Advantages of gas turbine engines

· Very high power-to-weight ratio, compared to reciprocating engines.

· Smaller than most reciprocating engines of the same power rating.

· Moves in one direction only, with far less vibration than a reciprocating engine.

· Fewer moving parts than reciprocating engines.

· Greater reliability, particularly in applications where sustained high power output is required.

· Waste heat is dissipated almost entirely in the exhaust. This results in a high temperature exhaust stream that is very usable for boiling water in a combined cycle, or for cogeneration.

· Low operating pressures.

· High operation speeds.

· Low lubricating oil cost and consumption.

· Can run on a wide variety of fuels.

· Very low toxic emissions of CO and HC due to excess air, complete combustion and no "quench" of the flame on cold surfaces.

Disadvantages of gas turbine engines

· Cost is very high.

· Less efficient than reciprocating engines at idle speed.

· Longer startup than reciprocating engines.

· Less responsive to changes in power demand compared with reciprocating engines.

· Characteristic whine can be hard to suppress.

From http://en.wikipedia.org/wiki/Gas_turbine

Exercise 16, p. 75.

How Do Piston Aircraft & Turboprops Compare?

Price

From a manufacturing and engineering perspective, the reciprocating engines found in piston aircraft are far less complex than their turboprop counterparts. This is primarily due to the high temperatures and forces unique to turboprop engine operation, which must be accommodated both in materials and engine design – and which come at a price. For this reason, piston aircraft almost always offer a lower cost of entry.

Maintenance

While piston engines are indeed a more simple design, turboprop engines have far fewer moving parts and the smooth, vibration-free operation of a turboprop engine typically offers greater reliability – and longer time between overhaul (TBO) – than a piston aircraft. In other words you can generally log more hours on your turboprop before you need to bring it in for inspection.

The typical TBO for a turboprop engine is 3,000 hours while TBO for a piston aircraft is generally 2,000 hours. So the higher upfront cost of a turboprop aircraft may be somewhat offset by lower maintenance costs over time.

Performance

The power of a turbine engine almost always allows turboprop aircraft to travel at higher speeds than piston aircraft. And as turboprop aircraft cabins are typically pressurized these planes also commonly fly at higher altitudes.

Turboprop aircraft are generally most efficient at altitudes of 20,000 to 30,000 feet and at average speeds of 250 to 300 ktas, while piston aircraft, which are typically not pressurized, are usually restricted to altitudes of 12,000 feet or less (unless an oxygen system is used). And as piston engines are less powerful than turboprops engines, piston aircraft speeds are generally limited to about 200 ktas.

Distance

Piston aircraft are generally smaller aircraft, seating no more than six passengers, and are well suited for relatively short missions of 300 miles or less, while turboprops tend to be larger aircraft with greater passenger capacity – and more fuel on-board – and are more likely to be found flying distances of up to 1,000 miles.

Both piston engine aircraft and small turboprop aircraft are right at home accessing smaller airports with shorter runways, making them the ideal way to reach many desirable final destinations, where larger aircraft simply cannot land.

Ultimately, there are many factors to consider when investing in an aircraft, but giving careful thought to the types of missions you intend to fly and understanding which aircraft will best support your intended use – while accommodating your budget – will help you arrive at the right choice!

From http://www.shorelineaviation.net/news---events/bid/50442/Piston-Engine-Aircraft-vs-Turboprop-Engine-Aircraft

 

APPENDIX B

 

AUDIOSCRIPTS

UNIT 1

The Steam Engine

It is tempting to think of the car or the airplane as among the most important inventions of the millennium. But these were merely evolutionary refinements of the steam engine – the first machine to convert burning fuel into mechanical energy on a large scale. The first steam engines were powered by wood, and later ones by coal. Steam engines transformed the industrial world as few other technologies have. This invention liberated people from the limitations of their own muscles and those of beasts of burden. It made possible the factories that drove the Industrial Revolution. And it was at the heart of the first form of high-speed mechanized transportation: the locomotive.

The steam engine is a classic example of an invention that came about in stages. In 1698 English engineer Thomas Savery developed a water pump that used steam from a boiler for its power. Savery obtained a broad patent on the idea, even though his engine design never proved to be very useful. This was followed by a significantly more effective steam engine, built by English blacksmith Thomas Newcomen in 1712. Newcomen's device was also used primarily to pump water from coal mines. However, Scottish engineer James Watt is the individual most commonly cited as the inventor of the steam engine. His machine, patented in 1769, was vastly more efficient than were previous devices, chiefly because of its separate cooling chamber for condensed steam. Watt added other improvements, such as oil lubrication and insulation. He also designed a system of gears that translated the up-and-down motion of the steam-driven piston into rotary motion, which was more useful in driving machinery.

Watt's engine freed factories from their dependence on water power and gave a huge advantage to nations with ready supplies of coal. Britain's great reserves of coal fueled the expansion of the British Empire in the 18th and 19th centuries. Steam-powered railroad trains played a huge role in industrialization and trade and were vital to the westward expansion of the United States. Internal-combustion engines and electrical generating stations have since taken over most of the jobs once performed by steam, but these creations had to stand on the shoulders of the steam engine.

From Encyclopædia Britannica

UNIT 2

The Watt engine.

While repairing a model Newcomen steam engine in 1764 Watt was impressed by its waste of steam. In May 1765, after wrestling with the problem of improving it, he suddenly came upon a solution — the separate condenser, his first and greatest invention. Watt had realized that the loss of latent heat (the heat involved in changing the state of a substance — e.g., solid or liquid) was the worst defect of the Newcomen engine and that therefore condensation must be effected in a chamber distinct from the cylinder but connected to it. Meanwhile, Watt in 1766 became a land surveyor; for the next eight years he was continuously busy marking out routes for canals in Scotland, work that prevented his making further progress with the steam engine. After Roebuck went bankrupt in 1772, Matthew Boulton, the manufacturer of the Soho Works in Birmingham, took over a share in Watt's patent. Bored with surveying and with Scotland, Watt immigrated to Birmingham in 1774.

After Watt's patent was extended by an act of Parliament, he and Boulton in 1775 began a partnership that lasted 25 years. Boulton's financial support made possible rapid progress with the engine. In 1776 two engines were installed, one for pumping water in a Staffordshire colliery, the other for blowing air into the furnaces of John Wilkinson, the famous ironmaster. During the next five years, until 1781, Watt spent long periods in Cornwall, where he installed and supervised numerous pumping engines for the copper and tin mines, the managers of which wanted to reduce fuel costs. In the following year Boulton, foreseeing a new market in the corn, malt, and cotton mills, urged Watt to invent a rotary motion for the steam engine, to replace the reciprocating action of the original. He did this in 1781 with his so-called sun-and-planet gear, by means of which a shaft produced two revolutions for each cycle of the engine. In 1782, at the height of his inventive powers, he patented the double-acting engine, in which the piston pushed as well as pulled. The engine required a new method of rigidly connecting the piston to the beam. He solved this problem in 1784 with his invention of the parallel motion — an arrangement of connected rods that guided the piston rod in a perpendicular motion—which he described as “one of the most ingenious, simple pieces of mechanism I have contrived.” Four years later his application of the centrifugal governor for automatic control of the speed of the engine, at Boulton's suggestion, and in 1790 his invention of a pressure gauge, virtually completed the Watt engine.

From Encyclopædia Britannica

UNIT 3

Modern steam engines

The next important development in the field of steam engines after Watt’s inventions was the introduction of practical noncondensing engines. Although Watt had recognized the principle of the noncondensing engine, he had been unable to perfect machines of this type, probably because he used steam at extremely low pressure. At the beginning of the 19th century the British engineer and inventor Richard Trevithick and the American inventor Oliver Evans devised successful noncondensing engines using the high-pressure steam. Trevithick used this model of steam engine to power the first railroad locomotive ever made. Both Trevithick and Evans also built steam-powered carriages for road travel.

Further improvement in the design of steam engines is afforded by the uniflow engine, which uses the piston itself as a valve and in which all portions of the cylinder remain at approximately the same temperature when the engine is operating. In the uniflow engine, steam moves in only one direction while entering the cylinder of the engine, expanding, and then leaving the cylinder.. The inherent advantages of the uniflow system are such that engines of this type were usually chosen for use in large installations, although the initial cost of the engines is considerably higher than that of conventional steam engines. One virtue of the uniflow engine is that it permits the efficient use of high-pressure steam in a single cylinder engine without the necessity of compounding.

The steam has the benefit of a large latent heat of vaporization. This is used in many ways, primarily using the change in phase for energy storage and energy release. As steam changes phase, it gives up energy without changing temperature – a phenomenon which is very useful in chemical processing, the development of power cycles and in heat exchanger design. Power systems utilizing steam also have the benefit of combustion external to the power prime mover. The obvious advantage of external combustion engines is enormous fuel flexibility. All varieties of biomass, waste fuels, MSW, and industrial byproducts can be burned in incinerators or waste fuel boilers to make steam. This includes industries like forest products, where opportunity fuels and the need for steam create ideal conditions. Finally, it is important to note the role of steam in combined cycle power plants. Combustion systems and gas turbines discharge heat at very high temperatures. This high temperature heat exhaust results in poor to modest heat engine performance. If that heat is used to make steam which is then used in a power cycle, the resultant discharge of heat takes place at a much lower temperature, increasing the efficiency of the combined power cycle. This principle appears in the design of nearly all new combined cycle power plants which can achieved thermal efficiencies greater than 50% without cogeneration.

From Encyclopædia Britannica

UNIT 4

Diesel Engine

Today, a new breed of Diesel engines fights an old metaphor. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The thermal efficiency of a power plant is a measure that reaches right into your pocketbook. When a power plant burns, say, coal or oil, thermal efficiency is the fraction of chemical energy in the fuel that reaches the electric generator. A big coal-fired steam power plant does well to reach forty percent efficiency. A lot more capital expense will buy fancy combined cycles with efficiencies as high as fifty percent. For years that's been as good as we get, and then only in huge plants producing hundreds of megawatts. Your automobile can reach only 20 or 25 percent efficiency, and then only under optimal conditions.

Now that 50 percent barrier is finally being broken by an unexpected contender: Today, huge 68-MW Diesel engines are reaching efficiencies over fifty percent. You can buy a single engine big enough to serve 40,000 households. Yet Diesels came into being as early variants on the lightweight internal combustion engine.

We should've seen great size coming long ago. As early as twenty years after Rudolf Diesel's patent in 1892, 3000 to 4000-horsepower Diesels were appearing in small ships. But steam kept providing the high power needed for fast-moving warships and liners. So we overlooked how well suited Diesel engines were to playing the large, slow-moving, high-efficiency role of the stationary power plant. We used both gasoline and Diesel engines to change transportation. Diesel engines began by displacing steam on smaller steamships. Then they replaced the old steam locomotives. The automobile industry has also used Diesel engines now and then. Maybe you're one of the few who drive a Diesel-powered car today.

Diesel himself built his first engines at Germany's Maschinenfabrik Augsburg-Nürnberg (or MAN). Today the same company (now MAN B&W) makes the biggest of these new engines. One of their twelve-cylinder plants is 80 feet long, and it stands 46 feet tall. It weighs over two thousand tons, and its great crankshaft turns at 100 rpm. Your car engine, turning at 3000 rpm, is like a wasp beside that elephant. When internal combustion appeared over a century ago, it shaped a new metaphor of lightness and speed. It revolutionized transportation. It made powered flight feasible and gave birth to automobiles and motorcycles. I once built model airplanes powered by both gas and Diesel engines that weighed scant ounces.

Internal combustion parted ways with the big steam power plants that began electrifying cities in the 1880s. That's why these huge new Diesel power plants are a surprise. They're not radical, but they violate the metaphor of lightness which internal combustion claimed at the outset. Once any metaphor takes root, it's almost impossible to escape. But these new monsters are escaping it. They're tiptoeing back across a metaphorical line that's separated them from major power production for a century.

From Encyclopædia Britannica

UNIT 5

Avery’s Turbine

Today, Avery's turbine. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Most of the power we use today is generated by nuclear energy or coal or oil. What's not so well understood is that all such power plants, like their cousin, the hydroelectric dam, ultimately use turbines to generate power. At the base of a dam are water turbines. Oil, coal, and nuclear energy are used to boil water and drive steam turbines. A burned fuel / air mixture is used to drive a gas turbine. In every case, fluid passes through some kind of turbine blading to provide a huge portion of the power we use today.

Any book on turbine origins begins with steam-powered toys in Egypt, two thousand years ago. But that's a bit of a red herring. Steam turbines didn't become commonplace until Charles Parsons began building them in the 1890s. So what came in between? Engineer Frederic Lyman tells about a little-known figure in the creation of the steam turbine. He was William Avery. Born in 1793, he grew up working as a mechanic in New York State.

In 1831 he and a friend were granted a patent for a steam-powered device very similar to one of those ancient Egyptian toys. Jets of steam, emitted from either end of a whirling propeller-like tube, drove it. By that time, Watt and others had already tried and failed at creating rotary steam-driven machines. And the French had just begun developing the water turbine. Modern steam turbines don't work the same way as Avery's. They direct steam through a succession of rotating blades. Each stage removes energy and reduces the steam pressure. The aerodynamics of those blades gets very complex.

In Avery’s turbine, steam flowed from the hub out through the tip where it escaped as a driving jet. The tips approached the speed of sound and, when one of Avery's cast iron rotors failed, its fragments tore through three floors of a building.

To get power out at useful speeds, Avery used a series of belts and pulleys. Yet the machine worked. By 1837, he'd built maybe seventy engines. They developed around twenty horsepower and were put work driving sawmills, cotton gins, gristmills.

Among his other engineering accomplishments, Avery also built what was probably the first steamboat on the Erie Canal. But he died at 47, and the company that manufactured his turbines went bankrupt soon after. Another half century passed before Parsons' turbines appeared.

Then the electric generator came on the scene, and it required a high-speed drive. The turbine was the natural mate to the generator, which was now the perfect means for putting its power to use. The ghost of William Avery could now smile at last. What Avery had begun Parsons could now bring to fruition and bring to resounding success as well.

From Encyclopædia Britannica

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