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Heat Engines


By John J. Furia, M.A.


Department of Physics, N. Y. University




The question often arises, "why is the hot air engine not used?" Air certainly is cheaper than gasoline. It takes less fuel to heat air than to convert water into steam. However, it does not follow that the hot air engine is the cheapest to run and at the same time obtain the necessary power. A little review of the gasoline and steam engines together with the principles of operation of the hot air engine will answer our question, why this type of engine is not used.

The Gasoline Engine

The gasoline engine (internal combustion) has attained great popularity by its use in the automobile industry. It is flexible, capable of delivering a large amount of power, it develops high speed and power almost instantaneously. It also has an efficiency of about 25 per cent. The engine and the fuel used are light in weight compared with other kinds of engines of similar power. The four cycle engine is typical.

Figure 1 is a sketch showing the conditions at each of the four cycles. The first cycle is started by turning over the flywheel (cranking the engine). This causes a suction in the cylinder. The inlet valve is either opened automatically or by the suction.

The properly mixed fuel (gasoline and air ) enters thru the inlet. ( During this cycle the exhaust valve remains closed.) After passing dead center, the inertia of the moving fly-wheel causes the piston to move inward (cycle 2). The inlet and exhaust valves are both closed during this cycle, and the explosive mixture is comprest, rendering it capable of delivering greater power when it is exploded. As dead center is again reached, and the mixture is comprest to a maximum, the mixture is ignited by means of an electric spark properly timed. The mixture disintegrates as a result of combustion and forms a gas of tremendous pressure, this pressure causing the piston to move outward.

This is the third cycle or power cycle. (Both valves are closed.) On again reaching dead center, the inertia of the fly-wheel causes the piston to move inward; the exhaust valve is automatically opened and the spent gases are expelled. (The inlet valve is, of course, closed during this cycle.) The four cycles are then repeated over and over again.

It is to be noted that during the whole four cycles, only one power stroke is secured; and that this power stroke is secured as a result of chemical action due to the combustion of the fuel mixture.

The four cycles occur during two revolutions of the fly-wheel. It is therefore necessary to have a fly-wheel with a large moment of inertia in order to cause a steady running of the engine. Smoothness is obtained by using several cylinders instead of one, thus securing more power strokes per revolution of the fly-wheel. The high price of gasoline together with the complications arising in the devices for securing the proper mixture and the right kind of spark at the right time are points against the gasoline engine. Because of the high temperatures developed as a result of the chemical action, the cylinders must be cooled by some means, usually water circulation (another complication).


Steam Engines

The steam engine is still very much used in spite of its low efficiency (usually ranging from 10 to 12 per cent). It possesses the disadvantage of not being capable of immediate starting; perhaps this is the chief reason for its not being used in automobile propulsion except in one or two special instances. Its bulk is, of necessity, very large comparatively as is the bulk of the fuel used. (A one horsepower engine requires about 2 1/2 lbs. of coal per hour.)

On the other hand it is simple in operation arid control. Once steam is "gotten up," the movement of one throttle is all that is necessary to cause a change in speed and power. No fuel mixing device is used nor any electrical system. Figure 2 shows the main parts of a typical steam engine. Steam is raised by heating water contained in a boiler (any kind of fuel can be used).

The steam is led into the cylinder thru the inlet valve. In the type illustrated below there are two inlets to the cylinder, one on each side. A slide valve operated by the flywheel moves in opposition to the piston, alternately admitting steam to the left and to the right of the piston, thus causing it to move back and forth.

It is therefore apparent that there are two power strokes for each revolution of the fly-wheel or four times as many as in the four cycle gasoline engine. The spent steam is usually exhausted into the atmosphere. In the more modern steam engine the spent steam is condensed and brought back to the boiler, thereby requiring water to be added to the boiler much less frequently besides saving fuel.

Because of the high price of gasoline, several automobile concerns have expended a great deal of time and money in research and development on automobile steam engines, and their "selling points" are extremely good ones. Getting up steam is reduced to a matter of only a few minutes at the beginning of the day and the steam pressure is controlled automatically.

Kerosene is used as the fuel which is considerably less expensive than gasoline. No shifting of gears is necessary. A turn of the throttle will give enough steam to climb any hill or to slow down to a crawl. No fussing with carburetor or ignition systems.

The spent steam is condensed and led back to the boiler so that a much smaller water supply tank is used. No cooling system is necessary. A pilot light using up a negligible amount of fuel keeps enough steam pressure all day, so that one can make an immediate start.

All gages operating on the fuel and steam are automatic, thus eliminating all danger. Why is it then that we do not see many of these steam cars. Two reasons—it takes a long time to educate the public—steam cars cannot be kept in the present public garages, since the open flame of the pilot light is liable to ignite the gasoline vapors present in all garages and cause fire.

No doubt the steam car has a great future (provided the oil companies do not boost the price of kerosene).

The Hot Air Engine

The hot air engine uses neither gasoline nor steam but plain ordinary air and that is pretty cheap. Let us examine the principles of its operation. Figure 3 shows an ideal engine consisting of a cylinder, piston, connecting rod, and fly-wheel.

Heat is applied to the lower end of the cylinder as in A, the cylinder being composed of non-conducting material, except the bottom which is a good conductor. The heat will cause the air in the cylinder under the piston to develop great pressure and eventually to expand, forcing the piston upward. If the heat is suddenly removed and replaced by cold, when the piston reaches the top of its upward motion, then the air under the piston will contract and the piston will move downward, due to two causes: (1) the suction under the piston and (2) the pressure from above the piston, since the air above the piston was comprest when the piston moved up. This then would constitute the complete cycle and if repeated indefinitely our engine would continue to run.

Let us look at Fig. 2 again. If the inlet were connected to a tank of air and the air heated, the air would expand, enter the cylinder and move the piston; if then new air were admitted and heated, this on expanding and entering the cylinder from the other side, because of the action of the slide valve, would cause the piston to move back again, giving us a motion similar to that of the steam engine.

The ideal hot air engine described above and this steam engine operated as a hot air engine, illustrate the two types of hot air engines in existence. The first typifies the closed cycle engine which operates continuously with the same mass of air (a fresh charge being occasionally admitted to compensate for leakage). The second typifies the open cycle in which a new charge of air is admitted and exhausted at each stroke. The closed cycle engine is analogous to the new type of steam engine employing a condenser for the spent steam, while the open cycle is analogous to the old type non-condensing steam engine.

Therefore it is readily seen that the closed cycle engine is more efficient than the open cycle, none of the fuel being wasted on the exhausted air. In all types of hot air engines, the power is secured by the pressure of the air produced by heat transferred by a separating metal wall. The air is admitted to the cylinder at a high temperature and pressure, it is allowed to do mechanical work on the piston as a result of which the pressure and temperature fall off.

There are two heating systems employed in hot air engines—the regenerative and the non-regenerative. A regenerative system is one which uses a device for absorbing the heat from the air as it passes in one direction and supplying it again on passing in the opposite direction. A non-regenerative system does not employ such a device.

One of the best of the hot air engines is the Stirling engine which has been actually used commercially, tho without much success. It is of the closed cycle type working with a constant volume of air, and requires about 2 1/2 lbs. of coal per hour horsepower ( i. e., more efficient than the steam engine). Its chief objection was that the heating plate, which takes the place of the boiler of the steam engine, burned out rapidly.

Figure 4 is a sketch of the essential parts of the Stirling engine. C is the brick foundation of the engine, B the fire box, A the draft, H is the working cylinder, E the displacer acting in opposition to the working piston, D the air-box, F metal gauze, and G coils of pipe thru which cold water flows. The total volume of the air contained in the air-box, the working cylinder and the connecting pipe remains constant. The lower end of D is a plate pierced with holes, thru which the air flows when the displacer E moves downward.

The upper part of D, thru which the coils G pass, is known as the refrigerator. The fire heats the air in the air box D. As the engine is started the displacer E moves downward, forcing the hot air thru the regenerator gauze F. Heat is absorbed by F and the air cooled. It is further cooled by the refrigerating pipes G. The pressure in H is therefore diminished and the piston moves downward. This forces the air back thru F into D, gaining heat from F and forcing the displacer upward.

The heat gained from F, in addition to that gained from the fire, causes the air to expand, raising the piston in H. This completes the cycle. The inertia of the fly-wheel causes the fly-wheel to push down the displacer E and the cycle is repeated.



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