Lesson Five

Text Fuel oil system

Fuel oil is thought to be one of the main factors having much to do with the operation and maintenance of an engine.

When selecting a fuel oil, there are more than ten factors to be considered, among those which affect combustion are the fuel oil’s viscosity, cetane number(十六烷值 ) and calorific value(热值 );

the sulphur( 硫分 ) in it and its carbon residues(残炭值 ) and ash(灰分 ) are factors that decide the formation of combustion products;

and the engine’s maintenance has a lot to do with its flash and setting points( 闪点和凝点 ), specific gravity(密度 ), viscosity, water content( 水分 ) and the mechanical impurities( 机械杂质 ) in it.

The work that fuel does in an engine not only depends upon the fuel oil itself, but also upon its working condition, which here means the fuel system.

•The fuel oil system for a diesel engine can be considered in two parts—the fuel supply and the fuel injection systems.

•Fuel supply deals with the provision of the fuel oil suitable for use by the injection system.

Fuel oil supply

A slow-speed two-stroke diesel is usually arranged to operate continuously on heavy fuel or residual oil(渣油 ) and have available a diesel oil supply for maneuvering conditions.

•The whole fuel system will then embody two almost entirely separate systems, one for diesel fuel and one for high viscosity fuel.

•The latter will embody heating arrangements and means to control the temperature of the fuel to ensure that it is adequately fluid when stored, transferred, cleaned and injected.

•In a typical fuel installation on board a ship for a modern diesel engine, separate filling connections for the diesel and heavy fuel tanks are provided.

•In use, the filling connections on deck are connected to shore hoses and the fuel oil bunkers( 燃料舱 ) are then distributed to various double bottom, deep, wing and side tanks(双层底舱，深舱，舷侧舱和边舱） as necessary.

•Steam heating coils( 蒸汽加热盘管 ) are installed in the various tanks to ensure that the temperature of the oil can be raised to reduce the viscosity to a level that will ensure easy pump-ability.

•Pipes used for transferring high viscosity fuel should be well lagged(加保温层 ) and long runs of piping(管道系统 ) should have steam-tracing pipes( 蒸汽伴热管 ) fitted.

•The fuel oil is drawn from the bunker tanks through a duplex strainer( 双联过滤器 ) by a transfer pump and thence to dirty oil storage or setting tanks.

•Leaving the fuel in the setting tanks whilst keeping it hot to reduce its viscosity allows the heavier impurities and any water present to settle to the bottom of the tank.

•The sediment( 沉淀物 ) and water are drawn off leaving the remainder ready for cleaning.

•It is then passed to a purification( 净化 ) system for treatment and from there to the clean heavy fuel tank, which known as the service tank.

•From the clean heavy fuel tank the oil gravitates through a heated buffer( 缓冲柜 ) 、 or balance tank to the booster pumps( 增压泵 )

•and is discharged through a fuel heater and filter to a viscosity regulator and from there to a duplex fine filter on the suction side of the main engine fuel pumps.

•Fuel excess to requirements is spilled back into the system via the buffer tank.

•The viscosity regulator controls the fuel oil temperature in order to provide oil at the correct viscosity for combustion.

•A pressure-regulating valve ensures a constant pressure at the fuel main. The valve can be opened to warm through the system by circulating with heated oil.

•Regular attention to the draining of heated settling(setting tank) and ready use tank(service tank) is advisable to eliminate the results of continuous condensation which can be very heavy under certain climatic conditions.

•Usually the settling and ready use tanks are lagged to reduce heating losses.

•The system will include various safety devices such as low level alarms and remote operation of tank outlet valves in the events of fire.

•The diesel oil supply system similarly uses transfer pumps to draw oil from the bunker tanks. The oil is then purified and stored in service tank.

•The diesel oil enters the system through a three way valve which permits the supply of only one type of oil to the system.

•The engine must be changed over to running on diesel oil for 30 minutes or more before a long period of standstill( 停止 ).

•Diesel oil required less heating, and thus the change from one fuel to another should take place gradually to allow temperatures in the system to stabilize.

Fuel Injection

The function of the fuel injection system is to provide the amount of fuel at the right moment and in a suitable condition for the combustion process.

•That means, the fuel injection system for diesel engines must be able to supply a metered( 定量 ) amount of fuel to each cylinder for each power stroke according to the load on the engine,

•and must include a timing mechanism to ensure that delivery of this fuel commences at the correct moment, and must have a means to atomize( 将…喷成雾状 ) the fuel.

•The type of injection system most commonly used in modern diesel engines is the jerk pump( 脉动泵 ) system.

•In the system, a separate injection pump is provided for each cylinder which operates once every cycle.

•The barrel and plunger ， together with the cam, are dimensioned to displace (discharge) fuel at the rate it is required in the combustion chamber.

•Ports in the barrel in combination with slots in the plunger, or separate mechanically operated spill valves( 回流阀 ), determine the amount of fuel delivered and the timing of its entry to the cylinder.

Plunger barrel

•Each pump is connected to the injector, or injectors, serving one cylinder.

•These injectors have spring loaded differential needle valves which are set to ensure that the fuel is raised to a sufficiently high pressure to cause atomization( 雾化 ) when they automatically open to admit it to the cylinder.

•Fuel Oil Quality

•Viscosity is not a measure of the fuel quality, but determines the complexity of the fuel heating and handling system, which should be considered when estimating installation economy.

•The standard engine fuel system is designed for fuels up to the viscosity class 55 mm2/s.

•When the density exceeds 0.991g/ml at 15℃, water, and to some extent solid matter, can no longer be removed with certainty by a centrifuge( 离心分离机 ).

•Centrifuging systems that are claimed to be able to clean fuel oils with densities up to 1.010g/ml at 15℃ are available.

•If such systems as the so called controlled discharge design are installed, fuels with densities up to 1.010 g/ml at 15℃ may be used.

•Note: Fuel oils having high density in combination with low viscosity may have low ignition quality.

•High sulphur content increases the risk for corrosion and wear, particularly at low loads, and may contribute to high-temperature deposit formation.

•The lubricating oil specification must be matched to this.

•High ash content causes abrasive( 研磨的 ) wear, and may cause high temperature corrosion and contributes to formation of deposits.

•The most harmful ash constituents （ 成 份 ） are vanadium( 钒 ) sodium( 钠 ) combinations( 化合物 ).

•High vanadium content causes hot corrosion on exhaust valves particularly in combination with high sodium content.

•The corrosion increases with increased temperatures (increased engine output).

•High Conradson carbon( 康拉特逊残碳值 ) may cause deposit formation in combustion chamber and exhaust system, particularly at low engine output.

•High content of asphalt ( 沥青 ) may contribute to deposit formation in combustion chamber and exhaust systems (at low loads).

•Asphalt may under certain circumstances, precipitate( 使 沉淀 ) from the fuel and block filters and/or cause deposits in the fuel system, precipitating asphalt may also cause excessive centrifuge sludge( 油泥 ).

•Heavy fuels may contain up to 1% water at delivery. Water may also originate from the installation in bunker tanks. To avoid difficulties in the engine fuel injection system water must be removed.

•Although low ignition quality produces long ignition delay, advancing the ignition timing makes things only worse; fuel is injected at a lower compression temperature and this will produce even longer ignition delay.

•Aluminum( 铝 ) and Silicon(硅 )

•Fuels may contain highly abrasive particles composed of aluminum and silicon oxides(氧化物 ) known as ‘catalytic fines( 催化粉末 )’ from certain refining(精练 ) processes.

•If not removed by efficient fuel treatment, wear down of high pressure fuel pumps, nozzles and cylinder liners in a few hours may expected.

•Reading Material

•A. Dual purpose plus(additive)

description

•Dual Purpose Plus is a concentrated combustion improver( 集中燃烧促进剂 ) for heavy fuel oils. It also has fuel conditioning properties.

•Product Properties

•The catalysts( 催化剂 ) in Dual Purpose Plus react with heavy fuel particles during combustion.

•The fuel ignition temperature is reduced, resulting in increased combustion efficiency with less carbon left to form smoke and soot( 烟灰 ).

•Engine and exhaust systems are kept cleaner with longer service life and less maintenance.

•Anti-polymerization( 反 聚 合作用 ) agents inhibit( 抑制 ) sludge formation, while dispersants( 分散剂 ) stabilize the fuel.

•This results in a cleaner fuel system and better fuel flow, giving improved fuel atomization and greater combustion efficiency.

•Sulphuric acid( 硫酸 ) corrosion caused by condensing exhaust gases may be seen in any of the cooler parts of the boiler or engine system.

•Typical problem areas are cylinder liners (cloverleaf( 苜 蓿 叶形的 ) corrosion), valve stems and funnel uptakes.

•Dual Purpose Plus catalytically( 催化 ) inhibits the formation of acid gases and so reduces the amount of acid present, thereby reducing acid corrosion.

•Directions for Use and Dose( 剂量 ) Rates

•Dual Purpose Plus is completely oil-soluble and should be added via a metering pump into the suction side of the booster pump.

•Alternatively, it can be added into the settling tank. If so, the dose rate should be increased by 10%.

•As a general guide, the average dosage( 剂量 , 配药 , 用量 ) should be 1:4000. Alterations can then be made according to operating experience and results obtained.

•B. Dieselite( 柴油助燃剂 ) description

•Dieselite is a multi-functional fuel treatment containing combustion catalysts and ash modifiers( 灰 分 调 节剂 ). It is intended for use in diesel engines and boilers burning residual fuels.

•Product Properties

•Carbon residue formation during combustion is inhibited by catalysts that lower the ignition temperature of heavy asphaltic particles.

•Combustion time is consequently increased, leading to a reduction of tarry( 焦油 ) deposits and carbonaceous( 炭 的 、 炭 质的 ) fire-scale.

•Ash modifiers combine with fuel combustion ash to raise the sinter( 烧结物 , 熔渣 ) and melting points of the ash above the engine or boiler normal operating temperatures.

•High temperature corrosion is minimized, reducing maintenance and extending service life.

•The majority of ash formed is ejected with the exhaust gases in a fine, solid state, and any ash remaining in the exhaust system is easily removed by light brushing.

•The conversion of fuel sulphur to potentially corrosive sulphur trioxide( 三氧化物 ) gas is also inhibited. Sulphur trioxide reacts with condensed steam in the exhaust trunking, funnel uptakes and other cooler zones to form sulphuric acid.

•Dieselite is a wide spectrum additive intended for continuous use.

•Directions for Use and Dose Rates

•For best results, Dieselite should be dosed automatically using a metering pump to dose into the fuel feed line as near to the injector or burner pump as possible.

•Where micro carbon residue (MCR)( 微量法残炭值 ) or vanadium/sodium analysis is available, use the tables given by the manufacturer. If no analysis is available, use an initial dose rate of 1:4000 and adjust according to results obtained.

•C. TEST OF FUEL INJECTION VALVE

•For injection test on fuel injection valve, perform with use of the fuel injection valve tester( 喷油嘴试验台 ).

•Adjustment of Fuel Injection Pressure

•Check fuel injection initial pressure by use of the fuel injection valve tester, and adjust this pressure to the designated value.

•For this adjustment, remove the cap nut first, and then adjust by means of the adjusting bolt.

•If components (needle Valve, spring, etc.) of fuel injection valve are replaced as well as the valve is disassembled, be sure to check the fuel injection pressure.

•If the injection nozzle is clogged, remove the carbon sticking to the nozzle part, using either carbon removing agent or cresol solution( 甲酚溶液 ); never use sandpaper( 砂纸 ) or the like.

•Remember that shape of the nozzle greatly affects atomization of fuel oil.

•Atomization Test

•Turn the hand lever of the fuel injection tester at the rate of 1-3 turns per second, and check for condition of fuel atomization under this circumstance.

•Confirm that fuel oil is evenly spread out as it is injected and that there is no abnormality( 异常 ; 变态 ) due to clogged nozzle.

•Since the tip part of this needle valve is in double cone shape, if the rate of the lever is slowed down, atomization becomes much coarser than a conventional needle valve.

•However, in actual engine operation, fuel oil is not supplied at such a low rate. Therefore, it is not necessary to check the atomization under such a low lever turning.

•Test of Valve Seat Oil Tightness

•Keeping the oil pressure at 20 kg/cm2 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( 污染 ) with a trace of test oil, the needle valve can be continuously employed.

•Determination for Nozzle Exchange Time

•To determine whether the nozzle can be continuously used, or not depends on hand test with use of fuel injection tester, sliding condition of needle valve, degree of flaw( 裂纹 ) damage, and abrasion( 磨损 ) of needle valve by visual inspection.

•Under normal handling however, life of the nozzle is about 2,000 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( 研 磨 ， 抛光 ), otherwise lapping of the valve will result in excessive valve lift and will decrease erosion( 腐蚀 ) preventive effect of the seat part.

•Do not misjudge on life of the nozzle on account of fact that with 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 of repairing by lapping the needle valve and valve case, replace them with the new ones.