The Still Steam-Diesel Engine

Updated: 10 Oct 2011
More info, new pic added
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The Still steam-diesel system was designed to increase overall cycle efficiency by combining a Diesel engine with a steam engine. There were double-acting cylinders, the top half working as a Diesel, while steam acted in the bottom half, working on the underside of the built-up piston. The steam was generated from the hot Diesel exhaust.
The system was invented by Mr William Joseph Still of London, who patented it in the USA in 1917. (US Pat no: 1,230,617) On 26 May 1919 in London he presented it to a meeting of the Royal Society of Arts, which as it happens was chaired by steam turbine inventor Charles Algernon Parsons.


William Joseph Still
The first Scott-Still engines were installed in the twin-screw cargo liner Dolius, owned by the Blue Funnel Line, also known as Alfred Holt & Co of Liverpool, in 1924. Her deadweight capacity was about 8000 tons and displacement was 11,533 tons. Scott's of Greenock, Scotland, were the shipbuilders. According to Hardy, on her maiden voyage from Cardiff to Algiers (carrying coal) she averaged 11.45 knots burning 8.4 tons of fuel per day, for propulsion and all services. This was a very economical performance for a motor-ship at that time; the testbed specific consumption at full load was 0.356 lb/HP/hour, well in advance of the most contemporary conventional diesels.

A smaller type of Still engine for coasters was developed by the Newbury firm of Plenty, but the project was abandoned.

Left: A small land installation of the Still system.

This picture (which appears to be an artist's impression) was used to illustrate an article on the Still engine in Popular Science, in September 1919.

One advantage claimed was that the reserve of steam in the boiler would deal with brief overloads which would defeat a straight internal combustion engine of the same power. Extra heating of the boiler, apart from the diesel exhaust, was occasionally used, to provide extra power to handle exceptional loads, and to allow the engine to start on steam power alone.

From the beginning there was controversy as to whether the greater efficiency and load-handling capacity was enough to justify the extra complexity. Such records as survive indicate that despite undisputably high efficiency the engines of the Dolius were intensely disliked by the engine-room staff as being over-complex and unreliable. Any fault with the steam side was likely to affect the diesel side, and vice-versa.

Left: This is a conceptual diagram of the Still system.

The steam cylinder is below the double-acting piston, and the Diesel cylinder above.

The Diesel exhaust C passes into a heat exchanger in one drum of a modified Yarrow boiler A, working at a steam pressure of 135 psi. The engine water jacket is connected to this boiler at full pressure by pipes Q. The boiler also has an oil burner N arranged to heat the boiler tubes in the usual way; steam is thus raised to preheat and start the Diesel.

Boiler feed is drawn by pump L from hotwell K, and passes through the economiser B, which is heated by the exhaust leaving the boiler.

Steam is taken from the top boiler drum at D and fed to the steam cylinder. The exhaust E from the steam cylinder powers a low-pressure turbine F, which drives the Diesel scavenge blower G of the diesel engine. The electric motor M is needed as part loads do not generate enough steam to drive the turbine; this could also be used as a generator when there was plenty of steam. The engine back-pressure was 19 in Hg vacuum, and the turbine exit vacuum 29 in Hg. I is probably some sort of jet condenser, with J either its injection pump or its air pump.

Various unique advantages were claimed for the shared-cylinder Scott-Still engine:

1) There is no heat loss caused by condensation of the admitted steam on the cylinder walls because they are preheated by the previous diesel combustion stroke.

2) The piston is kept from overheating by the presence of the relatively cool steam on its underside.

Two four-cylinder Scott-Still engines were installed on the Dolius. The steam cylinders had some sort of hydraulic valvegear, the details of which are currently obscure. The steam side was effectively triple-expansion; the aftmost steam cylinder was a high-pressure cylinder, the remaining three being run at intermediate pressure. Final LP expansion was done in the turbine driving the the scavenge blower. Overall efficiency was claimed as 38%, with 14% of the power coming from the steam side. That 14% involved a lot complication. Engine speed was 120 rpm.
The Dolius was lost during the Second World War; in 1943 she was torpedoed and sunk in the Gulf of St. Lawrence, Canada.

A second vessel was fitted with Scott-Still engines; the twin-screw cargo liner Eurybates built in 1928, and also part of the Blue Funnel line. The engines were more powerful than those of the Dolius, now giving 2500 bhp per shaft at 105 rpm, and had been drastically redesigned. The six forward cylinders were conventional two-stroke single-acting diesel units, of 17 in diameter by 45 in stroke, and the two aft cylinders constituted almost an independent steam engine, using Marshall valvegear and piston valves. This radical change appears to have been made to prevent oil from the diesel section of the engine from contaminating the boiler feedwater.
The steam cylinders were only converted to Diesel in 1948, so the principle must have been workable.

If anyone cares, Dolius and Eurybates are minor characters in Homer's Odyssey.

Left: The engine-room arrangements of the Dolius.

Each four-cylinder engine developed 1250 hp at 120 rpm. The diesel side operated on a two-stroke cycle with airless injection.

Unfortunately this diagram shows no real details of the engines themselves.

From "History of Motorshipping" by A C Hardy, Whitehall Technical Press Ltd
Above: Drawing of a Scott-Still engine-room arrangement for a single-screw cargo-vessel.

This drawing comes from an article contributed by Scott's themselves; it does not appear to relate to any particular vessel. The engine is a two-stroke Scott-Still type with four cylinders of 25 in diameter by 45 in stroke; it could run either ahead or astern on oil. They say "Operation of the steam inlet and exhaust valves is controlled by distributors driven by gearing from the crankshaft at engine speed, and reversing is accomplished by the movement of piston valves in the distributors. A pump of Hele-Shaw type, with automatic variable-delivery gear, supplies oil at 400 psi to the distributors for the operation of the steam valves. This pump is driven off the condenser circulating pump, which is arranged to suit this duty." It is not made clear what a 'distributor' is, but clearly hydraulic steam-valve operation is intended.

The steam boiler (labelled 'regenerator' on the drawing) had a steam generating surface of 1150 sq ft and worked at 150 psi. Apart from the Diesel exhaust heating it also had oil burners for raising steam, so it could be used as a donkey boiler as well as for starting the Diesel part of the engine.

From "The Marine Engineer & Naval Architect" March 1923. Note that this is a year before the first installation in the Dolius.

Left: This intriguing diagram shows the comparative efficiencies of steam and IC engines in 1920.

I should say at once that I have not the slightest idea how accurate these figures are. However, assuming they are accurate, they tell an interesting story. Steam engines are in the left column, and oil and gas engines in the right. The most efficient straight steam-plant is the 25000 kW Parsons turbine, coming in at 20%, which is easily beaten by the best Diesel at 36%. However, at the top of the left column come various versions of the Still cycle, with efficiencies from 31% to 41%.

Railway locomotives languish at the bottom of the steam column, giving between 4.5 and 6%.

Diagram taken from an article written by Captain Frank Acland, first published in The Journal of the Royal Society of Arts, and reprinted in The Journal of the Society of Automotive Engineers in June 1920.


The information below was kindly provided by Brian Dickson. I have edited it slightly. He writes:

"I served with the company concerned (Blue Funnel), but after those ships had gone, and my knowledge comes with talking to senior engineers who had served on them.

"The Scott-Still marine engines were of two designs, the first with diesel on the top of the piston,and steam on the underside of the piston, as illustrated; this was the approach used in the Kitson-Still locomotive. The second version had diesel power on five cylinders, and then two conventional steam cylinders on the same crankshaft providing power from the exhaust-heated boiler.

"The first design (fitted in the Dolius) suffered from the basic flaw of cross-contamination. Oil from the diesel part of the engine contaminated the boiler feedwater, (this seriously cuts down heat transmission in the boiler), and water contaminated the lubricating oil in the diesel engine crankcase.

"It was the second type of engine that was fitted in the Eurybates, and this avoided the cross-contamination problems as the steam cylinders were segregated from their diesel counterparts. It also meant that the two steam cylinders concerned could be, (and were) replaced by conventional diesel cylinders when other economic factors dictated it.

"These vessels required dual qualified senior engineering officers (Steam and Motor certificated), whereas conventional steam vessels only required Steam officers, and conventional Diesel motor vessels only required Motor officers. The career path of dual qualified officers, once qualified, was usually to quit seagoing, and go into survey\classification employment ashore.

"Thus, although they were a revolutionary concept, efficiency-wise, these engines exacted personnel and maintenance problems which offset the economy."


Left: An excerpt from "Modern Motor Cars and Commercial Vehicles" by Arthur W Judge

Judge was a well-known writer on internal-combustion engines.

I cannot comment on the accuracy of his figures, but it adds a little more background to this enigmatic machine.

The publication date is unknown but appears to be around 1930. Hopefully this extract is short enough to avoid copyright problems.


EXHAUST GAS BOILERS

The desire to make use of the heat in diesel exhaust gases did not disappear with the Still reciprocating system. As engines grew larger the amount of heat available increased until it could generate enough steam to run a turbo-alternator that provided all the electrical power required in the ship, with a surplus for heating purposes. The exhaust gases of modern Diesel engines contain thermal energy equivalent to more than 30% of the total heating value of the fuel. The temperatures of the exhaust gases vary from 270C to 320C for slow two stroke engines, and from 350C to 480C for medium speed four stroke engines. Modern engines have increased in efficiency to the point where the exhaust heat available is reduced, and enough electricity can now only be generated by relatively sophisticated steam-generating systems.

Left: This is a modern exhaust boiler system

Apologies for the poor image quality.

The boiler at the top generates steam in the usual way with water tubes and a water/steam drum. The steam passes through a superheater in the hottest part of the exhaust gases before going to the turbine-alternator set. The steam leaving this is condensed, and the water ingeniously preheated in the scavenge air compressor intercoolers. (compressing air releases a lot of heat, and removing it between each stage of compression makes the process more efficient) The feedwater is further preheated in the economiser at the top of the boiler, and enters the boiler drum again.

Note that the scavenge-air cooler produces enough heat to also run a hot-water heating system. and even then requires additional cooling with sea-water.


FURTHER READING

For more on the Scott-Still System see Marine Diesel Engines: Scott (External link)

The Still system was also applied to locomotives on several occasions. See also The Kitson-Still Locomotive and Russian Reforms.

Bibliography: Scientific American, May 1927

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