ECA impact on auxiliary engines

Ships not equipped with exhaust gas scrubber equipment – which means the vast majority of those not operating in emission control areas (ECAs) for the vast majority of their time – are having to use fuel with sulphur content of below 0.1%, which means, in most cases, low-viscosity distillate such as MGO or MDO, in both main and auxiliary engines.

Modern engines and systems are designed primarily to burn the relatively inexpensive HFO, with a much higher sulphur content. Switching to distillate as demanded by MARPOL Annex VI means that in order to ensure efficient and safe operation, there are certain operational issues to be considered.

First, the correct system oil must be used. Operators must refer to specific OEM requirements, but in general a lube oil of BN15 should be used if operating permanently on distillate, or for intermittent operation on both distillate and HFO then BN30-55 should be considered.

The lower viscosity of distillate fuels can impact on the hydrodynamic lubrication of fuel pumps, causing wear and scuffing, and can cause leakage between plunger and barrel, causing power loss and potential starting difficulties. Because of the difference in viscosity, it may be necessary to install a fuel cooler. Distillates also have different lubricity characteristics from HFO, which again can result in wear to fuel pumps. According to guidance published by CIMAC, operators have to guard against automotive specification distillate being supplied to marine users rather than the correct ISO 8217 specification marine distillate fuel. A further issue is that low-viscosity distillate fuel can leak into the system oil via the pump, degrading the lubricant properties of the lube oil, while incompatibility between some lube oils and distillate fuel can result in deposits being formed in the fuel pump, causing the pump to stick.

Modern four-stroke engines are designed to operate on a wide range of fuels, but if using mainly low sulphur distillate then operators will observe fewer combustion deposits on inlet valves. This is mainly beneficial, but the deposits can act as a lubricant to the valve seats, so some engine types have a valve seat lubrication system which injects a small amount of lube oil into the charge air to encourage production of a very slight layer of ash and coke on the valve seat to prevent wear.

If the engine is equipped with water cooled injection nozzles, the reduced viscosity and lower temperature of the distillate fuel could mean that the injectors are over-cooled resulting in deposits on the nozzles, and, if the nozzle temperature falls below the dew point of the acids in the combustion gas, the injectors may corrode. Therefore a means has to be provided of switching off the cooling water when operating on distillate for prolonged periods.

When changing over from residual to distillate fuel on four-stroke auxiliary engines, it will be necessary to observe similar precautions as on two-stroke engines to avoid problems resulting from temperature gradients, viscosity issues and incompatibility between fuel types.