big Ben. That clock uses a weight based system, and it's one of the biggest mechanical devices ever built, and that would be a reasonable power source for these large mechanical vehicles, perhaps paired with a fly wheel.
In "The Lord of the Rings" The One Ring could only be destroyed in the fires of Mt Doom where it was forged making it effectively indestructible, this property of indestructibility is commonly attributed to all enchanted items. An enchanted sword at the bottom of a lake will never rust, an enchanted robe will never fall apart at the seams or get eaten by moths, magical armor is always in pristine condition even if its occupant is killed, this is one of the great unstated universal laws of D&D.
So what about an enchanted flywheel?
In the real world you can only store so much energy in a flywheel of a given size before it's torn apart by its own momentum, if you want to store more energy you need a bigger flywheel and this quickly becomes impractical. But with an effectively indestructible flywheel we've got a kinetic battery with a practically unlimited storage capacity that can also be very small and lightweight.
Needless to say this makes high performance clockwork cars entirely feasible.