Trains Gallery 7

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Q - The recent thread about railroad tracks got me thinking about something else I've often wondered about. (Well, not that often, but once in a while.) Modern locomotives are diesel-electric, in which a diesel engine is used to generate electricity, which is used to power the electric motors that drive the wheels.

Why do they do this? Why not just use the diesel motors to drive the wheels directly? There must be some loss of energy in the electrical transmission and control equipment and in the electric motors. No matter how well the electrical system is designed, there will be *some* loss of power (1%? 10%? 25%?) with this two-stage design. Not to mention much greater equipment cost (motors, generators, transformers) and maintenance cost. I mean it is not like the old diesel-electric submarines that stored electric power in batteries so they could run quietly and underwater.

1) Electric motors can produce huge amounts of torque at very low speeds.
2) No need for mechanical clutch/transmission, which are hard to build for several thousand horsepower.

Mechanical transmissions have losses too.

And lest anyone be concerned about the added weight, it is an asset in a locomotive.

Steam locomotives have, if you look closely, a set of smaller "cylinders" above the big ones that drive the wheels. Located inside are the valves that control the timing and admission of steam the the main cylinders. What is not apparently obvious is that the steam is used to both "push" and "pull" the piston back and forth, in other words, it has power strokes in both directions, (actually, it pushes on both sides of the piston alternately) yielding two power strokes per revolution of the drivers.

Another non-obvious fact (since one rarely sees both sides of a locomotive at once) is that the drivers are "quartered", i.e. set 90 degrees apart in phase from the left to the right sides so that the power strokes for the whole locomotive are now 4 per wheel revolution, coming left-right-left-right.

After that little discourse, back to the main train of thought.....

The key to all of this moving steam around are the valves atop the cylinders. Looking at a steam locomotive, numerous smaller rods and cams and such are connected to the main drive axle via an eccentric crank (no, not ME, dammit!) and power the valve motion with respect to the main rod motion. We in the steam locomotive trade called this stuff "Valve Gear" and it comes in various flavors, such as Stephenson, Walschaert, etc. after the inventors of each particular type.

Now to the "transmission" bit:

Steam has the property, under pressure, of wanting to expand almost explosively. The engineer has a lever in the cab that is connected either directly (in which case it's one big motha lever(!) or indirectly (through a power reverse mechanism, usually run by compressed air) to the aforementioned "Valve Gear". It's function is to change the geometry of the gear by moving a pivot point or two, causing the timing of the steam admission via the valves to be controllable from the cab. Change the geometry enough, and the locomotive runs backwards (hence "power reverse" above).

Now, imagine that you are starting your train: you want the maximum power at a low speed. In a car, you use first gear, which is a lot of noise for a little movement, right? In a steam locomotive, you put the valve gear "in the corner" or full throw in the direction of travel. You are using the maximum amount of steam (because the intake to the cylinder is open longer through its' stroke) per power stroke. Standing near a steam locomotive and observing:

Engineer places valve gear in full forward position and releases brakes.

Engineer opens throttle valve (up at the top in one of those funny looking domes)

Steam rushes through big motha pipe to valves

Valves admit steam to one side of piston, it starts to move

After a period of time, valve closes input port and opens exhaust port

CHUFF!!! exhaust comes out the stack.

Repeat until train stops.

As the locomotive speeds up, so do all its' moving parts, and there will shortly come a time when the large volume of steam can't get out of the way in the cylinders fast enough. (Your car engine will only rev so high)

The engineer then pulls the lever back towards center some (called "hooking it up"). This allows the valve timing to adjust in such a way as to cut off the inflow of steam to the cylinder earlier in the process, thus letting the natural expansion of the steam do more of the work. (Think shifting gears in your car).

At high speeds the cutoff will be close to the center (think "overdrive") and as the load on the locomotive changes, the engineer will vary the amount of power to the engine by adjusting the cutoff/power reverse lever. (Think downshifting on a hill)