Fuel Injectors, its Nozzle & Fuel Filters.

Construction & Operation Of Fuel Injectors, its Nozzle & Fuel Filters.

FUEL INJECTOR (OR ATOMISER)

The purpose of the fuel injector is to inject a small volume of fuel in a fine spray and to assist in bringing each droplet into contact with sufficient oxygen to give quick and complete combustion.

 Fig. shows C.A. V. fuel injector. It consists of a needle valve which is pressed on its seating in the nozzle by a plunger or spindle. A compressionspring controls the pressure upon the plunger by which the needle valve opens. A nozzle is attached to the body of the injector by a cap nut. The fuel enters the nozzle through drillings in the injector body. The fuel may pass from a gallery down the sides of the lower parts of the needle valve, or it may enter an annular groove in the nozzle and then pass through drillings to a point just above the nozzle seat. The body or the nozzle holder provides access for the fuel and an outlet for the fuel that leaks into the area occupied by the spring.

When the needle valve is raised from its seat by the pressure of the fuel acting on the conical or stepped face of the valve, the injection of the fuel takes place. When the injection pressure falls below the spring pressure, the valve closes. This action tends to set up an oscillation of the valve during each injection and consequently breaks the fuel into small particles. Fuel leakage past the needle valve stem enters the upper part of the injector and is returned to the pump suction chamber or to the fuel tank. Fuel leakage provides lubrication also for the valve stem.

FUEL SYSTEM INJECTION NOZZLES AND NOZZLE HOLDERS 

The performance of a modern high-speed oil engine depends largely upon the proper functioning of its fuel injection system. For maximum efficiency in operation, it is essential that the engine be not only provided with tile: in quantities exactly timed and proportional to the amount of work it is required to do, but also that it should receive each charge of fuel in a condition such that it can be completely consumed, without causing smoke in the exhaust. This is the function of the nozzle, which is held in position in the cylinder head by the nozzle holder. As the nozzle may have to deal with many hundreds of fuel charges per minute. with widely varying conditions of pressure and temperature. the unerring precision necessary in the production of these parts will be appreciated.

Operation. The injector (Nozzle holder and Nozzle) is a spring loaded valve. The nozzle valve and Seat are manufactured to close limits and are selectively fitted in the factory. The nozzle uses a cone seat. The angle on the valve is slightly greater than the angle formed in the body near the top of the valve cone, The valve is Opened by oil pressure and by spring force.

The injection process is as follows :

 l. Spill cut-off is reached by the injection pump plunger.

 2 Pressure ofthe fuel Oil in the delivery pipe rises rapidly.

 3. Nozzle valve opens quickly.

4. Fuel oil is injected into the engine combustion chamber in a finely divided form.

 5. Spill port open position is reached by the injection pump plunger.

6. Pressure of the fuel oil in the delivery pipe falls rapidly.

7. Nome valve closes quickly.

 The injection process takes only a few milliseconds of time. Precise control is necessary for a clear exhaust gas. For good combustion and a long nozzle life, the valve must open quickly and when closed, should not allow oil to pass its seat. All internally opening nozzle valve have leak off connection. Oil which lubricates  the nozzle valve, flows from the leak off pipe and its then piped back to fuel  tank. The amount of oil flowing is small-an occasional drop only.

The type of nozzle to be used depends upon the particular requirements of the engine combustion and the nozzles vary considerably in respect of diameter and length of hole. Spray angle - valve lift. it is absolutely essential that only the correct type of nozzle is used.

 TYPES OF NOZZLES

·         Single hole nozzle. The single hole nozzle as shown in  Fig. has one hole drilled centrally through its body which is closed by the nozzle valve. The hole can be of any diameter from 0.2 mm upwards. A variation of this type, shown in Fig. is known as the conical end nozzle. In this case, the single hole is bored at an angle to the vertical centre line of the valve as required. 

·         Multi-hole nozzle. Multi-hole nozzle As shown  in Fig.  can have a varying number ofholes drilled in the bulbous end under the valve seating, their actual number, size and disposition being dependent upon the requirements of the engine concerned.

·         Long stem nozzles For direct injection engines where, owing to limited space between the valves in the cylinder head, it is not possible to provide adequate cooling for the standard short stem nozzle, an alternative form of nozzle with a small diameter extension has been developed.

·         Pintle nozzles. In the ease of the pintle nozzle shown in Fig.  which is designed for use in engine combustion chambers of the air cell, swirl chamber or pre-combustion type, the valve stem is extended to form a pin or pintle which protrudes through the mouth of the nozzle body. By modifying the size and shape of this pintle, sprays varying from a hollow parallel-sided pencil form up to a hollow cone with an angle of 60⁰ or more can be provideds.

Delay nozzles. Certain pre-combustion chamber  engines, While requiring a pintle nozzle, demand different spary characteristics in order to obtain quieter running at idling speeds. This is accomplished by modifying the design Of the pintle so that rate Of sired is reduced at  beginning  of the delivery, the resultant effect being to reduce the amount Of fuel in the combustion chamber, when combustion commences, thus diminishing "die knock”. The modified none is referred to a "delay" nozzle. It should be noted, however, that this type  of nozzle dœs not necessarily improve idling in every pre-combustiM1 chamber engine ; and should be selected only after prolonged tests.

Pintaux nozzle. The Pintaux nozzle is a development Of pintle type having an auxiliary spary hole to assist easy starting under cold conditions as shown in Fig. At engine starting Beds, the nozzle valve is not lifted sufficiently to clear the pinhole and the fuel is discharged through auxiliary hole. At normal running speeds, however. When pressures in the fuel system is higher. the nozzle valve is withdrawn from the pintle hole allowing the bulk of the fuel to discharged through it.

DIESEL FUEL FILTERS:

Dirt carried in the fuel is recognisedas prime source of trouble and inefficiency in diesel engine operation, as it is the cause of wear in the fuel injection pump and nozzles. The fuel injection pump is a sturdy, well designed piece of equipment that will give trouble-free service for many thousands of hours running, provided the fuel is clean, but if dirt, especially fine dust, is allowed to pass into the system in the fuel, then wear follows with it irregular running and loss of power : maintenance costs will become heavy and engines will need frequent attention. Where sedimentation in the supply tank can be efficiently carried out, this will greatly reduce the amount of dirt carried in the fuel. Under working conditions. however, it is absolutely essential that the fuel be properly filtered before entering the injection pump.

Method of Operation. The fuel enters the filter bowl through the inlet connection, then passes through the filter element and leaves the filter through the outlet orifice completely clean. In multi-stage (dual) filters, the fuel Oil flows first through the primary filter and then through the secondary micro filter before continuing to the fuel injection pump.

 Prior to putting into operation or after cleaning and changing of the filter element the filter mug flied with fuel oil through the filler plug orifice on the filter cover. After filling. the filler plug should immediately and the filter air vented.

    The importance of care in replacing the element, avoiding getting din on clean side of filter. cannot be over-emphasized, as many complaints Of fuel pump element Wear can traced to lack of care in the servicing of filters. When choking takes place, this is usually found to due to a waxy sludge which is deposited from the fuel. If filters are found to choke in an unreasonably short time this will probably point  to an unsatisfactory fuel supply or storage tank installation : and should be taken to find out how and at what point an undue amount Of impurities Can enter the system.

    Paper elements are not intended to be cleaned and must discarded When choked. The cost of replacement elements has been kept down to the lowest possible figure compatible with Special care in manufacture; and compares more than favourably with other Ins efficient filtering elements. The number of filters used for any engine installation will obviously depend the capacity of the particular engine and the conditions of operation. In all cases, the main object is to provide the highest possible degree of filteration consistent with long filter element life.

 

   Due to the Widely differing conditions of operation which have to catered for, precise instructions for servicing intervals cannot be stated for paper elements : and operators  are advised to follow in the first place the recommendations laid down in the vehicle, tractor or engine instruction book. The period at which it Will be found necessary to change the filter clement will naturally vary according to the type of fuel used, provision made for bulk storage, setting or prefiltering before filling the vehicle tank, and the local conditions under which the engine concerned is working. This may easily as short three months or as long as twelve, but it is not that it wilt be outside this period although exceptional circumstances may make it so.

   The filter inserts should never be cleaned. Every 5,000 km 125 hours operation. the filter bowls should be drained to remove sediments/water. Since, filter inserts attain maximum efficiency only after a film of din has deposited on the surface of the filter insert. the stage I and stage Il filter inserts should never be replaced at the same time. Sometimes. the filter get clogged in a very short time due to asphaltence or waxy compounds present in the fuel. In such eases, the filter insert  has to be replaced.