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The general proportions of this engine are good and the simplicity very marked. There is, however, a strong prejudice against electric ignition; but the makers supply this type

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of engine with tube ignition, and the charge is sufficiently explosive to be fired by an incandescent tube, so that equally good results can be obtained in this way.

CHAPTER V

FLYWHEELS

THE object of a flywheel is to reduce the fluctuations in the speed of an engine due to the inequality of power and resistance. In an 'Otto' engine, since the piston turns the crank only half a revolution during the explosion stroke, the energy stored in the flywheel must be considerable to keep up the rotation during the negative strokes, with the result that very heavy ones must of necessity be used.

The superiority of the graphical over the analytical methods for determining the weight of a flywheel is generally acknowledged, and deals with all the variable functions entering into the question with a simplicity, clearness, and accuracy perfectly adequate to the demands of practice. Yet, however advantageous its application, an amount of labour is involved not always at the disposal of the designer.

The first step to be taken in fixing the weight of any flywheel for a given engine is to ascertain the tangential pressure and turning effort on the crank pin, taking into account the effect of the reciprocating parts, which may be found as follows. Fig. 60 is an indicator diagram taken from an engine having a cylinder 19 inches in diameter and a stroke of 30 inches, running at 120 revolutions per minute. The weight of the piston and gudgeon in this engine is 546 lbs., the connecting rod = 786 lbs., making a total weight of the reciprocating parts of 1,332 lbs. This indicator diagram must be corrected, as the reciprocating parts-viz. the piston and connecting rod-are in a state of rest at the commencement of the stroke, and power is expended in accelerating or bringing them to the required velocity; then at the end of the stroke the inertia possessed by them must be added to the pressure behind the piston.

Therefore, taking the movement of the piston corresponding

F

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to 1° of turning of the crank, the extent of motion will be the versed sine of an angle of 1° with a radius of 1.25 foot =0001523 × 1·25·00019 foot in the time of

1

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720

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60

360 120

of a second, which is equivalent, as regards the ac

celerating force required to produce it, to a motion of '00019

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Whence f = ·00019 × 518400 × 2 = 196·99 feet per second, the space passed through, under a uniformly accelerating force, being in proportion to the square of the time; therefore this motion of 196-99 feet per second being 6-1 times the acceleration due to gravity (32.2 feet per second), the force required to impart this velocity amounts to 61 times the weight of the reciprocating parts = 1332 × 6·1 = 8125.2 lbs. 8125.2 = to a pressure of 283.5 (area of piston) inch, which equals the accelerating and retarding force due to the inertia of the moving parts. Although there is a slight difference in the effect produced at each end of the stroke, owing to the angularity of the connecting rod, it may be neglected for the purpose of this example.

= 28 lbs. per square

Fig. 61 is a diagram showing this accelerating and retarding force above and below the horizontal centre line A B, superposed for the purpose of correcting the original diagram (fig. 60) ; by joining these two forces we get a diagram which gives the pressure at any part of the stroke, and by subtracting and adding these pressures to the original diagram (fig. 60) we get the corrected diagram as shown in fig. 62 for the firing stroke, fig. 63 for the exhausting stroke, fig. 64 for the charging stroke, and fig. 65 for the compression stroke. Now take position 1 of crank pin and join 1', prolonging centre line of connecting rod beyond point 1 to C, making 1 C = 1' D pressure on piston, and neglecting angularity of connecting rod this equals pressure transmitted to crank pin by connecting rod. This pressure is now resolved into two forces, by drawing a perpendicular to centre line of crank arm to pass through point C; then

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