The maritime industry is rapidly evolving as shipping companies look for cleaner, safer and more efficient ways to operate their ships. Traditional marine fuels such as heavy fuel oil (HFO), marine diesel (MDO) and marine gas oil (MGO) are increasingly being replaced by alternative fuels such as LNG, methanol, ammonia and hydrogen. This transition has made secondary fuels an important concept in modern maritime engineering.
Simply put, secondary fuel is an additional fuel used in combination with the primary fuel. Depending on the engine design, ignition, combustion, fuel flexibility, and standby operation are facilitated.
Many dual-fuel marine engines use a small amount of liquid fuel as pilot fuel to ignite a gaseous or alternative primary fuel.
Secondary fuel is a fuel used together with the main fuel in an engine or combustion system.
For example, an LNG dual-fuel engine may use:
LNG = Primary Fuel
Diesel = Pilot/Secondary Fuel
The LNG provides most of the energy, while a small quantity of diesel is injected to initiate combustion.
It is important to understand that secondary fuel does not always mean backup fuel. In some marine engines, it has a direct and essential role in the combustion process.
The basic working principle is straightforward.
First, the primary fuel and air are supplied to the engine. A small quantity of pilot fuel is then injected into the cylinder. The pilot fuel ignites and provides the ignition source for the primary fuel.
The simplified process is:
Primary Fuel + Air → Pilot Fuel Injection → Ignition → Combustion → Power
This technology is particularly useful for alternative fuels that are difficult to ignite under normal compression-ignition conditions.
Dual-fuel engines can operate using two different fuels. This gives ships greater flexibility compared with conventional single-fuel engines.
Common combinations include:
For example, in an LNG dual-fuel engine, LNG can provide the majority of the energy while a small amount of liquid fuel acts as the ignition source.
Some dual-fuel engines can also operate completely on liquid fuel when the primary gaseous fuel is unavailable. This provides an important level of operational flexibility for ships.
LNG is one of the most established alternative fuels used in modern shipping. LNG is mainly methane and is stored in liquid form at approximately -162°C.
Marine LNG engines use specialised fuel storage, conditioning and supply systems.
Depending on the engine technology, a small quantity of liquid pilot fuel may be used to initiate combustion.
The basic concept is:
LNG → Main Energy Source
Pilot Fuel → Ignition Source
However, LNG does not automatically mean zero-emission shipping. Methane slip and the overall lifecycle of the fuel remain important environmental considerations.
Ammonia is receiving significant attention as a potential low-carbon or zero-carbon marine fuel because it contains no carbon.
However, ammonia has challenging combustion characteristics and is difficult to ignite. Therefore, pilot fuel can be used in certain ammonia dual-fuel engine designs.
Marine engineers working with ammonia systems must also understand additional hazards such as:
The IMO has identified pilot-fuel injection and combustion control as important considerations in ammonia-fuelled engine concepts.
Methanol is another alternative marine fuel being adopted by the shipping industry.
Depending on the engine design, methanol can be used with a pilot fuel to initiate combustion.
However, switching from conventional fuel to methanol requires changes to the complete fuel system, including fuel storage, piping, injection, control and safety arrangements.
Therefore, secondary fuel is only one part of a much larger alternative-fuel system.
Marine engineers play a critical role in operating and maintaining secondary-fuel systems.
They need to understand:
The engineer must also understand the specific operating instructions provided by the engine manufacturer.
Secondary-fuel technology provides several benefits:
Improved reliability: The engine may have an alternative operating mode.
Fuel flexibility: The ship can use different fuels depending on conditions.
Alternative-fuel compatibility: Pilot fuel can help ignite fuels with difficult combustion characteristics.
Operational security: A backup operating mode can be valuable during fuel-system failures.
Future readiness: Dual-fuel technology supports the maritime industry’s transition towards alternative fuels.
These systems also add complexity to marine engines.
Additional fuel tanks, pumps, valves, injectors, sensors and control systems may be required. Engineers also need specialized training to safely operate alternative fuel systems.
Fuel leaks, incorrect fuel pressure, poor fuel quality, or improper shifting procedures can affect engine performance and safety. For this reason, proper maintenance and strict adherence to the manufacturer’s instructions are essential.
Secondary fuel is an additional fuel used with the primary fuel. It may support ignition, combustion or provide an alternative operating mode.
Pilot fuel is a small quantity of fuel injected mainly to initiate combustion of the primary fuel.
Not always. Pilot fuel is specifically associated with ignition, while secondary fuel is a broader term that can include pilot or backup fuel.
Diesel has good ignition characteristics, making it suitable for initiating combustion in many dual-fuel engines.
Yes. Many dual-fuel LNG engines use a liquid pilot fuel, although the exact combustion technology varies between engines.
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