12 December 1997

How we once got work from water

While David Smith of the

Traction Engine Trust reveals

the inner workings of an

engine, Andrew Pearce

keeps soot from his notebook

NOW heres a question. How can ordinary old water – the stuff that drains uncomfortably down the back of your neck on a wet day – be persuaded to haul a load or pull a plough?

This might not be something to bother todays tractor drivers, cocooned in their cabs and whisked along by digitally controlled diesels. But the way of working that seemingly magic trick was clear to our great grandfathers great grandfather, and maybe his grandfather, too.

Although separated by a sea of time, agricultural traction engines and todays diesels have a common base – mini-lecture follows:

Both deliver usable work – trailer pulling or plough hauling – by heating something, which then expands to push down piston(s). Diesel fuel burns and its stored chemical energy turns to heat. This expands both the combustion gases and pre-compressed cylinder air, driving down a piston. Burning happens inside the cylinder, making the diesel an internal combustion engine.

In a steam engine, burning happens outside the cylinder. Chemical energy from the fuel (oil, coal, wood, old copies of farmers weekly or anything else to hand) is passed to water, which turns to steam. This, collected at high pressure and thus rich in energy, is led to the cylinder where it expands to drive the piston. So these are external combustion engines.

Contrary to popular belief theres no magic in steam. You can put heat into fluids as far apart physically as mercury and air, and these will equally happily change state, expand and drive a piston. Water is favourite as there is usually plenty of it, it is more or less free and is (generally) harmless.

The building blocks

Working in an industry where job costs had to be kept down, agricultural engines were generally low on complication and thus relatively cheap to buy and run. They stuck at the baseline of steam technology, with straightforward boilers and uncomplicated cylinders.

We will start with the overall picture, then zoom in on detail. Fig 1 shows the main working bits of an agricultural engine – a fire-box inside a vertical boiler section, a horizontal boiler barrel with its sealing end plates and fire tubes, a smoke-box and its stack. Crowning it all is the working cylinder with its connecting rods, which drive the mainshaft and eventually the wheels or a winch.

The fire-box has grate bars to hold the fuel, an ash can underneath and opens to the outside world. Air moves up through the fire-box, turns 90í and passes through the fire tubes, comes out into the smoke-box and exits through the stack.

With it go heat and smoke from the fire. When the engine is doing no work only smoke comes from the stack. When the piston(s) move, exhaust steam joins the party – more on that later.

The boiler is filled with water to a level above the firebox crown and fire tubes (fig 1). On the driving platform, a water gauge shows the crew how the level is faring. And with consumption at 18-20gal/mile, it pays to keep a close eye on it.

Low water means burnt fire tubes or fire-box damage, as temperatures well beyond 400C are on the loose. Should water level drop too close to the fire-box crown, a fusible plug in the top overheats. Its lead core melts, dumping boiler water into the fire and bringing proceedings to an entertaining stop.

The boiler barrel is not brim-full of water – there is a space left at the top. Hot air blasting down the fire tubes heats the surrounding water, bringing it to the boil. Steam bubbles upwards to collect in the air space, where it is kept boiling hot by the water below. To hold on to as much heat as possible the boiler barrel is jacketed with strips of wood, although you cant see it, as steel cladding presents a smart face to the world.

High pressure steam collecting in the boiler top is mainly dry, searing hot and invisible. Take a look at a boiling kettle; dry steam is in the clear area next to the spout, wet steam is the visible cloud.

Putting steam to work

Now we are coming to the nub. All that highly energetic steam needs to be brought to the heart of the engine to push a piston to and fro. This in turn drives a crank and eventually, via gears, the engines wheels or winch. A big flywheel on the crank smooths power delivery.

The cast iron ring-sealed piston lives in a cylinder, which is inside an insulated block bolted to the boiler barrel top. Steam fills a jacket round the cylinder to keep it hot (fig 1).

Even a simple traction engine is much cleverer than it looks. In a diesel or petrol motor the piston is only pushed one way. But the steam variety pushes its piston both ways, much like a double-acting ram. Heres how. Boiler-pressure steam arrives inside a chest alongside the cylinder, by way of a sliding regulator which the driver controls. Inside the steam chest waits a crank-driven slide valve – a flat plate which shuttles to and fro in the opposite direction to the piston, sequencing steam entry and simultaneously opening a way out for the spent exhaust.

Figs 2 and 3 show the slide valve in action. The cylinder has a port at either end which can let fresh steam in or spent steam out. At one end of its stroke, the slide valve lets fresh steam in to one side of the piston (A).

Full boiler pressure starts to push the piston across, driving spent steam out through B and into the central exhaust passage, which takes it off to the stack. The travelling slide then closes port A, trapping the fresh steam and giving it a chance to expand (fig 3).

As the slide moves to the other end of its stroke, the valves turns things round. Port A now connects to the exhaust, port B to fresh steam. So the piston is pushed back the way it came, blowing out the first lot of now-spent steam as it goes. Cunning valve timing lets the new steam in just before the piston comes to rest, cushioning its turn-round and maximising the big shove.

A single cylinder agricultural engine just pipes the exhaust into the stack, where its expansion lowers air pressure and so draws fresh draft through the fire. That is the "chuff" as an engine moves off or works under load, and explains why the stack belches a mix of smoke and steam.

Although a single cylinder engines exhaust has done its work, there is still a fair bit of expansion left in it. A compound agricultural engine takes the first cylinders used steam and ports it to a second (larger diameter) piston, while some marine and locomotive units take the residue from this on to a third or even fourth piston. That way, every usable drop of energy is wrung from the supply. Forwards or backwards?

The piston is powered in both directions. So it should be possible to choose which way it starts off, and thus which way the crankshaft turns. And once you can alter crank rotation at will, who needs a reverse gear?

Enter the Stephensons link. This simple but fiendishly clever device passes motion to the slide valve, picking it from a pair of offset eccentrics on the crank (fig 4).

It is much easier seen than described, so take a squint at the diagram. Drive rod ends are joined by a moveable link, whose vertical position can be changed by the driver using a quadranted reversing lever. Depending on where this is set, one or other of the rods takes priority in driving the slide valve, thus altering its start point relative to the piston and letting live steam first to one side of it or the other.

Lever setting also controls how far the valve moves, thus varying the time allowed for steam expansion. By juggling regulator and reversing lever settings, the driver can feed full boiler pressure to his engine and so make it deliver maximum torque at zero rpm (try doing that with a diesel), or make it work more economically by shutting the inlet port early and letting expansion do most of the work. &#42

Hot and steamy. Steam power, and the style of engineering it required, remains a fascinating subject for many enthusiasts.

WHY ALL THE HEAT AND PRESSURE?

High temperatures are vital to any engine. The hotter the expanding medium – gas and air for a diesel, steam for a traction engine – the more energy it holds and the harder it can shove down a piston. At sea level water boils at 100C. Once this temperature is hit, it cant take in extra heat; adding more only speeds up the boiling, rather than making the liquid hotter. To persuade water to take in more heat energy you need to stop it boiling, and the way to do that is to hold it in a sealed chamber and let pressure build. The higher the pressure over the liquid, the higher is its boiling point. By building a boiler strong enough to take pressures of 10.5-12.0 bar (160-180psi), water can take up much of a coal fires heat before it boils. Good Victorian engineering managed to roll plate more than 12mm (0.5in) thick, then lap and rivet it steam-tight. The prudent designer added a spring-loaded safety valve at the top to bleed off burst-threatening overpressure.

High temperatures are vital to any engine. The hotter the expanding medium – gas and air for a diesel, steam for a traction engine – the more energy it holds and the harder it can shove down a piston. At sea level water boils at 100C. Once this temperature is hit, it cant take in extra heat; adding more only speeds up the boiling, rather than making the liquid hotter. To persuade water to take in more heat energy you need to stop it boiling, and the way to do that is to hold it in a sealed chamber and let pressure build. The higher the pressure over the liquid, the higher is its boiling point. By building a boiler strong enough to take pressures of 10.5-12.0 bar (160-180psi), water can take up much of a coal fires heat before it boils. Good Victorian engineering managed to roll plate more than 12mm (0.5in) thick, then lap and rivet it steam-tight. The prudent designer added a spring-loaded safety valve at the top to bleed off burst-threatening overpressure.

STEAM, REST IN PEACE

If the idea of running an engine on anything burnable appeals and you are wondering why we dont do more of it, the reasons are hard to edge round. Turning water to steam takes much longer than turning a tractors start key; about 90min from lighting the fire to getting underway. And even the best steam engines only turn 15-18% of the fuels energy to useful work, while agricultural engines struggled to better 5-10%. Even a basic diesel manages about 30% efficiency. But when the oils runs out, we might be glad of steam power again.

1 Steam generation. Air drawn in below the fire is heated, passes through the fire tubes and out into the smoke-box. Water in the barrel surrounding the fire tubes turns to steam, which collects in the barrel top and cylinder jacket. A regulator (not shown) lets it to the cylinders slide valve (figs 2 and 3). Should the driver let water level drop too far, the fusible plug melts and puts out the fire.

2 (left) The working cylinder, minus steam jacket and regulator, showing the crank-driven slide valve (top) and piston (bottom). The valve shuttles to and fro in the opposite direction to the piston. As the slide opens, high pressure steam from the regulator rushes through port A and pushes the piston left, turning the crank. Exhaust steam from the previous stroke leaves through port B. Note: Piston sealing rings (generally two or three) are not shown. 3 (right) Expansive working. Once the valve moves far enough right to close port A, no fresh steam can get in. So the charge trapped in the cylinder expands. Using the Stephensons link (Fig 4) the driver can shift valve position relative to the piston, cutting off the steam supply later (for maximum torque) or earlier (for maximum expansion, and thus economy). Changing relative valve position when the piston is stopped decides whether steam comes first through A or B. This decides which way the piston goes and thus which way the crank turns, settling whether the engine moves off forwards or backwards.

4 Stephensons link. The crankshaft drives a pair of eccentrics (1 and 2). If the driver sets his reversing lever to position the vertical link (centre) as shown, eccentric 1 does the driving. Lifting the link would let eccentric 2 do the work, changing the relative slide valve/piston position and thus reversing piston travel. Towards the centre point of the link slide valve travel is reduced, cutting steam flow earlier and increasing expansive working.