More stringent limits on NOx emissions and the use of aftertreatment and dual-fuel engines are leading to unique challenges on turbocharger design
While turbocharger designers must adapt to the power and fuel efficiency requirements of shipowners, occasionally other factors emerge that throw up different design challenges. One example is the introduction of IMO’s Tier III NOx limits, which became a key focus for Mitsubishi Heavy Industries when designing its latest range of MET turbochargers.
Introduced in 2016, Tier III NOx regulations require newbuild vessels to meet strict limits in NOx emission control areas (NECA) – currently just the North American coastline but with the North Sea and Baltic Sea becoming NECAs in 2021. The conventional methods for compliance are either exhaust aftertreatment (through selective catalytic reduction of NOx or recirculation of exhaust gas) or the use of LNG in dual-fuel engines.
Both methods present challenges for turbochargers, says MHI senior deputy manager of two-stroke turbocharger design Yoshikazu Ito.
“For high-pressure EGR, for example, the engine operating point moves greatly when it is switched on or off,” he explains. “So, a wide range is needed within which the turbocharger efficiency does not change much when pressure and flow rate changes.”
In the case of dual-fuel engines, control of the air-fuel ratio is crucial. Turbochargers need to have high efficiency at a high load to make sure they can deliver the air needed to keep the engine firing within the safe combustion window.
Both factors contributed to the development of the forthcoming MET-MBII series, the successor to the MET-MB range (introduced in 2008) which has around 2,000 marine references. The design concept has been finalised and MHI is now preparing for testing, both with an enginebuilder and on board a vessel. The new series will then be launched next year. As with the MET-MB series, 10 frame sizes will cover engine outputs from around 3,400kW to 45,000kW.
The new turbocharger was designed for compactness and a high air flow, says Mr Ito. As a result, the casings are unchanged from the existing series, with development focusing on the rotating parts.
The impeller and related components were optimised for air flow, based on MHI’s in-house flow analysis technology. The blade profile was reviewed and the number of blades was changed from the current combination of 11 whole and 11 splitter blades to seven blades of each type. Reducing the number of blades both increases the air flow rate and simplifies the production of the impeller, reducing cost.
The design peripheral speed has also been increased and blade angle distribution adjusted. Blade thickness was revised to maintain the impeller’s overall size, even with the new more acute blade profile and the smaller number of blades.
“The demand for a wide operational range was realised by using flow analysis to balance air flow rate, pressure ratio and efficiency,” says Mr Ito.
To balance the high flow rate of the impeller, MHI also needed to adjust the turbine capacity of the MET-MBII. In response to the increase in the design peripheral speed, the turbine blade shape and the number of turbine blades were modified to maintain the same strength as the existing turbine blades. As a result, the number of blades increased from 35 in the MET-MB turbine to 42.
But increasing turbine blade capacity is not simple, says Mr Ito. It requires a corresponding increase in the opening between blades. But if the opening (or throat distribution) is increased too much, dynamic pressure at the outlet of the turbine increases and efficiency will decrease. MHI again turned to its flow analysis technology to resolve this issue. By improving the blade shape, exhaust chamber pressure recovery performance – and therefore turbine efficiency – was improved.
“Generators need to improve their load response for the stability of the electric power system, so turbochargers must have high responsiveness”
There were further design considerations, including reducing the diameter of the journal bearing and rotor shaft to reduce heat losses. The impact of thrust on the compressor as pressure ratios increase was also explored and a new solution formulated, changing the shape of gas piping on the turbine side to reduce the thrust force working on the compressor.
As a result of these changes MHI registered impressive performance, with air flow through the MET-MBII impeller 16% higher than that registered for the equivalent MET-MB unit, while maintaining similar turbocharger efficiency. This means that as well as offering stable performance for EGR use and the efficiency needed for gas engines, the MET-MBII can also cut capital costs for shipowners by allowing them to choose one frame size smaller than the current series.
The reduction of NOx is also an important factor for auxiliary engines, with four-stroke diesel engines often using Miller valve timings to limit emissions. Miller timings demand higher pressure ratios from turbochargers to compensate for the smaller volume of air that would otherwise be taken into the cylinder, due to early valve closing. For MHI’s latest range of four-stroke engine turbochargers then, as well as for its two-stroke units, managing NOx emissions presents design challenges.
Rotor design is crucial to improving air flow and has been a focus in MHI's latest models
IMO’s Energy Efficiency Design Index (EEDI) also had an impact when MHI began to look at the successor to its MET-SRC four-stroke engine turbochargers, explains senior deputy manager four-stroke engine turbocharger design Yushi Ono.
“New ships are subjected to EEDI evaluation, so it is necessary to improve the fuel efficiency of the engine reduce greenhouse gas emissions,” he says. “A high-efficiency turbocharger is required and energy-saving equipment, such as an exhaust heat recovery system, is adopted in some cases. Generators also need to improve their load response for the stability of the electric power system, so turbochargers must have high responsiveness.”
To meet these requirements MHI has upgraded pressure ratios for its MET-SRC turbochargers with every new update. The successor series, MET-ER, will boast a pressure ratio of 6.0. This has been achieved by a new type of compressor with optimised blade height and angle distribution.
The turbocharger response speed has also been improved, by about 25% compared with the existing MET-SRC series. A new turbine wheel with a smaller diameter, reducing inertia, contributed to this result, while allowing the new turbocharger to retain high flow rate and efficiency.
The combined impact of these advances was to reduce the turbocharger frame size needed for any given engine. For example, for a 1,000kW engine the old MET18SRC would have been needed, whereas the new MET16ER will now take its place – offering a size reduction of 40%. The number of parts has also been reduced by about 30%, reducing cost and making maintenance simpler.
The MET-ER range will be available covering engine outputs of about 500kW to 5,800kW.
MHI’s developments show that turbocharger efficiency and pressure ratios do not always need to be the focus of design efforts. In fact, the company believes that such a focus can be detrimental in some cases – for example, if turbocharging efficiency is too high for the overall engine system design, the heat being passed into downstream equipment such as economisers and boilers may make these systems work less efficiently.
The company’s efforts to identify the design parameters that matter – rather than a relentless focus on pressure ratios – are evident in the problems solved by its forthcoming new turbochargers, both for two-stroke and four-stroke engines.