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Steam engine (overview).

A steam engine is defined by its working fluid, steam obtained from boiling water. The heat required for boiling the water and supplying the steam is derived from a variety of processes, but mostly from burning some sort of combustible matter, solid, liquid or gaseous in a in a closed space with an appropriate supply of air. The heat is normally transferred to the water according to one of two principles (although the two are sometimes combined):

1) "the fire surrounds the water": water is contained in or run through one or several tubes surrounded by hot gases, (water tube boiler/generator) -

2) "the water surrounds the fire": the water partially fills a vessel below or inside of which is a combustion chamber or furnace and fire tubes through which the hot gases flow (firetube boiler).

Viewed as a complete system, there are two basic components to a steam engine: the boiler or steam generator and the motor unit, itself often referred to as a "steam engine" - or "engine part". The two components can either be integrated into a compact unit or else can work at a distance from each other, often in separate rooms or compartments. Steam generators and motor units can be combined in many ways and even have different careers with one or the other being independently replaced one or more times. The working of the two components combined is perhaps most easily understood as a sequence of events; each event can take place over more of less long period of time:

a) fuel is burnt in a closed furnace or firebox provided with an appropriate supply of air;

b) the heat of combustion is transferred to the water, boiling it and transforming it into its gaseous state in the form of saturated steam;

c) whatever the temperature of the heat source, steam in its saturated state, remains at the temperature of boiling water; the latter rises according to steam pressure increase on the water surface. The saturated steam exiting the boiler/steam generator can either be directly utilised according to step (d) or else may be further processed by raising its temperature (superheating it) in such a way as to reduce or eliminate suspended water droplets always present in saturated steam;this notably has the effect of economising feedwater and then go on to step (d);

d) the pressurised steam is transferred to a drive unit in order to obtain power to drive machinery or to displace the engine itself, often along with other non-powered vehicles; to do this it usually acts upon to one or more pistons or else upon the blades of a turbine;

e) finally the exhausted steam may be diverted and put to various auxiliary tasks such as to increasing the supply of air to the furnace; preheating boiler feedwater or it can to go to a condenser that turns it back to water ready to be recycled for further use. This can have the added benefit of to increasing temperature/pressure range and thus theoretical efficiency.

Certain of these phases such as superheating and condensing are optional and the cost of their application (including first cost of equipment and maintenance) has to be weighed against the actual benefits.

From the above it will be apparent that a steam engine cannot be fully understood in terms of internal combustion engine technology, which tends to be today's yardstick where heat engines are concerned; steam and internal combustion plant differ radically in their operating principles. Whereas in the internal combustion engine the action of combustion and power delivery are quasi-instantaneous, generally demanding the use of comparatively volatile, usually liquid fuels,this the working of the steam engine is perhaps best seen as a "long" process with a multiple sequence of phases occurring between combustion (which can be relatively slow) and power delivery. It has to be continually borne in mind when considering steam engine technology that combustion of fuel takes place separately from power delivery with the possibility of time lag between the two events; whilst these multiple phases tend to result in losses of efficiency, it is the relative independence of the processes that gives the possibility of independent development work on each phase knowing that each small improvement will contribute to the efficiency of the whole. This "organic" approach is the secret of the seemingly miraculous improvements achieved in the steam locomotive by André Chapelon and L.D. Porta.

Finally by isolating one or more selected phases, these can be readily incorporated into co-generation projects; for instance the steam generator can recuperate heat from another source, such as an internal combustion engine exhaust: another example would be a motor unit in isolation exploiting steam directly derived from a geothermal source[1].

In general usage, the term 'steam engine' may just as well embrace the whole integrated steam plant described above, (e.g.steam railway engine, portable engine mill engine) or else may be restricted to represent the motor unit alone (as in beam engine, oscillating engine,steeple engine...), regardless of the generator type supplying the steam. Moreover specialised devices such as steam hammers and steam pile drivers are also dependent on steam supplied from a separate, often remotely located boiler

The steam engine has sometimes been categorised External combustion engine, a category that also embraces the Stirling engine. Whilst gaining ground, this appellation is by no means universal usage; notably classifying the steam engine along with the Stirling engine fails to take into account the important possibility to store the working fluid in a pressure vessel for subsequent utilisation (e.g. fireless locomotive).

Confiiguration

In the case of stationary plant, boiler(s) and engine(s) or turbines are often housed in separate buildings; similarly in large ships, they frequently occupy separate compartments or "rooms". In this case the individual components may have independent histories, and may be replaced at different dates. Generally with smaller units, the two components are integrated into a whole ensemble in which case it is common to refer to the entire combined unit simply as an "engine", (e.g. "railway engine", traction engine, portable engine). Such can sometimes lead to terminological confusion. This integration has been carried to a high degree in the case of certain steam technologies recently researched[2] or at present (2008) at an advanced stage of development[3].

Applications

Since the early 18th Century steam Power has been set to a variety of practical uses. At first it was applied to reciprocating pumps, but from the 1780s rotative engines (i.e. those converting reciprocating into rotary motion) began to appear, driving factory machinery. At the turn of the 19th century, steam-powered transport on both sea and land began to make its appearance becoming ever more predominant as the century progressed.

Steam engines can be said to have been the moving force behind the Industrial Revolution and saw widespread commercial use driving machinery in factories and mills, pumping stations, as well as transport appliances such as locomotives, steam ship engines and road vehicles. The presence of several phases between heat source and power delivery has meant that it has always been difficult to obtain a power/weight ratio anywhere near that obtainable with internal combustion engines; notable this has made steam aircraft extremely rare. Similar considerations have meant that for for small and medium-scale applications steam has been largely superseded by internal combustion engines or electric motors, which has given the steam engine an out-dated image. However it is important to remember the plants that supply power to the electric grid, predominantly use steam turbine plant, so that indirectly the world's industry is still dependent on steam power. Recent concerns about fuel sources and pollution have incited a renewed interest in steam both as a component of cogeneration processes and as a prime mover. This is becoming known as the Modern steam movement.

Steam engines can be classified by their application:

Stationary applications

Stationary steam engines can be classified into two main types:

Winding engines, rolling mill engines, steam donkeys, (marine engines) and similar applications which need to frequently stop and reverse.
Engines providing power, which stop rarely and do not need to reverse. These include engines used in thermal power stations and those that were used in mills, factories and to power cable railways and cable tramways before the widespread use of electric power. Very low power engines are used to power model ships and speciality applications such as the steam clock.

The steam donkey is technically a stationary engine but is mounted on skids to be semi-portable. It is designed for logging use and can drag itself to a new location. Having secured the winch cable to a sturdy tree at the desired destination, the machine will move towards the anchor point as the cable is winched in.

Transport applications

Steam engines have been used to power a wide array of transport appliances:

Steamboat and steamship
Land vehicles:

Efficiency

The efficiency of an engine can be calculated by dividing the number of joules of mechanical work that the engine produces by the number of joules of energy input to the engine by the burning fuel. The rest of the energy is dumped into the environment as heat.

The yardstick of engine efficiency has long been the Carnot cycle established in 1826, in which heat is theoretically moved from a high temperature reservoir to one at a low temperature, efficiency depending upon the temperature difference. Hence, the aim of any engine development is to come as closely as possible to this ideal; steam engines thus should ideally operate at the highest steam temperature possible, and release the waste heat at the lowest temperature possible.

In practice, a steam engine exhausting the steam to atmosphere (open cycle) will have an efficiency (including the boiler) of roughly 1% to 15%, but with the addition of multiple expansion and a condenser and engines the efficiency may improved to as much as 25% or better. A power station with steam reheat, etc. will achieve 30% to 42% efficiency. Combined cycle in which the burning material is first used to drive a gas turbine can produce 50% to 60% efficiency. It is also possible to capture the waste heat using cogeneration in which the residual steam is used for heating a building. It is therefore possible to use about 90% of the energy produced by burning fuel— only 10% of the energy produced by the combustion of the fuel going wasted into the atmosphere ^{[citation needed]}.

The reason for varying efficiencies is because of the thermodynamic rule of the Carnot Cycle. The efficiency is the absolute temperature of the cold reservoir over the absolute temperature of the steam, subtracted from 1. As the temperature changes in seasons, the efficiency changes with it, unless the cold reservoir is kept in an isothermal state. It should be noted that the Carnot Cycle calculations require absolute temperatures.

One source of theoretical inefficiency is that the condensed water causes losses by being somewhat hotter than the outside world, However warm condensate returning to feed the boiler means that less heat will be required for vaporisation.

L.D. Porta gives the following equation determining the efficiency of a steam locomotive applicable to steam plant of all kinds: power (kW)=steam Production (kg h^-1)/Specific steam consumption (kg/kW h)^[1].

^ Porta, L.D. ‘’Advanced steam locomotive, development three technical papers’’ (Camden Miniature Steam Services, Somerset UK 2006 ISBN 978-0-9547131-5-7) pp.57; 61

[1] Porta, L.D. ‘’Advanced steam locomotive, development three technical papers’’ (Camden Miniature Steam Services, Somerset UK 2006 ISBN 978-0-9547131-5-7) pp.57; 61

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