greater the power the engine can be made to give out in regular work, the higher its mechanical efficiency will be i.e. B.H.P. =M.E.

I.H.P.

There is a difference between the friction of an engine with hot and cold water in the cylinder jacket, but this is seldom taken into account.

The most satisfactory data have been furnished by Professor Thurston. His experience proved that the friction of a steam engine was practically the same, no matter what power it was giving out.

A gas engine had to give a certain I.H.P. to drive itself, and this amount represented a constant amount, or dead load; all the power indicated in excess of this was effective or useful load put into the work actually done.

It must, however, be borne in mind that the temperature of the water jacket plays a very important part, the efficiency varying considerably with the temperature.

CHAPTER XXVIII

TESTING GAS ENGINES

THE first engine of each size made should be treated as follows at the works before delivery: Before heating the ignition tube, a cock should be fixed on the combustion chamber; turn flywheel round sharply, drawing in a few charges of gas and air, open this cock and apply a light, the object of which is to determine the quality of the mixture; if it burns with a characteristic blue flame it will be rightly proportioned, if it burns indifferently there will be too much air, whilst if it burns with a white cap there will be too much gas. In all cases the mixture should be regulated before attempting to start the engine.

Having made a start and got the engine fairly warm, a power card should be taken, which will show at a glance if the valve settings are correct.

Indicators are invariably fixed on the combustion chamber, and for this purpose a hole is tapped -inch Whitworth, and fitted with a plug. It is not advisable to have any connection between the indicator cock and the combustion chamber, the hole in which should not be less than inch.

When the indicator cock is closed, the indicator pencil should remain at rest; if not, a leak past the plug will be the cause of the pencil moving up and down, and preventing an accurate atmospheric line being obtained. As already explained, great care must be exercised in coupling up the indicator cord.

For practical purposes the following is the usual method of indicating a gas engine: Having fixed the indicator, it is advisable before taking a diagram to run the engine, say, fifteen minutes, to allow time to get rid of the moisture in the cylinder. Diagrams may then be taken, and will at a glance show the valve settings and the difference between the force of explosion before and after the cut-off is obtained.

It will be observed that the indicator diagram is the result of two motions: the vertical motion of the pencil due to the pressure in the engine cylinder, and the horizontal motion of the paper which represents the travel of the motor piston. Hence any point on the curve indicates the pressure in the cylinder corresponding to that position of the piston given by the horizontal distance along the line.

When we introduce the idea of time and compare amount of work done by an agent in a given interval of time, we use the term 'power' or 'rate of doing work.'

To find the horse-power of any agent it is only necessary to calculate in foot-pounds the work done by it in one minute and divide by 33,000.

The indicated horse-power of an engine is the rate at which work is being done on the piston by the working substance as calculated from the indicator diagram, the speed of the engine— that is, the number of turns the crank shaft makes per minute -not being taken into account, but chiefly the number of explosions per minute and average effective pressure (after deducting the pressure during back and charging stroke) and

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

E

=

explosions per minute.

A area of piston in square inches.

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 Tr 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.