A fine balance

Dual fuel is certainly not a new concept, it is already well established in the automotive industry, where OEMs are looking to reduce fuel consumption and offer a wider range of fuelling options to their customers.

This has resulted in a significant increase in the number of light-duty cars and vans and heavy-duty engines (including buses and trucks) with the ability to use both diesel and liquid petroleum gas (LPG) or gasoline and LPG. 

Defining dual fuel

In the marine and large engine industry ‘dual fuel’ options also exist. However, there are a number of different definitions of the term, and the potential for confusion is high.

One definition of this type of operation is where a spark ignited gas engine is designed with the flexibility to use a source of gaseous fuel or liquid fuel such as gasoline or alcohol.

Another definition is where a large marine diesel engine is designed with the flexibility to switch from using solely heavy fuel oil (HFO) to a combination of HFO and marine diesel oil (MDO). This option is very useful for vessels that need to operate both in open ‘non-controlled’ waters and within Emission Control Areas (ECAs) where they must meet more stringent emissions regulations.

A third definition is found in large marine diesel engines, which can operate entirely on liquid residual fuel (MDO or HFO). This fuel can also be used as a pilot fuel charge to ignite a further charge of gaseous fuel.

The pilot ignition can take place in a pre-combustion chamber from which the flame propagates before igniting the gas.  Or it can take place in the main combustion chamber where the pilot charge is injected and once it begins to burn the further charge of gaseous fuel is introduced for the main combustion cycle.

OEMs invest in dual fuel 

The Wärtsilä dual fuel marine engine is a four-stroke that runs on light fuel oil (LFO) or HFO, and can switch over from gas to LFO/HFO and vice versa smoothly during engine operation.

Wärtsilä says that the ability to switch to the most economical, or most readily available, fuel is of great value in a time of fluctuating fuel prices.

In their view, this technology offers real advantages including fuel and application flexibility, fuel economy over the entire engine and low exhaust gas emissions.

Last year, marine two-stroke engine manufacturer, MAN Diesel & Turbo (MAN), revealed its first commercial dual fuel engine.

The gas injection, dual fuel, low speed diesel engine can burn gas or fuel oil at any ratio, giving ship owners and operators the option to select fuel depending on relative price and availability, as well as environmental considerations.

The OEM sees a broad potential market for its dual fuel engine, extending from LNG and LPG carriers to other ocean-going vessel segments such as container ships as well as ships plying a fixed trade.

As fuel prices rise and exhaust emission limits continue to tighten dual fuel engines certainly seem to represent a highly efficient, flexible, propulsion plant solution.

OEM requirements

Marine OEMs offering dual (or even multi) fuel options have specific lubrication requirements.

In general terms the OEM’s prime concern is the protection of their engine and they have clear expectations of lubricant performance:

• Prevent ring sticking
• Control piston ring and liner wear
• Protect against corrosion and the deleterious effects of high sulphur fuels
• Limit the build-up of deposits on the piston crown and crown land

With these requirements in mind, in dual fuel applications, OEMs tend to recommend engine lubricants that are suitable for the most severe fuel in use.

For example, a four-stroke engine operating on MDO in an ECA and on HFO in non-controlled areas might be expected to operate on a 30BN or higher lubricant, depending on the fuel sulphur level.

Until now, two-stroke marine engines have generally operated using low or high sulphur HFO. However, changes to the global fuels market and tighter emissions legislation means there is increasing interest in fuelling these large engines with lower sulphur fuel such as MDO and gas using the third definition of dual fuel described previously.

Whichever fuels are selected, the demands on the engine are as high as ever. It is therefore essential for lubricants to provide good cleanliness, wear and corrosion control and deposit control on the piston crown and lands and behind the rings to prevent ring sticking.

In dual fuel applications OEMs tend to recommend engine lubricants that are suitable for the most severe fuel in use

Lubricant requirements

The lubrication of dual fuel four-stroke engines requires established fluids of known BN suitable for the most severe fuel used. However, the lubrication of two-stroke engines is a new environment requiring greater definition and consideration.

The nature of marine diesel cylinder lubricants (MDCL) is that they are injected onto the liner and consumed during use by combustion and through neutralisation with sulphuric acid. When gaseous fuel (with a pilot injection) is used however, the quantity of sulphur is dramatically reduced or eliminated, which means that the fundamental requirement of providing base to the oil is also removed.

This significantly impacts the balance of additives required to formulate lubricants optimised for two-stroke dual fuel engines. In these applications it is essential to carefully balance the lubrication requirements of the engine, and more specifically the piston and liner, with the types of fuel used. Ensuring that enough BN and ash is delivered to neutralise any acids produced while maintaining the cleanliness of the piston assembly requires different additive technology to be employed.

Future formulations

Certain unknowns such as: ‘What effect switching between MDO and HFO as the liquid pilot fuels might have?’ need further exploration. Understanding all the variables and delivering the desired performance will be key for success.

Infineum is investing in research, development and testing activities in order to design lubricants suitable for use in dual and multi fuel marine and large engines.

This activity is leading to new formulation approaches, which Infineum believes will help the industry to deliver these flexible, lower emissions engines to the market.

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