Injector Nozzles Chapter 21

1. Injector NozzlesChapter 21 DSL 131 – Fuel Systems

2. OBJECTIVES • Identify the subcomponents of a nozzle assembly. • Describe the injector nozzle’s role in system pressure management. • Identify four types of injector nozzles. • Describe the hydraulic principles of operation of poppet, pintle, multi-orifii, electrohydraulic, and piezoelectric nozzles. • Define nozzle differential ratio. • Describe a valve closes orifice (VCO) nozzle.

3. OBJECTIVES (Cont.) • Explain the difference between a low and high spring injector. • Bench (pop) test a hydraulic injector nozzle. • Disassemble, ultrasonically bathe, and reassemble an injector. • Test a nozzle for forward leakage. • Test nozzle back leakage. • Set injector nozzle opening pressure (NOP). • Evaluate the serviceability of a hydraulic injector nozzle.

4. INTRODUCTION • Diesel fuel is injected into an engine cylinder using an injector. • When the diesel technician refers to an injector, the nozzle holder assembly is generally being described. • In describing nozzle assemblies and their operating principles in this chapter, we will reference: • Integral injector assemblies • Subcomponent nozzles

5. INTRODUCTION (Cont.) • In general terms, we can describe injector nozzles as devices with the following functions: • Valves that open and close to begin or end fuel injection. • Define nozzle opening pressure (NOP) (hydraulic nozzles only). • Atomize fuel to the correct size for combustion.

6. FUNCTIONS • Injectors are classified by nozzle design. • Nozzles are simple hydraulic or electrically-switched valves whose functions are to open and close at the correct moment to atomize fuel as it exits the nozzle assembly, thereby injecting fuel into the engine cylinders. • There are four basic types of injector nozzles, two of which are effectively obsolete in medium and heavy duty truck and bus engine applications. • Pintle • Poppet • Multi-orifii • Electro-hydraulic.

7. FUNCTIONS (Cont.) • Our main focus will be on: • Multi-orifii nozzles used in most direct-injection (DI) diesel engines using inline, port-helix metering injection pumps. • Electrohydraulic injector (EHI) nozzles used in common rail (CR), EUP, and EUI fuel systems.

8. NOP IN HYDRAULIC INJECTORS • An injector nozzle is a hydraulically or electrically-actuated switch. • Switch status is changed either by hydraulic pressure rise or fall, or by an electrical signal.

9. EHI OPENING AND CLOSING • In more recently introduced EHIs, NOP is a soft value. • We use the term soft value, because both opening and closing of the injector valve are controlled by the engine control module (ECM) managing the engine.

10. SINGLE ORIFICE NOZZLES • Poppet and pintle nozzles should be of little significance to diesel technicians working with current trucks and heavy equipment. • However, they are addressed briefly here because they are integrated into some syllabi and certification testing. • While the vehicles/equipment they’re used in are old, they are still in service due to their value.

11. Poppet Nozzles • Poppet nozzles are the most simple of the hydraulic injector nozzles and, as such, were the least costly to manufacture. • Primary use is in prechamber or IDI (indirect injection) engines.

12. Poppet Nozzles (con’t) • They use a forward or otward valve opening principle. • All the fuel pumped to the nozzle is delivered to the engine (no return lines) • While cheap to manufacture, they are difficult to refurbish. • Maintenance is usually limited to bench testing to determine spray pattern and NOP • If found unsatisfactory it is usually simply replaced. • NOP is usually between 500 and 1800 psi. • Hydraulic pressure from the injector pump unseats the poppet valve and fuel then passes the valve and is sprayed from its single orifice.

13. Poppet Nozzles (con’t) • Shown is an injector pressure tester.

14. CAUTION • If ether is used to start a vehicle with glow plugs, be sure that the glow plugs are turned off. • Using ether in IDI could cause an explosion and damage the engine.

15. PINTLE NOZZLES • Like poppet nozzles, pintle nozzles are used mainly in IDI engine applications and seldom in truck/bus-sized compression ignition (CI) engines of the recent past. • You can find them in older automobile diesel engines manufactured by GM, Ford, and VW.

16. PINTLE NOZZLES (Cont.) • Like the poppet nozzle, the pintle nozzle body is designed with a single exit orifice. • The valve pintle protrudes through this single orifice even when in the fully-open position. • The fuel is forced around the pintle and makes a conical or fan spray pattern.

17. ACTION • Fuel is delivered to the pintle injector from the fuel injection pump by means of a high-pressure pipe. • The length of a fueling pulse is determined by the fuel injection pump and, when line pressure collapses, the spring reseats the nozzle valve, ending injection. • This type nozzle requires return lines for leak-off at the pintle

18. Multi-Orifii NozzlePintle Nozzle • Shown is a multi-orifice nozzle (a) and a pintle nozzle (b)

19. MULTIPLE ORIFICE NOZZLES • As mentioned in the introduction to this chapter, until recently most current truck and bus diesel engines used closed hydraulic injector nozzles. • Until 2007, an overwhelming majority of injector nozzles could be classified as multi-orifii. • Now they are primarily used as a subcomponent of EUI, EUP and HEUI injection circuits.

20. Sectional View of a Multi-Orifii Injector • Show is a typical Multi-Orifice injector nozzle.

21. DROPLET SIZING • The emitted droplet sizing from a hydraulic injector nozzle depends on two factors: • Flow area (size of nozzle orifii) • Pressure (managed by injection pumping apparatus) • Flow area is fixed, but the pressure can vary greatly. • As pressure increases, the droplet size decreases.

22. BACK LEAKAGE • Orifice nozzles use an inward opening valve principle and that requires leak-off lines. • The leakage that is occurring at the nozzle-valve-to-nozzle-body clearance and measured on the injector test bench is known as back leakage. • Where orifice nozzles are used in diesel engines, the following range of values is typical: • NOP (opening pressures) range from 1,800 to 5,500 psi (120 to 400 bar). • Peak system injection pressures range from two to ten times the NOP value.

23. NOZZLE DIFFERENTIAL RATIO • Nozzle differential ratio describes the geometric relationship between the sectional area of the nozzle seat and that of the pressure chamber or valve shank. • The whole sectional area of the nozzle valve is the sectional area of both the seat and that of the pressure chamber combined.

24. NOZZLE HOLDERS/INJECTORS • Injectors are simply mounting devices for hydraulic nozzles. • They come in many different shapes and sizes for a variety of reasons; for instance, long-stem nozzles are easier to cool.

25. PENCIL-TYPE INJECTOR NOZZLES • Pencil injector nozzles are a type of multi-orifii injector nozzle and, in fact, share common operating principles with a couple of small exceptions. • They are seldom found in any current truck engine applications. • Caterpillar used them in some of its older hydromechanical engines.

26. VCO NOZZLES • Valve closes orifice (VCO) nozzles have eliminated the sac. • The function of the sac in the nozzle is to provide balanced fuel dispersal, which is especially important in keeping the ignition lag time consistent. • Fuel left in the sac became unburned hydrocarbons.

27. ELECTROHYDRAULIC INJECTORS • Electrohydraulic injectors (EHIs) were introduced in the late 1990s and used on the first generation of CR, diesel fuel injection systems, such as those used in Mack Trucks light duty engines, Cummins ISB, and the GM/Isuzu family of engines. • We will take a look at the general operating principles of EHIs first and then examine piezoelectric actuators afterward.

28. ELECTROHYDRAULIC INJECTORS (Cont.) • EHI can be subdivided as follows: • Nozzle assembly • Hydraulic servo system • Actuator valve (either a solenoid or piezoelectric actuator)

29. EHI Operating Phases • When the engine is shut down, all injector nozzles are closed, meaning that their nozzle valves are loaded onto their seats by spring pressure.

30. EHI Operating Phases (Cont.) • In a running engine, injector operation takes place in three phases: • Injector Closed • Nozzle needle is held to its seat by a combination of spring and hydraulic pressure. • Nozzle Opening • The control valve allows hydraulic pressure on one side of the operating valve to drop, which allows the nozzle needle to open. • Nozzle Closing • Pressure is allowed to build in the control chamber to combine with spring pressure and closing the nozzle needle.

31. PIEZOELECTRIC INJECTORS • Piezoelectric actuators are based on the reversibility of piezoelectric effect. • When a piezoelectric crystal is squeezed an electrical current is generated. • When a piezoelectric crystal is charged with electrical current, it will expand. • Piezoelectric actuators can theoretically be used to replace any solenoid.

32. PIEZOELECTRIC INJECTORS(Cont.) • Piezoelectric actuators have the following characteristics when compared with equivalent solenoids: • They respond more quickly when a control voltage is applied. • They require much less electrical power to operate. • They are usually a little larger than an equivalent solenoid.

33. PIEZOELECTRIC INJECTORS (cont.) • May be switched at much higher speeds than solenoids (electromagnetic induction/counter current). • May pulse up to 7 times per fueling event (multipulse injection). • Quieter running. • Lower emisions.

34. NOZZLE HOLE GEOMETRY • EHIs use both sac-chamber nozzles and what are generally referred to as VCO nozzles. • VCO nozzles tend to be required in most current engine families. • Current injectors have 4 to 6 mm nozzle valve diameter.

35. SUMMARY OF EHIs • The advantages of EHIs include: • They can be integrated as a subcomponent of an EUI to provide it with soft (computer-controlled) opening and closing values. • They are designed to open and close precisely when the ECM electrically switches them so that they are not limited by factors such as cam profile. • They produce injection pressure values that remain relatively consistent through a fueling pulse. • They close almost instantly, eliminating the collapse phase that disadvantages comparable hydraulic injectors. • They may use either solenoid or piezoelectric actuators. • Piezoelectric actuators respond much more rapidly to control signals and are used in more recent fuel systems requiring multipulse injection.

36. TECH TIP • A modern injector nozzle functions at pressures that can exceed 35,000 psi (2,413 bar). • When a droplet of fuel is forced through a typically sized injector orifice at this kind of pressure, its speed is greater than 1,200 mph (1,930 km/h), exceeding the speed of sound. • Anything that alters the shape of an injector orifice can produce significant changes in the way fuel is burned in the cylinder. • Technicians should practice special care when working with diesel fuel injectors. Use protective caps to seal the nozzle and the injector input ports after removing an injector.

37. TECH TIP • Dirty/damaged injector spray pattern.

38. TECH TIP • Most OEMs prefer that EHIs are not field serviced. According to the OEMs, when a diagnostic routine condemns an EHI, it should be replaced and the defective injector returned to the manufacturer. • However, it has become commonplace to field repair EHIs, and a number of companies manufacture EHI bench test equipment.

39. NOZZLE TESTING AND RECONDITIONING • Injector nozzles are probably subjected to more abuse than any other engine component. • They are exposed to temperature peaks of 2,550°F (1,400°C) outside and to pressures exceeding 35,000 psi (2400 bar) internally. • Servicing this type of nozzle requires a simple bench test fixture (or pop tester) and ultrasonic bath.

40. CAUTION • Eye protection should be worn when working with or near high-pressure fluids. • High-pressure atomized fuel is extremely dangerous, and no part of the body should be exposed to contacting it.

41. CAUTION • Never touch the nozzle assembly on the pop tester when it is at any pressure above atmospheric—and remember that the entire injector assembly is under pressure. • Diesel fuel oil is a known carcinogen; that is, a cancer-causing agent. • Hands should always be washed after contact with it.

42. REMOVAL OF INJECTORS FROMTHE CYLINDER HEAD • The OEM-recommended method of removing an injector from a cylinder head must be adhered to, and failure to do so can result in failed injectors. • After removing the injector, clean the bore and blow out any fuel that may have drained into the cylinder using compressed air.

43. Seized Injectors • Never risk damaging a cylinder head attempting to remove an injector. • With the cylinder head removed, a seized injector can usually be removed from below with a punch and hammer.

44. TECH TIP • The reality of the trucking industry today is that it does not make economic sense to recondition injector nozzles in the general service facility. • It is a labor-intensive procedure that is not justified in modern diesel engines that have exacting requirements for atomized droplet sizing. • If a customer seeks your advice, recommend that it is considered cost-effective to replace rather than recondition a failed nozzle.

45. Assessment Procedure • Perform the following tests in sequence. • At the first failure, cease following the sequence and proceed to the next section.

46. Clean the Injector

47. Assessment Procedure (Cont.) • Locate the manufacturer’s test specifications. • Mount the injector in the bench test fixture. Build pressure slowly using the pump arm, watching for external leakage. • Bench test the NOP value and record. Use three discharge pulses and record the average value. Sticking or a variation in NOP value that exceeds 10 bar (150 psi) fails the nozzle.

48. Assessment Procedure (Cont.) • Test forward leakage by charging to 10 bar (150 psi) below the NOP value and holding the gauge pressure at that value while observing the nozzle. Any leakage evident at the tip orifii fails the nozzle. • Check back leakage by observing pressure drop from a value 10 bar (150 psi) below NOP. Pressure drop values should typically be in the range of 50 to 70 bar (700 to 1,000 psi) over a 10-second test period.

49. Assessment Procedure (Cont.) • Once again, actuate the bench fixture pump arm and observe the nozzle spray pattern, checking for orifii irregularity. • Ignore nozzle chatter (rapid pulsing of the nozzle valve); this can be regarded as a test bench phenomenon due to slow rate of pressure rise and the inability to drive pressure much above NOP.

50. Reconditioning • The function of the assessment procedure is to determine whether the nozzle should be reconditioned or placed back in service. • Reconditioning in the service shop of today seldom means regrinding of the valve nozzle seat or reaming nozzle orifii due to high labor and tooling costs in comparison with the cost of nozzle replacement.