Turbocharger Performance And Developments

A typical pressure-volume curve for a turbocharger is shown in Figure 7.9, with reference to the operating characteristics of a two-stroke diesel engine. The operating curve runs almost parallel to the surge

Air intake volume m3/sec

Figure 7.9 Typical pressure—volume curve

Air intake volume m3/sec

Figure 7.9 Typical pressure—volume curve line. Every value of the engine output corresponds to a point on the curve, and this point, in turn, corresponds to a particular turbocharger speed which is derived automatically. There is, consequently, no need for any system of turbocharger speed control.

If, for example, at a specific turbocharger speed the charge air pressure is lower than normal the conclusion may be drawn that the compressor is contaminated. By spraying a certain amount of water into the compressor the deposit can be removed, provided it has not already become too hard. A special duct for water injection is arranged on VTR turbochargers.

The operating curve and the behaviour of the turbocharger may vary, depending on whether constant pressure or pulse turbocharging is employed and also on how any volumetric (i.e. mechanically driven) compressors and turbochargers are connected together. By observing changes in behaviour it is usually possible to deduce causes and prescribe the measures necessary to rectify the problems.

Turbocharger design and charging system refinements continue to underwrite advances in diesel engine fuel economy and specific output. Turbocharger developments also contribute to improved engine reliability (lower gas and component temperatures) and operational flexibility (better part-load performance, for example). Rising engine mean effective pressure ratings call for an increase in charge air pressures, with turbocharger efficiencies remaining at least at the same level.

While satisfying increasing demands for higher efficiencies and charge air pressures, turbocharger designers must also address the requirement for compact configurations, reliability and minimal maintenance: qualities fostered by designs with fewer components. All servicing and overhaul work should ideally match the intervals allowed for the major engine components. In addition, safe engine operation dictates a maximum width of the compressor map in order to avoid compressor surging (liable to occur due to changes in operating conditions influenced by hull fouling which has the same effect as speed reduced at constant torque).

Demand for higher charge air pressures necessitates higher circumferential velocities of the compressor wheel and raises the sound level; this in turn has promoted the development of more effective intake silencers for turbochargers. Turbocharger development also seeks to raise the specific flow rate for a given frame size to achieve as compact a unit as possible for cost efficiency, space saving and ease of installation.

Turbocharger designers have pursued higher pressure ratios, overall thermodynamic efficiencies and specific volumetric flow rates, as well as lower noise levels, from more simple and compact modular configurations with uncooled casings, inboard plain bearings lubricated directly from the engine oil circuit and significantly fewer parts than earlier generations. Refinements have sought easier servicing and enhanced reliability and durability.

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