A tighter cost focus from shipowners is driving product and service development in the four-stroke engine turbocharger sector
For Kompressorenbau Bannewitz (KBB) a new cost sensitivity among shipowners’ is changing both how it presents the services it aims to provide and how it presents its business case. The company, which makes turbochargers exclusively for four-stroke engines, today sees operators demanding total cost of ownership calculations rather than just a sale price.
“Risk is being assessed in a more detailed manner and a shifting of the risk is sometimes sought,” explains KBB head of turbocharging technology Dr Silvio Risse.
Part of the company’s response has been the introduction of a more flexible service concept to increase the speed at which turbocharger spares or replacements can be made available, by evaluating the potential of each service station in its network to fulfil the order. A more tailor-made service will be welcomed as the company prepares to meet increasing demand for spares.
“Our field population’s average age is getting higher and as our products are getting more reliable, customers are aiming to maintain products longer, instead of replacing them,” says Dr Risse.
Another element of KBB’s response to the needs of cost-conscious customers is product development. In the past year the company has launched its ST27-EP turbocharger, offering a pressure ratio that now exceeds 6.0.
Demand for an increase in charging pressure is growing for gas and diesel engines. Dr Risse attributes this to the downsizing of new engines while performance demands continue to increase. The contribution of higher charging pressures to reducing emissions, by enabling valve timing adjustments, has also put new expectations on turbochargers.
Despite performance advantages, uptake of two-stage turbocharging has been slow. While two-stage turbocharging drove a leap in the charging pressure of large engines, the result has often been higher stress on the engine as well as higher costs, weight and installation space requirements. The impact of two-stage turbocharging on these important criteria has meant that demand for highly-charged, single-stage exhaust-gas turbochargers remains high.
“Risk is being assessed in a more detailed manner and a shifting of the risk is sometimes sought"
KBB’s established single-stage ST27 exhaust-gas turbocharger series has been improved to offer a higher charging pressure and maximum air flow-rate range in the new ST27-EP series.
“Since these requirements entail higher demands on both the compressor wheel and turbine, the entire exhaust-gas turbocharger had to be improved,” says Dr Risse. “This applies to all flow-guiding housings as well as the bearings and shaft seals.”
To increase the turbocharger’s pressure ratio from 5.5 to 6.0, KBB developed new design and optimisation methods to build a new compressor wheel family and a bladed diffusor. The air flow-rate range was also extended, to 10 compressor stages. These measures involved adding active compressor wheel cooling (to boost thermal management) and using more heat-resistant sealing materials.
Other performance requirements for the new series included improving acceleration behaviour and extending the flow-rate range while maintaining turbocharger dimensions. Each of these requirements challenged aerodynamic compressor design, says Dr Risse.
Available installation space and containment safety are also important design considerations for compressor stage components, including the compressor housing and silencer. The compressor’s volute housing was adapted to the extended flow-rate range for each size, resulting in an increase in total compressor-stage efficiency of around one percentage point in the upper flow-rate range over the entire operating range.
Dr Risse explains that this efficiency was achieved by winding the volute towards the outside and improving tongue geometry. Both measures led to lower total pressure losses in the compressor housing.
“KBB is aiming to use advanced monitoring to add more value for engine users”
Higher pressure ratios and increased compressor flow rates meant changes to the turbine were also needed. Three turbine stages cover 10 compressor stages, increasing the turbine's maximum flow rate. The turbine nozzle ring has also been extended.
During engine performance tests the new ST27-EP turbine showed a much better efficiency curve at full load. This higher efficiency at large expansion ratios means that decreasing exhaust-gas counterpressure gives two positive effects compared to the original ST27 turbine. First, the efficiency advantage increases because optimum efficiency for the selected turbine nozzle ring specification lies between 3.5 and 4.5. A higher turbine efficiency shifts the required exhaust-gas counterpressure further towards this optimum value. Second, charge-changing losses decrease because of lower exhaust counterpressure, causing a reduction in fuel consumption and emissions.
The first turbochargers from the ST27-EP series were delivered and commissioned to charge an engine in the marine sector in 2017. The turbocharger was subjected to a first assessment after around 5,000 operating hours. Dismantling showed noticeable deposits on the compressor and turbine wheel as well as on the components carrying the exhaust gas. These were not critical for operation and can be reduced by more frequent washing.
Dismantling also revealed the good condition of both the bearing and shaft seal. After cleaning and reassembling, and with an adjustment of the washing regime, the turbochargers were reinstalled on the engine and are now being operated until the next regular inspection.
Aside from designing its new turbocharger to accommodate higher demands on single-stage turbochargers, KBB is also aiming to use advanced monitoring to add more value for engine users, says Dr Risse.
A turbocharger is usually monitored by measuring the charge pressure and exhaust temperature before the turbine. If the intake conditions are known, basic conclusions can be drawn with respect to performance or possible damage. A speed sensor can be used to check whether the turbocharger is being operated within the allowed speed range, on the basis of higher intake temperatures. Proper lubrication and cooling of components can be ensured if oil inlet conditions are known.
All these values are collected and processed in an engine's control system (ECU). As the ECU only provides limited evaluation and storage capacity for turbocharger monitoring, the theoretical possibilities for analysing turbochargers have not yet been exhausted, says Dr Risse. More complete performance analysis – allowing condition monitoring and condition-based maintenance (CBM) – would require additional information, including turbocharger type, thermodynamic specification, operating hours and a history of performance-relevant service work on individual components.
The ideal solution for advanced turbocharger monitoring, says Dr Risse, is direct integration into the engine's control system or local implementation on a CBM server. Since no standards are available, close co-operation with the engine manufacturers is a priority.
He notes: “Advanced turbocharger monitoring schemes will ensure good performance over longer operating periods, the ability to recognise and prevent damage, the extension of component lifetimes by knowing real-load profiles, and the ability to adjust maintenance intervals depending on condition.”
KBB is working to develop monitoring concepts. Dr Risse presented some of the company’s ideas at CIMAC World Congress 2019 in Vancouver. Along with a continued focus on its single-stage products, KBB’s exploration of advanced monitoring provides another example of how customer focus on lifecycle costs is driving development in the turbocharger sector.