英国泽迪亚克实习报告题目及解答.doc

Contents
1) Sketch and describe a fuel injector for a modern marine diesel engine.
2) Detail what faults are likely to be experienced with the injector.
3) Explain the procedure for accurately settings the injector lifting pressure.
Answer for first question:
The fuel delivered the fuel pumps to the injectors. For the fuel to burn completely at the correct time it must be broken up into tiny droplets in a process as know as atomization. These tiny droplets should be penetrating for enough into the combustion space so that they mix with the oxygen. Fuel injectors achieve this by making use of a spring loaded needle valve. The fuel under pressure from the fuel pump is fed down the injector body to a chamber in the nozzle just above where the needle valve is held hard against it is seat by a strong spring. As the fuel pump plunger rises in the barrel, pressure builds up in the chamber, acting on the underside of needle. When this force overcomes the downward force exerted by the spring, the needle valve starts to open. The fuel now acts on the setting area of the valve and increase the lift.
As this happens, fuel flows into space under the needle and is forced through the small holes in the nozzle, where it emerges as an “atomized spray”. At the end of delivery, the pressure drops sharply and the spring closes the needle valve smartly.
In M/V HYUNDAI MECURY fuel injector is close type with mechanical operated closing needle by a spring loading which allows high pressure fuel to a nozzle which is provided with one or more atomizer holes. Injectors are located in the cylinder cover and three are fitted to produce effective combustion with an even heating effect. It consists of a valve head and valve housing. Fitted within the valve housing is a non-return valve and a needle and needle guide with a pressure spring and a nozzle. Fuel which leaks in consequence of the nozzle needle clearance is drained through leakage fuel pipe via connection to the leakage fuel pipe from the injection valve. Fuel oil which possibly leaks between nozzle body and nozzle holder rises and appears at the up-ears at the upper edge of the cylinder cover.
Working of fuel injector:
1. The electrical fuel oil primary pump circulates preheated oil through the fuel pump.
2. Fuel oil is led at high pressure to the three injection valves via injection control unit.
3. The fuel quantity required for injection flows through connection and bore to nozzle body.
4. The high fuel pressure lifts nozzle needle off it is seat against the force of compression spring, and finely sprayed fuel is injected in to the combustion chamber.
5. On completion of the fuel pump`s delivery stroke, the valve needle is pressed against its seat and the injection now stops.
Answer for second question:
Fuel injector is not working properly: if the fuel injector nozzle is not mounted correctly, the nozzle holes are clogged or dirt, broken spring.
Fuel injection pressure is abnormal: defective guide at the needle seat, as matter of the last importance is the fuel quality (replace fuel filter elements and drain off water and impurities).
Atomization is not good: if the fuel injector have defective nozzle causing improper atomization or a dribble after main injecting pulse, will lead to unburned fuel and black smoke. Confirm that fuel oil is evenly spread out as it is injected and that there is no abnormality due to clogged nozzles. Life of the nozzle is about 2000 hours, because the seat part of nozzle needle valve has been made to be of double cone shape, the needle valve should not be lapped, otherwise lapping of the valve will result in excessive valve life and will decrease erosion preventive effect of the seat part. Do not misjudge on life of the nozzle on account of fact that a low lever turning of the fuel injection tester, atomization naturally becomes coarse. If the needle valve is found to be defective by oil tightness test of valve seat or by visual inspection of needle valve, instead or repairing by lapping the needle valve and valve case, replace them with the new ones.
Fuel is not getting into cylinder: the needle had stuck in the spindle guide owing to accumulation of dirties, or insufficient cooling of fuel valves.
Valve oil tightness is not good: keeping the oil pressure lower than the designed injection initial pressure by use of the fuel injection tester, confirm that no test oil is dropping from the tip of nozzle. If however the nozzle tip is only stained with a trace of test oil, the needle valve can be continuously employed.
Answer for third question:
Pressure testing procedure, place the fuel injector in the test rig and secure it with the spring housing and nuts. Tighten the nuts until the top face of the pressure disc is flush with the top face of the spring housing. Mount the oil pipe between the pressure testing pump and the fuel injector. Remove air in the system and check the fuel jet. Slowly increase the working pressure until straight jets of oil are ejected from the nozzle holes. There is to be a continuous jet of oil through at least one of nozzle holes. Owing to the geometry of the internal part of the nozzle and because the height to which the spindle is lifted during pressure testing is lower than the height it is lifted during normal engine operation-the fuel oil will not necessary flow from all of the nozzle holes. Check the opening pressure. Increase the oil pressure until oil is admitted through the nozzle holes. Check the opening pressure on the pressure gauge. If the opening pressure is higher, replace the spring on the thrust spindle, and replace the complete thrust spindle; if the opening pressure is lower, the spring has LOST its tightness, replace the spring, or add a special thin disc. If a spring or a disc has been changed, the pressure testing procedure of the fuel valve must be replaced.