There is still much to be discovered about engine turbocharging as researchers extend industry understanding about their operation
Modern diesel engines use a range of technologies to minimise fuel oil consumption at lower loads but older vessels often have engines optimised for higher ship speeds. Lowering these speeds to reduce operating costs has been a common practice but other actions are needed to optimise engine performance in these conditions. One method to achieve this is through turbocharger cut-out and many manufacturers have developed systems specifically aimed at operators using slow-steaming to control costs.
But a paper published in the Journal of Mechanical Science and Technology suggests that turbocharger cut-out can have the effect of producing higher torsional exciting forces than are typical in engines designed specifically for lower load ranges. Concerns are based on the possibility of engine structural problems being caused by peak pressures higher than would usually occur in the cylinders, when the engine is operated at low load with turbocharger cut-out.
Higher peak pressures are also known to produce an increase in torsional forces on the engine body, risking potentially higher vibration in propulsion shafting systems at low loads compared to engines not using turbocharger cut-out.
Problems potentially caused by adoption of turbocharger cut-out have not been widely reported but research highlights the need to take all factors into consideration when changing the operating profiles of marine propulsion systems.
In ship enginerooms, turbochargers can be one of the major noise sources and emissions must be kept to necessary levels to meet exposure requirements. Studies into this have been carried out at Harbin Engineering University, in China, which has published a paper based on the performance of a radial-flow inlet silencer. Following initial estimation of attenuation levels by empirical formulae, the full acoustic performance was predicted using finite element analysis methods, followed by a series of trials.
Detailed results were presented at the 2017 International Congress on Sound and Vibration, held in London. These supported the principle that empirical formulae and numerical simulation could be used to predict the attenuation effects of an inlet silencer design. The study also concluded that a novel design of silencer based on dissipative principles, with a micro-perforated tube installed in the core and backed by a reacting cavity, would provide a highly compact solution.
There are many other instances of Chinese university research projects. One recent example is the study of two-stroke marine diesel engine operation with turbocharger cut-out, conducted jointly between contributors from Shanghai Jiao Tong University, China and Strathclyde University in the UK. A further study of a hybrid modular design exhaust, and its application on a turbocharged eight-cylinder marine diesel engine, has been carried out at Shanghai Jiao Tong University.
Another university providing cutting-edge technology to the industry is the University of Huddersfield, in the UK, which is working with BorgWarner Turbo Systems. This collaboration – like many others – is based on automotive-scale products but covers aspects such as higher boost pressures, advanced materials development, component reliability and wear, which are all equally applicable to larger scale turbochargers.
Another European collaboration has been established between the German turbocharger manufacturer KBB and the University of Dresden, having worked together on a number of aspects of turbocharger research and development. The Technical University of Delft in the Netherlands also promotes academic studies and research.
To assist with development, turbocharger manufacturers also work with companies such as Austrian specialist AVL, of Graz. AVL is currently working directly with manufacturers on new developments and also offers analytical software such as its AVL Boost, AVL Excite and AVL Fire products. These enable manufacturers to carry out advanced compressor and turbine component design and turbocharger matching to achieve the best overall engine and turbocharger performance. This integrated approach enables complex interactions between system components to be evaluated.
In the UK, Ricardo provides software and technical assistance and consultancy for engine and turbocharger manufacturers, including its well-known Wave simulation software. This enables performance and acoustic simulations to be carried out based on virtually any intake, combustion and exhaust system configuration.
Electric boosting to meet IMO Tier III engine needs
The impending introduction of IMO Tier III emissions limits has driven continued development in both engine and turbocharger designs and MAN Diesel and Turbo has selected two routes to meet the NOx levels required. For two-stroke engines, these involve high-pressure selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) systems.
The implication for turbocharging is the need to recycle cooled exhaust gas into the scavenge air receiver. This requires boosting due to the pressure difference between the exhaust and the scavenge air receiver. To meet the requirements, MAN has developed an Electrical Turbo Blower, (ETB), driven by a high-speed electric motor.
The compressor is optimised for efficiency at low pressure ratios and its wide compressor map allows operation across a wide range of volume flows. Its design can tolerate high levels of pressure pulsation, loss of stability or surging. Speed and temperature sensors are included in the unit and speed is controlled based on EGR rate demand.
Two frame sizes are currently available; the ETB30 and ETB40 with maximum powers of 200 kW and 300 kW respectively. The ETB30 is suitable for engine powers from 5 MW to 23 MW and the ETB40 suits engines starting at 15 MW rating and rising to the largest MAN 85 MW unit.
Wärtsilä and ABB work together to improve performance analyses
In agreement with turbocharger manufacturer ABB, Wärtsilä is taking steps to expand its analytical approach to performance. Under the agreement, specific service data will be shared to assist analysis of potential performance improvements and to develop upgrades. These advanced solutions are intended to help customers to maintain a competitive edge in their markets through improving the lifetime performance of marine and power packages, including the turbochargers.
“Being able to look at the whole installation increases its availability and efficiency, optimising service speed and agility,” said Wärtsilä Services vice president, four-stroke engine services, Tomas Hakala. “Customers can reduce their operational risk, ensure optimal tuning and save time by being able to overhaul their ABB turbocharger at the same time as the engine.”
As a result of this cooperative work Wärtsilä and ABB have already started to offer upgrade packages, delivering improvements to both performance and fuel efficiency, in addition to reduced operational downtime and costs.