FUNCTIONS
Fuel valve is one of the vital parts of the Diesel Engine due to their high precision in manufacturing and the intricate duties it has to perform, to atomiser, penetrate and distribute as swirling action achieved.
Atomisation
Atomisation is the splitting up of the fuel into very small droplets by the fuel injector, forcing fuel at high pressure through small atomiser holes.
The droplet size will depend upon the size of holes and the pressure difference between fuel pump discharge and that of the compressed air in the combustion chamber, and consequently the size of droplets may vary.
Atomised droplets have a high surface area to mass ratio, giving good heat transfer and causing efficient combustion with minimum of unburnt fuel.
Atomization – to ensure that each minute particle of fuel is surrounded by particle of oxygen with which it can combine.
Atomization. Is the break up of the fuel charge into very small particles it is injected into the cylinder.
Penetration refers to the distance that the fuel particles travel or penetrate into the combustion chamber.
Why fuel is required to atomise ?
To get complete (fully) combustion of charge fuel within a short time.
To ensure that each minute particle of fuel is surrounded by particle of oxygen
Excessive Atomisation
1) Smaller oil particles have insufficient KE, to go through combustion chamber
2) Dense compressed air has high resistance to the motion of oil particles
3) Smaller particles tend to cluster around injector tip, and oxygen-starved during combustion
4) Can cause after-burning
Insufficient Atomisation
1) Oil particles become larger and will have more KE and travel further into combustion chamber, and some may rest on cylinder liner and piston crown.
2) Carbon built-up around the top of cylinder and piston crown
3) Lower rate of combustion and after burning
Penetration
It refers to the distance the oil droplets travel into the combustion space before mixing with the air and igniting.
This will depend upon droplet size (atomisation), velocity leaving the injector and the conditions within the combustion chamber. It is desirable that fuel should penetrate into the whole of the combustion space for efficient space utilisation and good mixing, but droplets should not impinge on the internal surfaces before burning. The number of atomiser holes and their position will decide the spray pattern.
Low Penetration
1) Less intimate mixing of air and fuel particles in combustion chamber
2) Fuel cluster around injector tip causing after burning
High Penetration
1) Fuel particles travel further into the combustion chamber and some may rest on the cylinder liner and piston crown
2) Lower rate of combustion and after burning
Turbulence
It is the movement of compressed air and fuel within the combustion space before combustion occurs. This may have several causes. Swirl is imparted to the air during its entry at scavenge ports. It may be further agitated by the furl spray pattern and the shape and movement of the piston crown.
Turbulence will improve the mixing of fuel and air for effective and rapid combustion. It is particularly desirable for rapid combustion of heavy fuels in medium or higher speed engines.
Propose of cooling to nozzle tip
To prevent carbon trumpet (or) formation & needle valve seizure
To maintain the tip region temperature within acceptable limit
To avoid the malfunction of operation mechanism
How to check the fuel valve is OK or Not ?
Set pressure (on pressure gauge)
Atomization (visual check & listening jarring noise)
Spray pattern (by maker supply spray pattern plate, holes on it must not touch the oil or paper method @ right angle to valve)
Dribble (sensing hand feel after correct pressure test)
Effect of trumpet formation ?
1) Interfere the spray pattern
2) Increase fuel consumption
3) Poor combustion resulting in black smoke
4) High exhaust temperature
Fuel valve overhaul ?
1) Test & check the old valve
2) Slacken the lock nut & adjusting screw to release spring pressure
3) Make upside down & remove retaining cap nut, take out nozzle & dowel pin fitting
4) Make upright, remove lock nut, adjusting screw, spring, spindle and spring retainer
5) Clean all parts with kerosene especially on nozzle holes check all parts thoroughly & renew if necessary
Check point:
Nozzle hole diameter with pin gauge (Maker supply)
Needle valve surface
Valve lift
Spindle for straightness
Spring tension
Lock nut & pressure adjusting screw wear & tear
Body especially oil passage
6) Reassemble correct sequence and set the pressure on test pump and check the following
a. Set pressure (some holding pressure)
b. Atomisation
c. Spray pattern
d. Dribble
What point do you check when overhaul fuel valve ?
Needle valve lift
Seat clearance (tilted 30 ° from horizontal position needle must not fall down)
Nozzle atomisation hole
Spindle & spring
Fuel line & Cooling Water line to be clear, dowel pin fitting)
Contact surface of nozzle and body
Excessive needle valve lift
1) Spring failure
2) Impact damage between needle shoulder and thrust face & impact damage at the seat
3) Combustion gas may blow back into nozzle due to prolonging opening period
Too low/small needle valve lift
1) Restrict the flow
2) May overload the pump
Anti-dribbling arrangement
The angles of the needle valve and its seat are cut difference about 1 ° or 2 ° to achieve point contact thus preventing dribbling and sharp closing.
In other words, weeping and dribbling has been prevented by cutting the needle angle about 2 ° larger than the seat angle of about 60 °. This arrangement ensures sharp opening and closing of the fuel valve.
Worn nozzle hole
It is due to erosion of fuel oil which contains abrasive particles. That is due to insufficient filtration and purification or the fuel valve does not close snappily after injection.
It affects the spray pattern and atomization. It cause burning away of piston crown due to the fuel impingement and secondary burning and seriously effects engine performance.
How to decide nozzle is suitable for further used ?
By titling the nozzle body & needle valve assemble 30 ° from horizontal position.
If the needle valve falls from the nozzle body at only a slightly tilting angle the whole sprayer must be renewed.
Nozzle leakage
It is due to defective needle valve of partial opening of needle valve. It is due to defective delivery valve of fuel pump that causes dribbling of needle valve.
It may causes secondary burning, reduced combustion efficiency and high exhaust temperature. It may cause trumpet formation of carbon on the nozzle tip.
How to detect leaking fuel valve ?
Loss in power in affected cylinder (Power card)
Smoke at exhaust
Exhaust high temperature
There may be a knock or pressure wave in the injection system
Draw card show fluctuations of pressure during expansion process due to secondary burning of fuel leaking, higher expansion line.
Affect of leaking fuel valve
Increase fuel consumption
Engine over loading
Knocking
High exhaust temperature
Lifting of cylinder relief valve.
FULE VALVE TESTING
To pressure-test a fuel valve:
The valve is connected to a test pump with pressure gauge.
The passages are primed by pumping oil freely and all air forced through the valve air vent when the spring tension is low.
The correct lifting pressure is stated on the adjustment sheet for the engine.
The adjusting screw for the spring is now set so that the spindle lifts at this pressure: the screw is then locked in position and the lift pressure rechecked. If everything is in order, the needle valve will open suddenly when the correct pressure is reached.
Next, the nozzle is wiped thoroughly clean and pressure reapplied, this time to 10 kgf/cm2 below the injection working pressure. If the pressure remains steady for a few minutes, the valve is tight. A trace of oil at the nozzle holes is of no importance, as the valve will normally be worked-in completely after a few minutes of running. However, should the nozzle become wet or should drops appear, then replacement or regrinding of the valve is necessary.
Condition of the spray holes can be checked by placing a piece of cardboard just below the Tip (not held by hand) and depressing the tester handle briskly once.
The pattern (for a symmetrical nozzle) should be symmetrical.
Observe whether the nozzle ‘chatters’ whilst the fuel is discharged.
Injectors should be tested when removed, as a diagnostic check on their condition, and after recondition.
Opening pressures set too high can result in spring failure. If the opening pressure is adjusted low, combustion gas can blow back into the injector and build up lacquer and carbonaceous deposits.
Atomization test
Test handle is to be pushed hard two or three times.
To see fuel spray pattern is uniformly
Not dripping
Listing jarring noise.
Pressure drop test (Set pressure test)
300 kg/cm2 200 kg/cm2 (30 to 90 sec)
Dribbling test
Maintain oil pressure of 10 kg/cm2 below opening pressure
Check nozzle tip
Spray pattern test
Original test bench has hole plate (same number with nozzle hole) when the spray can enter into the hole without touching the plate.
Paper method fuel valve and paper right angle position.
Incorrect spring pressure setting
It is due to
1) Broken spring
2) Loose pressure adjusting nut and loose lock nut
3) Incorrect initial pressure setting
4) Sticking of spring
5) Worn out spring
It may cause
Early injection (low pressure setting)
Late injection (high pressure setting)
Early injection may causes knocking the engine unit. Shock heavy load on bearings.
Late injection may causes high exhaust temperature with reduced engine output and fouling of exhaust system.
How to know fuel valve spring is broken ?
Over fuelling
Engine knocking
Black smoke from funnel
Exhaust temperature high.
Chocked nozzle cooling space
It is due to insufficient pressure of cooling liquid (if in separate system). It is due to choked cooling line due to internal corrosion that is resulted from failure of anti-corrosive liquid to be added into cooling water system.
It may be due to incorrect alignment of nozzle holder and nozzle. The choked nozzle cooling carbonization of nozzle tips and clogging of nozzle holes.
* scale formation, sticking of nozzle needle*