Electric assistance will give MTU engines more effective turbocharging, reducing fuel consumption
German engine builder MTU, part of Rolls Royce Power Systems, has always been proactive in turbocharger development, seeing this as an essential element in overall engine design. Its ZRT turbochargers have contributed strongly to the success of its engines, with class-leading power to weight ratios. Rolls-Royce has now taken a further step by acquiring technology for electrically assisted turbocharging, aiming to bring products to series production on MTU engines by 2021.
It has acquired G+L innotec, giving it exclusive rights for this new technology, which was originally developed for off-highway engines with power outputs above 450 kW. G+L innotec, based in Germany, had already successfully demonstrated hybrid electrical turbocharging technology in its Cross-Charger concept, also referred to as ‘turbo by wire’. Its principles are protected by patents and, to date, the technology not been available to the engine marketplace.
Rolls-Royce now plans to apply electrical assistance to turbocharging its MTU engines. “Electrically-assisted charging is a milestone on the way to hybridising the engine,” said MTU director of development, turbocharging and fluid systems, Johannes Kech. “Using this technology, it will be possible for us to develop agile, low-consumption engines.”
Its electrically-assisted turbocharging system incorporates an electric drive coupled with a traditional MTU turbocharger design. This enables the turbocharger to accelerate more quickly, speeding the build-up of charge and reducing the phenomenon often referred to as turbo-lag. It will make MTU marine engines capable of more rapid acceleration when power demand increases, particularly from levels of low engine power or speed.
This introduces the possibility of reducing both engine fuel consumption and exhaust emissions in a range of applications. Engine response speed is of great importance for many high-speed marine applications but the system will also enable more rapid start-up and loading of emergency diesel generating sets. It is equally suited to engines running on gas fuel.
The electric motor used to boost the turbocharger is a permanent magnet unit, installed outboard of the air compressor wheel, with electrical windings integrated into the compressor casing. The induced air is not subjected to any restriction, because of a large gap between the rotor magnets and the field windings. This ‘media gap’ motor is controlled by a purpose-designed power electronics system, which optimises the electrical boost and delivers power rapidly to the motor in response to sudden load demands.
To date, the collaboration between MTU and G+L innotec has resulted in MTU turbochargers being equipped with this electric drive and successfully tested to evaluate their operational potential. The next stage for the companies is preparation for series production. The first applications are planned to be for marine vessels and emergency generating sets, along with land vehicles.
This latest MTU development follows a history of innovation in turbocharging high-speed engines. In addition to the turbochargers themselves, as used in traditional single-stage applications, MTU has introduced a range of configurations to maximise the performance, flexibility and responsiveness of its engines. One example of this is in sequential single-stage turbocharging systems, where individual turbochargers are configured in parallel and brought into operation sequentially, as required by engine speeds and loads.
MTU has also developed two-stage turbocharging systems to provide its engines with even higher boost pressures, enabling increased power outputs and improved emissions performance. Sequential two-stage turbocharging systems operate in a similar manner to single-stage systems but with a pair of turbochargers being added to or removed from operation as necessary.
More advanced regulated two-stage configurations have also been applied, where the exhaust gas flow from the cylinders is split, with part passing through the high-pressure turbocharger turbine and the balance diverted through a bypass, using a controllable wastegate valve. Downstream of this the full exhaust mass flow then re-combines to drive the low-pressure turbine stage.
Such systems, combined with the efficient design of the individual MTU turbochargers themselves, have enabled the company to keep pace with the demands for good emissions performance and IMO compliance. To meet IMO Tier III emissions levels, MTU has further adapted its approach for its Series 4000 marine engines, introducing further enhancements to the turbocharging system, combustion process and fuel injection. By combining these with a new selective catalytic reduction system, NOx emissions have been reduced by 75% compared with IMO II limits. Furthermore, particulates have been reduced by 65% compared with the US Environmental Protection Agency’s Tier 3 limits, without the need for additional diesel particulate filtration.
Woodward buys L’Orange
MTU has traditionally produced its own turbochargers but, in 2016, the company increased the involvement in MTU’s turbocharging business of injection specialist L’Orange, also part of Rolls-Royce Power Systems. This move has since been followed, in April 2018, by an agreement that Woodward, of Colorado, USA, is to acquire the L’Orange business. However, L’Orange will continue to be a partner and key supplier for both MTU and Rolls-Royce Bergen engines with an initial long-term agreement for 15 years.
Woodward chairman and chief executive Thomas Gendron said at the time that “L’Orange is an excellent strategic and financial fit for Woodward and this transaction exemplifies our acquisition strategy to invest in markets with solid long-term fundamentals.” He said the acquisition “brings innovative technology, bolsters relationships with key customers and enhances the profitability of our Industrial segment.”
The current L’Orange turbocharger portfolio has been mainly developed for MTU engines and is based on small, medium and large series products. These start with ZR1 turbochargers, capable of delivering up to 0.9 m3/s of charge air at maximum pressure ratios of 5.2:1. ZR3 products have higher capacities, offering between 0.8 and 2.0 m3/s with higher pressure ratio capabilities of up to 5.5:1. The largest ZR5 turbochargers in the current range extend capacities further, to a maximum air delivery of 3.5 m3/s.
All products feature wide compressor maps to enable maximum flexibility in engine operation and rotor components are designed for high numbers of load cycles, a key requirement for many high-speed engines. The milled centrifugal compressors are available in aluminium or in titanium where high temperature operation is required. Designs feature vane-less diffusion and the turbine section is also nozzle-free.
Turbine wheels are of radial or mixed-flow design and the rotor is mounted in maintenance-free plain bearings. A double piston ring seal system ensures low blow-by and the overall turbocharger designs have features for containment protection, to maximise equipment safety in operation.
Well-suited to marine, power generation and industrial applications, L’Orange also offers a remanufacturing option for these products. This effectively gives the turbochargers a second life-cycle and improved economics for operators.