The Napier Deltic Marine Engine
THE NAPIER DELTIC MARINE ENGINE
E. E. Chatterton, B.Sc.(Eng.), M .I.M ech.E ., F.R .A e.S.,* and D. D. P. Fuller**
Introduction
The recent amendment to the Merchant Shipping Act has two principal effects. In the first place, it gives scope for smaller main propelling machinery installations disposed so as to occupy the minimum possible space consistent with the efficient and safe operation of a vessel. In the second place, the amendment offers incentive to marine engine designers to develop lighter and more compact units which will permit further saving of space than is possible by merely changing the position and design of engine rooms. In this paper proposals are submitted designed to take full advantage of the new regulations by the application of the Napier Deltic marine engine, which is now being manufactured for the Royal Navy and for the navies of several foreign powers. The recent amendment to the Act has given rise to still more active consideration of the application of the Deltic engine to merchant ships, and the invitation from your Papers and Transactions Committee to embody some of the conclusions in this paper is welcomed.
As always, consideration of a question of this nature must be based on the requirements of the shipowner, namely that a cargo should be transported at the required speed at the lowest possible cost per ton. The introduction of propulsion machinery of advanced design, thus saving space and weight, offers an opportunity to reduce substantially the cost of transporting cargo at a given speed, and in considering any proposals it is essential that full consideration should be given to this aspect, which may be broadly subdivided as follows : —
1. A reduction in the capital cost of the machinery.
2. A reduction in the capital cost of the hull in relation to cargo capacity.
3. A reduction in the cost of the hull due to the lighter machinery.
4. A reduction in power (or fuel consumption) in relation to cargo capacity.
The relative importance of each of these factors on operating cost will, of course, depend on the operating schedule of the ship, but in general the most important factors will be those which affect the capital cost of the vessel. It is submitted that by the use of multi-engine installations using compact engines of modern design, advantages can be shown on each of the foregoing counts. The detailed features of construction referred to in the following proposals have already been widely published and a paper devoted to the operation of Deltic engines will be presented to the Institute by Mr. Chatterton during next session. For ready reference, however, those features of design and performance which are relevant to the application of the engine to merchant ships are repeated here.
The 18-cylinder Deltic engine comprises six triangular banks. Each bank is in the form of an equilateral triangle with a main crankshaft journal at each apex, as illustrated in Fig. 11. Thus the engine comprises, in effect, three six-cylinder uniflow scavenged, opposed piston, two-stroke engines.
The triangular nature of the engine exploits, to the full, the advantages and simplicity of the opposed piston design without any of its disadvantages, and also results in almost complete balance of the reciprocating forces and masses within the engine frame. The major rotating masses are balanced and this leads to the production of a smooth-running engine in which the frame is almost free from vibration and no major pulsating forces are transmitted to the bearers. It is standard practice to employ anti-vibration mountings at the four bearing points to the hull structure.
The smoothness of operation of an 18-cylinder two-stroke engine is well illustrated by the close ratio of maximum to mean torque—about 106 to 1. This eliminates the destructive effects responsible for gearing troubles sometimes encountered.
The essential reliability and maintenance-free operation demanded by marine duty are achieved in this engine, which has components of a size which permit the use of modern manufacturing techniques and high grade materials in the important working parts such as forged steel liners, chromium plated in the bore, fully hardened crankshafts, lead bronze thin wall bearings and case-hardened and ground gear wheels.
One feature of the triangular designs which has demonstrated its value during the development of the engine, and which should be mentioned here, is the absolute rigidity of the main structure and this, coupled with the high grade materials employed, is undoubtedly a most important contributory factor in the reliable operation that has been achieved at rotational speeds up to 2,000 r.p.m. For use in merchant ships a maximum crankshaft speed of 1,500 r.p.m. is recommended.
* Chief Engineer (Piston Engine Division), D. Napier and Sons, Ltd.
** Manager of Deltic Engine Sales and Contracts, D. Napier and Sons, Ltd.
Description of Engine
The Deltic engine, as illustrated in Fig. 12, is produced as a self-contained power plant, complete with all pumps and filters, and control of both the engine and reverse gear is effected by a single lever. The leading features of the construction are as follows: —From the geometrical layout, two important technical advantages arise. Firstly, there is a phase-angle difference of 20 degrees between exhaust and inlet crankshafts, which leads to a most efficient porting layout for scavenging and cylinder charging. The second advantage is that each crankpin carries one inlet and one exhaust piston, so that the loading on each crankpin and the power transmitted through each crankshaft are identical.
Construction of the Triangle
The triangulated unit is built up from three identical cast-aluminium six-cylinder blocks, which form the sides, and there are three cast-aluminium crankcases at the corners. The resulting structure is held together by high-tensile steel through-bolts, which extend from each crankcase through the cylinder block to the other crankcase. These bolts carry all the combustion loads, the cylinder blocks remaining in compression.
Cylinder Liners
The cylinder liners are of the wet type and are machined from hollow steel forgings. The bores of the liners, in addition to being chromium plated and lap-finished, are so treated in the plating to have a closely spaced, dimpled surface arranged in a definite pattern. The depressions caused by the dimpling process fill with carbon, which absorbs enough lubricant to ensure, even when starting, that the piston will not operate in a dry condition.
Exhaust ports are machined around approximately two-thirds of the circumference at one end of each liner, while, at the other end, inlet ports are provided around the whole circumference and are machined with a partially tangential direction of entry, thus imparting a “swirl” to the inlet air.
Injection System
Two injectors of the outward-opening valve type are provided for each cylinder and both are fed with fuel from a single injection pump. The nozzles inject at a wide angle from the axis of the injectors and deliver fuel at the outer edge of the combustion chamber, the fuel being transmitted to the centre by virtue of the air swirl.
The pumps are of normal C.A.V. type, modified to suit the design requirements of the engine. The six pumps for each cylinder bank are carried on a camshaft casing, which extends for the length of the cylinder block. Rack control is by a rotating shaft, the pumps being connected together by couplings which are torsionally stiff but axially flexible, thus eliminating differential expansion effects. The design allows any one pum p to be removed and replaced by another, correct timing and matching being automatically obtained.
Pistons
The pistons are constructed in two parts, held together by a spring circlip. The inner member is machined from a Y-alloy forging and carries the hardened-steel gudgeon pin. The outer
member is of cast aluminium and has an unbroken skirt.
It has a cast-in austenitic-iron insert, in which the two top
gas rings are mounted. Between the outer and inner members
there is an annular chamber extending from the bottom of the
skirt to behind the gas rings, and communicating with grooves
in the top face of the inner member which pass across the
underside of the piston crown. F or cooling purposes, oil
is fed into this chamber from the connecting rod.
Connecting Rods
The connecting rods are manufactured from high-tensile steel stampings and are machined and polished all over. The pairs of rods on each crankpin are of the fork and blade type. The big-end of the forked rod is fitted with a steel shell, nitrided on its outer surface and carrying in its bore a Vandervell steel-strip bearing, lead bronze-lined and lead-plated, which runs direct on the crankpin. The outside sur face of the shell provides the bearing surface for the blade rod, the big-end of which is also fitted with a steel-strip, lead-bronze coated bearing.
Crankshaft
The seven-bearing crankshafts are nitrided all over, the bearing surfaces being finished by lapping. The main journals run in steel-strip, lead-bronzed and lead-plated bearings, which are carried direct in the cast-aluminium crankcases.
Lubrication
Lubrication is by pressure feed to each bearing and uses a dry-sump system and a sea water-cooled heat exchanger is included in the scavenge circuit for oil-cooling purposes. Thermostats are provided to control the oil temperature.
Cooling System
The cylinder jackets form part of a closed system in which distilled water is circulated through a heat exchanger by a centrifugal pump on the engine. Sea water is circulated through the cool side of the heat exchanger by a centrifugal pump.
Crankshaft Phasing
The three crankshafts are coupled together through gearing, the power from each crankshaft being transmitted through a tributary gear train into a common output shaft at the centre of the triangle. This latter shaft runs slightly above crankshaft speed. The connecion between chrankshafts and gear train includes a flexible quill shaft on the output end of each crankshaft.
By alternative assemblies of idler gear in this phasing system the output shaft rotation can be reversed the crankshaft rotation remaining unchanged.