class-23-baby-deltic

The BR Class 23 'Baby Deltic' was a series of ten Bo-Bo diesel-electric locomotives built by English Electric and fitted with a single Napier T9 'Deltic' engine in 1959–60 for the Eastern Region of British Railways, representing an attempt to evaluate a lighter, less powerful version of the full Deltic engine that had been developed for the successful Class 55 Deltic locomotives. The Baby Deltic — as the class was inevitably nicknamed — used one of the two Napier engines from the full Deltic configuration, giving approximately half the power in a smaller and lighter locomotive body suited to secondary express and fast freight working rather than the top-link Class 55's principal express duties.

The single Napier T9 engine in the Class 23 was the 'half-Deltic' configuration: the distinctive triangular Napier opposed-piston two-stroke diesel, which in full Class 55 form used two such engines to produce 3,300 bhp, here produced approximately 1,100 bhp from one unit. The engine was technically fascinating but mechanically demanding — the opposed-piston two-stroke design, with its pistons working in opposite directions in the same cylinder to compress the charge between them, required precise timing and careful maintenance to keep all the moving parts in synchronisation across three crankshafts.

The Class 23 proved no more reliable in service than their Napier engine's complexity suggested, and the entire class was withdrawn between 1968 and 1971. One example, D5910, is preserved — the sole survivor of this fascinating experimental class.