P0 5 4 3 2

5000 10000 20000 -►Engine rating (kW)
V300 [m3/s]

Figure 7.13 (a) Range of application of VTR..4E turbochargers and (b) the performance map of a VTR 564E model

Development goals for the VTR..4D series were defined as: a compressor pressure ratio of 4 while improving efficiency; the highest level of reliability despite an increase in mechanical load; and the retention of key elements of the proven VTR design and its external dimensions, and hence facility for full interchangeability.

A turbocharger combining the pressure ratio of the VTR..4/4P and the high efficiency of the VTR..4E was targeted. The thermodynamic potential of existing compressors and turbine designs led to a strategy for developing a new series optimally blending well-proven components and matching the fitting dimensions of the VTR..4E. The high pressure compressor of the VTR..4P turbocharger was selected for thermodynamic and economic reasons, the designer citing its:

• High efficiency level for the whole range of competitive volume flow rate.

• Greater margin for further increases in mean effective pressure and overload margin.

• Use of aluminium as the impeller material instead of the significantly more expensive titanium; a full load pressure ratio of 4.2 is underwritten.

Two compressors of different diameters are available to cover a competitive volume range at the required high efficiency level within the same overall turbocharger envelope.

The turbine of the VTR..4E turbocharger was chosen for the VTR..4D series since it offers efficiency advances of up to around 3.5 points over the VTR..4/4P turbine. This is due to better recovery of kinetic energy at the turbine wheel outlet, resulting from an optimally adapted outlet diffuser in combination with a new enlarged exhaust gas casing. The latest anti-friction bearing technology was tapped to benefit the VTR..4D series, particularly on the turbine side. Improved mechanical reliability is promised from a fully machined steel cage with surface treatment, an optimized oil feed for better lubrication, optimized smaller rollers for less slip and a larger cage guiding area.

Like other ABB turbochargers, the VTR..4D is designed to ensure that in the worst case scenario—bursting of the impeller or turbine— all the parts remain within the casings. Casing material was changed from grey (laminar) cast iron to ductile (nodular) cast iron and the turbine enclosed in a protective shroud. Burst tests on the compressor and turbine confirmed the containment capability.

ABB Turbo Systems' VTR..4P series addresses the needs of enginebuilders seeking to raise the output of medium speed designs. Such engines may run with meps of up to 24-25 bar, requiring a turbocharger compressor ratio of 3.8-4.0 and an overall turbocharger efficiency exceeding 60 per cent. Meps of up to 28 bar, required by some four-stroke engine designs, dictate a compressor ratio of around 4.2-4.5 while maintaining or even improving the overall turbocharger efficiency. The VTR..4P series offers ratios up to 5:1 from a singlestage turbocharging system whose simplicity is favoured over more complex two-stage arrangements.

Advanced flow calculations, stress analyses, CAD/CAM methods and laboratory tests resulted in a one-piece compressor wheel with backward-curving main and splitter blades discharging through an airfoil diffuser. An aluminium alloy impeller will suit most needs but titanium can be specified for extreme applications. The turbine disc is of a high grade nickel alloy, with new configurations applied for the blade roots and damping wires.

The VTR..4P programme was evolved to cover the demands of engines with outputs ranging from around 1000 kW to 10 000 kW. The high performance units are designed to be physically interchangeable with VTR.. 4 models for convenient retrofitting as well as for application to new engines. The turbocharger can be flanged directly onto existing air and exhaust gas ducting to improve engine performance by increasing the boost pressure. The pressure-optimized series complemented the established efficiency-optimized VTR..4E series in the ABB Turbo Systems' portfolio.

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