By the start of the 1960s, BR had appreciated that diesel locomotives more powerful, yet lighter in weight, than those currently being built were needed. A specification for a 2,750bhp Type 4 with a 19 ton axleload was published to serve as an invitation to the British locomotive industry to come up with proposals. Three companies built prototypes against the specification, one being the English Electric Company (EE).
EE’s development of the ‘V’ engine used in Class 20 and ‘40’ had raised the output per cylinder from 125 to 169bhp at 850rpm. This meant that the 16-cylinder version could be offered at 2,700bhp, instead of 2,000bhp as in Class 40. The uprated diesel was already on order for BR in EE’s Class 37 but with the 12-cylinder unit rated conservatively at 1,750bhp at 850rpm.
EE was fully committed with contracts for equipment for both the home and overseas markets, notably Class 20, ‘37’, ‘40’ and ‘55’ under TOPS notation. In order to produce a new locomotive design in the shortest time and with the minimal drawing office input, the company arranged for its Vulcan Foundry workshops at Newton-le-Willows north of Warrington to use the underframe and bogies then in production at the factory for Class 55 as the basis.
The resultant locomotive emerged in May 1962 and carried the designation ‘DP2’, short for Diesel Prototype 2, where the original ‘Deltic’ was EE’s first prototype. By now, however, BR had already ordered Type 4s to a design produced by Brush Electrical Engineering Co Ltd (Brush), which used the Sulzer 12LDA28-C engine, rated at 2,750bhp at 800rpm, designated Class 47 under TOPS.
Although EE submitted tenders against enquiries for further Type 4s, its 16CSVT unit was regarded by BR’s Chief Mechanical & Electrical Engineer (CM&EE) as unproven and rejected.
Nevertheless, BR and EE entered into a hire agreement for DP2 and it worked initially on the London Midland Region (LMR) and later the Eastern Region (ER) in a competent manner.
The National Traction Plan produced in February 1965 examined the quantity of diesel and electro-diesel locomotives that were required in the light of the 1963 Reshaping (Beeching) Report. The Plan identified a deficit of 48 Type 4s and, when the BR Board gave authority for a batch of this type in April to address the shortage, stress cracking issues began to emerge with the engines in Class 47.
Prior to this approval and also in April, BR had made a comparison between the 3,300bhp Class 55 ‘Deltic’ and Class 47 for the WCML north of Crewe and concluded the balance of advantage lay with the former for reliability and scheduling.
In June, BR wrote to the Ministry of Transport (MoT) to seek authority for the 50 additional Type 4s authorised by the Board in order to eliminate steam by 1970 and diesel was the cheapest option. The Minister was reluctant to accept this view and wanted to consult with the Ministry of Power. This was because, concurrently, the question of extending West Coast Main Line (WCML) electrification to Glasgow was being debated, both within BR and the MoT and between them.
The BR Chairman pointed out to the Minister that the National Coal Board said it could no longer supply all the coal BR required. In his blunt style, he said a decision was needed whether to electrify to Glasgow and so give the public what they wanted, or whether the first consideration should be saving money by buying diesels.
BR’s calculations showed that the financial advantage lay with an order for 50, rather than 110 Type 4s, both quantities having been specified in the April tender. This was because the lower number would purely permit the elimination of steam, while the extra ones would create a reserve pool for possible growth in liner and block trains, which were still in their infancy at the time.
The MoT also queried why BR did not want the new order to be as powerful as the electric traction delivered for WCML electrification then nearing completion between Euston, the West Midlands and North West. BR responded that more powerful units might be difficult to redeploy as and when full WCML electrification was sanctioned.
It took until August 20 for the Minister to give his approval and, in line with BR’s submission, the quantity was 50, which left the railways with a Type 4 traction shortage by the early 1970s.
The question now confronting the BR Board was whether to order additional Class 47s or look at an alternative. On October 8 the Board Member for Engineering, whose subordinates included the CM&EE, presented a lengthy paper describing the multiple issues being faced with the Sulzer LDA28 diesel in the various Type 4 classes in which it was fitted. Performance was regarded as unsatisfactory and would take several years to resolve.
Two options were put forward, one being to order more ‘47s’ and the other options for to buy 50 DP2-type locomotives. Arguments in favour of the former were:
- Considerable experience had been gained with the Sulzer engine through living painfully with its technical development.
- There would be continued standardisation and continuity in production.
- Ordering a different design would give the impression that a serious mistake had been made in opting for the Sulzer engine.
- While DP2 had run 138,000 miles in less than 3½ years, which included a long spell out of service, it was a single prototype.
The case for buying 50 DP2s was:
- DP2 was a direct development of locomotives that had generally given good service.
- There was unlikely to be a problem with the engine. This view was a volte face of the opinion previously expressed that it was unproven.
- DP2 shared a significant number of major components with other EE classes in BR service, examples being bogies and traction motors.
- DP2 was 9% cheaper than Class 47.
- EE was prepared to give guarantees concerning reliability and performance.
- An order for 50 DP2s would give experience and help to guide future procurement.
- EE would be responsible for the whole of the design, whereas three contractors were involved with Class 47.
- Delivery dates were likely to be the same whichever design was chosen.
The memo concluded with a recommendation to buy DP2s, which received Board approval on October 14. In view of the problems with previous designs of diesels, the Board wanted guarantees from EE on locomotive availability.