Compound engine

A compound steam engine or multiple - expansion engine (English compound engine) is a steam engine having at least two steam direction successively connected units of work. Anatole Mallet reported in 1874 to the use of the composite principle in locomotive for a patent.

Discontinuous composite, reciprocating engines

Function

In a composite piston steam engine the high pressure steam expands in the first cylinder and piston to a lower pressure. The partially expanded steam is not (as in the single machine ) released after this first expansion into the atmosphere, but is supplied with its residual pressure for the performance of additional work a second cylinder unit that operates with a larger diameter for the use of lower pressure. Because the reduction in temperature of the steam is distributed in the composite operation on spatially separated parts of machines, reduces the surface damage; the steam loses less heat to cool walls than in a single-stage expansion ( = cooling).

This two-step use of the vapor pressure provides a better utilization of the energy contained in the specific vapor ( enthalpy). The composite steam engines are therefore in the fuel and water consumption cheaper than comparable steam engines with single-stage steam expansion. At the output of the last cylinder is usually a capacitor, in which the steam is condensed and the water is fed back into the circulation.

Use

The principle of composite steam engine was applied for steam locomotives with two-, three - and four-cylinder engines. There are also more than two series-connected stages of the steam pressure in piston machines use feasible, but not become common in the art. For reasons of space came the three-stage steam expansion, ie a triple - expansion engine, almost exclusively in stationary mode or as the ship's engine to the application, such as in the coastal defense ships of the Siegfried class.

Quadruple expansion engines were due to their size and operating characteristics very rare. The first machine of this type was installed in 1898 in the Kaiser Friedrich. She pointed to five cylinders, which had three cranks. The low pressure was processed in two cylinders, all other stages consisted of only one cylinder. The plant did not reach the required performance and fell by strong vibrations. The fast steamer Germany HAPAG was also equipped with quadruple expansion steam engines, but which possessed six cylinders. Also these machines fell on by excessive vibration; the ship was therefore referred to as " cocktail shaker " from the traveling public.

Construction

In the development of composite steam engines has been recognized that there was a larger area of the low-pressure cylinder for the partial relaxation of the vapor in the high pressure cylinder required for different lengths of the piston rods as a result of differences of practical considerations. First, the surface of the low-pressure cylinder is doubled in relation to the high pressure cylinder, because of simplicity emanating from a halving of the vapor pressure. In three- cylinder compound engine with double steam expansion, the favored the development of designs with a high - and two low-pressure cylinders in each case the same diameter. With experience in the construction and operation of composite steam engines the effective area of low pressure was then further enlarged to optimize the energy yield. In triple expansion steam, the ratio of the cylinder surfaces was treated accordingly. Means for calculating the area of ​​the printing cylinder with respect to the low pressure cylinder is treated as if the pressure cylinder, a high pressure cylinder.

In terms of numbers, there is the following ratio of the cylinder surfaces and piston diameter:

• At double steam expansion:

• With triple expansion steam:

The crank displacement was chosen so that the crank forces of the various cylinders acted in approximately the same thickness on the drive axle, with two- and four-cylinder engines, it stood at 90 °. In three- cylinder engines ruled obvious uncertainty about the optimal crank displacement; some machines were built with 120 ° crank offset, such as the locomotives of the Swiss B Series 3/4. This creates a nearly uniform torque and a good mass balance was achieved. A first patent for a locomotive with such an engine was a French engineer Michel Andrade in 1875, but failed its construction to mechanical problems. But there were also designs which 90 ° crank offset between the low-pressure cylinders and 135 ° between low-and high-pressure cylinder had at double steam expansion, such as the freight locomotives of the Württemberg series G. It even existed whole series, which were otherwise identical, some with 120 ° crank offset, in part with 90 ° 2 × built 135 ° crank offset and put into service.

The advantage of the increased efficiency are some disadvantages of the composite principle against: firstly, the construction cost of several cylinders, pistons and the slider or control devices. One problem with the juxtaposition of several different sized expansion units is also the scheme. At varying load cases (steam locomotive ), it is almost impossible to set both the high-pressure stage and the low-pressure stage optimal. One of influencing consumer behavior, the amount of steam controller always acts on both stages, as an intermediate outlet of the steam does not occur. Was usually dispense with a separate influencing of the individual thrusters, so as not to make the control mechanism is too complex. Also for getting such a steam engine special devices were required.

Continuous composite, turbomachinery

Almost every modern steam turbine is a compound steam engine. The expansion of the vapor is in the longitudinal direction of the turbine in a diameter graduation from small Schaufelraddurchmessern ( for the boiler pressure) toward increasingly larger Schaufelraddurchmessern ( close to ambient pressure) instead in order to realize as completely as possible thermodynamically possible conversion of the forms of energy ( heat energy to mechanical energy).