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