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Modern hydraulically operated diesel fuel injectors are fitted in large two stroke diesel marine engines. The main difference in these new type of fuel injectors used in the main engine fuel injection system is their cooling system. This is provided by the fuel oil; not as in normal cooled injectors by a separate pump and cooler system.
The fuel is usually heavy fuel oil (HFO) that is a byproduct of refining of crude oil; being tar-like and very viscous. Because of this fuel must be treated on board to remove solids and water before being supplied to the fuel injectors where it is injected as an atomized mist to the combustion chamber.
As we have seen fuel injectors can be classed as cooled or uncooled and in the next few sections we will examine one type of uncooled fuel injector.
The first section gives an overview of the HFO treatment and the operating principles of the fuel injector.
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The heavy fuel oil is received on board ship and stored in the bunkers; however when abroad, the engineer must be careful when taking HFO bunkers as in the past these have had additions of used lube oil and oil from ships sludge tank.
A sample from the bunker barge tanks should therefore be taken and analyzed to check for these, before accepting the bunkers.
Once the bunkers are on board steam is supplied to the heating coils in the tanks to maintain the oil at around 100°F, as the water and solids in the HFO settle in the bottom of the bunker tanks.
In order to prevent damage to the fuel injector needle valve and seat it is imperative that small particles are removed by HFO on-board treatment plant that incorporates equipment to process the oil, rendering it suitable for use in the main engine.
Once the HFO in the bunker has become viscous, it is then pumped from here through several sets of filters, heaters and settling/header tanks becoming cleaner and more viscous. It is now passed through the centrifuges; a purifier and clarifier where water and solids are removed. The HFO is now ready for supply to the final heater and viscometer and into the fuel pump.
Basically, the principle of the uncooled fuel injector is based on the recirculation of the HFO.The fuel oil is supplied to the common rail where high pressure fuel pump(s) supply it to the injectors. When the fuel is not required to be injected, it is recirculated through a chamber in the injector body. When it is required to be injected, the recirculation ceases and the fuel is injected into the combustion area.
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Fuel Injectors Cooled by Recirculation
These fuel injectors are the preferred type for use in the modern marine diesel engine, that along with common rail fuel supply and engine management systems add to the engines efficiency.
This modern injector does not have any of the normal cooling water passages to keep it cool. Instead it relies on the recirculation of the HFO to maintain injector temperature, along with the cylinder head cooling water that passes close to the injector pocket..
The cylinder head design is modified to accommodate numerous cooling water drillings around the fuel injector’s pocket and this assists in the cooling of the injector body.
So these two methods combine to keep the injector cool without the use of an external fuel valve cooling system. The resulting reduction in power through not using this system improves the engine efficiency.
The injector itself consists of a steel body and nozzle, the nozzle containing the needle valve, its seat and atomizer holes; the body containing the spring and actuating rod.
Instead of the usual drillings in the injector body to direct the HFO to the nozzle chamber, there are two chambers; an upper and lower one. The upper chamber is continuously supplied with HFO when the fuel pump cam is at the bottom of its stroke, in this stage the fuel in this chamber is constantly recirculated.
When the cam in the fuel pumps rotates to the top, the increase in pressure from the fuel pump operates a relief valve, supplying high pressure fuel from the upper chamber to the lower chamber. The high pressure fuel acts on the shoulder of the needle valve lifting it and injecting atomized fuel through the nozzle holes into the combustion chamber.
A simplified flow diagram of a typical recirculating oil cooled fuel injector is shown below.