The uniflow type of steam engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

 uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency

Thermal efficiency

In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...

 is increased in the compound and multiple expansion types of steam engine by separating expansion into steps in separate cylinders; in the uniflow design, thermal efficiency is achieved by having a temperature gradient along the cylinder. Steam always enters at the hot ends of the cylinder and exhausts through ports at the cooler centre. By this means the relative heating and cooling of the cylinder walls is reduced.

Design details

Steam entry is usually controlled by poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

s (which act similarly to those used in internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

s) that are operated by a camshaft

Camshaft

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

. The inlet valves open to admit steam when minimum expansion volume has been reached at the start of the stroke. For a period of the crank cycle steam is admitted and the poppet inlet is then closed, allowing continued expansion of the steam during the stroke, driving the piston. Near the end of the stroke the piston will uncover a ring of exhaust ports mounted radially around the centre of the cylinder. These ports are connected by a manifold and piping to the condenser, lowering the pressure in the chamber below that of the atmosphere causing rapid exhausting. Continued rotation of the crank moves the piston. From the animation the features of a uniflow engine can be seen, with a large piston almost half the length of the cylinder, poppet inlet valves at either end, a camshaft (whose motion is derived from that of the driveshaft) and a central ring of exhaust ports.

Advantages

Uniflow engines potentially allow greater expansion in a single cylinder without the relatively cool exhaust steam flowing across the hot end of the working cylinder and steam ports of a conventional "counterflow" steam engine during the exhaust stroke. This condition allows higher thermal efficiency. The exhaust ports are only open for a small fraction of the piston stroke, with the exhaust ports closed just after the piston begins traveling toward the admission end of the cylinder. The steam remaining within the cylinder after the exhaust ports are closed is trapped, and this trapped steam is compressed by the returning piston. This is thermodynamically desirable as it preheats the hot end of the cylinder before the admission of steam. However, the risk of excessive compression often results in small auxiliary exhaust ports being included at the cylinder heads. Such a design is called a semi-uniflow engine.

Engines of this type usually have multiple cylinders in an in-line arrangement and may be single- or double-acting. A particular advantage of this type is that the valves may be operated by the effect of multiple camshafts, and by changing the relative phase of these camshafts, the amount of steam admitted may be increased for high torque at low speed and may be decreased at cruising speed for economy of operation, and by changing the absolute phase the engine's direction of rotation may be changed. The uniflow design also maintains a constant temperature gradient through the cylinder, avoiding passing hot and cold steam through the same end of the cylinder.

Disadvantages

In practice the uniflow engine had a number of operational shortcomings. The large expansion ratio required a large cylinder volume. To gain the maximum potential work from the engine a high reciprocation rate was required, typically 80% faster than a double-acting counterflow type engine. This caused the opening times of the inlet valves to be very short, putting great strain on a delicate mechanical part. In order to withstand the huge mechanical forces encountered, engines had to be heavily built and a large flywheel was required to smooth out the variations in torque as the steam pressure rapidly rose and fell in the cylinder. As there was a thermal gradient across the cylinder, the metal of the wall expanded to different extents. This required the cylinder bore to be wider in the cool centre than at the hot ends. If the cylinder was not heated correctly, or if water entered, the delicate balance could be upset causing seizure mid-stroke or, potentially, destruction.

History

The uniflow engine was first used in Britain in 1827 by Jacob Perkins

Jacob Perkins

Jacob Perkins was an Anglo-American inventor, mechanical engineer and physicist. Born in Newburyport, Massachusetts, Perkins was apprenticed to a goldsmith...

 and was patented in 1885 by Leonard Jennett Todd. It was popularised by German

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 engineer Johann Stumpf

Johann Stumpf (engineer)

Johann Stumpf of the Charlottenburg Technical College in Berlin is best known for popularising the uniflow steam engine, in the years around 1909, and his name has always been associated with it...

 in 1909, with the first commercial stationary engine produced a year previously in 1908.

The uniflow principle was mainly used for industrial power generation, but was also tried in a few railway locomotives in England, such as the North Eastern Railway

North Eastern Railway (UK)

The North Eastern Railway , was an English railway company. It was incorporated in 1854, when four existing companies were combined, and was absorbed into the London and North Eastern Railway at the Grouping in 1923...

 uniflow locomotives No 825

NER Class S

The North Eastern Railway Class S were a 4-6-0 type of steam locomotive designed for express passenger workings. The first example was built in 1899. They were very similar to the NER Class S1, except for the smaller wheels of the Class S. They were designed to reduce double heading on the East...

 of 1913, and No 2212 of 1919, and the Midland Railway Paget locomotive

Midland Railway Paget locomotive

The Midland Railway's Paget locomotive, No. 2299, was an experimental steam locomotive constructed at its Derby Works in 1908 to the design of the General Superintendent Cecil Paget ....

. Experiments were also made in the USA and Russia. In no case were the results encouraging enough for further development to be undertaken.

The final commercial evolution of the Uniflow engine occurred in the USA during the late 1930s and 1940s by the Skinner Engine Company with the development of the Compound Unaflow Marine Steam Engine. This engine operated in a steeple compound configuration and provided efficiencies approaching contemporary diesels. Many lake freighter

Lake freighter

Lake freighters, or Lakers, are bulk carrier vessels that ply the Great Lakes. The best known was the , the most recent and largest major vessel to be wrecked on the Lakes. These vessels are traditionally called boats, although classified as ships. In the mid-20th century, 300 lakers worked the...

s and car ferries

Train ferry

A train ferry is a ship designed to carry railway vehicles. Typically, one level of the ship is fitted with railway tracks, and the vessel has a door at the front and/or rear to give access to the wharves. In the United States, train ferries are sometimes referred to as "car ferries", as...

 on the Great Lakes

Great Lakes

The Great Lakes are a collection of freshwater lakes located in northeastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface, coming in second by volume...

 were so equipped, several of which are still operating.http://www.boatnerd.com/pictures/fleet/stmaryschallenger.htmhttp://www.boatnerd.com/pictures/fleet/badger.htm Notable among these is the SS Badger. The Casablanca class escort carrier

Casablanca class escort carrier

The Casablanca class escort aircraft carriers were the greatest number of not only escort carriers, but also any size aircraft carrier ever built to a like-design by any nation at any time. Fifty were laid down, launched and commissioned within the space of less than two years - 3 November 1942...

, the most prolific aircraft carrier design in history, used 2 5-cylinder Skinner Unaflow engines.

In small sizes (less than about 1000 horsepower), reciprocating steam engines are much more efficient than steam turbines. The Whitecliffs solar steam power plant

White Cliffs Solar Power Station, New South Wales

White Cliffs Solar Power Station was Australia's first solar power station. It is located at White Cliffs, New South Wales, which was chosen as it has the highest insolation in New South Wales, and in 1981 when the station was constructed had no grid connection.Constructed by a team from Australian...

 used a three-cylinder uniflow engine to generate about 25 kW electrical output.

The single-acting uniflow steam engine configuration closely resembles that of a two-stroke internal combustion engine, and it is possible to convert a two-stroke engine to a uniflow steam engine by feeding the cylinder with steam via a "bash valve" fitted in place of the spark plug. As the rising piston nears the top of its stroke it knocks open the bash valve to admit a pulse of steam. The valve closes automatically as the piston descends, and the steam is exhausted through the existing cylinder porting. The inertia of the flywheel then carries the piston back to the top of its stroke against the compression, as it does in the original form of the engine. Also like the original, the conversion is not self-starting and must be turned over by an external power source to start. An example of such a conversion is this steam-powered moped, which is started by pedalling.
The Whitecliffs solar steam power plant

White Cliffs Solar Power Station, New South Wales

White Cliffs Solar Power Station was Australia's first solar power station. It is located at White Cliffs, New South Wales, which was chosen as it has the highest insolation in New South Wales, and in 1981 when the station was constructed had no grid connection.Constructed by a team from Australian...

also used a three-cylinder uniflow engine with "bash" type admission valves.

Sources

• Teach yourself heat engines by E. de Ville, published by The English Universities Press Limited, London, 1960, pp 40-41

External links

• The Museum of Retro Technology – Uniflow Steam Engines