TOPICS ON MECHANICS

The most important feature of all turbines is the shape of the blades and the angle they present because the speed may be largely affected by an alteration in the shape of the blades or in the direction of flow of the incoming fluid.

Turbines are primarily classified according to the fluid employed in: water or hydraulic turbines, steam turbines, gas turbines, mercury-vapour turbines and others.

According to the principal direction of fluid flow, they are classed into parallel-flow turbines or axial-flow turbines, radial-flow turbines (including the outward and inward-flow types), mixed-flow turbines, and so forth.

Turbines are also classified considering the method of steam expansion in: impulse turbines and reaction turbines. The former have steam expansion only in the stationary blades or nozzles and the latter have steam expansion both in the stationary and in the moving blades.

Finally, turbines may be classified according to the method of subdividing the available energy in : pressure stage turbines and velocity-stage turbines. In the pressure- stage turbines the steam pressure drop is subdivided among two or more successive sets of rotating blades, each set taking care of a part of the pressure drop; while in velocity-stage turbines, the steam at constant pressure passes through two or more successive sets of rotating blades starting with a high steam speed and decreasing the same from set to set.

Steam turbines are high-speed prime-movers and are most efficiently used in direct connection to electrical generators, centrifugal pumps, centrifugal compressors and ship propellers.

If we compare turbines with the other prime movers we shall see that they require less floor space, lighter foundation, less attention, a lower consumption of oil and maintenance cost and excellent regulation. They have extreme overload capacity and great reliability on account of their simplicity of construction.

A turbine is a device in which the steam from a boiler is directed by jets, or by guide vanes, against blades fixed to the outer surface of a drum, the energy of the escaping steam causing the drum and its blades to rotate.

The steam is allowed to expand in a number of separate stages. The steam from the boiler passes first through jets or a set of fixed blades where it expands slightly, and a fraction of its pressure energy is used to set the steam in motion. The moving steam now falls on a set of moving blades and in passing through them gives up its kinetic energy to the blades.

The steam now expands passing by another set of jets or stationary blades and is directed against a second set of moving blades. Thus, the expansion of steam is so controlled that its speed at all stages, is that required for efficient work.

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In order to aid the inlet and outlet of the charge, the cylinder is fitted with two valves operated by their cams and a camshaft.

We say the engine is on its bottom dead center when the piston is at its lower position whereas it is on its top dead center when the piston reaches its upper point.

We call stroke the motion of the piston from one end of the cylinder to the other. The first stroke constitutes the admission of the proper amount of explosive mixture; in the second stroke this mixture is compressed; in the third one it burns, while the exhaust of the burned gases takes place in the fourth and last stroke. These operations constitute what we term cycle.

We can classify internal combustion engines according to the number of strokes of the piston in one complete working cycle. Thus, we speak of two-stroke-cycle engines and four-stroke-cycle engines.

The complete cycle of events of the former group, that is suction, compression, explosion and exhaust, is accomplished in a single revolution of the crankshaft or in two strokes of the piston, as the compression and expansion of the charge take place during one stroke, while the admission of a fresh charge occurs during the other stroke simultaneously with the escape of the burnt gases.

In the four-stroke cycle engines the explosive mixture is drawn into the cylinder on the induction stroke, compressed on the compression stroke and ignited. It burns and performs useful work on the expansion stroke and the products of combustion are exhausted in the exhaust stroke.

Internal combustion engines may be classified according to the kind of fuel used in four main groups: gas engines, heavy oil engines, light oil engines and special engines.

There is still another classification according to the process of combustion: explosion or constant-volume combustion engines and constant-pressure combustion or Diesel engines.

The thermal efficiency of internal combustion engines is higher than that of external combustion engines, where loss of heat occurs during the raising of the steam and its passage to the cylinders.

The principal fuels used in them are petrol, as in the motor-car engine, coal-gas and similar gases, as in the gas engine; and heavy oil as in Diesel engines. The essential elements of all these fuels are carbon and hydrogen and the action that takes place when they are burned, is the combustion of their elements with atmospheric oxygen to form carbon dioxide and water, respectively, with liberation of considerable heat.

In an explosion or constant-volume-combustion engine, a mixture of volatile liquid or gas and air, compresses to a moderate pressure during the compression stroke of the piston, ignites usually by an electric spark, explodes and expands with constant volume.

Constant-pressure-combustion or Diesel engines operate on the Diesel cycle, and mixed-cycle engines, in which the combustion is partly explosive and partly at constant pressure, operate on what is termed Sabathe's cycle.

In mixed-cycle engines the fuel-air mixture is ignited by a flame of gas or by some form of hot tube, hot bulb or uncooled portion of the surface of the combustion chamber.

In supercompression engines, a portion of the compressed charge is compressed to a higher pressure than the remainder during the last steps of the compression stroke, in this way raising this portion of mixture to a temperature above the one of ignition.

Precombustion engines have a small uncooled precombustion chamber or cup separated from the cylinder in which a portion of the mixture is ignited, and in turn, this portion ignites the cylinder charge.

Some mixed-cycle engines are very similar to Diesel-cycle engines and are called semi-Diesel engines.

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