Turbochargers must be able to withstand frequent load fluctuations, writes GasKraft Engineering owner Prof Dr Hinrich Mohr
Working either in a harbour or offshore, tugs have very specific operating profiles. The majority of their working time is spent waiting, idling or in low-load operation. Typically, less than 10% of their operations are at high power. In most cases, the power ramp up or power down has to occur quickly in a short time, requiring the installed propulsion system to be very responsive.
In most cases, tugs are equipped two or four prime movers, either large high-speed or small medium-speed diesel engines. Pure gas or dual-fuel engines are seldom used in tugs, since the required load ramping times are challenging for gas operations. Additionally, the space constraints onboard a tug make it more difficult to accommodate LNG fuel storage.
When it comes to the specific operating profiles of tugs, turbochargers are essential system components. Turbochargers have to react quickly to provide the proper amount of air for the engine combustion. This is especially critical in the ramp-up phase when a lack of air can lead to a hot, incomplete combustion, generating a great deal of black smoke. For these reasons, the inertia of the turbochargers should be as small as possible, combined with a good efficiency over the entire load range. This can be achieved best by a turbocharger designed with radial turbines.
“Turbochargers have to react quickly to provide the proper amount of air for the engine combustion”
Additionally, the turbocharger components, especially the aluminium-alloy compressor wheel, must be designed to withstand the frequent load fluctuations of a typical tug operation.
The development of these and other turbochargers is the result of extensive simulation work performed as the basis for the final design. The simulation includes detailed 3D computational fluid dynamic (CFD) and finite element (FE) studies. Other important success factors are today’s available materials and machining capabilities.
Once newly designed and produced, the turbocharger will be put on a test rig at the manufacturer, driven by a combustion chamber providing the required exhaust gas streams. After completing these tests, the turbocharger is mounted on a test engine to check its performance under near real-world conditions.
There are a growing number of new tugs being built with hybrid propulsion systems. This adds a battery and e-motors into the propulsion package. This technology allows several improvements, such as a pure electric operation during waiting/idling times, a load pick-up support during ramp-up phases, a boosting at maximum load to increase the bollard pull and an optimised engine operation during low load phases by charging the battery.
As a result of this operating regime, the combustion engines are faced with more frequent starts and stops – and the turbochargers as well. These frequent transient operating conditions, extended by more starts and stops, could impact the lifetime of the compressor wheel.
Among the benefits are a reduction of long idling times and low load operation, which should result in reduced fouling of the turbine area.
Another aspect to be considered is the installation of exhaust aftertreatment equipment. This can occur in the newbuilding stage or as a retrofit of an existing tug. In most cases, this is related to the installation of a selective catalytic reduction (SCR) catalyst to reduce NOx emissions.
The catalyst itself will generate an increased back pressure. Additionally, the installation will cause more bends in the exhaust system – especially in the case of a retrofit solution – further impacting the back pressure. The Increased exhaust back pressure might affect engine operation in general and especially the performance of the turbocharger. Based on experience, it is highly recommended to evaluate the effect of an exhaust aftertreatment system on engine and turbocharger performance before installing it.