the mean of the first explosion after cut-off and, say, two others being taken.

Therefore, let P = mean effective pressure of diagram,

S = length of stroke of piston in feet, and explosions per minute.

E

A

area of piston in square inches.

P x A x S× E indicated H.P.

33,000

=

Of the total indicated horse-power in an engine part is spent in overcoming the frictional resistance of the mechanism, and the remainder that is available for effective work is called the actual, brake, or effective horse-power.

Undoubtedly the simplest and most accurate method of gauging this is by means of a brake as shown at fig. 174. It consists of an endless rope, in two turns kept apart from, as well as from slipping off the brake wheel by wooden crosspieces. These should be laced to the ropes, and kept well clear of the rim of the wheel. It is not advisable to fasten the rope to the block by screws or nails, as these are liable to touch the

rim of the wheel and make it excessively heated by the friction so as to burn the rope. Grease, tallow, paraffin, and plumbago are often used to lubricate the rope and blocks. The more lubricant is used, the greater will be the variation on the spring balance.

It is possible and practical to run a wheel of 60 inches diameter with a load equal to 12 H.P. on the rope for ten hours with safety without lubricant; in fact the author has run an engine fitted with a 48-inch wheel, and having 30 lbs. on the rope, for 231 hours without lubricant of any description. Providing the wheel is turned smooth on its face and a good close strand of rope is used, lubricants are unnecessary, and there is only slight variation on the spring balance.

Engines developing 14 to 30 B.H.P. should be tested with flywheels split at the boss. If not, there is a great danger of the wheel seizing on the shaft. Above 20 B.H.P. all wheels should be arranged with a water-trough rim.

To find the actual horse-power

Let W = gross load

w = average pull on the spring balance, which must be deducted from W to give the

nett load.

r = radius of brake wheel and rope to point of suspension, the effective circumference 2 πr feet.

N = revolutions of wheel per minute.

Therefore the work done against friction is
(Ww) 2 πr N foot-pounds per minute.

THE principal use of the indicator is to give a graphic representation of the varying pressures in the cylinder of gas engines, enabling engineers to determine the amount of work done

by the working fluid, and to detect faults in design which might otherwise escape notice. But the correct interpretation of the diagram is by no means the simple matter it appears at first sight, as it involves most careful study and long experience. The spring and moving parts constitute, of course, the most important details of the instrument. Each indicator is supplied with a number of springs of different strength or stiffness to suit different pressures and speeds, the object always being to obtain the largest possible diagram which can be had under the conditions of speed and pressure. The different springs are arranged in terms of their compression in fractions of an inch, under a pressure on the piston of 1 lb. per square inch, or a direct load of lb.; and the springs are numbered accordingly, the limits of pressure for which the spring is available being marked on its boss. The scales used are generally multiples of 4 or 8, so as to allow for conveniently using an ordinary inch scale divided into eighths for measuring up the diagrams. The divisions of the scales supplied with each spring are uniform throughout the range, on the assumption that the deflection of the spring is always for this range directly proportional to the load. This is not strictly true; and it would certainly be more accurate to divide each scale separately from the spring, instead of forcing the spring, as it were, into a certain predetermined scale. The error under the present system is, however, not large, amounting, according to the investigations of Professor Reynolds and Mr. Brightmore, to a maximum of about 1 per cent. in ordinary good springs, and this is quite insignificant compared with other errors to which the diagram is subject. For ordinary purposes this slight disadvantage is more than counterbalanced by the great convenience of having a fixed scale.

The most recent improvements in the indicator have been in the direction of stiffening the spring and reducing the inertia of the parallel motion, so as to obviate as far as possible the oscillations of the pencil, and render the instrument applicable at the higher speeds which are daily finding more extended application.

Before using any indicator a motion card should be taken.

To do this all that is necessary is to take out the piston and spring, draw the atmospheric line, and allow the drum to return. to its original position; then lift pencil. The line drawn should be a horizontal straight line.

When using an indicator for taking diagrams from a gas engine very great difficulty is experienced, owing to the sudden rise of pressure, and abnormally high cards are often obtained, in some cases reaching 400 to 600 lbs. above atmosphere. The strain is invariably severe: the temperature may affect the spring, and there is considerable risk of the piston sticking; therefore it is not desirable to place too much reliance upon the indicated horse-power of a gas engine.

Diagrams taken when the engine is cold are very different from those when the engine is hot. Then, again, the first diagram after a cut-off-after the cylinder and combustion chamber have been cleared of the products of combustion-is invariably larger than the succeeding ones.

Thompson's Indicator

Fig. 175 is a part sectional elevation of a small Thompson indicator, as made by Messrs. Schäffer & Budenberg, the chief distinguishing feature of which is the novel arrangement of light levers carrying the pencil.

The parallel motion is carefully designed to ensure that the pencil point describes a straight line, and the motion of the pencil point should be precisely proportional to that of the indicator piston throughout the stroke.

The link connecting the pencil arm to the piston rod has a ball-and-socket joint at the loose end to allow free motion.

A movable collar around the upper part of the cylinder swings round against a stop to prevent undue pressure of the pencil on the paper. This collar carries a fixed standard with radius bar, as well as a link with the fulcrum of the pencil arm at its upper end.

The parts which move with a high velocity are made as light as possible, consistent with strength and stiffness, in order to reduce their inertia to a minimum.

The paper drum is shown separately in section, fig. 175.

By removing the outer cylinder, loosening the nut on the top of the drum spindle, and by turning the disc holding the flat spring inside, the latter may be readily adjusted to suit any speed of engine up to 400 revolutions per minute.

A

FIG. 175. THOMPSON INDICATOR

When using this indicator for taking diagrams with exceptionally high maximum pressures a special piston A, one-half the area of the ordinary piston, is used, thus reducing the stress on the levers and motion bar and effectually overcoming the wave oscillation.