Turbo technology the key to clean low speed diesels

31 Jul 2010
Model showing the compact architecture of two-stage turbocharging with intermediate cooling

Model showing the compact architecture of two-stage turbocharging with intermediate cooling

MAN has completed the reorganisation of its engine and turbo businesses and the former MAN Diesel and MAN Turbo companies are now joined as a single enterprise, known as MAN Diesel and Turbo SE.

The company believes this will help growth and will strengthen its position in the market. CEO Klaus Stahlmann says that the merger means that expertise can be pooled in large bore diesel engines and enable it to offer its customers solutions like waste heat recovery systems for maritime applications. Combining production, purchasing and service activities should achieve better control of costs. MAN Diesel and Turbo will, along with Renk AG, form a new Power Engineering division within the MAN Group.

Most of MAN’s recent development has centred on preparing for stricter emissions limits, notably in turbocharging of large-bore low speed diesel engines. The company has recently announced its latest TCX turbocharger generation, offering two-stage turbocharging as a means of increasing charge air pressure to provide higher specific engine output with lower emissions.

The TCX incorporates two turbochargers of different size connected in series. The exhaust gas coming from the engine drives the turbine of the smaller, high-pressure turbocharger which in turn drives the turbine of the larger, low-pressure turbocharger. The low-pressure turbocharger’s compressor draws in ambient air and sends it via an intermediate cooler to the high-pressure turbocharger’s compressor. Here, the air is compressed once again and, via a further charge-air cooler, sent to the engine. The system adapts to varying operating conditions either through controlled turbine bypass or by variable nozzle rings (VTA).

The demands placed on the individual turbochargers in the high- and low-pressure stages vary considerably from each other. The high-pressure stage is charged by the full exhaust, however only receives a comparably low air volume (of previously compressed air) from the low-pressure stage. For this reason the high-pressure stage employs a smaller compressor. In contrast, the conditions for the low-pressure stage are similar to those encountered in single-stage turbocharging though at lower pressure ratios.

The TCX design is based on the existing TCA/TCR-series with uncooled casings and durable plain bearings, but where the TCA/TCR series use axial and radial turbines respectively, the TCX employs a new diagonal turbine that is suited to lower-pressure ratios. Two-stage turbocharging poses a number of challenges to the engine design as well as the turbochargers themselves. Besides the space and piping requirements that an additional turbocharger stage requires, an optimised intercooler is included. In response to this, MAN Diesel & Turbo has delivered a compact solution where the turbochargers are arranged at 90° to each other

Environment award

The TCA variable turbine area technology gained a 2010 Seatrade award for MAN Diesel & Turbo. The company received the main award for protection of the marine and atmospheric environment. The chairman of the judging panel, secretary general of the International Maritime Organization (IMO), Efthimios E Mitropoulos, stated that the annual awards, which also include categories for safety at sea, innovation in shipping, and investment in people, are all directly related to the goals and objectives of IMO, i.e. safe, secure and efficient shipping on clean oceans.

The VTA matches the volume of charge air to the quantity of fuel oil injected at any operation point of an engine’s load and speed range. This results in improved engine response, fuel savings and reduced emissions.

A conventional turbocharger with fixed nozzle rings is usually optimised for a pre-determined engine load point. At any other engine load, the turbocharger does not work with optimal efficiency, which results in poorer combustion, and higher fuel consumption and emissions. MAN has successfully applied VTA technology to both axial and radial turbines for installation on large diesel engines burning HFO, which the company describes as probably the most technically challenging engine fuel in daily use.

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