Steam engine: principles, history, design, and uses

The piston steam engine converts thermodynamic energy (steam pressure) from steam generators into mechanical rotational energy. In the process, a piston moves back and forth in the associated cylinder, performing an oscillating motion. However, a rotational movement is usually required for the useful mechanical energy.

The outward movement of the piston is carried out with the pressure of the steam as a power stroke. The return movement of the piston is performed from stored rotational oscillation energy when the piston is pressurized on one side. In the case of a piston pressurised on both sides, however, the return movement of the piston is also performed as a power stroke, but now on the underside of the piston using steam pressure control.

The steam supply into the cylinder is controlled by a slide valve. The piston is first moved downwards or in the direction of the crankshaft with the pressure. The linear movement of the piston is converted into a rotational movement by means of a crosshead and connecting rod as a coupling element on the crankpin of the crankshaft. The connecting rod then (in one-sided operation) uses the rotational energy stored in the flywheel and crankshaft to push the piston linearly back from the lower position to its upper starting position.

The working process of a steam engine is thus divided into two cycles and is therefore a two-cycle process.

Atmospheric steam engine

In an atmospheric steam engine, the cylinder space under the piston is filled with steam. In the next working cycle, water is injected into the cylinder so that the water vapour cools down and condenses in the process. A vacuum is created so that the piston is forced into the cylinder by the external atmospheric pressure. The extending movement of the piston takes place with the steam valve open and by means of a flywheel mass attached to a lever arm, the so-called balancer.

The best-known representative of this design was the atmospheric steam engine by Thomas Newcomen from 1712. The steam engine was mainly used for keeping water in coal mines. The energy efficiency of this engine was less than 1 % until it was further developed by James Watt. Watt transferred the condensation of the steam from the working cylinder to a downstream water-cooled container, the condenser. This eliminated the need to constantly cool and reheat the working cylinder in each working cycle, a cause of considerable energy losses.

The illustration on the right shows how the negative pressure or atmospheric pressure in the atmospheric steam engine does the work when the hot steam condenses and contracts sharply. Most of the water that comes out of the cylinder is liquid. This method of operation is not very economical, as much energy is used in heating and cooling the cylinder and piston at each cycle. To allow the steam to condense quickly enough, cold water was injected into the cylinder near top dead center (not shown here).
In the expansion steam engine, the fill valve opens only briefly at the beginning of the cycle. In contrast to the full pressure machine, the steam flowing in under high pressure loses part of its pressure during the working cycle. Although this also leads to a cooling of the cylinder, considerably less steam is consumed than with the full pressure machine. To limit cooling, the steam is not completely expanded to atmospheric pressure. Compound steam engines also take advantage of the pressure remaining when the steam exits by directing the steam into another cylinder of larger diameter. (Note: Contrary to what is shown here, the steam hardly cools any further during the downward movement of the piston).

Low pressure steam engine

In the low-pressure steam engine, the steam is applied at a slight overpressure of a few 100 mbar. In contrast to the Newcomen steam engine, work is done not only during condensation but also during filling of the cylinder. This leads to an increase in efficiency and was the starting point for the further development of the steam engine to higher steam pressures. The best-known representatives of this design were the steam engines of James Watt from about 1769 (see below).

Watt also developed the single-acting steam engine, which acted on the piston from one side only, into a double-acting steam engine, in which the piston was acted on alternately from the top and bottom. This resulted in a reduction in mass and also increased efficiency and output to a certain extent, as the idle stroke was eliminated.

High pressure steam engine

In high-pressure steam engines, the steam is heated to well over 100 °C, so that a higher pressure builds up. Cooling of the steam leaving the cylinder can be dispensed with, as the atmospheric pressure is no longer a factor compared to the significantly higher operating pressure (exhaust operation). The condenser can thus be dispensed with, which, in conjunction with the higher energy density of the pressurised steam, makes this type of engine considerably lighter and thus made the use of steam engines in steam locomotives possible in the first place. Representatives of this type are practically all piston steam engines in vehicles since Oliver Evans and Richard Trevithick from about 1802 (see below). High-pressure steam engines also made it possible to use steam expansion. Whereas atmospheric and low-pressure steam engines are usually full-pressure engines with steam flowing into the cylinders during the entire piston stroke, the cylinders of an expansion engine only receive steam at the beginning of each piston stroke. Further movement is caused by the expansion of the steam as the pressure falls. The energy stored in the steam is thus utilized much better

Compound Steam Engine

A compound steam engine or multiple expansion engine is a steam engine with at least two working units connected in series in the direction of steam flow.

As a vehicle drive, steam engines have largely been replaced by internal combustion engines, which start without warm-up time, are more efficient, offer greater power with less weight and are more convenient to operate. Furthermore, due to the widespread supply of electrical energy, the steam engine has lost its function as the central energy source of an industrial enterprise, which it held for a long time. In coal mining, steam engines were and still are used in hoisting plants, for there the steam engine can serve both as a hoisting machine for lifting coal and as a brake for lowering backfill material. When braking, the energy is used to heat the steam.

Although the era of the piston steam engine seems to be long gone, a renaissance is not out of the question. One of its advantages over the internal combustion engine is its continuous combustion process, which can be designed with lower emissions. Another advantage of the steam engine is its extreme overload capacity when power peaks are demanded. The closed circuit of steam and feed water, which is common today, results in low-emission lubrication of the engine's cylinder and piston. In this sense, the steam engine has been developed as a modernized steam engine.

In the late 1990s, IAV GmbH was commissioned by Volkswagen AG to develop such a modern "steam engine", which uses extremely low-emission external combustion to generate a certain supply of high-pressure steam, which is then injected via nozzles according to energy requirements, as in a diesel engine. At the end of 2000, this gave rise to the company Enginion, which further developed the current SteamCell from the ZEE (Zero Emission Engine) prototype, which achieved an efficiency of 23.7%. This engine operated in a two-stroke process and also managed without the usual lubricants because the wearing parts were made of modern carbon components. However, Enginion had to file for insolvency in 2005.