A steam locomotive is, at heart, a self-propelled kettle that turns the energy locked in coal into motion. It can look bafflingly complex, but follow the journey from fire to wheel and the whole machine falls into place. The cutaway below shows that journey in one picture; the sections beneath it follow the same path, step by step.
1. The fire and the boiler
Coal burns on a grate inside the firebox, reaching well over 1,000°C as the fireman feeds and spreads it. A brick arch across the firebox forces the flames to travel forward and mix properly before they leave, which burns the fuel more completely. The hot gases are then drawn forward through dozens of fire-tubes running the length of the boiler, a long steel barrel filled with water. As the tubes glow they boil the surrounding water, and the steam gathers in the steam space at the top of the barrel. Nearly the whole barrel is kept full of water, with that vital pocket of steam left above it; keeping the water at the right level is one of the fireman's constant cares, because uncovering the firebox crown can be catastrophic.
2. Making and controlling the steam
Pressure builds until the boiler holds steam at perhaps 200 to 250 pounds per square inch, kept in check by safety valves that lift and blow off if it climbs too high. The steam is collected at the highest, driest point, usually a dome on top of the barrel, where the regulator (the driver's main steam valve, in effect the throttle) lets it through. When the driver opens the regulator, high-pressure steam rushes from the boiler towards the cylinders. On most twentieth-century engines it first passes through the superheater, a nest of small tubes threaded back inside the largest, hottest fire-tubes. This re-heats the steam far above boiling point so it stays bone dry and expands far more powerfully in the cylinders, a major gain in efficiency over plain “saturated” steam.
3. Cylinders and pistons
The steam enters the cylinders, where it drives a piston back and forth. Locomotive cylinders are double-acting: steam is admitted to each end of the cylinder in turn, so the piston is pushed on both the forward and the return stroke, with no wasted movement. With (typically) two cylinders, one on each side set a quarter-turn apart so the engine can always start, you get the familiar four-beats-per-revolution chuff, chuff, chuff, chuff. Many larger engines used three or even four cylinders for a smoother, more powerful drive.
4. The valve gear: the clever bit
Timing the admission of steam is the job of the valve gear, with Stephenson's and Walschaerts' being the common types. A sliding valve in the steam chest above each cylinder uncovers the ports at exactly the right instant, admitting fresh steam to one end of the cylinder while letting the spent steam escape from the other. Crucially the gear also controls the cut-off: how early in the stroke the steam supply is shut, so the steam already in the cylinder can expand and do its work economically rather than being thrown away half-used. The same gear lets the driver reverse the locomotive and “notch up” for efficient running once the train is rolling, much as a motorist changes up through the gears.
Types of valve gear
Several ingenious mechanisms have been used to drive the valves, all doing the same job of opening and closing the ports at the right instant:
- Stephenson link motion: the classic early gear, driven by two eccentrics on the axle working a slotted link. Simple and dependable, it was the mainstay of Victorian engines and was usually tucked away between the frames.
- Walschaerts valve gear: the gear most associated with the later steam age, taking its drive from a single return crank plus a connection from the crosshead. Its great virtue is that it sits outside the frames where it can be seen, oiled and maintained, and it gives a clean, even cut-off. Most preserved main-line engines carry it.
- Joy and Baker gears: alternatives that did away with the eccentrics, the British Joy gear taking its drive straight from the connecting rod, and the American Baker gear using a train of levers.
- Poppet valves (Caprotti): instead of sliding piston valves, some later engines used mushroom-shaped poppet valves worked rather like those in a car engine, driven by the Caprotti gear for free running and sharp, independent control of admission and exhaust.
The reverser and cut-off
Whatever the gear, it does two things for the driver. It sets the direction of travel, forward or reverse, and it sets the cut-off: the fraction of the piston's stroke for which steam is admitted before the valve shuts it off. The driver controls both from the cab with the reverser, either a long lever working in a notched quadrant or a screw and handwheel.
This is the heart of economical steam working. Starting a heavy train, the driver winds the reverser into full gear, admitting steam for most of the stroke (a long cut-off, perhaps 75 per cent) for maximum effort. Once the train is rolling he notches up, shortening the cut-off so steam is admitted for only the first part of the stroke. The steam already in the cylinder then expands against the piston for the rest of the stroke, going on doing useful work without drawing any more from the boiler. A short cut-off does the same job on far less steam, much as a motorist eases off the throttle and lets the car run on. Working an engine as expansively as the load allowed was the art of good, economical driving.
5. From piston to wheel
The piston's thrust travels along the piston rod to the crosshead, which slides in guides and keeps the motion perfectly straight, and from there through the connecting rod to a crank pin on the driving wheel. That converts the straight push into rotation, exactly like the pedals and crank of a bicycle. Coupling rods then link the driving wheels together so they all turn as one, spreading the effort across several wheels and multiplying the grip on the rail. Watch the cutaway: the connecting rod swings as the crank pin sweeps round, while the coupling rod stays level and simply orbits in a small circle.
6. The blastpipe: the engine breathes through its chimney
Here is the elegant trick that ties it all together. The “used” steam leaving the cylinders is funnelled up the blastpipe and out through the chimney. As it shoots upward it drags the firebox gases with it, sucking fresh air through the fire. So the harder the engine works, the more steam it exhausts; the more it exhausts, the fiercer it draws the fire; and the fiercer the fire, the more steam it makes. It is a beautifully self-regulating cycle, and it is why a locomotive working hard barks so sharply from its chimney while one drifting along is almost silent.
7. Water, coal and brakes
The tender behind the engine (or the side tanks of a tank locomotive) carries the coal and the water. Fresh water is forced into the pressurised boiler against its own steam pressure by ingenious devices called injectors, which use a cone of steam to drive the cold water in with no moving pump. And because a train of several hundred tons takes a long distance to stop, it is held by a continuous vacuum or air brake that runs through every vehicle and applies automatically if the train ever parts, so a breakaway brings itself safely to a stand.
Put it all together, fire, water, steam, piston, rod and wheel, and you have a machine that hauled Britain's goods and passengers for more than a century, and still draws the crowds today on our heritage railways.
More in this section: The Parts of a Steam Locomotive · Wheel Arrangements Explained