Before passing on to other aspects of this subject it may be helpful to introduce a table giving particulars and measurements of the chief types of Diesel engines.
I have already stated that the two-cycle engine finds greater favour in this country and engineers have devoted considerable :Attention to its construction. The improvements which have been made in this type may be shown in the statement that when the two-cycle was first constructed the engine had a fuel consumption of about .5-1 lbs. in b.h.p. the hour, whereas it now stands at .41 lb. in b.h.p. the hour. The pressure of scavenging air has also been reduced from 5-6 lbs. the mivare inch to about 1 lbs. the square inch, resulting, of course, in a correspondingly smaller scavenging pump, without sacrificing, but rather increasing, the maximum efficiencr.
The fact that the two-cycle engine requires only four cylinders, while the four-cycle requires at least six for manoeuvring purposes, thus adding considerably to its cost and weight, and necessitating 50 per cent more working parts, has no doubt largely influenced British engineers in their preference for this type. Its greater mechanical efficiency, in spite of other disadvantages, has also no doubt been an attractive feature, while at the same time greater cargo space is permitted. The four-cycle type is, however, rigidly adhered to in Continental practice, and is mostly in favour among American engineers. It is probable that this preference is due to the fact that the engine is stressed to its maxi mum for only l2i per cent of its cunning time, compared with 25 per cent in the case of the two-cycle engine, and that long experience has confirmed its reliability. Another favourable feature of this type is that its parts are readily accessible, and lend themselves to easy repair. If, for instance, any trouble arises with the exhaust or fuel valves, the engine can be stopped, and the defec tive valves replaced in about half-an-hour. Experience has shown that during a voyage of six months it is necessary in this type of engine to overhaul and regrind the exhaust valves twice. The fuel valves should, to
keep them in condition, be overhauled twice, while the starting air valves can endure a year's service without attention, though it is preferable to overhaul these once in every six months. The inlet valves can carry on for a year, at the end of which period Lloyd's demand a survey. In regard to compressors—on the main engine compressors it has been found in practice unnecessary to touch the valves between cleaning and regrinding, that is about once in six months.
The four-cycle engine is more economical in fuel consumption, and in tests which have been carried out it has been proved that the two-cycle engine consumes, approximately, 100 per cent more oil the square inch of cylinder than the former type. On the other hand, the mechanical efficiency of the four-cycle engine ranges between .75 and .85, while the two-cycle type varies between .68 and .78. Moreover, the weight of the former type is less than that of the latter, as, for instance, 1200 b.h.p. four-cycle engine weighs 200 tons, compared with 220 tons for a two-cycle engine of the same size. Another point against which objections are raised to the four-cycle type is that scavenging involves two lost strokes out of the four, and the piston and running gear, which is designed for maximum working-pressure, must be operated through one complete revolution for the purpose of scavenging the cylinder. This, of course, means a loss of power. In the two-cycle engine the charging and scavenging are carried out through pumps and gear provided for the purpose, though this does not increase its comparative efficiency either mechanically or from the point of view of fuel consumption. An improvement in scavenging arrangements has been introduced into the opposed-piston type of engine. These are of a simple character, the air ports surrounding the cylinder at one end and the exhaust at the other, enabling the scavenging air to sweep through the whole of the cylinder. This form of scavenging induces low fuel consumption, ability for slow running, and a high output by unit of cylinder volume.