Physics Part 36

The common gasoline or gas engine is called a four-cycle (better four-part cycle) engine (see Fig. 164), since it requires four movements of the piston to complete one cycle or series of changes. This is ill.u.s.trated in Fig. 165 =1=, which represents a cross-section of the _cylinder_ of the gasoline engine with the _piston_ moving downward. At the upper end of the cylinder are two _ports_ or openings. One, the _exhaust_ port, is closed, the _inlet_ port is open and a mixture of gas and air is entering. Fig. 165 =2= shows the piston returning; both ports are closed and the "charge" of air and gas is being compressed. As the piston reaches the end of its stroke in compressing the charge, an electric spark explodes or "fires" the charge of gas and air. The hot burning gas expands suddenly driving the piston downward with great force (Fig. 165 =3=). The piston rod is attached to the crank of a heavy fly-wheel and this is given sufficient energy or momentum to keep it going through the next three strokes. Fig. 165 =4= represents the returning piston pus.h.i.+ng out the burnt "charge" through the open exhaust valve _e_. On the next downward motion of the piston the valve _e_ closes. It opens, and new charges of gas and air enter and the "cycle"

is repeated.

[Ill.u.s.tration: FIG. 165.--The four strokes of a gas engine cycle.]

In order to make the motion more even and continuous and also to secure more power, more than one cylinder is attached to the same shaft and fly-wheel. Two, three, four, six, eight and even more cylinders have been attached to one shaft. Four or six cylinders are commonly used in automobile gasoline motors. To lessen the sound of the "exhaust," the latter is sent through a "m.u.f.fler" which often reduces the noise to a low throbbing. (See Fig. 166.) The gasoline engine is more efficient than the steam-engine, since the fuel, gas, is burned in the cylinder and not in a separate furnace. The combustion of the fuel in the cylinders makes some special cooling device necessary to prevent their overheating. This usually consists of a casing about the cylinders.

Between the cylinder and this casing is water which on being heated pa.s.ses to a tank or radiator. In the radiator the water cools and then returns to the s.p.a.ce between the cylinders and casing thus keeping up the circulation.

[Ill.u.s.tration: FIG. 166.--An efficient automobile m.u.f.fler. (_Courtesy Popular Science Monthly._)]

=197. Efficiency of Gas Engines.=--One may test the _efficiency_ of a gas engine by determining the amount of power developed and comparing it with the mechanical equivalent of the fuel burned. Illuminating gas is sometimes employed to drive gas engines. One cubic foot of illuminating gas should produce 600 B.T.U. when burned. The efficiency of the gas or gasoline engines is sometimes as high as 25 per cent. This engine is free from smoke and is also compact and quickly started. While the fuel, gas or gasoline, is somewhat expensive it is light and easily carried.

Suppose a gas engine produces 1 horse-power and uses 20 cu. ft. of gas an hour, what is its efficiency? 1 horse-power-hour = 550 60 60 = 1,980,000 ft.-lbs. 20 cu. ft. of gas = 20 600 778 = 9,336,000 ft.-lbs.

Efficiency = work out/work in = 1,980,000/9,336,000 = 21.2 per cent.

[Ill.u.s.tration: FIG. 167.--The principle of the steam turbine.]

[Ill.u.s.tration: FIG. 168.--Path of steam in DeLaval steam turbine. (_a_) and (_c_) movable blades, (_b_) stationary.]

=198. The Steam Turbine.= One form of the steam-engine that is coming into general use is the turbine. (See Fig. 167.) This consists of a shaft to which are attached blades, the shaft and blades being contained in a closed case. Steam is admitted by nozzles and strikes the blades so as to set them and the shaft in motion. There are also stationary blades (see Fig. 168), which a.s.sist in directing the steam effectively against the rotating parts. The steam turbine is used for large power plants.

(See Fig. 293.) It is very efficient, makes very little vibration, and occupies about one-tenth the floor s.p.a.ce that a reciprocating engine of equal power uses. Some large ocean steamers are now driven by steam turbines.

Important Topics

1. The gas engine, its construction, action and efficiency.

2. The steam turbine.

Exercises

1. If coal costs $4.00 a ton, and gas, $0.80 per 1000 cu. ft. what amounts of heat can be secured from 1 cent's worth of each?

2. What will it cost to heat 30 gallons of water (1 gal. of water weighs about 8-1/3 lbs.) from 40F. to 190F. with coal costing $4.00 per ton and yielding 12,000 B.T.U. per lb. if the heater has an efficiency of 50 per cent.

3. What will it cost to heat 30 gallons of water from 40F. to 190F.

with gas at $0.80 per 1000 cu. ft. if the heating device has an efficiency of 75 per cent.

4. Construct a cardboard working-model showing the action of the gas engine and be prepared to explain the action of the various parts.

5. If 500 lbs. of iron should fall 2000 ft. and all of the resulting mechanical kinetic energy should be transformed into heat, what would be the amount of heat produced?

6. What are the special advantages of (a) the gasoline engine? (b) the turbine? (c) the reciprocating steam engine?

7. Do you burn coal or gas in your kitchen stove at home? Which is for you the more economical? Why?

8. What are the advantages of using a fireless cooker?

9. What is the efficiency of a locomotive that burns 3.2 lbs. of coal per horse-power-hour?

10. A gas engine developed in a test 0.34 horse-power for 1 minute. and 50 seconds, 0.5 cu. ft. of gas being used. The heat of combustion of the gas was 600 B.T.U. per cu. ft. Find the efficiency of the engine.

11. Find the horse-power of an engine, the diameter of the piston being 19 in., stroke 26 in.; it uses steam at an average pressure of 200 lbs.

per square. inch and makes 100 strokes a minute.

12. What is the efficiency of an engine and boiler that develops 200 horse-power, while burning 390 lbs. of soft coal per hour?

13. If a locomotive has an efficiency of 6 per cent. and develops 1700 horse-power how much coal is burned in an hour?

14. If an automobile engine burns 1 gallon of gasoline in an hour and develops 10 horse-power, what is its efficiency?

15. The A.L.A.M.[J] formula for horse-power is (_N_ _B_)/2.5 when the piston speed is 1000 ft. per minute, _N_ being the number of cylinders and _B_, their diameter. Find the horse-power of a 4-cylinder engine, the cylinders having a diameter of 4 in.

[J] American League of Automobile Manufacturers.

16. Find the horse-power of a 6-cylinder automobile engine, if the cylinder diameter is 4.5 in.

17. A 4-cylinder automobile having 4-in. cylinders, uses 1 gallon of gasoline in 1 hour. Find its efficiency, if its average horse-power developed is 6.

18. The motor boat Disturber III, has 24 cylinders each with diameter 3.5 in. If the piston speed is 1000 ft. per minute, what is the horse-power? (See problem 15.)

Review Outline: Heat

Heat; sources (4), effects (5), units (2).

Temperature; thermometer scales (3), absolute temperature, 9C/5 + 32 = F.

Expansion; gases, Law of Charles (V_{1}/V_{2} = T_{1}/T_{2}), liquids, peculiarity of water, solids, coefficient of expansion, uses, results.

Heat Transference; conduction, uses of good and poor conductors, convection, in nature, heating and ventilating systems, radiation, 3 peculiarities, value of sun's radiation.

Heat and Moisture; relative humidity, dew point, formation of dew, fog, rain, snow, etc., evaporation, effects, conditions.

Heat Measurement; specific heat, heat of fusion, of vaporization, combustion.

Vaporization; Boiling point, laws of boiling, distillation, artificial cooling.

Heat Engines; steam, gas.--construction, action, efficiency, mechanical equivalent of heat. Heat equivalent of fuels.

CHAPTER IX

MAGNETISM