Could a mainspring be used to generate train acceleration during starting-up from the kinetic energy stored during its slowing down?

Question

When waiting for the underground, I've always been wondering why we could not absorb the kinetic energy of the train to slow it down at the station and then to release this energy to help it accelerating during the starting up. This would also reduce air pollution due to braking at underground stations.

Indeed, in frequently-stopping, trains such as underground trains, most of the energy is used to accelerate, so I am wondering whether the train could be slowed down by a large spring that would be compressed while arriving a stop station, then blocked when the train is stopped for the travelers to get out/in, then released to help the train accelerate to continue its route.

Of course, the train should not go to the opposite direction when releasing the spring, so a system should be designed so that the spring can be released in the correct direction.

This idea looks simple and I'm sure engineers has already thought about it, however, I've not found any discussion on this on the web. I found that on some Spanish lines, the kinetic energy during braking is converted to electricity and used by other trains from the same line (research paper), but why not keeping the mechanical energy to release it directly with a spring method?

Answer 1

I found that on some Spanish lines, the kinetic energy during braking is converted to electricity and used by other trains from the same line (research paper), but why not keeping the mechanical energy to release it directly with a spring method?

Because there is already a motor on the axle and there is no room for your spring. This and all the other problems that would be associated with controlled collection and release of huge amounts of energy. The KE of a 28 tonne London Underground car doing 20 m/s (72 kph) would be $ \frac 1 2 mv^2 = \frac 1 2 28000 \times 20^2 = 5.6 MJ $. Your wind-up train would require engage / disengage mechanisms, have to allow running in both directions, have to allow free-wheeling during shunting and require some means of discharging any stored energy before maintenance. If the spring storage couldn't be mounted on the bogie some flexible drive arrangement would be required to connect the axles to the springs.

Instead, many underground systems combine regenerative braking with placing the stations at the high points on the line so that they get free deceleration into a station and free acceleration out of the station.

Regenerative electric braking is more efficient, the control gear requires only slight modification, it is tried and tested and avoids major mechanical mechanisms.

Answer 2

On the London Underground, many stations use gravity. The line runs uphill into the station, and then departs downhill. This gives a large amount of energy storage, for free, with no additional moving parts requiring maintenance. It's somewhat inflexible, the amount of rise needs to be tied into the train's operating speed.

It obviously can't be applied on surface railways, or for road vehicles, but underground you have that extra degree of freedom.

Answer 3

Note also that springs are subject to eventual (and very sudden) failure as they develop cracks with age and a very sudden 5.6MJ energy release inside a railroad car would be exceptionally dangerous!

Answer 4

Mechanical energy recovery has been tried on rapid-transit systems, in the form of flywheel storage.

This is known as KERS in the motorsport world, but usage on a railway requires much higher reliability, so the cost, and the need for a containment system that can handle the rapid release of energy if the flywheel fails, means that the system hasn't been widely adopted.

Feeding current back into the supply (regeneration) is simpler & cheaper, though it recovers less energy when the line is unreceptive, i.e. at off-peak times, when there are few electrical loads in the vicinity of the regenerating vehicle.

Answer 5

Existing systems for trucks use hydro-pneumatic accumulators as the springs. As Eaton Corp said in 2010: "The system is best suited for vehicles that operate in stop and go duty cycles such as refuse trucks—the initial commercialization target."

For more information, search for 'hydraulic regeneration'. (But note that hydraulic regeneration is also used for things other than just energy saving).

Trains don't stop and start as often as garbage trucks, and have other restrictions on the permissible acceleration, deceleration and jerk. More recently, with new battery technology, interest in electrical regeneration has extended to trucks and buses.

For some applications, an alternative is to just use the recovered energy to provide compressed air or hydraulic power, without using the power for 'launch assist'.