Turbocharger systems and strategies are being used to both reduce the installed power on ships and to overcome the acceleration challenges that reduced power requirements can bring
As the energy efficiency challenges on shipping escalate, more sophisticated measures for reducing emissions are coming into play. The turbocharger has a role on both improving engine efficiency and in recovering energy potentially wasted through heat.
Mitsui Engineering & Shipbuilding first developed its turbo hydraulic system (THS) to recover waste heat from turbochargers in 2008. The first engine with THS was delivered in 2014 and the company has since delivered the system for 19 sets of engines.
While many waste heat recovery systems aim to return recovered heat energy in the form of electrical power generation, the idea behind THS is to reduce fuel consumption by assisting engine rotation through the use of hydraulic technology. The main circuit consists of hydraulic pumps connected to the shaft end of the turbocharger by a reduction gear. The high-pressure oil pressurised by the pumps is delivered to a hydraulic motor directly connected to the engine crank shaft. Having driven the motor, the hydraulic oil is then returned to the pumps to be pressurised again.
According to tests on Mitsui’s test engine and at shop trial on commercial engines, THS delivers a fuel oil consumption cut of up to 3% without increasing NOx emissions.
“Tests showed a 2-3 g/kW improvement of fuel oil consumption for engine loads from 50-100%, while NOx emissions are nearly unchanged”
But the world has moved on since Mitsui first launched THS. Electronically-controlled engines, using hydraulic power to actuate valves and the injection system, have become more popular. A new version of the system, THS2, aims to recover heat to pressurise hydraulic oil for these functions too. The company believes that the updated system can recover twice the hydraulic power needed for engine valve actuation and fuel injection, meaning it can continue to assist engine rotation by using the engine-driven pumps as hydraulic motors.
Mitsui has tested the new system using a MAN Energy Solutions TCA55 turbocharger on a S60ME-C engine. Tests on a turbocharger test bed and on Mitsui’s 4S50ME-T test engine showed a 2-3 g/kW improvement of fuel oil consumption for engine loads from 50-100%, while NOx emissions are nearly unchanged.
Unfortunately, the goalposts have moved again for Mitsui and its turbo hydraulic system. Since IMO’s NOx Tier III regulation entered force in 2016, those building new ships have had to introduce counter-measures to reduce NOx. One of these measures, high-pressure exhaust gas recirculation (EGR), poses a challenge to waste heat recovery by reducing the exhaust gas energy available for the turbocharger. So THS2 offers reduced recovery when EGR is used to reach Tier III limits.
But the system can be used with another function, delivered on MAN’s two-stroke engines, whereby EGR can be run to provide an efficiency boost while an engine is required to meet Tier II NOx limits. Under EcoEGR mode, introduced by MAN last year, the engine tuning is adapted towards greater fuel efficiency, even though this would push it beyond Tier II limits without the intervention of EGR. The EGR system then enables the engine to stay within Tier II.
Although THS2 cannot offer full energy recovery benefits when an EGR is being used to meet Tier III, it can provide full assistance when EcoEGR is used, resulting in an even bigger fuel saving.
The THS system can also offer benefits when a shipowner has installed an engine with a lower power output – to meet IMO’s Energy Efficiency Design Index (EEDI) requirements, for example. Reduced engine output causes the slow acceleration of the ships due to less torque being available during acceleration. If power output is too low, this can mean that ships pass too slowly through the ‘barred speed range’, the speeds between which they are advised to pass quickly because of the risk that torsional vibrations will damage the shaft line.
Using Mitsui’s turbocharger waste heat recovery system, the hydraulic pumps installed on the turbocharger and the hydraulic motor on the engine crank shaft can be used to reverse the flow of hydraulic power, helping to turn the turbocharger turbine. For large two-stroke marine engines, auxiliary blowers are normally installed to supply enough air for the engine on lower load, typically up to 35%. Using the turbocharger assist function, THS can perform this service and improve engine performance on lower load as well as acceleration capacity.
The technology that Mitsui needed to monitor and control the air-fuel ratio was developed during engine tests for the four-stroke, lean-burn MD36G gas engine, which requires strict air-fuel ratio control for stable combustion. A turbocharger assist test has been carried out at Mitsui’s test engine to confirm how much scavenging air pressure can be increased by the THS 2 turbocharger assist function; an improvement of about 0.1 bar has been confirmed from 15-35% engine load without affecting engine performance.
UK team develops turbocharger energy recovery
A consortium led by Bowman Power Group has developed an energy saving and emission reduction system that converts waste heat from the exhaust stream into electrical power.
The electric turbo compounding system will recover electricity from the heat of engine exhaust gases
The project, completed in 2018 after four years of study, indicated the potential of the technology – known as electric turbo compounding (ETC) – to achieve fuel savings of up to 7.8%. Besides the indicated fuel savings achieved with the ETC 1000 installation on the MTU Series 4000 engine, predicted optimum payback time on system investment was 2.3 years.
Bowman teamed up with Rolls-Royce Power Systems, Lloyd’s Register and University College London (UCL) to conduct the research. Innovation accelerator and funding agency Innovate UK contributed around £1M (US$1.3M).
The objective was to deliver a design and prototype hardware to demonstrate the application of ETC at full scale on marine diesel and gas engines. The core technology already has a record of reliable performance in land-based power generation.
Applied in-line with an MTU 4000 M93 engine’s turbocharger, Bowman’s ETC 1000 has a turbo generator which harnesses energy from the exhaust. Through the power electronics element of the system, the high frequency electricity is converted into grid-quality electricity at 50/60Hz, as suited to a shipboard net.
Rolls-Royce provided key information and simulation results for the study using an MTU Series 4000 M93 high-speed, four-stroke engine fitted with a twin ETC 1000 arrangement. The Marine Research Group at UCL devised a system model to explore the benefits, performance limits, secondary impacts and expected results. The software gave insights over complete operating profiles.
Bowman is moving forward with prototyping and testing. Seven different turbo generator and power electronics unit prototypes have been built and tested in different applications.
As a result of the work, validated with testing, the participants have delineated a road map and route to market for the system in marine applications. Bowman is in discussions with two large engine manufacturers and a major ferry operator to bring the solution to market.