or Bunsen flame, it is diluted with air, and should be allowed to burn until the flame becomes the colour of an ordinary gas jet. It may so happen that, although the engine may run fairly well with a light load, yet with a full load the gas supply may be found insufficient; so that, the gas bag being emptied, the supply is drawn direct from the main, with explosions in the air and exhaust pipe following as a consequence because the gas and air are not mixed in the proper proportions in cylinder. In case the supply of gas is throttled-i.e. the main is not large enough to suit the pressure, an accumulation of water, or obstruction in the pipe; sometimes by negligence in making joints. On the other hand, it may be all right at night, when the pressure is higher, the explosions taking place only during the day. All lights connected to the same main will fluctuate considerably, so that the Bunsen flame for heating the tube may persist in lighting at the air holes. If the gas is insufficient, the simplest remedy is to increase the size of the main. The increase in size will, of course, depend on the size of the main gas supply, and the distance the engine is fixed from this supply. However, the lowest pressure should always be known before the engine is fixed. Engines placed at the top of a building are better off in this respect.

CHAPTER XXV

SCAVENGING METHOD

THE first successful engine using a positive scavenger was the six-stroke engine invented by Linford in 1881; but the first four-stroke Otto cycle engine with positive scavenger was that invented by Mr. J. H. Hamilton, B.Sc., and exhibited by Messrs. Wells Brothers at the Crystal Palace Electrical Exhibition in 1891 and 1892. This engine was a 14 H.P. NOM., and used 16.5 cubic feet of gas per I.H.P. per hour when developing 27 B.H.P. The compression of the charge before ignition was 65 lbs. per square inch, and although using a differential piston the work absorbed in forcing the air through is, owing

to free passages, very slight, a reduction of lb. to 1 lb. per inch from the indicator card will well cover it.

square

Although the advantages gained by exhausting the products of combustion have been disputed and re-disputed over and over again, there is no doubt that to get rid of the products as quickly as possible after the working stroke is completed is the right thing to do. Not only is the average pressure of the indicator diagram maintained more uniformly, but a greater power is developed for the same size of cylinder, with a marked decrease in the temperature of the cooling jacket water, and thus making the engine more suitable for hard continuous work. Under certain conditions, such as using a long exhaust pipe without a silencing chamber, when the pipe has become heated a partial vacuum is created (even with the ordinary valve settings this is very noticeable when the air and exhaust valves have been arranged in close proximity to each other). Several methods of eliminating the exhaust gases without the use of separate pumps have been used, the most noticeable being the Crossley-Atkinson Scavenging Method,' patented in 1893, and described in the 'Engineer,' December 1894.

Fig. 172 shows various positions of the crank pin with the valve settings, and fig. 173 is a weak spring diagram.

Assuming the crank pin to be in the position A at the time the exhaust valve opens, towards the termination of a working stroke, the exhaust valve is kept open until position B is reached. The exhaust valve in an ordinary 'Otto' engine would be closed about the point C when the piston is at the end of its stroke.

Fig. 172 shows approximately the crank pin in its various positions. The relative periods are also shown developed.

In the ordinary' Otto' engine valve settings the air and gas enter during a half-revolution, the exhaust valve opens before the end of the outward stroke, and closes at the end of the instroke; but referring to fig. 172 it will be seen that in the scavenging engine the air valve opens before the end of the instroke and closes at the end of the out-stroke, and the exhaust valve opens before the end of the stroke but does not close until the piston has passed the dead centre, the air and exhaust valves being thus open together for about a quarter of a

с

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 T

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

has recently tested a Forward engine having a cylinder 7 x 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