The Deltics & Baby Deltics: A Tale of Success and Failure

By Andrew Fowler

Andrew Fowler is a well known writer of railway history, with a regular feature in Railway Herald Magazine. The Deltic class 55 Locomotives were some of the most successful, first generation diesels introduced to British Railways, being constructed from 1960-1962 and numbering twenty two, in the production class. The prototype machine was constructed in 1956 and was tested extensively on express trains on the London Midland and Eastern Regions of B R, until 1960. The interest and enthusiasm, for the class is reflected in the fact, that six examples of the class are preserved, including the 1956 prototype. The Baby Deltics, were a derivative Locomotive design, using one rather then two engines, for use on outer suburban and short main line semi fast services. Only ten Baby Deltics were constructed between 1961-1962, for use on services out of London Kings Cross. The Baby Deltics were all withdrawn within a decade, as they were not very successful in main line service.

• Language: English
• Publisher: Open Road Integrated Media
• Release date: Jun 30, 2021
• ISBN: 9781473870116

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Prelude to the ‘Deltics’

It was late in 1947 that the first main line diesel locomotive was introduced in Britain. At this time, the country was still recovering from the ravages of war, but both H.G. Ivatt of the LMS and O.V.S. Bulleid of the Southern Railway were working on new diesel designs. Though the design and production of the Bulleid machine commenced earlier, it was Ivatt’s design, LMS Co-Co No. 10000, that entered traffic first, rolling out of Derby Works on 5 December 1947 and being officially handed over for service on the 18 of the same month. It was joined by its twin, No. 10001, in July 1948. The latter emerged in British Railways black livery.

LMS Co-Co Diesel Electric No. 10000 near Eastleigh on 2 July 1954, powering the down ‘Royal Wessex" express. (Brian Morrison)

LMS Co-Co Diesel Electric No. 10001 in Brighton Works Yard on 2 October 1954. (Brian Morrison)

Power units and electrical and traction equipment for the pair were supplied by English Electric, which at the time was already manufacturing similar locomotives for the Egyptian State Railways and used this experience to produce acceptable motive power for the LMS. Although both successful and reliable, the locomotives’ English Electric 16SVT power units only developed 1,600hp at 750rpm, despite which they weighed in at almost 130 tons each. With a top speed of 90mph, they were capable in theory of hauling some of the fastest expresses, but when operating singly their acceleration was poor, particularly on heavier trains. As a result, Nos. 10000 and 100001 usually operated in pairs, but even so their power output was only roughly equal to that of an LMS ‘Rebuilt Scot’; singly they were more evenly matched against a run-down ‘Black 5’. Moreover, the distribution of weight over only six axles, with a resultant axle loading of over 21 tons, only slightly lower to that of a ‘Duchess’, severely restricted the routes that they could operate over. It was clear that diesel traction had a long way to go before it would be a suitable replacement for steam.

SR 1Co-Co1 Diesel Electric No. 10201 heads an unidentified express at Battledown in the early 1950s. (Brian Morrison)

The first Bulleid machine had been under construction since 1946 but was delayed due to a lack of financial approval for such an innovative project, the Company instead preferring to concentrate on the production of Bulleid’s Battle of Britain and West Country class steam locomotives. The relative success of the LMS ‘Twins’ was convincing enough for the completion of the first locomotive to be authorised, along with the construction of a second. Again, weight was a major problem and the Southern Railway got around this by adopting a 1Co-Co1 wheel arrangement; this added a non-driven carrying axle on the outer end of each bogie to reduce the axle loading, as the Southern’s civil engineering department had imposed severe weight restrictions on a number of bridges all over the network. The locomotive weighed in at a whopping 133 tons, but the addition of the outer carrying axles, each carrying 12 tons 16cwt, reduced the maximum axle load of the driven axles to a more acceptable 18 tons 9cwt. The leading axles had ample free movement within the bogie frames to allow the locomotives to negotiate sharp curves, but reduced the overall adhesive weight available for starting heavy trains.

Bulleid/English Electric 1-Co-Co-1 Diesel Electric No. 10202 rounds the Bickley-Petts Wood loop line on 11 February 1954, powering the down ‘Golden Arrow’. (Brian Morrison)

No. 10201, as it became, emerged from Ashford Works in 1950 under the auspices of British Railways, with its classmate No. 10202 entering traffic the following year. The delay in manufacture actually worked out as a benefit for the Southern Region, as developments in turbocharger design meant that the same English Electric powerplant used in the LMS ‘Twins’ now developed 1,750hp at the same 750rpm and the difference was obvious if not overly so. The Southern’s own ‘Twins’ were initially designed for a top speed of 110mph, firmly identifying them as an express passenger design, but again lack of power meant that this was not readily achievable in service and they proved to have insufficient ability to handle heavy trains on their own. Consequently they were modified in 1952 and re-geared for a top speed of 90mph, enabling them to work heavier trains; they were able to achieve much higher mileages than steam locomotives but their performance never quite matched other forms of traction.

English Electric & Bulleid 1-Co-Co-1 Diesel Electric No. 10203 on display at the Willesden Exhibition on 4 June 1954. (Brian Morrison)

Nevertheless they were successful enough that construction of a third locomotive, No. 10203, was authorised in 1953 and construction began that February at Brighton Works with the locomotive emerging in March 1954. By this time, real improvements had been made to the English Electric 16SVT engine and it now developed a more acceptable 2,000hp at a higher speed of 850rpm. When put to work with its classmates on the Waterloo-Exeter route it clearly demonstrated its superiority, and was at last a more suitable contender for the steam locomotives that worked the semi-fast expresses; however, it was still not quite good enough a match for the best SR Pacifics, and when the trio were transferred to the London Midland Region in 1955, they proved to be lacking against the comparable ex-LMS motive power as well.

Notwithstanding its obvious shortcomings, following the 1955 Modernisation Plan, No. 10203 was selected as the basis for what would become the standard London Midland Region express passenger diesel locomotive, the English Electric Type 4 (later Class 40) with the first ten pilot scheme locomotives introduced in 1958.

The obvious drawback to these early diesel electric locomotives was their excessive weight, with the power units, generators and traction motors all being heavy items. Although English Electric’s own equipment proved to be rugged and reliable, the drive to reduce axle load was apparent. The Western Region made its own attempts by opting for hydraulic traction to eliminate the weight of generators and traction motors, but its D600 and D800 ‘Warship’ classes, and even its D1000 ‘Westerns’, all proved disappointing in terms of actual performance.

Relatively new Class 40 No. D206 passes Holloway South signalbox on 7 October 1958, with the King’s Cross-bound all-Pullman ‘Master Cutler’ express from Sheffield.

The prototype ‘Deltic’

While the early main line diesel prototypes had concentrated on English Electric’s own straight six and V16 powerplants, the firm had other prime mover choices available in its stable. English Electric had taken over the D. Napier & Son aero engine company, and there was a novel and lightweight power unit available in its arsenal that was yet untried in a rail application.

The origins of the engine went back to the 1930s, to the Junkers aircraft company in Germany. Instead of using a conventional four-stroke diesel engine, Junkers hit upon the concept of a two-stroke, opposed piston design for its aircraft. The engine was lightweight and flat enough to fit inside an aircraft’s wing; considerable weight was saved by avoiding the need to include a cylinder head, the two pistons inside each cylinder fulfilling the role instead, while use of the two-stroke process did away for the need for valves and camshafts. The only drawback was the need for two separate crankshafts, one on either end of the engine, which needed to be coupled together in order to drive the output shaft. In the mid-1930s, Napier obtained a licence to manufacture and develop the engine and had begun work on its own version, known as the Culverin.

Diesel engine production and development was halted at the outbreak of war, with Napier concentrating on its petrol driven aero engines instead. The project was revived in in the post war period, when the Admiralty was seeking a long-range power unit for torpedo boats. Up to this point, the boats had been petrol-driven and not only did this give them a lower range than the German diesel-driven equivalents, it also made them more flammable. Having learned of the Napier Culverin, the Admiralty placed a contract with English Electric early in 1946 to develop a more powerful variant that would be both compact and lightweight as well as powerful.

The Napier team, led by Sir George Nelson, eventually hit on the idea of using three banks of cylinders in a triangular formation. The Admiralty contract included for the development of a single cylinder version, a three-cylinder unit and six full-sized versions of what would become the familiar 18-cylinder version. Thanks to the opposed piston arrangement, the 18-cylinder version of the engine was in effect three separate V12 units, without the added weight of cylinder heads, and was thus a very compact and powerful unit. Only three crankshafts were required, arranged on each corner. The triangular unit was akin to the Greek letter Delta, thus giving rise to the ‘Deltic’ name that is now so familiar to enthusiasts.

Detailed design of the single cylinder unit was undertaken first, as might be expected; this was completed by the end of 1946 and a test unit was constructed in January 1947 and ran successfully. The three-cylinder unit, however, was a different story. It was first tested in October 1947 but was very sporadic in its operation due to issues with balance, and it was subsequently abandoned altogether. Junkers had itself experimented with the Delta formation for its own engines, but had difficulty in making the pistons move in the intended manner. Napier also encountered these issues early in the development process, but the issue was overcome thanks to input from Admiralty draughtsman Herbert Penwarden, who suggested that one crankshaft should be designed to turn the opposite way to the other two in order to produce a suitable piston phasing arrangement. This required a very complex gearing arrangement for the unit, but Napier engineers successfully overcame the issue.

As the engine had no inlet or exhaust valves, fixed ports had to be included, and to simplify operation, each crankshaft ‘paired’ pistons operating in different cylinders, with each pair using a fork and blade connecting rod arrangement to share the same big end. By this method, one piston could open and close the inlet or outlet ports for the other; with the triangular arrangement chosen, this was very effective indeed. Further improvements in operation resulted in the decision to have each cylinder’s exhaust piston to lead its inlet piston by 20 degrees of crankshaft rotation, allowing the exhaust port to open well in advance of the inlet port, making the efficiency of both exhaust and inlet air much greater. This also led to the very distinctive exhaust note of the power unit, as the 18-cylinder engine ‘fired’ once for every 20 degrees of crankshaft rotation. The end result was a very powerful and smooth running arrangement, and the lack of valves meant that the engine’s design speed was a peak of 2000rpm, with continuous rating of 1700rpm, double that of a conventional diesel engine; the corresponding power outputs were a maximum of 2,500hp or 1,875hp continuous. Despite the lack of valves, three camshafts were provided for the sole purpose of driving the necessary fuel pumps, adding further to the complex gearing at the output end. The benefit of the ‘Deltic’ design was in its flexibility, as it could be made with varying numbers of cylinders, with 9 and 18 being the most common.

Development of the engine began in 1947 and the first unit was completed in 1950. Six had been produced by 1952, and