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The Admiralty required a light weight high powered diesel engine for their new MTB’s and high speed rescue craft. Napier having produced the Culverin a 6 cylinder 930 HP opposed piston diesel aero engine pre WW2 were selected in 1946 to design and produce six test engines to meet the Admiralty specification. Their design team under Gordon Murray (not the Brabham/McLaren one -asst ed) used the Culverin as a starting point being a six cylinder opposed piston diesel engine with two crank shafts. By adding one crank shaft they could produce a triangular layout with three banks of six cylinders with a crankshaft at each apex. However with this configuration if all three crankshafts rotated in the same direction, the inlet and exhaust pistons in one bank simply chased each other. This problem was solved by an Admiralty engineer Herbert Penwarden who realized one crankshaft must rotate in the opposite direction.

The Deltic was born and ran for the first time in 1948.


The opposed piston engine operates on the two stroke cycle and has a number of advantages such as no cylinder heads, large area of inlet and exhaust ports resulting in excellent volumetric efficiency. The inlet port is a series of openings in the cylinder liner which are opened and closed by the inlet piston opposing the exhaust piston which similarly opens and closes the exhaust ports in the same extended cylinder. The pistons move towards each other on the compression stroke forming the combustion chamber at about TDC when fuel is sprayed in to the air compressed during the compression stroke to about 550 PSI and 600 C. The fuel is burnt rapidly and the hot high pressure gas at about 1600C forces the pistons apart on the power stroke with the exhaust port opening first then the inlet port opens just before the exhaust port closes. Hence the two stroke cycle.

The inverted triangular configuration of the Deltic resulted in a compact very rigid assembly of three banks of 6 cylinders with 36 pistons, three crank shafts and crank cases, with a phasing gear box mounted on the output end of the engine connecting the three crankshafts. The two top crankshafts rotated clockwise and the bottom crankshaft anti clockwise.

Each crank pin carries a forked con rod connected to an exhaust piston and a blade big end con rod connected to an inlet piston. The big end bearing being formed by the blade con to fitting inside the forked con rod so that both cylinders are on the same axis. The exhaust piston has a 20 degree lead over the inlet piston so that the exhaust port opens before the inlet port and closes just after the inlet port opens to allow some blow through of the charge air. When the exhaust piston is at TDC the inlet piston is at 20 degrees before TDC which results in the exhaust piston having a longer power stroke hence generating more power than the inlet piston. Each crank pin carries an inlet and an exhaust piston hence all the crank pins transmits the same power.

The phasing of the crank pin positions are the same on the two top crank shafts, however the bottom crankshaft rotates in the opposite direction hence the crank pin phasing is reversed. Three gear driven cam shafts rotating at engine speed are located on each bank of cylinders driving the conventional plunger type fuel pumps one per cylinder. The driving end of the engine is mated to the phasing gear casing which contains precision straight cut gears carried in roller bearings connecting the three crankshafts to the power output shaft. The two top crankshafts have one intermediate gear, while the bottom crankshaft has two intermediate gears so as to have the opposite rotation. The free end of the engine carried the engine driven scavenge blower or rotary supercharger with a twin entry compressor feeding compressed air to the three intake ducts. The various power ratings of the engine have turbo blower of various sizes fitted to the free end where the exhaust turbine feeds power back to the engine at high power.

The general specification of the Deltic is as follows.

Cylinder Bore 5.125” Stroke 7.25”
Swept Volume 88.3 Litres Effective 86.9 Litres.
Compression Ratio 19.26 : 1
Piston Speed at 2,000 RPM 2,416 ft/minute
Overall Dimensions Height 7ft 1in. Width 6 ft 2.5 in Length 8ft 2 in.
Weight 8,725 lbs


The Deltic was available in 18 cylinder or 9 cylinder versions with a number of power ratings depending on the customer’s requirements. During the development period the following types were tested.

Napier’s had produced the Nomad diesel compound aero engine and used their experience in designing the compound Deltic.

The E185 Deltic Compound was designed in 1956 using the basic 18 cylinder engine. A 12 stage axial compressor was housed inside the triangle supplying compressed air to the cylinders and combustion chamber, with the exhaust gas driving a three stage axial turbine. Unlike the Nomad the gas turbine was not coupled to the engine and acted as a super turbo charger. This version of the Deltic produced 5,300 BHP and excellent fuel consumption. However the Admiralty decided to use gas turbines despite their high fuel consumption, and lost interest in the compound Deltic.

The CT18-42K version was designed in 1955 which was the basic engine with a larger turbo charge plus charge cooling. This version produced 3,700 BHP later increased to 4,200 BHP. This was more power than the Admiralty needed at the time so this version of the Deltic was not put into production until the early 1970’s Three of these engines are used in Japanese fast patrol boats with a top speed of some 50 Knots.

The D18 7A version of the Deltic was selected by the Admiralty for use in the Dark Class MTB and later the Ton Class of mine sweepers. This produced 2,200 BHP max at 2000 RPM with a continuous rating of 1,800 BHP.

A nine cylinder version was selected to power the generators on the mine sweepers.Two nine cylinder T9 59K and one T9 59B are currently in use in the Hunt Class minesweepers due to bow out of service in 2015 some 66 years from inception.

The T18 37K Deltic produced 3,100 BHP at 2,100 RPM and was used by the Norwegian Navy in the Nasty Class MTB. The US Navy selected the Nasty Class for use during the Vietnam War. Napier’s set up a base at Subic Bay in the Philippines to maintain the Deltics. A number of other Navy’s also used the Deltic in fast patrol crafts.

The D18 25 was selected for use in the class 55 express locomotive by British Rail. This produces 1,600 BHP at 1,800 RPM with a timed service life of 4,000 hours between overhauls, at which time the engine was lifted out of the loco and replaced with an overhauled unit. Hence the loco had a good availability for service. Two D18 25 engines were used on the locomotive with a total of 3,600 BHP driving DC generators which in turn drove the traction motors in the bogies. A total of 22 class 55 Deltic locomotives were produced in the late 1950’s replacing over 60 steam engines used on the service between Kings Cross London and Edinburgh. They remained in service for some 25 years with three locos now preserved in running order regularly on special trains. Click here to see a film of one hired in to haul a contemporary train. British Rail also used a T9 29 Deltic producing 1,100 BHP in their class two locos. This loco is known as the Baby Deltic with one left being restored by enthusiasts.

A number of Deltic’s were produced for use in a variety of applications where high power and low weight are required.

The New York Fire Department had a requirement for a very powerful water pump for fires in tall buildings. The Deltic Super Pumper was produced with a T18 37K Deltic driving a Delaval water pump mounted on a Mack truck.

Another application was for the US Air Force who required a compact light weight unit to produce liquid Oxygen for use on their rocket bases. Napier together with Alfalaval took on the design and build contract on the understanding that if the two prototypes were satisfactory they would get the production contract for some 200 units. Within one year we had designed and produced two prototypes which were in those days were amazing machine. They had a Deltic and a special Alfa pump with associated equipment all packed in to a portable box which could be transported by air. It was all automatic. The operator just pressed a button and it all roared in to life and within ten minutes delivered liquid oxygen. The US Airforce engineers were very impressed, but when it came to the production contract Napiers lost out due to a ruling by Congress. General Electric were awarded the contract.


The Deltic was an audacious design by engineers with vision who only had a slide rule, pencil, paper, and their imagination to produce this unique and very successful engine.
No one expected the complex Deltic to remain in service for some 60 years - a truly remarkable achievement.

D.NAPIER & SONS (and David gtaylor's part).

The company was established in 1808 as a family business designing and producing machinery to meet customers requirements, such as an advanced printing press for bank notes, machine tools etc. They became renowned for the high quality of their designs which were produced to the highest engineering standards of the day. They were approached by S.F.Edge to improve his competition car in about 1901 and ended up designing their own car for Edge to win the 1903 Gordon Bennett Trophy. They then went in to designing and manufacturing a variety of cars up to 1925.

At the start of WW1 they commenced manufacturing aero engines under licence. They soon designed their own aero engine The Lion. A W12 cylinder, with four valve heads and twin overhead camshafts which was so far advanced it soon became the world leader for aircraft, and world speed records on land, sea and air. The family business was taken over by English Electric Group in 1942 and became part of a large corporation.

I was fortunate to join D.NAPIER & SONS at their Liverpool works in 1963 as my bride to be lived in Crosby North Liverpool and already had a job working as a Speech Therapist for the NHS. My first job was with Les Wade in the Deltic Development department with our office next to that of Jim Shortall and the Deltic assembly bay.
Les Wade was a lovely chap; very laid back and jovial having been with Napier’s all his working life and we became good friends.The company's head office was in Acton north London and the Liverpool works was built to produce the Napier Saber aero engine used in the Typhoon and Tempest fighters in WW2.

I spent some of my time in the Deltic Strip bay where customer’s engines were dismantled for overhaul recording piston ring wear, and other defects. Most of the engines were time expired and came from the Royal Navy, Norwegian Navy, and British Rail. The Navy engines were doing about 2000 hours and the railways engines 4,000 hours between overhauls. Most of the moving parts showed very little signs of wear and could be built back in to the engine. However the pistons were unreliable on the railway and Norwegian engines, and the main cause of engine failure. The Navy piston was made up from a forged alloy skirt and crown and separate alloy gudgeon pin housing with oil cooling slots. This was still used in the low powered version and seems to be reliable until more powerful Deltic types were introduced. The exhaust pistons then suffered overheating and would fail resulting in a broken con rod.

A new design of piston was introduced with the skirt and gudgeon pin housing from alloy forging as before but with a heavy crown made of a copper alloy Hidural. The alloy skirt and Hidural crown were screwed together with a torque of 1000 LBS FT later increased to 1,200 LBS FT. The crown would become detached from the skirt but surprisingly the engine could still run for hundreds of hours due to there being gas pressure above the crown at all times. Sadly in most cased the skirt failed resulting in a con rod through the side. The reputation of the Deltic was under threat and the design team led by Chamberlain and in turn the development team led by Reggie Taylor was under pressure to resolve this major problem. Various piston thermal profiles were tested at length with no improvement. I remember going to Accrington works with Les Wade to collect the first set on totally redesigned pistons. The skirt was from forged alloy with the Hidural crown attached by steel studs to the flat alloy crown with the separate alloy gudgeon pin housing modified to suite. This seemed to solve the problem, but not quite, as the copper alloy worked hardened and the crown cracked.

I discussed the piston problem on many occasions, as I felt that the piston needed to be light, mechanically strong with a good heat path from crown to skirt. To achieve this, the piston should be a single alloy forging with strong bosses for the gudgeon pin and plenty of metal connecting the crown to the skirt to create a good heat path. In addition a multi jet spray of lubrication oil to the underside of the crown and skirt would enhance cooling the crown. Les Wade tended to be of the same mind however we were in no position to question our superiors.

Finally Coffey the metallurgist came up with a revised mix for the copper alloy which solved the problem, and saved the Deltic which is still in use in the Hunter Class of mine sweepers of the Royal Navy after some 60 years from inception which is a testament for this fine engine.

Jimmy Galne was the works manager and we got on very well. so I joined the production team working closely with Jim sorting out some of the problems. One job I remember was a further up rated type of Deltic was being introduced and the design team required the manufacturing limits of the gear tooth profile of the main power train or phasing gears to be tightened. The hardened gear teeth has an involute profile with tip and root relief produced on the MAAG grinding machine. Jim Galna was not pleased, as to achieve the tighter limits required special gear grinders operating in a temperature controlled and air conditioned room. I went to head office with Jim Galna to explain to the design team the consequence of their request. I found out that Dr Zanovitch, who was the expert on torsional vibration, had raised the need for this change so I met him in his office. I asked him to explain the reason for this costly change, and he showed me a few sheets of calculations which I could not understand. He eventually explained that these calculations showed that under overload conditions the deflection of the gear teeth would result in scuffing. I suggested that at worst the engine could only operate at these extremes of torque for a very short period as the engine’s hydraulic governor would reduce the power automatically. After I explained the difficulty and cost resulting from his request, he said he would review the position and discuss it with his colleagues. After a few weeks Jim Galna called me to say the design request had been withdrawn to his delight.

Big corporations tend to get reorganized from time to time and so it was for English Electric . Napier’s Liverpool works was closed at the end of the 1960’s. The Deltic went to Paxman in Colchester, and the Napier turbo blowers went to Lincoln. In recent times the Napier Turbo blowers were bought out by their management and continue under the Napier name. The final resting place for the Deltic is in the hands of Rolls Royce special engines at Crewe who maintain the remaining engines. A lasting memory of the Deltic was a new T18 37K for the US Navy on test with the test house exhaust silencer remover for maintenance. At max power the high pitched bark from the eighteen cylinders coupled to the scream from the exhaust turbine sounded just like a racing engine. Marvelous!

Left:Cutaway of Deltic locomotive.


Right: "Proof of concept" 3 cylinder E130 3T test engine


December 2012

Disclaimer: The above represents only the unofficial view of the writer and not of the Monoposto Racing Club in any way whatsover. Subheadlines and captions are not originated from the named author. We are unable to reproduce results due to copyright reasons. If any pictures are copyright and the owner wishes them removed please email us.

Pictures from internet, but mainly IMechE, whose website is glorious


General layout


Cross section


Jim Shortall who was the assembly shop superintendent standing by the new Super Pumper engine for the New York Fire Department

Deltic triangle during assembly

Phasing gears during assembly

Crankshaft and connecting rods during assembly

Nasty class MTB

New York Fire Dept Super Pumper

Deltic pulling East Coast Mainline express train

Eagle comic illustration of a Dark Class MTB. 2 were used for making a film in Malta after decommissioning and the hulks were there into the 2000's, now sadly both broken up.

HMS Ledbury, a Hunt Class minesweeper

Vospers built this 2x18cylinder high speed passenger launch for Shell Venezuela. One can't help thinking there might have been a ready second hand market amongst the South American Independent Pharmacutical Distribution profession.