Design effort increases for two-stage turbocharging
Turbocharger manufacturers are focusing on further NOx reductions with new two-stage developments to boost engine charge pressure, says Wendy Laursen.
ABB’s new Power2 800-M turbocharging system is now ready to complement the first generation released in 2010. The new generation two-stage system is designed to achieve pressure ratios of up to 12 and efficiency levels of over 75% on medium speed gas and diesel engines. The pressure ratios achieved are suitable for reducing NOx emissions by up to 60% compared to single stage systems and the turbochargers can be combined with exhaust gas recirculation or selective catalytic reduction systems to meet IMO Tier III and EPA Tier 4 requirements.
Designed from the start to be a two-stage solution, ABB’s Power2 turbocharging system has a high-pressure and a low-pressure stage that work together to compress air to much higher pressures. The setup enables strong Miller Cycles to eliminate the combustion temperature peaks responsible for over 90% of NOx formation. The Miller Cycle cools the engine’s charge air by shortening the opening period of the inlet valve to promote expansion and cooling in the cylinder. The high turbocharging pressures produced by Power2 compensate for this shorter opening period by forcing a greater amount of air into the cylinder.
An axial turbine design was chosen for the low pressure turbine. This combines high specific volume flow with excellent efficiency at part load and good acceleration behaviour due to a low mass moment of inertia. The specific volume flow associated with the low pressure compressor stage’s impeller wheel has been increased by about 10% compared to the low pressure compressor stage of the first generation. The new components have enabled a compact design for flexible arrangement on engines and low vibration levels. An optimisation of the air inlet duct with numerical simulations has allowed the design to achieve a 20% reduction in blade vibration potential and a 7% reduction in pressure drop compared to earlier designs.
The high pressure turbocharger also has an axial design turbine as it needs to cover a wide range of volume flows due to the large range of possible pressures and temperatures at the turbine inlet. Keeping the system compact, ABB has achieved specific volume flows about 20% higher than previous single stage turbocharger designs.
The complex flow field in the gas outlet casing has been optimised to minimise total pressure loss and increase the recovery of static pressure in the gas outlet channels of both the high and low pressure turbochargers. Additionally, the pressure loss of the gas inlet casing has been reduced by about 25% by making the circumferential distributions of mass flow and inlet flow angle at the turbine inlet the same.
The space requirements of the gas inlet casing have also been reduced as a result of the more compact flow channel. This allows the application of a new type of turbine cleaning nozzle. Using two-phase flow CFD simulations, the location and shape of the injection holes along the nozzle body have been optimised, and the nozzle body can be accessed easily from the outside.
A new injection system has been developed to facilitate regular compressor cleaning, and a new shaft seal design reduces overall blow-by leakage of the two-stage system to acceptable levels. The new seal concept will be used for both low and high pressure turbocharges and achieves a leakage rate of about 25% of that of latest model labyrinth seals. This means overall blow-by leakage rates are below that of single stage turbochargers.
ABB has reduced service time for the complete two-stage system to below that of current single stage turbochargers. An extended cartridge system includes the entire interior of the turbocharger so that, during service, only the air inlet casing and the insert wall of the compressor needs to be removed. There is no need to remove the insulation of the turbocharger or to handle oil connections, air outlet flange connections gas inlet or gas outlet flange connections. A new tool has been developed to facilitate quick exchange of cartridges, and a crane is only needed to lift the cartridge on to the transporting pallet and back again. ABB service stations also offer inspection by endoscopy to check the condition of compressor and turbine stages without the need to remove the cartridge.
The Power2 800-M has four system sizes for medium speed engines with bore sizes between about 300mm and 500mm.
KBB is currently developing a two stage turbocharger series to meet IMO Tier III exhaust gas regulation with pressure ratios of 6 to 10. Apart from two turbochargers, this type of turbocharger system also contains an intermediate cooler and a bypass control for the high-pressure and low-pressure turbine. Complexity is further increased if an exhaust gas recirculation system is included, and KBB is rising to the challenge with further investments in simulation technology under the project title K2B – Knowledge to Boost.
A new generation of compressors and turbines is being developed for the large map widths and best efficiencies required at low pressure ratios. New design solutions have to be worked out to ensure oil and gas tightness under the very high absolute pressures.
KBB is working on the development of a low-pressure turbocharger series and a high-pressure series. The turbochargers in the low-pressure series are based on the HPR series and are complemented by a size 7 axial turbine charger. The turbochargers in the high-pressure series will have changed outline dimensions due to the higher absolute pressures which have to be controlled. The first prototypes of the two series were built in 2012, and engine trials are now underway.
Meanwhile, KBB’s 7th generation single stage, high pressure turbochargers from the ST27 series are suitable for engines with an output range from 300kW to 4,800 kW and have been developed for operation with pressure ratios of up to 5.5:1 to satisfy the demands of IMO II engines.
Shaft motion measurements were carried out to confirm the rotor bending characteristics of the two rigid body modes. The rotors’ stability was investigated in experiments by varying the rotor imbalance and the oil parameters in the bearings. As a consequence and following evaluation of the experiments, KBB had to abandon its previous standard design for a compact bearing and change to a radial bearing with squeeze film dampers. The profiled, stationary bushings in the squeeze film damper bearings showed some very good stability characteristics in the experiments. The oil parameters and the balance state of the rotor were varied in numerous other trials. Even with 10 times the permissible unbalance, the dynamic radial shaft motion is considerably below the KBB limit. Endurance tests with Wärtsilä engines were carried out for the new series. An inspection after 16,000 running hours in the field has shown all turbocharger parts in an excellent condition.
BorgWarner has launched its Tier 3 turbocharger product on Caterpillar 32-litre and 18-litre engines and is currently in the process of launching further models on several engine platforms including Navistar’s 13-litre and 9-litre engines.
“Up till now, we have quite easily been able to meet the torque requirement of the engines with single stage turbocharging either with a single or twin turbo configuration,” says Kevin Fogarty, applications engineer for BorgWarner. “However higher power densities and increased compressor boost pressure requirements will make us look to two stage systems.”
This need is not only coming from higher power densities and higher horsepower, but also from the desire for faster transient responses that wastegates alone cannot accomplish. “We as a company have a lot of success and experience with two stage turbocharging in other markets and a lot of that will be carried over when employing these on the marine applications.”
BorgWarner currently provides turbocharging solutions for four stroke diesel engines with engine displacements ranging from 1.5 litre up to 52 litre and power ratings from 48bhp to 3,000bhp. The company also offers solutions for generator sets.
“Our new impellers have been optimised to provide higher efficiencies and also a wider operating range. Turbine wheels for our new bearing systems (Gen2 B-series) have greater efficiencies as well and also enable higher turbo speed limits,” says Mr Fogarty. The new bearing systems also have improved oil sealing and increased load capacity thrust bearings.
Water cooled turbine stages are used to limit the skin temperatures of the hot turbine stage which sees exhaust gases at temperatures over 750°C. Each new design of these water cooled turbine housing undergoes a CFD and heat transfer study through the use of computer aided simulation tools to determine the effectiveness of the water cooling and ensure the water jacket designs are optimally cooling the housing.
The company has adopted pneumatic wastegates with a newer poppet valve design in addition to using conventional linkage driven actuation. The advantages of this are simplicity and compactness of the whole wastegate system into a single poppet valve. BorgWarner offers a wide range of turbine volute sizes, and the smaller sized variants are usually wastegated to help the engine achieve good transients and emissions levels.
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