Figure 267 shows how the steam may be passed from the steam nozzles N through three sets of moving blades M by

the use of two sets of stationary blades S. By this arrangement the energy of the steam is used with a high degree of efficiency. The steam turbine is used to drive the dynamos in some of the largest plants for the generation of electric current.

314. Gas Engines. In a gas engine the motive power is the expansion caused by the explosion of a mixture of gas and air in a cylinder. The expanding gases work against a moving piston as the expanding steam does in a recipro

cating steam engine. The cycle of operations in a fourstroke or four-cycle engine is briefly as follows (Fig. 268):

INLET

OUTLET

SPARK PLUG.
ARK

FIG. 268

On the first, outward, stroke of the piston the inlet valve is opened and a mixture of gas and air enters the cylinder through the inlet. On the return stroke the mixture is compressed. When the piston begins its third stroke, which is the second outward stroke, the gas is exploded by an electric spark and expands, and on the fourth stroke the outlet valve is opened and the products of the combustion are driven from the cylinder through the outlet. The third stroke is the only one from which power is obtained.

D.

CRANK-
SHAFT

FIG. 269

CRANK

PIN

E

In the two-cycle engine the mixed gases enter the crank case B through the port A, Fig. 269, when the piston C is at the top of its stroke during which the gas in the cylinder D is compressed. The

spark explodes the gas, driving the piston downward and com

pressing the gas in the crank case. The gas from the crank

case passes through the transfer port as shown by the arrows, and forces the exploded gas out of the exhaust port E. Both ports are closed as the piston begins its upward motion and the rest of the stroke

completes the compression.

Since the power is applied for short intervals only, the fly

wheel is made very heavy, in order that its momentum may keep the speed of the engine more nearly uniform. Sometimes two flywheels are provided, as in Fig. 270. Gasoline engines are explosion engines which differ from gas engines only in certain details of construction required by the fact that the fuel is a liquid. They are extensively used in automobiles.

315. The Energy Stored in Coal. While we look upon the sun as the present source of the heat received by the earth, we must not forget that in the vast deposits of coal found in the earth we have a storehouse of the sun's energy in the past preserved for our present use. Whenever a gram of carbon is burned, it unites with 23 grams of oxygen, and gives out 8080 calories of heat. Since the percentage of carbon in anthracite coal is very high, we may say that if we could use without waste all the heat given out by burning one gram of coal to heat water, we could raise 8000 grams through 1o C., or we could raise 80 grams from zero to the boiling point. The mechanical equivalent of this heat is about 3415 kilogrammeters of work. The burning of only