UK-based Napier Turbochargers, now part of the Siemens group, offers its Napier 7-series and 8-series turbochargers for application to two- and four-stroke engines developing up to 11 000 kW per turbocharger. Napier 7-series turbochargers are suitable for engines requiring a single turbocharger installation from 1500 kW to 6500 kW. Developed from the company's earlier generation, the 297 and 357 models deliver a 5:1 pressure ratio and an overall efficiency exceeding 70 per cent.

Development of the 8-series was stimulated by the need to offer turbochargers capable of yielding pressure ratios in excess of 5:1 and serving engines with outputs up to 11 000 kW. The new axial-turbine series (Figure 7.17) was headed by the NA298 and NA358 models with a turbocharger efficiency exceeding 70 per cent and a 5.5 pressure ratio promoting lower engine fuel consumption and thermal loading. Enhanced levels of component durability are also claimed, while the air-cooled design contributes to a reduced overall weight. Inboard bearings were applied for the first time on a Napier turbocharger of this size, the hydrodynamic units reportedly securing stable rotor dynamic performance and underwriting a long life. The new bearing arrangement facilitates the use of axial and radial exhaust gas flow into the turbocharger, extending the options for the enginebuilder.

Figure 7.17 Cross-section of Napier 458 turbocharger for medium speed engines

Two separate compressor wheel designs are offered to provide differing characteristics for optimized efficiency in diverse applications. Other features include a circular exhaust flange (eliminating the need for transition pieces) and a mounting flange incorporating both the lube oil inlet and outlet. Another concept pioneered by Napier—

cartridge construction—benefits serviceability of the 7-series and 8-series, this configuration allowing operators to remove and repair the rotor and bearings through a simple procedure without disturbing the exhaust connections. Good access is also provided to key functional areas, such as the nozzle ring.

Titanium compressors, offering enhanced aerodynamic potential and mechanical properties, were initially planned to replace the aluminium components but design refinements enabled the required performance to be attained with aluminium.

Smaller engine requirements are addressed by the Napier 047, 057 and 067 all-radial air-cooled turbochargers in the 7-series, which are suitable for engines developing outputs from 500 kW to around 1700 kW and yield pressure ratios up to 5. Typical applications include the Wartsila 20 engine and other circa-200 mm bore genset engines. The 047 model features inboard bearings and is uncooled. In common with other turbochargers in the series, the bearings are designed to run on the engine's lube oil system and to yield an extremely long life, even when operating on contaminated lube oil. The 047 model incorporates 50 per cent fewer parts than its predecessor, the Napier CO45. Cost savings result from using a reduced number of components; and maintenance is simplified, with no special tooling required or clearances to set. Simple clamps are used to hold the casings together and provide total indexability of connecting flange positions.

Earlier axial-flow turbine Napier 457 and 557 models, which serve engines from 4000 kW to 10 000 kW output, featured long-life outboard plain hydrodynamic bearings fed directly from the engine lube oil system, and water-cooled casings.

High efficiency at all pressure ratios was sought from the 5- and 7-series through advanced compressor and turbine aerodynamic designs. The compressor stage features a one-piece aluminium alloy wheel manufactured on five-axes milling machines (Figure 7.18). Vanes with sweepback and rake are specified to secure high performance, and a divergent circular arc (DCA) diffuser design yields maximum pressure recovery from a compact configuration. A high efficiency fabricated nozzle with profiled blades serves the turbine section of the small and large axial-flow turbochargers.

Napier turbochargers exploit the HF bearing design to secure a long service life, even when operating with the poorest quality engine lubricating oil: a bearing life exceeding 20 000 hours is reportedly common.

Figure 7.18 A 3D CAD visualization of the Napier 457 turbocharger impeller, showing the one-piece intervaned swept-back design with tip rake
Napier turbocharger series performance data
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