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THE MAINTENAN F, OF OIL-ENGINE INJECTION SYSTEMS

28th December 1934
Page 42
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Page 42, 28th December 1934 — THE MAINTENAN F, OF OIL-ENGINE INJECTION SYSTEMS
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FROM the point of view of maintenance of a compression-ignition engine, it is principally necessary to realize that the fuel pump and injector replace the carburetter and ignition apparatus and perform

similar functions. It is particularly the case that the function which, in an oil engine, corresponds to ignition in a petrol engine, is effected by the pump and injector. . . .

The entry of the finely sprayed jet of fuel into the compressed air in the cylinder head, at or about the termination of the compression stroke, causes combus tion to begin in just the same way as the passage of an electric current through the sparking plug, causing a spark at the plug points, ignites the mixture in a petrol engine. The operation of the pump is actually timed in a manner similar to that of the magneto, and the in jection point advanced or retarded, as the case may be, in accordance with the conditions of running. Efficient operation of a compression-ignition engine can therefore be maintained if a reasonable amount of attention be given to these particular engine components.

Hints Applicable to a Widely Used Pump.

In the maintenance hints which follow, the C.A.V.Bosch type pump and. injector are mainly in mind.

The instructions are generally applicable, but opportunity will be taken at a convenient date to deal in detail with some of the special maintenance features of particular types of engine.

One factor which promotes efficiency is the main tenance of the compression pressure in the cylinder head. Spontaneous combustion of the oil fuel is effected at a minimum temperature of round about 300 degrees C. The air in the cylinders must be compressed sufficiently to attain that temperature.

In the ordinary way there is an ample margin. The sompression pressure is rqund about 400 lb. to 450 In. per sq. in. and the corresponding temperature is 600 de 1332 grecs C. That temperature, however, drops fairly rapidly as the pressure falls, hence the importance of maintaining the pistons, cylinders and valves in good condition.

The need for efficient compression is naturally emphasized at starting. In the petrol engine, with magneto ignition, a quick . turnover of the crankshaft is necessary for starting. The same need exists in the case of the compression-ignition engine for a different but analogous reason. A rapid turn of the crankshaft is desirable in order to retain the heat of compression and ensure combustion. This is, of course, more necessary in the case of engines which are not equipped with heater plugs.

Many of the maintenance operations needed for the oil engine are the same as those for the petrol engine. The valves and tappets must have the appropriate clearance, the valves must be ground in to a good seating, the cylinders must occasionally be decarbonized, and so on. It is imperative to change the crankcase oil at the periods specified by the manufacturers of the engine. It is doubly imperative to use the grade of lubricant recommended by the engine makers.

The injectors on a compression-ignition engine corre spond, as has been stated, to the sparking plugs on a.

petrol engine. They do not require much more attention than is recommended for sparking plugs, to function efficiently. They should be taken out aild cleaned.

about once a month, more or less according to the mileage, say, once every 2,000 miles. The nozzles should be dismantled, and the feeler pin and valves carefully cleaned in petrol or paraffin until they are perfectly free and capable of being rotated or reciprocated within the nozzle. If this be done regularly, the chances of trouble developing with injectors are most remote.

At the same time, if there be any trouble, the injectors, should be suspected and the procedure followed as hereinafter described, to determine whether one of them is at fault. The symptoms are blue smoke in the exhaust, when the engine is hot and pulling, knocking or misfiring.

If there be knocking, the first thing to do is to locate the knock. To that end the union nuts for the fuel pipes should be slackened off, one at a tithe, in turn,

and the engine speeded up while each is tested. The knock will cease when the union nut for the faulty injector is loosened.

Having located the faulty injector, remove it and reconnect it to its fuel pipe. Restart the engine and observe the character of the spray from the nozzle. If -the

injector be in good order, the spray should appear as a fine mist, not as a jet, and with no dribble.

If there be a jet of fuel in the centre of the spray, one of the four following faults is the cause :—(a) There is dirt under the nozzle valve. (b) The nozzle valve is not seating properly. (c) The spring may be broken. (d) There is maladjustment of the spring. .

If the fault be in the adjustment of the spring, 'this

will usually be apparent on removing the end cap. A broken spring can likewise be discovered by dismantlement. If both the adjustment of the spring and the

spring itself be in good order, then a thorough cleaning of the injector is probably all that is required, it is of the utmost importance, when cleaning injector valves and parts, that no emery paper or abrasive material be used, and that the greatest care be taken in scraping the valve or feeler pin ; only the finger nail or

a pen-knife, very carefully applied, should be employed. Another important point is to observe absolute cleanliness when handling any pump or injector part. A clean sheet of paper should be laid down as the first operation and the hands should be rubbed clean.

Care Needed to Avoid Damaging the Parts.

Sometimes a nozzle valve slicks and cannot easily be moved. Care must be exercised in freeing it The projecting stem of the valve can be held in the vice between lead clamps or similar soft material and the nozzle body worked carefully with the fingers. A good soaking in paraffin or penetrating oil is a help.

It is important not to disturb the adjustment of the spring. When dismantling for examination or to replace a broken spring, the spring cap nut should be removed • without touching the adjusting screw or lock nut. The spring can be lifted out, examined, and replaced without the proper adjustment being lost, provided that the adjusting screw or lock nut has not been touched.

A good spray from the injector accompanied by a dribble may be due to dirt, to the valve sticking, or, possibly, to a cracked nozzle body.

In order to discover which of these faults is present, wipe the nozzle dry and repeat the test, noting whence the dribble comes. If it be from the nozzle orifice, then the fault is either dirt or a sticking valve, but if from between the nozzle and the nozzle cap nut, a cracked nozzle is probably the cause.

A jet of fuel with little or no spray is probably due

to a valve sticking, to a broken spring, or to faulty adjustment of the spring. When about to replace an injector, after one or other of the foregoing attentions, the 'effects of the operation should be tested as above described.

It is also important, when replacing an injector, to eliminate air from the pipe. To that end, after the injector is put back into the cylinder, leave the fuelpipe union slack and run the engine or turn it over by other means until fuel flows from the union.

If, after carrying through the foregoing operations and ensuring that the suspected injector is in good order, dribble still persists, it is probable that the fault lies.in the pump. To test this, change the injector on -to another fuel line and try again, because it is not likely that all four or six fuel lines will be faulty.

Correct Lubrication and Thorough Filtration.

So far as the pump is concerned, the two principal essentials to ensure freedom from trouble are, first, ample lubrication of this component with the recommended grade of oil, and, secondly, adequate filtration of the fuel." If the pump be disclosed by the .test just described as.being 'faulty in operation, the cause may be a stioleing delivery valve, a broken plunger spring, or the presence of an air .lock in the pump chamber.

The first of these troubles is most likely due to dirt, hence the importance of care in filtration of the oil fuel. The cure is to remove the valve, clean it carefully arid replace it. Here, again, we would emphasize the iniportance, when handling either the pump or the injector, of absolute cleanliness.

The-delivery valves, incidentally, must on no account be changed one with another. Each is a pumping fit in its guide, and the limits of these fits are so fine that absolute interchangeability is impracticable. If a pump plunger spring has broken, the work of repair is a job to be entrusted to a properly equipped service station.

As regards the third possible cause of pump trouble, an air lock, it should • be noted that it is most essential that all air be removed from the fuel supply system of a compression-ignition engine.

Air may enter in any of the following circumstances: The fuel tank may have been allowed to run dry; some parts of the fuel system may have been disconnected, allowing air to enter ; the engine may have been run with the fuel tap closed, or the filter may have become clogged and may have prevented the passage of fuel.

If the tank has been allowed to run dry (or the Autovac tank, where one is fitted), open the air vent cock on the pump and fill the tank with fuel, continuing to pour until the oil issues in a steady stream from the overflow pipe. Next slacken off all the pressure-pipe unions on the injectors, fully depress the accelerator, and rotate the engine until oil flows from every union.

In the case of faulty delivery from the pump to an injector, as described above, remove the delivery valve holder, lift the valve from its seat to enable air to escape;and allow fuel to flow until free from bubbles. If the.. fuel does not flow of its own accord, crank the engine until it does.

Individual Elimination of Air Locks.

This procedure should be repeated for each delivery valve which does not appear to be operating as it should.. On completion, couple up the pump ends of the -delivery pipes and then slacken off each injector union in turn sufficiently to allow fuel to flow and to clear any air bubbles from those pipes.

It is sometimes recommended that the vent on the pump should be opened periodically while the engine is running, to get rid of any airthat may have collected. In all cases,after an injector has been removed, the fuel pipe should be rid of air as described.

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