Power and Propulsion

Operating in sequence

Malcolm Latarche
Malcolm Latarche

05 December 2018

Operating in sequence

Most large ships have engines with multiple turbochargers because of the need for sufficient air to support the combustion of the engine when running at full power. If slow steaming is being practised, not all of the turbochargers are needed and it has become common practice to shut off one of the turbochargers on a semi-permanent basis.

To make for a more flexible system, variable turbochargers have been developed and an alternative – known as sequential turbocharging – has been borrowed from other uses and developed for Diesel engine use.

Last year ABB Turbocharging launched its new FiTS2 system, which is an example of this technology. Standing for Flexible integrated Turbocharging System for two-stroke engines, this sequential turbocharging system offers significant fuel savings whilst maintaining flexibility in engine and vessel operation. Developed with key engine designers, FiTS2 is available to all two-stroke engines.

The typical engines for large tanker, bulkers, and feeder container vessels with conventional turbocharging systems run with two same-type turbochargers, which are always in operation in high loads, but as well in low and part engine loads. To optimise engine efficiency via improved turbocharging in low and part load, the engine with FiTS2 runs in lower loads with only one turbocharger in operation, whereas at higher loads (typically above 50 to 60% engine load) two turbochargers are operating simultaneously. The same principle is applied for very large engines – with FiTS2, they will run with two turbochargers in lower loads and with all three turbochargers for higher load operation.

Large two-stroke engines are also often fitted with auxiliary blowers but in many cases FiTS2 will be able to support slow steaming without these running. Auxiliary blower switch-on can typically be reduced from around 35% engine load to 25%, thus saving energy.

For large container vessels, which might use three turbochargers, one unit, can be adapted to shut off at lower power demands, typically below 50% load. Engine settings are optimised as part of the FiTS2 design, and the system can be returned automatically and rapidly to its original full power capabilities. High load performance is therefore available combined with enhanced efficiency performance at lower loads.

FiTS2 benefits include a high potential of lower engine fuel consumption by up to 6 g/kWhr when operating at part or low load, but with flexibility of operation maintained up to full engine power.