How it Works: The Steam Engine

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How It Works: Steam Engine

1903 Jaxon Steam Car

Stanley, Locomobile and White

Turn the clock back a century or so and the automotive industry was still sorting out which would become the favoured mode of engine design. It was a time when coal was cheap, and steam cars vied with petrol and battery-stored electricity for dominance. In a time when steam engines had already enjoyed a full century of development and the internal combustion was new and almost untried, the steam car was strongly competitive: the best remembered of the early steam cars-the Stanley, the Locomobile and the White - and all were smoother and faster than most petrol cars.

A Stanley with a stream-lined body and a special 0.5:1 axle took the speed record at Daytona Sands, and a White known as Whistling Billy won many races on primitive American oval tracks. Nevertheless, the steam car even then showed certain severe drawbacks: its boiler was potentially dangerous, it was very heavy, and slow to start from cold. Moreover it used vast amounts of water, which limited its range: the 30 hp Stanley used water at 25Ib/hph, yet it was not until after 1914 that it was given a condenser for recycling the water.

By that time, the petrol engine was dominant and in mass production, whereas the Stanley (most popular of the steamers) never exceeded its 1906 record of 650 cars built in a year. By the 1920s steam was virtually off the roads, except for steam wagons in Europe and the Doble car in the USA. The Doble was a legend in its own time, a classic among steamers. It had a flash boiler in a closed circuit, could generate full steam pressure in a minute from cold, and accelerate smoothly and very fast to a top speed of 90 mph. The most impressive figure was the price: it cost US$10,000 dollars, and in its eight years career only a few dozen were built.

Abner and Warren Doble

In the early 1930s the Doble firm was bankrupted: the Dobles themselves, Abner and Warren, became steam consultants to other automotive firms, and the other assets of the company were acquired by the Besler Corporation. Besler did some amazing things. In 1933 they built and flew a steam-powered aircraft, confirming the predictions of Erasmus Darwin. It had a 150 hp condensing plant that for its power was probably the lightest yet built, weighing only 675 lb. Fuel and water were more burdensome, but the water rate (a frequently used index of steam engine efficiency) was down to 10 lb of water per horsepower hour, when anything less than 7 was exceptional and unsophisticated engines operated nearer the 30 mark.

The Besler Steam Plane

The most extraordinary thing about the Besler steam plane was its quietness: the pilot could engage in shouted conversations with people on the ground! Thereafter Besler concentrated on steamers for the railways and the US Navy, though an unsuccessful conversion of a Chevrolet car attracted some attention in 1969. Meanwhile, Abner Doble was acting as consultant to the Sentinel company, who had been making steam wagons in England since 1906. Their last production model, introduced in 1930, only disappeared from the catalogues in 1952: it was a huge beast of a thing that could draw on its furious boiler (at 14Ib/hph) for an overload performance of 120 bhp for brief uphill struggles against a heavy payload.

The McCulloch Paxton

The water load diminished rapidly, at the rate of 4 gallons per mile, and coke fuel at the rate of 20 miles per hundredweight; but Doble designed a condensing compound engine that was expected to halve this figure. The Doble cars had burned gasoline or kerosene. In Germany they preferred fuel oils derived from coal, and Warren Doble did valuable work for Henschel, putting steam engines in trucks and buses - in which latter the drivers were thus saved about 4000 gear-changes each day. Abner Doble went from Sentinel back to the USA, where McCulloch (makers of chainsaws and lightweight petrol engines) fancied a steam engine for their proposed Paxton car.

Doble designed a generator producing a steam pressure of 2000 psi, probably the highest ever used in a car, and its associated 6-cylinder compound expansion engine could sustain 120 bhp or reach 150 briefly. The complete power plant weighed 953 lb but the intended steam temperature of 1200°F had to be dropped to 900°F because of its effect on lubricants. It was just one example of enthusiasm for steam being checked at an expensively late stage by the realisation that it creates more problems than it solves; but the enthusiasts kept trying to improve the steam engine in competition to the petrol driven version, encouraged by a new wave of politically-induced public hysteria about the evils of the petrol-engined private car.

Amedee Bollee's first 1878 steam-powered car
Amedee Bollee's first 1878 steam-powered car.

Amedee Bollee's steam-powered Rapide of 1881

Amedee Bollee's steam-powered Rapide of 1881 was capable of 60 km/h.

1898 Stanley two-seater
1898 Stanley two-seater.

1904 Stanley Steamer
1904 Stanley Steamer.

Stanley Wogglebug
In 1906 Fred Marriott set the Land Speed Record of 127.66 mph the famous Stanley Wogglebug. He also drove a different steam car over 150 mph the following year, but the vehicle crashed before the official speed was recorded.

The Keen and Lear Steam Cars

But by the middle of the 20th Century it was obvious to all that petrol was the victor, but still there were those that persevered with steam, In the late 1960s there was the Keen steam car, the Williams, the Pritchard, the Saab, and odd new engines like the Gibbs & Hosick Elliptocline ... these all appeared to sue for a brief and unprofitable notoriety. Most notorious and most interesting of them all was the Lear steam car which was entered for Indianapolis in 1969, but which was abandoned as a failure before it ever ran in public. The failure of the Lear steamer focussed the limelight on the steam engine in a way that must have made Lear the envy of many.

Soon Ford and General Motors and even British Leyland were boasting of, or admitting to, research programmes looking not only for steam's refinement of running but also for a power unit whose exhaust products did not pollute the atmosphere. Unfortunately the advantages of steam can only be gained at a sacrifice in thermal or mechanical efficiency - that is not to say that steam can not be efficient - in electrical power generation applications it can reach thermal efficiencies as high as 44 per cent, better than even than the diesel engine can manage. But the problem was always with the scale - that is, the size restrictions of what could be applied to an automobile. Steam is superheated, reheated, recirculated, condensed; the heat abstracted in the condensation process is used to preheat the boiler feed water, and as like as not the condensate itself is sent around yet again for turning into steam.

No Gearbox Required

The reciprocating steam engine has some things in its favour, but not as many as you might think. Arguably the best feature is that the steam engine needs no gearbox: by varying the valve timings it is possible to vary not only the magnitude but also the direction of the torque, so that constant horsepower at all speeds and maximum torque when accelerating from zero are theoretically available both forwards and in reverse. The occasions for driving flat out backwards being statistically insignificant, this retrogressive ability may be dismissed as of no real value - and so the reciprocating steamer may be seen to have no particular advantage in respect of transmission requirements over, say, the differential diesel, which can in theory do without a multi-ratio transmission although in practice a two-speed gearbox has been found useful.

In any case the weight represented by the deleted transmission might be more than countered by the weight of a steam engine's ancillaries, which is considerable. Things like the boiler feed pump, the air pump, the cooling air fan, the forced draught fan and the fuel feed pump all have to be energised even when the propulsive unit is stationary. This makes nonsensical the often quoted advantage of the steam engine, that it is only working when there is work to do and does not have to waste a lot of fuel by ticking over when the car is stationary in traffic. The power consumption of the steam engine's auxiliaries is really oppressive, and even though they may be steam-powered most of the time (a duty for which the steam turbine is ideal), they need some sort of electric drive during the phase of raising steam from a cold start.

Mastering the Cold Morning Start

The fan that blows air through the combustion chamber needs a big motor for its impulsion, one which will drain a lot of electricity in the vital seconds of fire-up and warm-up before the steam drive to the auxiliaries can take over: the motors and battery system will weigh as much as any normal four/five or six-speed gearbox. The original steam engine boilers took minutes or even hours to warm up, and even Doble's sixty seconds was considered by many critics to be too slow. By the mid 1960s steam systems had progressed to the point where they could go from switch-on to drive-away in thirty seconds from a stone cold start, or as little as ten seconds in less forbidding circumstances.

The other objection to steam power was the waste of energy up the funnel. In locomotives, steam jets provided the induced draught for the boiler: the steam exhausted from the working cylinders was directed by a blast pipe up to the smoke box in conditions which aided and even automatically regulated the breathing of the entire apparatus. The. whole process was very wasteful, and if this thermal inefficiency is to be checked then the steam must not be allowed to escape: it must be captured, condensed and used again. This highlights one of the great problems of steam, that of heat rejection. All heat engines rely on temperature differences in the working fluid, be it water or air, mercury etc.

In the internal combustion engine this heat is liberated actually in the working fluid as a result of the combustion that takes place within the cylinder or turbine. Steam, however, relies on external combustion, so the heat can only be fed into the water through some separating wall, conventionally the material of the boiler tubing. The heat ultimately has to be taken out through a similar barrier if it is not to be wasted by venting steam to the atmosphere, and this means that the heat must be fed through a heat exchanger that functions very much like the radiator of an ordinary car. This is part of the condensing system. In simplified terms, the steam exhausted from the working cylinder is passed through a heat exchanger where it is cooled and condensed into water which may then be led back to the boiler for conversion into steam. The heat removed from the steam in the process of condensation has to be shed, passing through the walls of the the heat exchanger or radiator and being carried away by an induced air flow.

The Importance of the Condenser

Petrol engines find it difficult enough to have to shed heat at a temperature not very far above that of boiling water; yet the performance of the condenser is far more critical to a steam engine, because the process of condensation creates a vacuum which augments the efficiency of the exhaustion of steam from the working cylinder - or, to put it in thermodynamic terms - encourages the continued expansion of the steam fed into the cylinder, the expansion ratio being equivalent to the compression ratio in a petrol engine. The condensing steam engine depends not only on its boiler, nor on its valve gear, but mainly on its condenser for its performance, since it is the condenser that governs the rate of heat rejection. Therefore it has to reject far more heat through its radiator than a petrol engine has to, and yet must evaporation condensation volume.

The Rankine Thermodynamic Cycle

This is the Rankine thermodynamic cycle of a steam engine. The active phase is that in which the steam expands and forces the piston, or turbine, to move. maintain a lower temperature. In theory, the steam engine could show increased efficiency if the valving were arranged to cut off the introduction of steam to the cylinder at an increasingly early point in the cycle as the operating speed rises. As cut-off advances like this, expansion ratio increases; correspondingly retarding the cut-off allows a steam engine to give brief overload power quite out of proportion to its size, at alarming cost in thermal efficiency and boiler pressure recovery, though these aspects are never mentioned by steam power enthusiasts who point to the bhp of a tiny reciprocator which is only a small fraction of the total power-generating system. If cut-off could be advanced to the point where the pressure of the steam fully expanded in the cylinder were equal to the condenser pressure, efficiency would be very high.

Unfortunately things can never work out as well as that, for heat tends to be transferred in the wrong places within the engine, .cooling the fresh steam and heating the exhaust . One answer to the problem is to compound the engine in the classical way, by providing double, triple or even quadruple expansion of the steam in successive cylinders, each of which is larger than the one before it. This can be done most effectively in a turbine, but it is not compatible with high mechanical efficiency in a piston engine, and is dismissed by most engineers as too inefficient and cumbersome for the needs of an essentially cheap and simple car.

Another solution is to use the uniflow cycle, in which high-pressure steam is admitted to the cylinder through mechanically operated valves in the cylinder head, the expanded steam being exhausted through piston-controlled ports uncovered in the cylinder walls as the piston approaches the bottom of its stroke. In this case the expansion ratio is fixed by the exhaust port position (though variable valve timing could modify it) and the engine becomes dependent as no other on the proper working of the condenser: if condenser vacuum fails, the engine explodes. The third solution to the problem lies in running the engine at very high speeds. Such an engine will not be efficient at low speeds, so one of the supposed advantages of steam engines disappears.

The high-speed engine also runs into problems of high frictional losses reducing its mechanical efficiency, and into problems of lubrication that are a real bugbear in a condensing steam cycle. The oil gets into the steam, is carried into the condenser, travels thence back to the boiler in the water, and is there deposited at the point where the water is vaporised afresh into steam. At best, the accumulated deposits impair efficiency; more commonly they cause the tubes to burn through and the boiler to fail. Solid carbon can be used on rubbing surfaces, having unique abilities to act as a lubricant in water at high temperatures, and the substitution of the refrigerant Freon for water as the working fluid may be a costly palliative too; but the problem remains.

Saving the Planet

California is the one US state that has always been compulsively drawn to a campaign against atmospheric pollution that has made the petrol-engined car its particular scapegoat. It was with the Californian authorities that Lear hoped to do good business. The trouble with the Lear steam engine was that it was too clever to be good. All sorts of engineers and critics acclaimed it a progressive piece of engineering, and so it was. Its shape was that of a delta six, whose three banks of two cylinders were united by three crankshafts located at the corners of the equilateral triangle formed by the cylinders (just as in a Napier Deltic diesel) with two opposed pistons in each cylinder. This layout had a great deal to commend it, but resulted in each pair of opposing pistons working 60 degrees out of phase, rather to the detriment of the rapid expansion necessary in an engine built to turn at very high speeds.

High speeds helped prevent the transient heat losses, though they introduced mechanical losses that would be aggravated in the Lear delta by the extremely short connecting rods that were part of the design - possibly with the notion that the high piston accelerations consequent upon the rods' shortness would to some extent offset the unfortunate effects of the 60 degrees phase shift. On balance the advantages were in compactness and mechanical simplicity, as it was possible with such an engine configuration to govern the admission of steam to all six cylinders through a simple rotary valve passing down the centre of the delta. The shaft on which this valve was mounted was geared to the three crankshafts, and formed the output shaft, emerging from the geometrical centre of an extraordinarily compact engine.

The California State Highway Patrol

All steam engines look remarkably compact in relation to their potential power output. It is the ancillaries that tip the scales. The police car prepared by Lear for the California State Highway Patrol, required to reach 130 mph in three miles weighed three tons. The proposed Indianapolis racer would have been much more difficult; perhaps it was designed with a view more to publicity rather than with any serious competitive intent. Nevertheless there was plenty of interest in the Lear design, especially with heat rejection. Recognising that, to get rid of heat effectively, a really steep temperature gradient is desirable, the radiator was arranged to contain water at very high temperature and therefore very high pressure.

The high temperature was a very good thing, since it enabled the heat to be dissipated into the minimum quantity of cooling air; but the high temperatures and pressures within the radiator required it to be built very strongly and inevitably heavily. From this it follows that it must be kept as small as possible, and they argued that the condensation process would have to be carried out upstream of the heat exchanger. What they proposed was a jet condenser in which the steam was expanded through a nozzle, the condensate thus formed being run to a highly pressurized heat exchanger. But there was a snag: high condenser temperature means a smaller pressure difference between the working cylinders and the condenser, and thus a lower operating efficiency because of the lower expansion ratio.

Why Steam Will Never Replace Petrol

To compensate for this, the Lear's steam was super-heated to raise it to very high temperature and pressure, though at 2000 psi it was still nowhere near the critical level. Obviously the water passes through a regenerative cycle which sees it being vaporised and condensed over and over again, so there was also the lubricant worry that (despite the use of Freon instead of water) was never really overcome satisfactorily. It seems doubtful whether a satisfactory compromise can ever be found in all the conflicting demands of the many components that make up a steam power plant and so critically affect its efficiency. There are too many efficiencies involved: the efficiency with which the combustion heat is transferred to the water so as to evaporate it, the efficiency with which the steam is fed to the working cylinders, the efficiency of its expansion, the efficiency of the condenser vacuum which in turn governs the expansion ratios so critically, followed by the efficiency of the condenser and the heat exchanger doing their respective jobs so as to recover all the heat from the steam and to reject as much of it as has to be shed, after using as much of it as possible for such worthwhile contributions as preheating the feed water.

The list seems never-ending, and everyone of these items can have a serious effect on the overall efficiency of the steam plant, not to mention the mechanical efficiency of the reciprocating engine itself. Even allowing very optimistic efficiencies as high as 80 per cent for as few as five basic stages in the superheating/condensing steam cycle, you still end up with a thermal efficiency below 33 per cent, a level at which petrol engines have been working for years. In fact steam efficiencies are generally much lower than this, especially in condensers. The twenty per cent efficiency at the driving wheels of the condensing Besler engine used on the New Haven Railroad's partially experimental 'Turbo train' was an exceptional performance.

So what hope is there for the steam car? With several centuries of development, it seems unlikely the steam engine will ever be a suitable alternative to the petrol driven internal combusion engine. As an alternate souce of power, our money is on hydrogen.
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