LESSON 6:  ENGINES

In the previous lessons we have found that while energy may be converted from one to another of its forms it is never destroyed.  We also found that there is a fundamental tendency for all other forms of energy to change into heat, and for all heat to come to the same temperature.  When a difference of temperature exists it is possible to convert heat into work, but if no temperature difference exists no heat can be converted into work even if, literally, oceans of heat exist.

Definition of an Engine.  An engine may be defined as any type of machine which takes energy in any form and converts it into work.

The initial form of the energy converted may be mechanical, as in the case of wind and falling water; it may be chemical, as in the case of coal, oil and wood; it may be electrical, as in driving an electric motor from a power line; or it may be radiant energy, as in the case of using the sun’s heat to drive an engine.

TABLE 2

ENGINES CONVERTING MECHANICAL ENERGY INTO WORK:

Engine                                                                                        Energy Used

Windmill …………………………………………………………………           Kinetic energy of the wind

Sailing vessel ……………………………………………………………           Kinetic energy of the wind

Water wheel ………………………………………………………………           Potential energy of water

ENGINES CONVERTING CHEMICAL ENERGY INTO WORK:

Engine Energy Used
Steam Engine

(a)           Reciprocating type (piston)

(b)          Steam turbines Internal combustion engines:

 Fuel—Coal, oil or wood
        (a) Natural Gas engine (Methane) ………………… Gas

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48                                                 TECHNOCRACY STUDY COURSE       

(b) Gasoline engine ……………………………………..                          Gasoline

EXAMPLES OF ENGINES CONVERTING ELECTRICAL ENERGY INTO WORK:

Engine                                                                                        Energy Used

Various forms of electric motors …………………………………..Electrical energy from power lines or from electric batteries

An engine which makes the initial conversion of energy into work is called a prime mover.  In electric power systems mechanical or heat energy is converted first into work which is used to drive the electric generators.  These convert work into electrical energy.  The engine which drives the generator in this case is the prime mover.  Electric motors converting this electrical energy back into work are not prime movers, but ‘secondary movers,’ instead.

Efficiency of Engines.  The efficiency of an engine is defined as the ratio of energy converted into work, to the total energy initially supplied.

Efficiency= work output .

Energy input

Therefore, in order to measure the efficiency of an engine it is necessary to know both the total energy taken during a given time and the work done in that time by the engine.

In the case of a waterfall, the available energy per unit of time is determined by the amount of water passing through the water wheel in that time, and by the height of the fall.  Suppose the fall is 100 feet high, and that 990 pounds of water per minute fall through the water wheel.  In this case the energy input would be 990 x 100, or 99,000 foot-pounds per minute.  Since 33,000 foot-pounds per minute is 1 horsepower, then the input into this wheel would be 3 horsepower.

Suppose the output of the wheel were only 2 horsepower due to frictional losses or to poor design of the wheel.  Then the efficiency of this wheel would be:

Efficiency=2h.p.=66.7 percent.

3h.p.

The maximum efficiency possible in this case would be 100 percent, with an output of 3 horsepower.   Modern hydro-turbine installations such as the 70,000 horsepower units at Niagara Falls have an efficiency of approximately 92 percent.  That is, they convert into electrical energy 92 percent of the energy supplied by the water.

Efficiency of Heat Engines.  In order to measure the efficiency of a heat engine we have to measure the heat supplied to the engine as well as the engine’s output of work.  We cannot measure the heat directly, but we can measure the fuel that is used; then we can determine the heat input if we know the amount of heat that is produced by a given amount of fuel.

Heat Value of Fuel.  It was pointed out in Lesson 3 that when certain chemical reactions take place heat is evolved.  Also, for the same number of substances taking part in a given reaction, the same amount of heat is always produced.

Now, the production of heat by the burning of a fuel results from the chemical reaction due to the chemical combination of that fuel with oxygen.  Fuel plus oxygen equals waste products plus heat.  If the fuel be of a particular grade, then the number of calories of heat produced by burning 1 gram is the same for all the fuel of that grade.  The number of gram calories produced by burning 1 gram of the fuel, or the number of British thermal units produced per pound, is called the heat value of that substance.

Heat values are obtained by placing a measured amount of fuel surrounded by compressed oxygen in a gas-tight container.  This is placed in a heat insulated vessel of water and the fuel ignited by an electric spark.  When the spark occurs the fuel burns and the heat which is released is taken up by the water.  The amount of water is known, and the rise of temperature is measured.  From this the number of calories or British thermal units is obtained.

TABLE 3

HEAT VALUE

FUEL

Coal

 Gram Calories per Gram British Thermal Units per lb.
Bituminous, low grade ………………………………… 6,000 11,000
Bituminous, high grade ……………………………….. 8,000 14,000
Anthracite, low grade ………………………………….. 7,000 12,500
Anthracite, high grade ………………………………….  Liquid fuel 7,500 13,500
Gasoline …………………………………………………….. 11,000 20,000
Fuel oils ……………………………………………………..

Wood

 10,500 18,500
Oak …………………………………………….

Pine ………………………………………………..

 4,500

5,000

 8,500

9,000

The average consumption of coal by central power stations in the United States in 1938 was at a rate of 1.41 pounds of coal per kilowatt-hour.  This was a drop from a rate of 3.39 pounds in 1920.  These figures are based upon a heat value of 13,100 British thermal units per pound of coal.  At this value 1.41 pounds of coal contain 18,470 British thermal units.  Since a kilowatt-hour represents 3,411 British thermal units, the average efficiency for the year 1938 is given by

efficiency= workdone = 3,411 =18.5percent.  energyused 18,470

The corresponding figure for 1920 is 7.7 percent.

TABLE 4

ENGINE                                                      EFFICIENCY

Water wheels                                                                                  70 to 92%

Steam engines

  • Locomotives ………………………………………. 5 to 10%
  • Stationary reciprocating engines …………… 10 to 17%
  • Steam turbines ……………………………………. 15 to 30%
  • Hartford mercury vapor station …………….. 10%
    Average of all central power stations in U.S.  in 1932 –    17.30%

Internal combustion engines

(a) Gasoline engine (automobile type) ………..
15 to 28% (b) Gas engines ………………………………………..
25.00% (c) Diesel engines …………………………………….   29 to 35%

The above discussion of engines has been presented in some detail not because we are interested in having the reader become an engineer, but because this, it is hoped, will help to clarify the relationship between matter and energy.  It was stated at the outset that all the matter on the earth is composed of 92 chemical elements, and that, whether this matter is in the form of living organisms or rocks, its movement involves a degradation of energy.

Engines do not create work or energy; they are instead converters of energy —they convert energy from one form to another.

In our next lesson we shall show that the human body is itself an engine that converts energy into heat and work in strict and exact accordance with the laws of thermodynamics.

 

References:

  • The Story of Energy, Mott-Smith.
  • A Textbook of Physics, Grimsehl (Vol. II, Heat and Sound, pp.  153-173).
  • Thermodynamics, Planck.