POWER UNDER PRESSURE
Steam engines have huge power and torque from effectively zero revs, but how does that translate into reality? Who better to explain than Cayman owner Peter Maynard, who has experience of many different types, from diminutive 0-6-0s to massive 2-10-0 frei
Most road vehicles have internal combustion engines of one form or another, but the steam locomotive has at least two external combustion engines, fed with steam from a boiler. Instead of generating energy within each cylinder, the steam locomotive creates the energy required to move it by heating water by fire, most often using coal, but sometimes oil.
Each ‘engine’ on the locomotive typically takes the form of a cast-iron cylinder block with an integral valve chest located above the cylinder. Some engines have two cylinders, some larger ones three. The valve employed may be of the slide type or, on more powerful locomotives, a piston valve sliding to and fro, admitting and exhausting steam to and from each end of the cylinder in turn. (The steam engine scores a point over its internal-combustion rivals by being double-acting. Every piston stroke counts.) Steam engineers flirted with poppet valves – as in modern internalcombustion engines – but seemed always to return to the trusty piston valve.
Steam locomotives don’t have a gearbox but they do have a ‘reverser’, which not only controls the direction of travel (in theory the engine can travel as fast in ‘back gear’ as it can in forward gear) but also the amount of steam admitted to the cylinder during each piston stroke. This can be as much as 75 per cent (steam is admitted for the first three-quarters of the piston travel, and then ‘cut off’ for the remaining quarter) to as little as none, in which case the loco is in ‘mid-gear’, ie not in forward or back gear.
The work done by expanding steam in the cylinder is converted to motion along the track by a connecting-rod and a crank attached to one of the driving wheels (or driving axles in the case of a cylinder inside the locomotive’s frames). Power – typically expressed in pounds of ‘tractive effort’ – is a function of boiler pressure, cylinder diameter and driving-wheel size. Higher boiler pressure: more force to drive the pistons. Big cylinders: able to accommodate more steam. Small driving wheels: the work carried out during one rotation moves the train a smaller distance than would be the case with a big driving wheel. Freight engines that needed to move heavy trains at low speeds had small drivers, whereas the ‘racehorses’ like Flying Scotsman, designed to work faster and lighter passenger expresses, had much larger ones – effectively a higher final drive. So-called ‘mixed traffic’ locomotives had a compromise somewhere in between.
Assuming full boiler pressure – on a large, modern locomotive over 200 pounds per square inch (say around 1400kpa or 14 bar) – the driver has at his disposal maximum power and torque at maximum (ie 75 per cent) cut-off. Thus when starting from rest judicious application of the regulator, which controls the flow of steam from the boiler to the cylinders, is called for in order to avoid wheelslip. ‘Traction control’ is the driver’s hand gripping the regulator handle, deftly reducing the flow of steam through the regulator valve. Large regulator openings and long cut-offs, though, are hugely wasteful: in car terms it would be like cruising at 60mph in second gear. The correct approach, once nicely on the move, is to reduce the cut-off so that steam is admitted to the cylinders for a shorter length of the piston stroke; it takes only a relative puff of steam to keep the train moving. And at the same time the regulator can be opened more widely; the reduction in torque due to the shorter cut-off reduces the likelihood of wheelslip.
Assuming sufficient traction is available (and you are dealing with narrow steel ‘tyres’ on possibly wet and greasy steel rails, remember), maximum acceleration is with full regulator and 75 per cent cut-off; the car equivalent is a wide-open throttle in first gear. Once up to speed, full regulator delivers maximum power but at a short cut-off a lower amount of torque. Think of it as being akin to your foot flat on the floor in sixth or seventh gear. And, just as you change down (for more torque) to climb a hill, so the engine driver lengthens the cut-off to achieve the same effect.
With steam locomotives, then, there is no ‘rev-drop effect’. They actually have a continuously variable transmission, except that the variability comes from the valves that control the admission of steam to the cylinders. And steam locomotives are certainly not ‘automatics’.