A case study of the design related aspects of the introduction of a new turbocharger

The design related aspects of the introduction of a new turbocharger - the Napier NA355. Napier Turbochargers Ltd. manufacture a range of turbochargers suitable for most medium and slow speed diesel engines above about 1000 hp. This work describes the many and varied aspects associated with the design of a new turbocharger during the early 1980's. The company was producing two similarly sized products designated the SA105 and the NA350, intended for engines developing about 3000 hp. A replacement for these two turbochargers was required. The demands of the engines in the marketplace were determined together with the extractable performance of the major competitor's products. The performance levels of the two Napier models was examined, and was found wanting, particularly with respect to the compressor efficiency. Also the mechanical limitations of the two models were less than desirable. A number of technical proposals were considered, with the aim of deciding how best the needed improvements could be introduced. The SA 105 offered more items that could be utilised in a new turbocharger, although both compressor and turbine required improvement. It was necessary to avoid lengthy and expensive development wherever possible, therefore only proven technology was employed. The major casings and rotor forgings were to be retained, if possible, due to the lcngthy, expensive, and often troublesome procurement cycles. Only those components that had a lack of performance or had a mechanical limitation would definitely be replaced. A new backswept compressor and an improved turbine stage were designed. A new rotor assembly incorporating the new impeller, together with many detail improvements, was also designed. The work describes the analytical techniques that were used to carry out these designs. The turbocharger was rig tested, as an open cycle gas turbine, to simulate service duty and environment. The first turbocharger was built and tested with encouraging results. A satisfactory number of turbochargers have been manufactured and are now in service. Although some problems occurred that necessitated modifications the basic concept was clearly satisfactory. The NA355 can be considered as a particularly successful turbocharger.