TPS series

Advances in small medium speed and large high speed diesel engine technology called for more compact turbochargers with increased pressure ratios at higher overall efficiency levels. ABB Turbo Systems addressed the market in the mid-1990s with the TPS series, designed to serve four-stroke engines with outputs from 500 kW to 3200 kW per turbocharger (Figure 7.14).

Figure 7.14 ABB Turbo Systems' TPS turbocharger for small medium speed and large high speed engines

To ensure the full range of pressure ratios required by such engines, the TPS..D/E turbocharger is available with two completely different compressor stages. One stage is designed for pressure ratios up to 4.2 (TPS..D) and the other for pressure ratios up to 4.7 (TPS..E). Thanks to mechanically optimized single-piece designs incorporating bore-less compressor wheels, pressure ratios up to 4.5 can be reached for a wide range of applications using aluminium alloy as the compressor material. Both compressors feature a single-piece aluminium alloy wheel with splitter-bladed impeller and backswept blades for high efficiency and a wide compressor map. Peak efficiencies of more than 84 per cent are obtainable.

Experience with the RR-type turbocharger was tapped by the designers in creating the TPS series. The resulting configuration is modular and offers a high level of flexibility with a minimum of sub-assemblies. A significantly reduced number of parts is used in comparison with the RR type, itself a simple design. A modular configuration fosters the easy integration or fitting of options, such as a jet assist system. Special attention was paid during design and development to ease of maintenance and simple mounting of the turbocharger on the engine.

Criteria for turbine development included the ability to operate at gas temperatures of up to 750°C and yield a long lifetime on heavy fuel operation and under extreme load conditions. The requirement for a large volumetric range had to be met with a minimum number of parts. Considerable aerodynamic potential was offered by the turbine of the RR..1 turbocharger which had also demonstrated excellent reliability and the highest efficiencies. Operation at pressure ratios far above today's levels was possible with the turbine, for which a nozzle ring was developed to satisfy the goals for the TPS turbocharger. A new scroll casing and nozzle ring were designed using proven inverse calculation methods that ensure losses are kept very low and turbine blading is not too strongly excited. Thermodynamic measurements showed that the turbine efficiency of the new design was as high as that of its predecessor. Blade vibration excitations are even lower.

Previously, stress distribution for the turbine wheel was optimized by trial and error: a geometry was defined and the resulting stresses calculated, the geometry then being varied until the stresses lay at an acceptable level. Using the latest computer-aided tools for optimizing structures, the stress level was reduced by around 30 per cent over the earlier version.

Turbocharger bearings must deliver a good load-carrying capability with respect to both static and dynamic forces, good stability and minimal mechanical losses from a cost-effective design fostering modest maintenance. The TPS turbocharger bearings are lubricated directly from the engine lube oil system.

An increase in turbocharger pressure ratio means a rise in the load exerted on the thrust bearing. A turbine wheel with a high back wall was chosen to counteract the main thrust of the compressor as far as possible. Extensive calculations and temperature measurements were performed on the thrust bearing, both in steady state operation and with the compressor in surge mode, to optimize its dimensions and to achieve the lowest possible bearing losses without compromising on load-carrying capability.

The key design factor for the radial plain bearings is their stability characteristic. Considered in terms of this criterion, ABB Turbo Systems notes, a squeezed film damped multi-lobe plain bearing would be the best choice. But such a bearing has higher losses than a bearing with freely rotating bushes. The latter, on the other hand, exhibits an inadequate stability characteristic. A new bearing design with rotating hydraulically braked multi-lobe bearings was therefore developed to blend the respective merits of the two types.

By providing a tangential lube oil supply it is possible to reduce the speed at which the floating bushes rotate by around 20 per cent compared with freely rotating bushes. This measure, the designer says, has ensured a stability which is effectively as good as with the squeezed film damped multi-lobe plain bearings; and mechanical losses can be reduced by more than 20 per cent.

Development of the TPS..-F33 series, introduced in 2001, focused on increasing the pressure ratio (up to 4.7 with a compressor wheel made of aluminium alloy) and the flow capacity, while retaining high efficiency, reliability, long lifetime and ease of maintenance (Figure 7.15).

Figure 7.15 Cross-section of ABB TPS57-F33 turbocharger

The new series comprises four different sizes, with the same outline dimensions as the established TPS..D/E range and hence representing an increased power density. The interchangeabilité offers good solutions for upgrading engines. Among the options are variable turbine nozzle geometry and an air recirculation system (a flow recirculating device in the compressor inlet that substantially enhances the surge margin and thereby broadens the compressor map). The recirculating flow is driven by pressure differences between the downstream and the upstream slots in the compressor casing.

The rise in brake mean effective pressure ratings of engines in recent years has dictated a considerable increase in the power density of turbocharger rotors. Burst tests are performed to ensure that even in the worst case scenario (for example, a fire in the exhaust system) — in which the compressor or turbine wheel can burst—all parts remain within the turbocharger casings and do not endanger personnel. The turbine casing design, which includes inner and outer burst protection rings, seeks optimum containment.

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