steam turbine

Since it is a steam jet and no more a water jet who meets the turbine now, the laws of thermodynamics are to be observed now. The modern steam turbine is an action turbine (no reaction turbine), i.e. the steam jet meets from a being certain nozzle the freely turning impeller. There's a high pressure in front of the turbine, while behind it a low pressure is maintained, so there's a pressure gradient: Steam shoots through the turbine to the rear end. It delivers kinetic energy to the impeller and cools down thereby: The pressure sinks.
"Steam"Steam turbines are operated today of course no longer with normal water vapour only, but depending on the field of application also with other materials, e.g. with freons). Steam is produced in a steam boiler, which is heated in power stations by the burn of coal or gas or by atomic energy. Steam doesn't escape then, but after the passage through the turbine it is condensed in a condensor and then pushed back into the steam boiler again by a pump. This has the advantage that for example in nuclear power stations work- and cooling water are clearly separated.Multi-level steam turbinesIn modern steam turbines not only one impeller is propelled, but several being in a series. Between them idlers are situated, which don't turn. The gas changes its direction passing an idler, in order to perform optimally work again in the next impeller. Turbines with several impellers are called multi-level. The principle was developed 1883 by Parsons. As you know, with the cooling gas expands. Therefore it is to be paid attention when building steam turbines to a further problem: With the number of passed impellers also the volume increases, which leads to a larger diameter of the impellers. Because of that, multi-level turbines are always conical.Coupling of several turbines
Grafic: Coupled steam turbine. source: Helmut Hütten, "Motoren", Motorbuchverlag Stuttgart, S.379 In power stations today, different types of turbines are used in a series, e.g. one high pressure -, two medium- and four low pressure turbines. This coupling leads to an excellent efficiency (over 40%), which is even better than the efficiency of large diesel engines. This characteristic and the relatively favorable production make the steam turbine competitionless in power stations. Coupled with a generator and fired by an atomic reactor, they produce enormously much electric current. The strongest steam turbines achieve today performances of more than 1000 megawatts.

Wankel engine

Rotary enginesIn rotary engines the emphasis of the rotor (turning piston) rests or it describes a circular course.When we have a look at a lifting cylinder engine, we notice rapidly that many parts don't run evenly, they get immensely accelerated and braked again in short time (e.g. piston, connecting rod, valves, valve rods, etc.).

Animation: Pappenheimpump.This lack was tried to be recovered for a long time by letting the strength not affect an oscillating system but a turning one. Like that the enormous acceleration forces in the lifting cylinder engine could be avoided, which set a boundary to the numbers of revolutions of the crankshaft. Already around 1636, a German called Pappenheim sketched a rotary pump, which was used about 150 years later in Watt's steam engines for the first time in practice. Unfortunately the basic difficulties could not be solved at this time: The sections could not be sealed, the efficiency of these rotary engines/pumps was by far worse than in lifting piston engines. Many engineers tried in vain to solve these problems and it didn't seem possible that once an efficient rotary engine will be developped.
source: Helmut Hütten, "Motoren", Motorbuchverlag Stuttgart, UmschlagThe rotary engines can be constructed, in contrast to the lifting piston engines, which are bound to a more or less fixed form, in enormous variety. The trochoid's tracks allow a big selection of shapes. Some of them run in a suitable four-stroke cycle with compression. The theoretically possible engines didn't work in practice.Felix Wankel (1902 - 1988) brought finally the turn. The German autodidact (he never completed a study) was occupied since 1924 with the rotary engines. He finally succeeded in solving to most difficult seal problems and he could also answer the question of the best shape. The German firm NSU supported Wankel a lot and 1967 the fist car using a Wankel engine, the NSU Ro 80, was produced (See biography Wankel).

Grafic: Wankel rotary engine.enlarge (13 K, 800 * 600), slow animation (13 K, 390 * 390), without gas (4 K, 195 * 195) mode of operation of the NSU wankel engineIn the center of the epitrochoidal housing is an eccentric cam (emphasis and fulcrum not in same place). The three-edged rotor inside is turning thereby around the eccentric cam (with ball- or roller-bearings, cogwheel-transmission). In the form of the external housing the rotor describes now the typical course. Consider: While the runner makes one revolution, the eccentric cam turns three times. The rotor and the housing form three spaces, whose volume changes periodically. The engine shown beside is designed as a four-stroke engine: Through the right opening the air/fuel mixture is sucked in by the rotor. Then the mixture is compressed and ignited when it is max. compressed by the spark plug.
Ro-80 NSU Wankelmotor. source: Helmut Hütten, "Motoren", Motorbuchverlag Stuttgart, S.340The developed pressure pushs the rotor now. When the eccentric cam is on the right, the mixture has expanded on the left maximally and in the following clock the exhaust is ejected through the left ejection pipe. This mode of operation has many advantages: The piston does not swing, additionally no valves are needed. But also the problems mentioned above are not missing: Not only the solved questions of sealing and shape cause difficulties, but also the unfavorable combustion chamber, which sets limits to the performance. DevelopmentThe wankel engine was developed in the 70's by many companies, e.g. by General Motors, Daimler Benz, Peugeot and Mazda. They built altogether over one million wankel engine-operated cars.
Wankel engine. Quelle: http://web.ukonline.co.uk/Members/jr.marsh/wankel2.htmlBut because of the arising environmental regulations and in the consequence of the oil crisis no more investments were done. Only Mazda continued to develop at the rotary engine (e.g. in 1999 they produced a new car with a wankel engine inside). The efficiencies - so far rather bad - and the bad exhaust quality could be improved with courageous investments in the future. But the supremacy of the dominating lifting cylinder engines seems insurmountable, and it also hardly dares someone to shake at proof of worth. Thus the future of the wankel engine looks rather dark.

two-stroke engines

The two-stroke engineCouldn't an engine be constructed, that needs less than four strokes and that performs the same power as the Otto engine? Couldn't you reduce or even replace the complicated valve mechanics? These questions led to the development of the two-stroke otto engine. It needs only one revolution of the crankshaft to indicate a new ignition. Additionally, the piston has at the same time the function of a valve, which saves many mobile parts: The two-stroke engine consists of only three mobile parts: Piston, connecting rod and crankshaft. Let's have a look at the mode of operation of this engine:

Mode of operation of the two-stroke engine1st stroke: The piston is at the bottom of the cylinder. A pipe at the left side is opened and lets the fuel mixture, which is already compressed a bit, flow from the lower to the upper part of the cylinder. The fresh gases expulse now the exhaust through an ejection pipe, which is not closed by the piston at this moment.2nd stroke: After being hurried upward, the piston now covers the pipe on the left side and the ejection pipe. Because there is no way out any more, the upper, fresh gas mixture gets compressed now. At the same time in the part below fresh gas is taken in by the piston driving upward through the open suction pipe. At the upper dead-center, the compressed fuel mixture is ignited by the sparking plug, the piston is pressed downward while he compresses at the same time the fresh gas below. The process begins again as soon as the piston arrives at its lowest point.
Animation: two-stroke engineenlarge (21 K, 850 * 650), slow animation (21 K, 260 * 510) DevelopmentThe idea to build a two-stroke engine goes back to the year 1879. But this engine became a qualitatively good product only after many years, when the German DKW company accelerated its development. Because of its disadvantages compared with the four-stroke engine, the two-stroke engine is used practically just in a small range of capacity, e.g. in small motorcycles. Formerly the engine was even used to power tiny cars.Problems of the two-stroke engineActually the two-stroke engine should perform twice the performance of a four-stroke engine with the same cubic capacity. Though it is just possible to gain a performance that is about 50% better. The reasons are obvious: The cylinder can't be filled up with the same amount of fuel as in the four-stroke engine, because the individual strokes are seperated not so clearly. If more fuel is induced, it leaves the combustion chamber through the ejection pipe without being burnt. Many concepts were developed to provide a better expulsion of the exhaust in way that the fresh gas doesn't leave the combustion chamber (as for example the "nosepiston" you can see in the animation above, which causes turbulences of a certain type). Though all these inventions, the filling of the two-stroke engine is always worse than in the four-stroke engine, which loses fresh fuel only because of the "overlap" of the valve times (both valves are open for an instant). Beside these performance-technical problems, there are also increasing difficulties with the environment. The fuel mixture of the two-stroke engine often gets shifted with a certain quantity of oil because of the necessary lubrication. Unfortunately the oil gets burnt partly, too, and harmful gases are expulsed by the engine.

what's a gas turbine?

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of hot gas produced by combustion of gas or fuel oil in a stream of compressed air. It has an upstream air compressor (radial or axial flow) mechanically coupled to a downstream turbine and a combustion chamber in between. Gas turbine may also refer to just the turbine element.

Energy is released when compressed air is mixed with fuel and ignited in the combustor. The resulting gases are directed over the turbine's blades, spinning the turbine, and, mechanically, powering the compressor. Finally, the gases are passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure.

Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, electrical generators, and even tanks.

This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings.

history of gas turbines:

• 60: Hero's Engine (aeolipile) - apparently Hero's steam engine was taken to be no more than a toy, and thus its full potential not realized for centuries.
• 1500: The "Chimney Jack" was drawn by Leonardo da Vinci which was turning a roasting spit. Hot air from a fire rose through a series of fans which connect and turn the roasting spit.
• 1629: Jets of steam rotated a turbine that then rotated driven machinery allowed a stamping mill to be developed by Giovanni Branca.
• 1678: Ferdinand Verbeist built a model carriage relying on a steam jet for power.
• 1791: A patent was given to John Barber, an Englishman, for the first true gas turbine. His invention had most of the elements present in the modern day gas turbines. The turbine was designed to power a horseless carriage.
• 1872: The first true gas turbine engine was designed by Dr F. Stolze, but the engine never ran under its own power.
• 1897: A steam turbine for propelling a ship was patented by Sir Charles Parsons. This principle of propulsion is still of some use.
• 1903: A Norwegian, Ægidius Elling, was able to build the first gas turbine that was able to produce more power than needed to run its own components, which was considered an achievement in a time when knowledge about aerodynamics was limited. Using rotary compressors and turbines it produced 11 hp (massive for those days). His work was later used by Sir Frank Whittle.
• 1914: The first application for a gas turbine engine was filed by Charles Curtis.
• 1918: One of the leading gas turbine manufacturers of today, General Electric, started their gas turbine division.
• 1920. The then current gas flow through passages was developed by Dr A. A. Griffith to a turbine theory with gas flow past airfoils.
• 1930. Sir Frank Whittle patented the design for a gas turbine for jet propulsion. His work on gas propulsion relied on the work from all those who had previously worked in the same field and he has himself stated that his invention would be hard to achieve without the works of Ægidius Elling. The first successful use of his engine was in April 1937.
• 1934. Raúl Pateras de Pescara patented the free-piston engine as a gas generator for gas turbines.
• 1936. Hans von Ohain and Max Hahn in Germany developed their own patented engine design at the same time that Sir Frank Whittle was developing his design in England.

gas turbine's cycles and operations:

Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.

In practice, friction, and turbulence cause:

a) non-isentropic compression: for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.

b) non-isentropic expansion: although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.

c) pressure losses in the air intake, combustor and exhaust: reduces the expansion available to provide useful work.

As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production.

Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternative-rotor assembly (see image above), not counting the fuel system.

More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.

As a general rule, the smaller the engine the higher the rotation rate of the shaft(s) needs to be to maintain tip speed. Turbine blade tip speed determines the maximum pressure that can be gained, independent of the size of the engine. Jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm.

Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. This is giving way to foil bearings, which have been successfully used in micro turbines and auxiliary power units.