Turbo developments major on increased airflow and greater efficiency

31 Dec 2011
Model illustrating the compact architecture of MAN two-stage turbocharging with intermediate cooling

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

Advances in turbocharger design allow smaller frame sizes, energy savings, higher efficiency, reduced emissions and cost savings, writes David Tinsley

Since turbocharger design and performance is pivotal to improvements in diesel engine efficiency and environmental compatibility, the level of commitment to turbocharging research and development (R&D) has a fundamental bearing on advances in marine engineering technology.

As pointed out by engine and propulsion system supplier MTU Friedrichshafen: "Turbocharging is an integral component of the engine design concept. It shapes the characteristics of the engine more than almost any other system, as it affects its economy, dynamics and emission characteristics."

The competitive impulse and technical need for producers, whether dedicated manufacturers of turbochargers or engine makers, to maintain the R&D momentum in this sphere, has never been greater, ultimately driven by the intensifying economic and legislative challenges confronting shipowners and operators. Such research endeavours, with their attendant costs of resourcing, in terms of qualified personnel, equipment and facilities, must be sustained despite fluctuations in financial returns and market conditions from year to year.

The ploughback into R&D, geared to clear objectives as to greater efficiency, higher turbocharging pressure, operating safety, long service life, compact construction and low production costs, is highlighted by Swiss specialist ABB Turbo Systems: "Measured against sales, ABB Turbo Systems invests a substantial amount in R&D in areas such as thermodynamics, aerodynamics, computational fluid dynamics (CFD), particle flow, acoustics, structural mechanics, blade mechanics and dynamics, bearing technology and rotor dynamics, materials science, surface treatment, coatings, computer-aided design (CAD), production and metrology. These investments pay off."

Given the considerable expense of research, and need for its continuity, there are perhaps added attractions in participating in publicly-sponsored, joint industry technological research projects that typically qualify for upwards of 50% in state or EU support. The core practical merit, of course, is the broad swath of know-how that these pre-competitive initiatives draw in across the various disciplines. In particular, the most ambitious, multi-partner undertaking of publicly-backed marine engine research ever implemented, the Hercules programme, has MAN and Wärtsilä as its lead players.

The three-year ’Beta’ stage of Hercules, which is drawing to a close at the end of 2011, has produced valuable test results with regard to two-stage turbocharger technology, for instance, among its multifarious sub-projects. This technology is viewed as a groundbreaking advance which could lead to a new era of environmentally sound solutions and competitive lifecycle costs for large marine diesel engines.

The principal aims of the Hercules-B integrated project have been the development of new technologies to reduce gaseous and particulate emissions from marine engines, and improvements in both engine efficiency and engine reliability, reducing specific fuel consumption, carbon dioxide (CO2) emissions, and lifecycle costs. The multinational project consisted of 54 sub-projects grouped into 13 task packages and seven work packages covering the entire spectrum of marine engine technology.

One of the main arguments for studies to be perpetuated under the envisaged, follow-on Hercules-C project is the need for further integration of the multitude of new technologies identified in Hercules-A and Hercules-B, and for the development of new optimisation techniques. Other proposed research areas are the factors affecting engine reliability and engine lifetime. If Hercules-C gets the go-ahead for financial support within the terms of the EU’s Seventh Framework Programme, the new phase will run from 2012 until 2015.

While leading the Hercules initiative, MAN Diesel & Turbo and Wärtsilä are involved in other collaborative research schemes with industry specialists, complementing unerring in-house R&D focused on key technologies such as turbocharging.

Wärtsilä and ABB Turbo Systems have been following a joint programme to develop two-stage turbocharging for large diesel engines. Wärtsilä’s objective is the combination and integration of two-stage turbocharging with advanced engine technology to optimise engine performance, benefiting fuel efficiency and emissions.

In the new technical solution introduced into the Wärtsilä four-stroke engine portfolio, two turbochargers are arranged in series to generate increased charge air pressure, increased airflow, and a more efficient turbocharging effect. The increased combustion technology, offers power output improvements up to 10%. At the same time, both fuel consumption and CO2 emissions are reduced.

Testing of the two-stage turbocharging concept has been carried out using four-stroke Wärtsilä 20- and 32-series engines at the company’s Vaasa test facility in Finland. It is planned to extend the technology to two-stroke engines.

MAN is prepared for two-stage turbocharging of large-bore diesel engines through the company’s development of the TCX generation of turbochargers, which employ the innovative variable turbine area(VTA) technology successfully applied to both axial- and radial-flow units. While based on the proven design philosophy of MAN’s TCA and TCR series, the TCX types use a novel, diagonal turbine especially well suited to the low-pressure ratios. Moreover, the TCX system is compact, wherein the turbochargers are arranged at 90 degrees to each other.

Earlier this year, MAN introduced two-stage turbocharging to the stationary power market by way of its potent 18V48/60TS medium-speed diesel. This employs tried and tested TCA88 and TCA77 standard turbochargers in sequence, each achieving pressure ratios in the range of 6bar at significantly higher efficiencies compared to a single-stage system. Despite the two-stage turbocharger modular system being an engine add-on, the 48/60TS does not require any more space than an engine with a single-stage turbo system.

In a marine engine context, the key point of interest surrounding two-stage turbocharging is the potential it offers in helping the industry reduce oxides of nitrogen(NOx) emissions to the much lower limits to be mandated in 2016 under IMO’s Tier III edict. It facilitates the use of a different type of combustion process, the so-called Miller cycle, which has the effect of lowering peak temperatures within the combustion chamber, thereby producing less NOx.

The basic principle underlying the Miller process is that the effective compression stroke can be made shorter than the expansion stroke by suitably shifting the inlet valve’s timing. Since the inlet valves are open for a shorter period than normal, less air will be drawn in to the combustion chamber. By applying two-stage turbocharging, however, the high boost pressures force sufficient air into the chamber to realise the full benefits of Miller timing. As a consequence, the two technologies form an optimum combination.

Two-stroke savings

MAN has recently unveiled a further advance in the design of its TCA axial turbocharger series which provides for an increased air flow within the same outline dimensions as before, enabling the use of smaller turbochargers in two-stroke engine installations.

In its debut installation, the uprated TCA55-26 turbocharger is to be fitted to a six-cylinder, MAN low-speed engine of the S50ME-B9.2 type chosen to power a handysize bulker under construction in China. Although this design and cylinder number has an L1 maximum continuous rating of 10,680kW, the engine for the bulker has been specified at a de-rated power of 8,750kW so as to benefit from lower fuel consumption.

The TCA55-26 turbocharger was delivered from Germany to the licensee engine builder, Hudong Heavy Machinery (HHM). The recipient 35,500dwt vessel is one of a series on order at Cheng Xi Shipyard.

Key to the performance enhancement offered by the -26 specification of TCA turbocharger is the adoption of a new compressor wheel geometry (RCQ45) with higher capacity and improved efficiency. Furthermore, the compressor incorporates an internal recirculation (IRC) device to extend the surge margin and optimise performance. IRC is a standard feature of the uprated unit.

The -26 development has been applied to four TCA models. With their extended application range, the TCA55-26, TCA66-26, TCA77-26 and TCA88-26 units thereby provide the means of turbocharging many two-stroke engines with a smaller turbocharger size than was previously possible. Examples are listed in the accompanying table. The selection of smaller turbochargers yields savings in the initial cost of engines and in maintenance and spare part expenses, and is also beneficial as regards machinery room space.

The -26 designation follows MAN’s model identification system, whereby the first figure ‘2’ signifies applications for two-stroke engines--as opposed to the use of ‘4’ to denote four-stroke applications-- and the second figure ‘6’ describes the design status.

Conceived as a completely new generation to succeed the NA family, the TCA (TurboCharger Axial) series was launched in 2002. As the initial design status was ’0’, the first version for use with two-stroke machinery(’2’) carried the -20 suffix. The developmental evolution to the present -26 stage is summarised in Table B.

Increases in airflow are generally achieved by extending the compressor wheel diameter. Compressor wheel geometry limitations are material strength, casing capacities and sonic speed within the compressor, given that maximum mass flow is at sonic speed.

The 6S50ME-B9.2 engine, featuring electronic fuel injection control, is the first of its kind to have been produced by HHM. The company is China’s largest manufacturer of marine diesel engines and MAN’s oldest Chinese licensee, with over 30 years’ experience in building the group‘s two-stroke designs.

The ‘2’ suffix in the engine designation indicates the new fuel-optimised version of the original, IMO Tier II emissions-compliant S50ME-B9 design. It was developed to meet the market demand for improved part-load and low-load SFOC (specific fuel oil consumption). Hence, the SFOC of the S50ME-B9.2 is 2g/kWh lower than that of the original-SFOC version, while still complying with the Tier II NOx limit.

MAN lists the key features of its TCAxx-26 uprated turbocharger as:.

  • enlarged compressor stage;
  • new compressor geometry(RCQ45);
  • higher airflow capacity; and
  • higher efficiency.

Seagoing debut for hybrid

Japan’s maritime industries have promoted an advance in hybrid marine turbocharger technology through the installation of an innovative, home-grown system on a 2011-built bulk carrier. The entry into service of the 182,000dwt Shin Koho, has provided a seagoing verification platform for the Mitsubishi MET83MAG turbocharger generator.

The hybrid unit has been designed to meet the vessel’s entire at-sea electrical power needs by utilising the exhaust gas from the main engine not only for driving the turbocharger compressor but also for power generation. The concept promises overall savings in fuel consumption and associated CO2 emissions. Constructed by Universal Shipbuilding’s Tsu shipyard, the NYK-controlled Shin Koho is claimed to be the world’s first vessel to have adopted a hybrid turbocharger power supply system for the main engine.

The maximum power output of the MET83MAG is 754kW (at 9,500rpm). Since the generated output power is a high frequency three-phase alternating current (AC), this is first rectified into direct current (DC) and thereafter converted to the appropriate voltage and frequency for the shipboard consumers. To accomplish this, the system utilises an IGBT (insulated gate bipolar transistor) for active rectification, as well as an inverter. Since these two elements also function in reverse, power from the ship can be supplied to the generator, so that it acts as a motor to accelerate the turbocharger rotor.

The new system accordingly has the further advantage of permitting the generator to function as a motor to add power to the turbocharger when the engine speed is low, thereby serving as a substitute for an auxiliary blower. Turbocharging performance can thereby be improved at part-load.

The generator is integrated within the turbocharger main unit, such that the overall space requirement is essentially the same as for a conventional turbocharger, with no major modifications required on the engine side. In a presentation of the concept last year, MHI technicians gave the length of the MET83MAG, excluding the connection box, as 4,013mm, compared to 3,700mm for the MET83MA standard turbocharger. Total width (2,250mm) and height (1,188mm) are the same, and overall weight was given as 15,700kg relative to 11,100kg for the MET83MA turbo.

In the new hybrid, the generator is positioned within the turbocharger silencer, and a two-part cast steel shell is attached to the compressor scroll to provide extra rigidity.

The power supply system has been developed by Mitsubishi Heavy Industries in cooperation with shipping group NYK Line, Monohakobi Technology Institute (MTI), Universal Shipbuilding Corporation. Hitachi Zosen Corporation also participated in bringing the project to the commercial stage.

The technical project has received subsidy funding from Japan’s Ministry of Land, Infrastructure, Transport and Tourism under the programme entitled ‘Support for Technology Development for Curtailing CO2 from Marine Vessels’. It has also been supported by Nippon Kaiji Kyokai (ClassNK) under the provisions of a joint research scheme. Evaluation of system performance and efficiency in service aboard the NYK bulker, currently employed in the transport of iron ore to Japan, will contribute towards further improvements in the concept.

MHI had originally developed a hybrid turbocharger prototype using a MET42MA model turbocharger, designed for diesel engines of about 5,000kW, and subsequently proceeded with the larger MET83MA model as the basis for the new product.

Shin Koho’s turbocharger generator is fitted to an electronically-controlled, seven-cylinder MAN diesel of the S65ME-C type, giving the ship a top speed of just over 15 knots. The L1 maximum continuous rating for this engine is 20,090kW.

Among the attributes of the MET83MAG design is its compactness, given the integration of generator and turbine. This feature is considered to be conducive to retrofit applications as well as newbuild installations.

Shin Koho encapsulates a high degree of design optimisation promising gains in hydrodynamic efficiency and payload capacity. She is the third of a new series wherein increased deadweight has been achieved within Dunkirk-max dimensions, one of the key criteria being the 45m hull width limit imposed by the port’s eastern harbour lock. The bulker incorporates the Leadge-bow form, reducing wave resistance, plus Super Stream Duct (SSD) and Surf-Bulb (a rudder fin with bulb) appendages forward and abaft the propeller, respectively.

Investment in production

As one of the cornerstones of the long-established engineering industry in Lincoln, UK, turbocharger production and development looks set fair for the future as a result of major expenditure by Napier in manufacturing and allied facilities.

Re-emergent as an independent force after its 2008 acquisition from Siemens Industrial Turbomachinery by investment firm Primary Capital, Napier Turbochargers has expanded its factory and office footprint by some 50%. The product portfolio is being augmented by the long-anticipated 8 Series generation of high-efficiency turbochargers, building on the design and reputation of the 7 Series.

Key elements of the programme at Lincoln have included the opening of a new turbocharger assembly line and a dedicated line for large frame-size models, a turbocharger paint booth and a state-of-the-art production cell for turbine blades, production training workshop and classrooms, plus new office space.

The company manufactures axial turbochargers for diesel, heavy fuel, dual-fuel and gas engines in the 500kW-20MW power range in marine, power generation and rail traction applications, and the substantial installed base creates parallel business activity in the provision of spares and aftermarket services.

The 7 Series, made up of the NA297, NA307, NA357 and NA397 models, represents the company’s core range, suitable for engines requiring single turbocharger outputs from 2,000kW to 6,500kW, and up to about 20MW in multiple-unit installations. Based on the original Napier cartridge construction design, the 7 Series achieve a pressure ratio of 5:1 in a single stage using an aluminium compressor.

The latest 8 Series generation offers an increased pressure ratio of 6:1 and a claimed efficiency of 70%. The main design focus in the development of the new product range has been to allow for medium-speed diesel engines to operate at higher powers, with higher overall efficiency, helping to reduce fuel consumption, harmful exhaust emissions and through-life costs for the operator.

The latest aerodynamic and mechanical advances, including bearing technology and gas path components, have been combined with feedback from engine manufacturers and end-users. The cartridge concept has been retained in the new series because of its well-proven benefits as regards serviceability.

The 8-Series is currently offered in four frame sizes, with the NA498 at the upper end of the spectrum in terms of power rating, to serve engines of 8,000-12,000kW output. The NA398 is designed for engines of 5,000-8,000kW, with the NA358 and NA298 catering to 3,500-5,500kW and 2,000-4,000kW ratings respectively. Turbocharger speed ranges from 19,400rpm in the case of the NA498, to 33,000rpm for the NA298.

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