C

revolution. When the exhaust valve opens there is a pressure of 30 to 40 lbs. left in the cylinder; the burned products on passing the exhaust valve begin to compress the gases immediately in front against the inertia of the column beyond, and this compressed area passes rapidly up the pipe until the whole column is in motion. Fig. 173 is a weak spring diagram

from one of Messrs. Crossley Brothers' engines, and it will be noticed that a period of wave-like oscillations is shown. The exhaust line rises above the atmospheric line; but had the exhaust not set the column of air in the exhaust pipe in motion, the back pressure would, of course, have been greater. As the piston nears the end of the in-stroke, the pressure falls below the atmospheric line, due to the momentum stored up in

the exhaust column, and the scavenging is effected during the time the air valve is open along with the exhaust.

It is, however, beyond dispute that very economical results have been obtained without 'scavenging,' and Mr. Lanchester Stop on Indicator

Atmospheric

Line

SCALE

FIG. 173.-WEAK SPRING DIAGRAM (SCAVENGING METHOD)

has recently tested a Forward' engine having a cylinder 7 × 14 inches stroke, running 195 revolutions per minute with a mean pressure of 84 lbs. per square inch, and the gas consumption per I.H.P. hour was only 17.25 cubic feet.

CHAPTER XXVI

LARGE POWER ENGINES

ALTHOUGH increase in compression before ignition has to a very large extent been the means of increasing the power and economy of gas engines, the author is inclined to think that this increase of compression has been overdone for large engines. In fact, without compensating advantages, increased expansion is essential, because there is less trouble with the exhaust.

With engines having large bore and short strokes, increased expansion is obtained with very high compression.

The difficulties are not merely in the construction of an engine which will withstand the heavy explosion pressures, but which will withstand them when aggravated by the differential stresses which result from the high temperature of combustion and the unequal cooling effect of the water in the jacket. There are also serious difficulties connected with the cooling of the piston, and with the ignition of the charge of poor gas in large cylinders. With such large dimensions and great forces the questions concerning the jacket, the exhaust valves, and piston become difficult, and great care has to be

taken to prevent overheating, and to secure as near as possible a regular mean temperature of the cylinder. The cooling surface is much less in proportion to the volume of the gases burned in the cylinder; the consequence is that the products of combustion retain heat to a far greater extent than in small engines, and the high temperature in the larger engines is liable to ignite the incoming charged, the result being that large engines at full power are more liable to back firing and pre-ignitions than those of smaller size.

The tendency is to construct engines of large power having a large bore of cylinder and short stroke-in some cases the stroke is little more than the bore of cylinder. These engines are run at not less than 800 feet piston speed per minute.

From a commercial' point of view it will be understood that the object is to construct an engine so that the cost of it per unit of power is low; but from a 'mechanical' and even scientific' point of view this is wrong, notwithstanding the fact that the theoretical efficiency increases with compression.

It must, however, be borne in mind that by constructing an engine with ordinary compression and increasing the expansion a large sized engine is required, and although, as already stated, in keen competition this class of engine may suffer, nevertheless the wear and tear is considerably reduced, and it is very questionable whether in large sizes there is any gain in the long run by adopting high speeds and short strokes combined with very high compression.

CHAPTER XXVII

FRICTION OF GAS ENGINES

It is generally accepted that a steam engine has practically the same amount of friction when driving all the moving parts and doing no other work as it has when doing its maximum duty. This applies equally to a gas engine, and since this is so, a gas engine of any dimensions will require a certain amount of power to keep it moving at its normal speed; all additional power is practically effective horse-power, consequently the

greater the power the engine can be made to give out in regular work, the higher its mechanical efficiency will be i.e.

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 2-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 enginethat 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