Alternative fuels and two-stroke engine lubrication
In recent years the number of alternative fuels to oil that are being used, or may be used in the future, by ships has grown to include LNG, LPG, methanol, ethanol and ethane with hydrogen and ammonia being other recent newcomers. That all of these fuels could, with some modifications, be used in an engine that can also burn mineral oil fuels and biofuel variations is a testament to the versatility of the diesel engine and a good reason why its demise is not seen as being something that will happen anytime soon.
Operations with LNG in four-stroke dual-fuel engines are nothing particularly new but there is much less experience gained on dual-fuel two-stroke engines. While quite a number of these have now been fitted – almost 250 are in service at mid-May 2019 – many are only considered as gas-ready with no gas fuel system in place so are presently running as typical fuel oil burning engines.
The dual-fuel engines that can run on LNG are not the only two-stroke power units using alternative fuels. There are for example 10 ships in service that are using methanol as a fuel – nine of them with two-stroke engines and the Stena Germanica as the only four-stroke conversion so far. Methanol is a liquid fuel that is considered a ‘drop in’ replacement for oil fuels but which obviously has very different characteristics and lubrication needs.
All of the fuel types mentioned have little or no sulphur content and no other obvious components that would increase the acidity of the fuel so a very low BN lubricant would normally be needed. Another positive benefit is that they would also be very unlikely to contain any cat fines unless contaminated at some point during storage and delivery.
From the point of view of CO2 emissions, LNG performs best with ethane, propane and butane, each having one more carbon atom than its predecessor. The latter two are collectively referred to as LPG. As regards lubrication all have similar properties as LNG and can be considered as essentially the same.
Under most circumstances, if operating on one or more of the mentioned fuel types and even current ECA-compliant fuels, the lubricant should be either a 25 or 40BN product – although WinGD suggests that for LNG the BN could even be as low as 15. However, it should be borne in mind that the ship could also be running on other fuels at different times especially in the early days after the new global sulphur cap becomes effective.
By their very nature dual-fuel methanol engines are subject to an even wider range of sulphur conditions. As things stand, they could be burning 95% methanol, which has zero sulphur, along with either a low sulphur or high sulphur pilot fuel; they could be burning a 0.10% sulphur distillate fuel for ECA compliance; they could be burning up to a 3.5% maximum sulphur heavy fuel oil; or they could be burning a mix of 70% methanol along with either a high or low sulphur oil product.
Since none of the ships presently burning methanol has a scrubber fitted, they will be obliged to use another compliant alternative when not running on methanol, although it is entirely possible that if no compliant fuel is available then something else may have to be used instead. If that is the case, the cylinder lubricant will need to be changed for the duration of the voyage on non-compliant fuel and the again when reverting to compliant fuels.
It is entirely possible that the operators of some of the increasing number of ships now being fitted with scrubbers may in the future also decide that methanol is a good choice as a fuel. That would further complicate matters as to determining the best lubricant to use.
The range of potential fuel options for ships now able to use methanol already means that decisions are needed as to which level to limit the cylinder oil to the liners, and what the BN of the cylinder lubricant has to be.
Use of alternative fuels has so far been extremely limited and consequently kept under careful observation by the owners, engine makers and lubricant suppliers. Crews have therefore been well supported in managing lubrication changes and monitoring the effects of new fuels on the engine. If there is a wider take-up of alternative fuels, the crew involved may have less support and care should be taken to investigate requirements in conjunction with lubricant suppliers and manufacturers and whatever other sources of assistance are available.