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MAN Diesel has announced that it is using the 23rd SMM Shipbuilding Machinery and Marine Technology International Trade Fair to exhibit what it calls its latest 'fuel saving' VTA turbocharger technology.

VTA, or Variable Turbine Area, denotes a system of adjustable nozzle ring vanes which replace the fixed vane nozzle rings fitted in MAN Diesel’s standard axial TCA and radial TCR turbochargers. Adjusting vane pitch regulates the pressure of the exhaust gases on the turbocharger turbine to vary the output of the compressor.

With VTA, MAN Diesel says the quantity of charge air entering the cylinder can be more precisely matched to the quantity of fuel injected, so that combustion can be optimized at all pointson the engine’s operating profile.

In this way, MAN Diesel claims specific fuel consumption and related emissions can be minimized at all engine speeds and loads, combined with improved dynamic behaviour of the engine-turbocharger system, i.e. better engine response.

Control of vane position is said to be fully electronic and a wide range of control signals can be used in both closed-loop control systems with feedback or open-loop systems with mapped vane adjustment.

MAN Diesel claims that a comprehensive range of control signals can be used. It is therefore confident that VTA turbochargers can be precisely tailored to a wide range of applications, including both mechanically and electronically controlled two and four stroke engines.

The company also stresses that VTA technology can be readily retrofitted to turbochargers already in the field. For such applications the Turbocharger Business Unit offers complete upgrade packages including the VTA nozzle ring and the associated actuation and control systems.

With regards to "slow steaming" - a practice where shipowners and operators reduce vessel speeds in order to save costs - MAN Diesel claims that with VTA technology charge air delivery can be precisely matched to the power output demanded from the engine at a given load condition.

By doing this, the company says there is no deterioration in the emissions behaviour of the engine and fuel consumption is saved not only by running the engine at a lower output, but also by optimising combustion at the appropriate load point.

The first turbocharger with VTA technology to be tested in a marine application was on a slow speed, two stroke, six cylinder, 46 cm bore MAN B&W brand 6S46MC-C engine.

The engine burns Heavy Fuel Oil (HFO), was built by MAN Diesel’s Croatian licensee Brodosplit and features mechanically controlled fuel injection and mechanical exhaust valve actuation. It is one of two engines installed in the twin engine propulsion system of the 70,000 ton, shallow draught tanker Stena President.

In order to be able to carry out a performance comparison, the Stena President’s second 6S46MC-C engine was fitted with a standard TCA55 turbocharger. MAN Diesel reports that the inclusion of VTA technology on the axial TCA55 turbocharger allows up to 0.5 bar variation in compressor output pressure at part load.

The company says that the results with VTA technology during field tests has been "extremely favourable", both in its effect on engine operation and its resistance to fouling and wear in the hostile environment of an HFO engine exhaust.

Commenting on the performance of its VTA technology so far, MAN Diesel said "The VTA system aboard the Stena President has operated for over 5,000 hours and is delivering even higher fuel savings than expected - depending on engine load, the reduction in SFOC on the engine fitted with VTA has been measured at as much as 4.4 g/kWh compared with the engine fitted with the standard turbocharger - or well over 2.5 percent."

MAN Diesel reports that no major problems were encountered prior to a 3,000 hour inspection in early 2008. At the inspection, all the components of the TCA55-VTA turbocharger were visually inspected and all components were found to be in good working order.

"There was no abnormal wear, in fact very little wear was measured at all. The power needed for vane actuation has been monitored during the trial and in spite of operation in HFO exhaust gases, the vane adjustment mechanism still moved freely. Indeed, it could be actuated using hand pressure only," MAN Diesel said.

"Overall results show the expected improvements at part load in terms of fuel consumption, as well as reductions in emissions of soot and unburnt hydrocarbons and improved engine response under load changes," added the company in a statement.

Accordingly, MAN Diesel says its VTA turbocharger technology is creating considerable customer interest at a time when slow steaming is becoming more and more common amongst shipowners and operators.