Onboard fuel testing is carried out during the bunkering operation (fuel loading) so that the ship’s crew can obtain immediate information about the quality of the fuel before it is used in the engine system. This practice provides several important advantages.

1. Rapid Verification of Fuel Quality

Onboard testing allows the crew to quickly check whether the fuel supplied complies with the basic specifications stated on the Bunker Delivery Note (BDN). Important parameters such as density, viscosity, and water content can be verified immediately.

2. Improved Operational Safety

Early testing helps in detecting major contaminants, such as excessive water content or catalytic fines (cat fines). Identifying these contaminants at an early stage prevents serious damage to engines and fuel injection equipment, as well as blockages in the fuel system.

3. Detection of Fuel Incompatibility

Onboard testing can also indicate whether different batches of fuel are incompatible. If incompatible fuels are mixed in storage tanks, they may react with each other and produce sludge, which can lead to fuel purification problems, filter clogging, and poor engine performance.

A representative fuel sample is usually obtained using the continuous drip sampling method.

Sampling Location

The sample is taken at the ship’s bunker manifold, which is the point where custody of the fuel is transferred from the supplier to the ship.

Sampling Process

A sampling flange fitted with a needle valve is installed at the manifold. During bunkering, the valve allows a small and continuous stream of fuel to drip into a sample collection container.

This process continues throughout the entire bunkering operation, ensuring that the collected sample represents the overall quality of the entire fuel batch, rather than only the fuel supplied at the beginning or end of the transfer.

The formation of various exhaust emissions depends on the chemical composition of the fuel and the combustion conditions inside the engine cylinder.

(i) Oxides of Nitrogen (NOx)

Oxides of nitrogen (NOx) are mainly formed when nitrogen and oxygen present in the intake air react at very high temperatures and pressures inside the cylinder.

When the combustion temperature rises above approximately 1300°C, nitrogen and oxygen molecules dissociate and combine to form nitric oxide (NO) and nitrogen dioxide (NO₂). These gases together are referred to as NOx.

Thus, high combustion temperatures and pressures promote the formation of NOx emissions.

(ii) Carbon Monoxide (CO)

Carbon monoxide (CO) is produced as a result of incomplete combustion of carbon in the fuel.

Under ideal conditions, carbon in the fuel should completely oxidize to form carbon dioxide (CO₂). However, if there is insufficient oxygen, incomplete mixing of fuel and air, or poor combustion conditions, carbon is only partially oxidized and forms carbon monoxide (CO) instead.

Incomplete combustion may occur due to:

• Poor fuel atomization
• Low combustion temperatures
• Incorrect air–fuel ratio

(iii) Oxides of Sulphur (SOx)

Oxides of sulphur (SOx) are formed when sulphur present in the fuel reacts with oxygen during combustion.

Sulphur is naturally present in many fuel oils. During combustion, it combines with oxygen to form gases such as sulphur dioxide (SO₂) and sulphur trioxide (SO₃).

Since sulphur is a fuel-bound element, the amount of SOx produced is directly proportional to the sulphur content of the fuel oil. Therefore, fuels with higher sulphur content produce greater SOx emissions.

Sulphurous combustion products can lead to cold corrosion, where sulphuric acid forms and attacks engine components such as cylinder liners. Several measures can be taken to reduce these harmful effects.

1. Use of High TBN Cylinder Lubricating Oil

Cylinder oils with a high Total Base Number (TBN) are used to neutralize acidic products of combustion, particularly sulphuric acid formed in the cylinder.

2. Control of Engine Temperatures

Maintaining high jacket water temperatures helps keep the cylinder liner surface temperature above the acid dew point. This prevents the condensation of sulphuric acid on the liner surface, thereby reducing corrosion.

3. Use of Low-Sulphur Fuel

Using Low Sulphur Fuel Oil (LSFO) or Ultra-Low Sulphur Fuel Oil (ULSFO) reduces the initial sulphur content entering the engine, thereby lowering the formation of sulphur oxides during combustion.

4. Exhaust Gas Cleaning Systems

Exhaust gas scrubbers can be installed to remove SOx from exhaust gases before they are discharged into the atmosphere, thereby reducing both environmental pollution and sulphur-related corrosion effects within the system.