At the “Turbocharger Technical Update” event held in Augsburg in mid December 2007, the Business Unit Turbocharger gave an exposé of its current development activities in the area of advanced turbocharging for large two- and four-stroke diesel engines. A traditional turbocharging pioneer, MAN Diesel has never relinquished technological leadership since it began turbocharger manufacturing in 1934. For example, only a few years later, in 1940, MAN Diesel devised the basic rotating group concept that would come to predominate in all sizes of turbocharger – i.e. a core group consisting of the turbine and compressor mounted either end of a shaft supported in a central “inboard” plain bearing, lubricated from the engine lubricating system. This concept was – sooner or later – adopted by all major manufacturers.
Emissions and fuel consumption
At the centre of present development activity is planned legislation to further limit emissions of oxides of nitrogen (NOx) from large diesel engines, as exemplified by the second Tier of emissions regulations from the International Maritime Organisation (IMO) and similar limits for stationary engines, as well as efforts to reduce specific fuel oil consumption (SFOC), both for economic reasons and as a route to reduced emissions of the greenhouse gas carbon dioxide (CO2). Indeed, the link between fuel efficiency and emissions has gained new importance as emissions of CO2 have come to share equal focus with noxious emissions like NOx and oxides of sulphur.
While, essentially, exhaust emissions are in direct proportion to fuel consumption, a special challenge in reciprocating engines is the so-called “trade-off” between specific fuel consumption and NOx – i.e. the fact that reducing NOx formation in diesel or gas engines is normally bought at the expense of fuel efficiency.
This trade-off was, hence, a recurring theme in the presentations of the MAN Diesel Turbocharger event and is illustrated by the embedded graph. As shown in the example, the aim of the latest MAN Diesel turbocharger developments is to move this curve towards the “origin” of the graph. The optimization of the SFOC / NOx trade-off is achieved by advanced turbocharging as a method of simultaneously reducing specific fuel consumption and NOx formation via reduced combustion temperature combined with Increased thermal efficiency.
Advanced air management
In the recent past MAN Diesel announced its “VTA” (Variable Turbine Area) technology on its axial turbochargers in a two stroke marine engine application and on radial turbochargers employed on its four-stroke type 32/40 PGI gas engine with Otto combustion process. The company is also currently developing the STC Sequential Turbocharging System for its high power density 28/33D marine engine, initially for application in naval vessels.
The STC system offers optimum engine-turbocharger matching for special requirements and gives the type 28/33D engines an extended torque envelope, resulting in economical operating modes and improved engine acceleration characteristics. These modes are especially useful in naval applications and include cruising with a controllable pitch propeller set at optimum pitch for noise while still retaining high acceleration capability; operating a single engine at twice the propeller law in multi-engine systems (e.g. twin input / single output gears, CODOG etc).
MAN Diesel’s STC system is derived from well proven equipment used on the Pielstick PA6 engine and in contrast to other sequential turbocharging systems, the MAN Diesel STC system is essentially – and intentionally – simple, consisting of two identical, standard turbochargers, one providing copious charge-air at low and medium speeds with the second cutting in at higher speeds.
The VTA system consists of a nozzle ring, equipped with adjustable vanes which replaces the fixed-vane rings used in MAN Diesel’s standard TCA and TCR turbochargers. Adjusting vane pitch regulates the pressure of the exhaust gases impinging on the turbine to vary compressor output. The quantity of charge air can be more precisely matched to the quantity of injected fuel, resulting in reduced specific fuel consumption and emissions, in combination with improved dynamic behaviour of the engine-turbocharger system.
In detail the VTA system consists of a nozzle ring equipped with adjustable vanes which replaces the fixed vane nozzle rings fitted in MAN Diesel’s standard TCA turbochargers. In this way, VTA technology can be readily retrofitting to turbochargers already in the field. By adjusting the pitch of the vanes, the pressure of the exhaust gases can be regulated and the output of the compressor optimised at all points on the engine’s performance map. In order to minimize thermal hysteresis and improve adjustment accuracy, each vane has a lever, which is directly connected to a control ring. The control ring is actuated by an electric positional motor with integrated reduction gear whose development was an integral part of MAN Diesel’s VTA solution. The adjustable vanes are manufactured in heat and erosion resistant steel alloy, and careful selection of fits and materials ensures operation under all conditions without sticking, especially in applications on engines burning heavy fuel oil (HFO).
Control of vane position is fully electronic with feedback or open-loop control with mapped vane adjustment. A comprehensive range of control signals can be used, including charge air pressure after the compressor and exhaust gas temperature before and after the turbocharger. In this way, MAN Diesel states, it can offer control packages precisely tailored to a specific application, including both mechanically controlled engines and engines with electronic management. For retrofit applications, MAN Diesel will offer complete packages including the VTA nozzle ring, the actuator and the associated control system.
First VTA applications
The first application for an axial turbocharger with VTA technology is a two-stroke, low-speed marine engine, while a radial turbocharger with VTA technology is being tested on MAN Diesel’s revolutionary 32/40 PGI gas engine. In the stationary 32/40 PGI application with radial turbocharger, MAN Diesel’s VTA technology has been verified as an effective alternative to a charge-air by-pass system for the precise control of air: fuel ratio. With the VTA system, turbocharger output can be precisely matched to engine air demand instead of blowing off excess compressor output into the atmosphere, resulting in improved engine efficiency.
The VTA system on an axial turbocharger is under test on a six cylinder, 46 cm bore 6S46MC-C engine built by MAN Diesel’s Croatian licensee Brodosplit. The HFO-burning 6S46MC-C features mechanical controlled fuel injection and exhaust valve actuation and is one of two engines installed in a twin engine propulsion system aboard a 70,000 ton, shallow draught tanker. The vessel, the Stena President, was built at the Brodosplit shipyard for the Stena Concordia Maritime shipping line.
Inclusion of VTA technology on the axial TCA55 turbocharger allows up to 0.5 bar variation in compressor output pressure at part load. Overall results show the expected improvements at part load in terms of fuel consumption, as well as considerable reductions in emissions of soot and unburnt hydrocarbons, as well as improved engine response under load changes. It was also demonstrated that VTA technology gave a useful new dimension to the mechanically controlled engine. The effects are comparable to the use of variable valve timing and
electronic engine control. To attain the best possible comparison the engine with VTA turbocharger runs alongside a second 6S46MC-C engine with conventional turbocharging.
Specifically the benefits of the higher scavenging pressures in part-load operation provided by the VTA turbocharger include lower SFOC at part-load, improved torque and engine acceleration, lower combustion chamber temperatures and the exacted savings in electrical energy to drive the auxiliary-blowers, depending on the engine load-profile.
High pressure turbocharging
On a slightly longer timeframe, MAN Diesel is also pursuing single and two-stage high-pressure turbocharging. In its single stage, high-pressure turbocharging concept, MAN Diesel employs optimised Series compressor wheels to achieve pressure ratios up to 6 bar at 80 per cent turbocharger efficiency.
The MAN Diesel two-stage concept consists of two turbochargers in tandem with an intermediate charge air cooler and is presently capable of producing scavenging pressure ratios of 6.5 to 7. The second, smaller turbocharger is fitted with the VTA control system to increase control of charge air output. The system has already been tested in prototype form on a four stroke 32/44CR engine with common rail fuel injection and the MAN Diesel VVT variable valve timing system.
By employing the inter-stage cooler between the two turbocharging stages, the energy required to compress the intake air to high pressure is considerably reduced compared to a system without this feature.
These high-end turbocharging techniques offer decisive improvements to engine performance data, especially by enabling strong Miller valve timing to improve the trade-off between SFOC and low NOx emissions. As the comparative table shows, mean cylinder pressures over 30 bar are possible while the strong Miller process allows NOx reductions in excess of 30 per cent savings with no SFOC penalty.
At the same time an increase of up to 8 per cent is possible in thermal engine efficiency combined with a 2 per cent improvement in fuel efficiency, while future potential for SFOC and NOX savings is also considered substantial. These improved efficiencies are expected to be of special value in the stationary applications of engine from MAN Diesel i.e. electrical power generation and cogeneration.
Complete article with illustartions, see Skipsrevyen 1/08.