Catnellaird-Fullagar Diesel engines are of the two cylinder opposed piston type and of a very novel design which, whilst permitting the use of opposed pistons, calls for the addition of few extra reciprocating parts.
The unit consists of pairs of vertical side-by-side cylinders carrying a straight liner, and affording straight-through scavenging by means of ports in the walls.
Two pistons are provided for each cylinder, the lower one in each case being coupled with a connecting rod to a two-throw crank-shaft. Each of the upper pistons is connected by means of a diagonal rod to the lower piston in the adjoining cylinder. When combustion takes place between the two pistons the upper one misses and thus exerts an upward pull on the adjoining lower piston, whilst at the same time the lower piston in the cylinder in which combustion occurs is forced down, bringing with it the upper adjoining piston.
Instead of the flywheel being called upon to overcome compression in a cylinder, the direct combustion force is used, and thus, instead of the crank-shaft's having a downward load on one throw and an upward one on the other, it is constantly subjected to a positive rotating force. Each cylinder, therefore, directly utilizes a part of its potential driving power to overcome the period of inactivity in its neighbour. The driving couple present in all other types of internal combustion engine is thus eliminated, and a rocking force on the crank-shaft is eliminated in favour of a constant load.
The disadvantages naturally associated with diagonal rods are overcome by the provision of suitable guides to overcome the heavy side thrust which is inseparable from the design. It must be remembered that from the nature of the lay-out the diagonal members are always in tension, and, therefore, a complexity of forces is not in evidence.
Fuel valves are, of course, in the centre of the cylinder and between the pistons. They are cam operated from a horizontal skew-driven cam-shaft.
Reverse is direct and is easily effected by altering the valve timing by means of a slight rotation of the cam-shaft. Air starting being employed, the reverse is very readily brought into action.
The novelty of the design of Camellaird-Fullagar engines is justified by many advantages. The hull space which is occupied by one of these engines and a more conventional type is h.p. for h.p. considerably less ;
the engine can be run at a very low speed, greatly facilitating manoeuvring in harbour, etc., the weight of the unit is extremely low, and both first cost and running expense are, owing to the absence of complexity, extremely low.
Doxford oil engines for marine and stationary work are a combination of the Diesel and the Hot Bulb type, embodying the advantages of the former in regard to mean pressure and consumption, and, in regard to the latter, for simplicity. They are of the two-cycle type, working on a low compression pressure, and burn Mexican fuel oils.
The cylinders consist of a plain tube close-grained casting one inch thick for the largest diameter, with scavenging ports at the lower end, and exhaust ports at the upper end controlled by the respective pistons, permitting the most efficient scavenging.
The piston heads exposed to the flame of combustion are of ingot steel forgings, retaining a high temperature to assist combustion of the heavy fuel oils. They are easily removable without breaking high pressure joints.
No stresses are passing through the frame structure and main bearings excepting those on the crosshead guides, the loads on the pistons being transmitted direct to the crank-shaft through steel rods.
Fuel valves are of special design, conspicuous for the absence of glands and springs, and are so arranged that the oil sprays do not come in contact with the pistons or cylinder walls.
Fuel pumps, one to each cylinder driven direct off the crank-shaft, and have no glands.
Reversing is effected by ahead and astern cams, the cam-shaft being moved longitudinally by hand and lever. Compressed air is used to start rotation of the engine, then changing over to oil, which is ignited at the lowest revolution.
Marine engines are of exceptionally long stroke, and therefore suitable for single screw drive of the highest power. The normal speed for 3,000 horse-power is 77 revolutions, manoeuvring down to 16 revolutions. No air compressor is used for injection, as they operate on " Solid Injection." The Vickers Diesel engine is of the four-stroke cycle, this type having been adopted as being the most economical and practical for the purpose of propelling merchant ships.