Whatever its type or origins, fuel is intended to end its journey in the combustion chamber of an engine or in the burner unit of a boiler. It might be imagined that after its journey from the service tank and through the conditioning system, the fuel will no longer be subject to checks, treatment or any other process. This is far from the truth and more to the point, the treatment process it has gone through may not have been sufficient to overcome all potential problems.
Not all of the fuel from the conditioning system makes it immediately to the engine as operational circumstances may mean that conditioned fuel is not required. Any fuel will then be diverted back to the service tank for when it is required.
Where vessels make use of different oil fuel types for the main engine, auxiliaries and boilers, there may be multiple conditioning systems since at time two or more fuels may be required for different purposes. In some more modern vessels, both main and auxiliaries may run on the same fuel type and here there may be just once conditioning module that can operate in various ways. These systems can easily handle the switch from heavy to distillate fuels when entering ECAs.
Dual-fuel engines that run on oil or gas obviously do not share a common conditioning system because the two fuel types need very different processes before reaching the engine. Furthermore, the engine itself has different delivery systems for oil and gas fuels. It is for this reason that most dual-fuel engines including the two-stroke types such MAN Energy Solutions’ ME-GI and WinGD’s XDF types, are able to switch seamlessly between fuel types.
It should be mention at this point that in most fuel systems there is a buffer tank, that sits between the first set of filters after the service tank or mixing module and the fuel conditioning module. This tank is also fed with unused fuel returned from the engine and which has therefore already passed through the conditioning module. The feed to the buffer tank from the first set of filters is where any flow meters used to measure fuel consumption are located.
Fuel returned from the engine to the buffer tank does not pass through the flow meter as this would give a higher and false figure for consumption. However, the return fuel line from the engine does have a valve to return fuel back to the service tank although under most usual circumstances the valve would remain closed.
Aside from its primary role of merely measuring fuel consumption over time, the flow meters may also be connected to performance monitoring systems which combine the data with inputs from other systems and devices such as shaft power, ship speed, draught, trim, weather and more to give a full picture of how efficiently the ship is being operated. Fuel use is obviously at the heart of efficiency calculations and in performance systems where immediate use is made of the data constant monitoring is essential.
In fuel systems in older ships, the circulation pumps that move fuel from service tanks to fuel conditioning systems tend to operate at maximum most of the time. When the engine is running at low load – perhaps because of a slow steaming policy – then the fuel being delivered is much in excess of what is used so some is recirculated. In more modern ships, the feed rate might be reduced by variable speed pumps or the lower demands of the engine can be accommodated by keeping fuel passing through separators for longer periods. This also has the additional benefit of removing more contaminants – especially cat fines – from the fuel. A means of synchronising demand and separator dwell time is therefore required.
Returning to the fuel flow from the conditioning module to the engine, there is a point at which the fuel system of the engine takes over from the ship’s fuel systems. The engine fuel system comprises a number of important components most notably the high-pressure pump, the common rail and injectors. The fuel lines of the engine are necessarily of finer bore than the rest of the fuel system in order to achieve the higher pressure necessary for atomisation of the fuel as it is delivered to the combustion chamber.
There will be a fine 10 micron filter installed either before the engine high-pressure pump or at the individual pumps for fuel injectors in older systems. This is to protect pump plungers and barrels from any debris remaining in the fuel. High-pressure diesel injection pumps are very sensitive to minute particles of debris which can cause micro-seizures and finally total failure of the pump plunger and barrel.
Today high-pressure fuel lines must be of the double wall type so as to reduce the risks of leaks and the dangerous consequences such as fires caused by leaking fuel being exposed to hot surfaces. The very high pressure needed for injection – up to 1,000bar – also makes any leak extremely dangerous to the human body which could easily be penetrated by a stream of high pressure, high temperature fuel.
The high-pressure pump at the beginning of the engine fuel supply system may be driven mechanically by the engine or it could be electrically operated. On some engines both methods may be employed. The pump moves the fuel into the or to the injector depending on the age of the engine. Early engines without a common rail system will have separate fuel lines and jerk pump for each cylinder. However, most modern engines have a common rail system where the fuel for all cylinders is accumulated in a single rail and kept at high pressure until the injector opens.
Injection can either be controlled mechanically by a camshaft or electronically. In the first case, the camshaft actuates the valve for the appropriate injector and fuel will be injected. At slow speeds, a camshaft-controlled engine will inject more fuel and consequently cause the engine to smoke.
On an electronically controlled engine, the amount of fuel injected can be more strictly controlled and suited to the load of the engine. This overcomes the issue of smoke which is caused by unburnt or partially burnt fuel in the exhaust and improves efficiency. Importantly, the electronic control can also determine when the fuel is injected. On a mechanical engine, the timing is always controlled by mechanical contact but with an electronic engine injection can be made early or late as required. Adjusting timing in this way can help reduce NOx emissions.
Another benefit of electronic engine timing is that it is much easier to shut down one cylinder if felt necessary. This might be because of damage to rings, liners or to the piston itself. This can also be done in a mechanical engine but less easily.
The more precise engineering of components in the engine fuel system, is the main reason why fuel needs to be conditioned properly before it reaches the engine. Any temperature changes needed when switching between HFO and distillate should be kept to a gradient of 2°C per minute so as not to induce thermal shock to any of the components. Any cat fines that reach as far as the engine’s fuel system have the ability to damage the components in the engine fuel line and beyond extended as far as the cylinder liner and pistons.
Even without any cat fines in the fuel, incorrect viscosity or temperature caused by over or under heating. If the viscosity is too high, as would be caused by under heating, the resulting extra pressure could cause mechanical damage to pumps, fuel lines and injectors. In addition, there will likely be increased wear in cylinder liners and wrings and damage to valve seats.
Only when the fuel has passed through the injectors into the combustion chamber of the engines can the final test take place. If for any reason the fuel has been contaminated by chemicals or has unusual combustion characteristics and this has not been discovered beforehand – the fuel conditioning process would probably not detect or remedy this – the result could be a major engine breakdown.
Assuming any mechanical damage was minimal and repairable, the engine can be restarted and used but only after all of the affected fuel has been moved to a suitable tank for storage. As a precaution it would normally be considered appropriate to clean any components in the fuel line so as to ensure no recurrence when the engine restarts.