Fuel Injector is also called as fuel valve. It’s a cylinder head mounting and quintessential part of fuel oil system (Fuel Injection System).
It delivers the fuel oil in the combustion chamber of engine.
Each droplets of injected oil spray must have sufficient energy to penetrate the air mass.
It should not open (inject) till pre-set pressure is built up.
Also serves as non-return valve, so that doesn’t allow the combustion gas to enter in fuel system.
Function: It injects and disperse the fuel oil evenly into the cylinder in a finely atomized spray with good penetration.
Cooled Injector: Older design, having internal cooling passages for cooling water circulation.
Uncooled Injector: Modern design, without cooling passages, using fuel oil circulation and bores for cooling of injector.
Hydraulically operated: It’s an uncooled type, modern fuel injector and widely used. So, our discussion topic will be limited to this type only.
Chief components of a fuel injector are: needle valve, needle valve guide, valve seat, nozzle, spring, set screw, valve body.
Operation of opening and closing of fuel valve is performed by the fuel pressure, delivered by the fuel pump to the fuel valve (injector).
Valve opening pressure is about 320 – 380 Bar & Injection pressure is about 1000 – 1500 Bar. Injection pressure is generally high because it should be more than pressure inside the cylinder, so that fuel can be injected.
High pressure fuel oil from fuel pump acts on the shoulder of the needle, thereby lifting the needle against the spring force and the high pressure fuel is injected through spray nozzle into cylinder.
At the end of injection, the oil pressure drops and spring will force the needle down and needle will close in the seat terminating the fuel injection.
Isolate the system (Engine or respective cylinder unit) and remove the injector from respective unit.
Bring injector in workshop or injector testing room and clean it.
Check the pressure testing machine and do the pressure test, (If it’s good then we don’t need to overhaul, but if it’s not satisfactory, then we have to overhaul and troubleshoot).
Mount injector on vice for dismantling, open the locking nut or union nut of injector and slack the nozzle holding nut.
Remove nozzle assembly and keep it safe, as it’s costly.
Remove spindle, needle, spring, non-return valve (Main Engine fuel injector) & other possible parts and keep it in clean diesel for cleaning purpose.
Check all dismantled parts including nozzle.
Lapping: In fuel injector, lapping of needle valve and needle face is done for smooth seating by lapping paste.
Now, assemble all the parts and do the pressure test.
Injector pressure test: Put the injector on pressure testing device properly and increase the pressure gradually. Note the pressure at which injection starts. Pressure can be adjusted by pressure adjusting bolt. Cracking sound of injector during testing indicates the satisfactory condition of spring.
Drip test: If injection pressure is 310 bar (let’s assume), bring the pressure up to 290 bar and wait to observe the dripping of the valve. Dripping is nothing but the drop by drop falling of oil (fluid). Lapping should be proper to avoid dripping.
Leak test: Leakage between needle and guide of the valve is more common. After injection, if pressure drops suddenly then it means valve is leaking. Generally it takes 30 seconds to 1 minute for pressure drop due to leakage.
Spray pattern test: Keep a card board or hard cover below the injector and give the stroke (start injection). If spray is in inside direction, then it means hole is not clear.
Improper cooling: Cooling should be proper otherwise too much cooling creates sulphur corrosion and too less cooling creates carbon deposits at tip.
Dribbling nozzle: Fuel burning at nozzle tip induces poor seating of fuel valve due to impurities in the fuel causing abrasive wear of seat and poor cooling.
Carbon trumpets: It influences the spray pattern of injection. And due to wrong spray pattern, there will be smoky exhaust and increase in exhaust temperature and poor combustion.
Wrong spring pressure: It leads to wrong opening and closing of injection which causes poor combustion and smoky exhaust gas. It is due to wrong setting of pressure during overhauling of injector.
Nozzle hole diameter, depth & number: It influences penetration, atomization and combustion. Nozzle hole is generally chocked with impurities, carbon trumpets, burning of sac area, trapped fuel, prolonged running at low loads etc.
Length of the nozzle hole is thrice the size of the diameter of the hole.