THE INTERNAL commusalos /INCISE combustion engine is a generic term which comprises all descriptions of engine$ in which air and oil are brought together in specific quantities, and by their mixture are exploded. There are thus internal combustion engines in which light oils are used, as in the motor car and aeroplane engine, and those in which heavy oils are used, as in the Diesel and Hot-bulb types of engine. The principle of combustion is the same in both types of engine. The term has, how ever, become almost wholly associated with the Diesel type.
It was not until Dr. Diesel designed an engine in which it was possible to bring about the combustion of heavy oils that the application of the internal combustion engine became applicable to industrial and transport work. The original design was for its use in stationary work, and it %MS not until the engine was taken by the various marine engineers in different countries that its applicability to propelling ships was recognized. The latter direction now appears to offer the widest field for the use of this type of engine.
These engines now fall into two distinct categories, namely, the two stroke and the four stroke, but, before referring in detail to the differences between these, it may be advisable to describe briefly the actual opera tion which takes place in the Diesel engine of any type. This operation consists of four %vell-defined actions ; first, the suction stroke, by which the piston sucks air and oil through a valve into what is termed the vaporizer; second, the compression stroke, which merely com presses the mixture ; third, the explosion stroke, during which the gas is exploded or burnt, and fourth, the exhaust stroke, which ejects the burnt gas through the exhaust valve. This, very simply, embraces the whole process by which the Diesel engine works.
Reference may now be made to the two distinct types of this engine, namely, the two cycle and four cycle, in each of which, however, precisely the same operations take place. The difference between the two types is that in the former an explosion occurs with every revolution, while in the latter an explosion occurs on each alternate revolution.
There is, hoWever, another description of the internal combustion engine, known as the hot-bulb or semi Diesel engine. In this the explosion of the gas is pro
duced artificially, and not, as in the Diesel, as a part of the actual operation, and further reference will be made to this type later. For the moment we will restrict our attention to the variations which have been introduced by the different firms into the original design. It is unnecessary in this little work to describe the character istics of each Diesel engine now available. I will, therefore, restrict this review to those types which are in greatest use at the moment, and to those which embody some distinct feature, differentiating them from previous types. At the present time, the majority of engines constructed are on the four cycle principle, though in Great Britain the two cycle type is more largely favoured.
The principle exponents of the four cycle type are Messrs. Burmeister & Wain, of Copenhagen, and the largest motor ship now running is equipped with engines constructed by Messrs. Harland & Wolff, who are licensees of this engine in this country. In order to give a clear and concise description of this engine I cannot do better than reproduce that which appears in the Petroleum Year Book. It states that this engine is built according to the four-stroke cycle system, whereby during—each second stroke cold air is sucked into the cylinder, which during the stroke cools the internal surfaces. As this suction of cold air takes place in the cylinder itself, it is not necessary to use scavenging pumps, which form a necessary part of the Diesel engines working according to the two-stroke cycle system. The result is that the four-cycle engines are not so compli cated, and, as they are not fitted with scavenging pumps, the cylinder dimensions can be increased to such an extent that within the same length, height and engine weight, the b.h.p. will be, at least, the same, or in most cases larger, than for a corresponding engine of the two cycle system. The ordinary opinion that a two cycle engine gives double as much horse power as a corresponding four-cycle engine is therefore not quite correct.