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Fuel Subsystems Chapter 19

Fuel Subsystems Chapter 19

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Fuel Subsystems Chapter 19

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  1. Fuel SubsystemsChapter 19

  2. OBJECTIVES • Identify fuel subsystem components on a truck or bus chassis. • Describe the components used in a diesel engine fuel subsystem. • Define the functions of internal and external fuel tank components. • Troubleshoot a fuel sending unit. • Define the role of primary and secondary fuel filters. • Service primary and secondary fuel filters. • Describe the three ways water can be suspended in fuel.

  3. OBJECTIVES (Cont.) • Explain how a water separator functions. • Service a water separator. • Define the principles of operation of a transfer or charge pump. • Prime a fuel subsystem. • Test the low-pressure side of the fuel subsystem for inlet restriction. • Test the charge side of the fuel subsystem for charging pressure. • Identify some typical sensors used in diesel fuel subsystems.

  4. INTRODUCTION • The diesel fuel subsystem on a commercial vehicle is best defined as the group of components responsible for fuel storage and its transfer to the injection pumping apparatus.

  5. INTRODUCTION • The main fuel system components used on C7 and C9 common rail (CR)–fueled engines

  6. INTRODUCTION Navistar fuel subsystem schematic.

  7. FUEL SUBSYSTEM OBJECTIVES • If you take a look at the fuel systems shown in the preceding slides, you will observe a clear divide between the low-pressure side and the charge side of the fuel subsystem. • A primary filter is most often located on the suction side of the transfer pump, while the secondary filter is located on its charge side.

  8. FUEL SUBSYSTEM OBJECTIVES (Cont.) • However, there are some fuel systems, notably some manufactured by Cummins, in which all movement of fuel through the fuel subsystem is under suction. • When such a fuel system uses multiple filters, the terms primary and secondary tend not to be used.

  9. FUEL SUBSYSTEM OBJECTIVES (Cont.) • We can summarize the objectives of the fuel subsystem as follows: • Stores fuel in tanks until required. • Removes moisture from the fuel. • Filters fuel to remove abrasive particulates. • Delivers fuel to the injection components at the proper temperature.

  10. FUEL TANKS • Fuel is stored on commercial vehicles in fuel tanks. • In most highway trucks, fuel tanks are mounted to the frame. • It is common to use a pair of fuel tanks. • This balances the considerable weight of onboard fuel as it is consumed.

  11. FUEL TANKS (Cont.) • Many diesel fuel management systems are designed to pump much greater quantities of fuel through the system than that required for actually fueling the engine. • This excess fuel factor varies from a minimal amount to values exceeding 60% of pumped fuel. • The excess fuel is used to: • Lubricate high-pressure injection components. • Cool high-pressure injection components (especially those exposed to extreme temperatures). • Cool electronic components such as engine control modules (ECMs) and injector drivers.

  12. FUEL TANK RECOMMENDATIONS • Engine and fuel injection system manufacturers tend not to manufacture most of the fuel subsystem components but often make specific recommendations to the chassis OEM.

  13. FUEL TANK RECOMMENDATIONS (Cont.) • Essentially, a vehicle fuel tank will function most effectively as a heat exchanger if the following is true: • Located in the airflow. Truck fuel tanks tend to be mounted in cradle brackets bolted to the outboard side of laddered frame rails. This ensures fairly good airflow around the tank into which heat removed from the cylinder head by the fuel can be dissipated.

  14. FUEL TANK RECOMMENDATIONS (Cont.) • Cylindrically shaped. A cylindrically shaped vessel helps maximize the surface area of the tank exposed to the airflow. • Aluminum construction. The coefficient of heat transfer of aluminum is high, enabling heat to be transferred efficiently from the fuel to atmosphere. • Aluminum is less susceptible than steel to water corrosion at the base of the tank and is much lighter. • Maintained 25% full or better. • At low levels the bypass fuel may get warm enough to reduce fuel lubricity.

  15. DUAL TANKS • Most heavy highway trucks use multiple fuel tanks (usually two) to increase onboard fuel capacity and also to distribute fuel weight evenly . • May use a “y” type pickup tube or crossover tubes. • Each tank requires a fuel cap and must be filled separately when no crossover pipe is used.

  16. PICKUP TUBES • Fuel pickup tubes are positioned so that they draw on fuel slightly above the base of the tank and thereby avoid picking up water and sediment. • Fuel pickup tubes seldom fail, but when they do it is usually by metal fatigue crack at the neck.

  17. FUEL TANK SENDING UNITS • Most medium and heavy duty commercial truck fuel subsystems use remote (from the tank) fuel transfer pumps and not assemblies that incorporate the sending unit and a transfer pump. • Variable resistance type fuel sending unit problems can be diagnosed by disconnecting the terminals and using a digital multimeter (DMM) in resistance mode. • Normal readings for a 240 ohm sending unit: • Full Tank - 20 to 50 ohms • Half Tank – 80 to 120 ohms • Empty tank – 220 to 260 ohms • Normal Readings for a 90 ohm sending unit: • Full Tank – 86 to 94 ohms • Half Tank – 40 to 50 ohms • Empty Tank – 0 to 4 ohms

  18. Venting • Currently, most jurisdictions in North America permit venting of diesel fuel tanks to atmosphere but some OEMs prefer that ingested air be filtered to prevent the admission of road dirt and excessive moisture into the fuel. • A plugged fuel tank vent or breather will rapidly shut down an engine, creating a suction side inlet restriction value the transfer pump will not be capable of overcoming.

  19. Breather Filters • As fuel is drawn out of a fuel tank, ambient air is drawn in. • This ambient air contains whatever dust and dirt contaminants happen to be suspended in the air. • As a consequence, the OEM recommends the use of a high-efficiency fuel tank breather similar to that shown in.

  20. WIF Checking • Diesel fuel tanks should be periodically checked for water in fuel (WIF). • To check for the presence of water in fuel tanks, first allow the fuel tanks to settle, and then insert a probe (a clean aluminum welding rod) lightly coated with water detection paste (Gasoila WF25) • WF25 is a mustard yellow until it contacts water when it turns to red. • This test will give you some idea of the quantity of water in the tank by indicating the height on the probe where the color has changed.

  21. TECH TIP • Allow a mobile fuel tank to settle for at least an hour before testing. This will allow you to determine how much water is in the tank. Always expect a trace quantity at the base of the tank.

  22. FUEL TANK REPAIR • Generally, OEMs recommend that a defective fuel tank be replaced rather than repaired. • However, corroded and punctured fuel tanks are going to be repaired regardless of OEM recommendation, so the following suggestions are offered (Tech Tip), not to endorse fuel tank repair practice but to promote some safety awareness.

  23. TECH TIP • When evacuating fuel tanks, check with the tank OEM for the safe vacuum test value. • A high vacuum value may collapse a fuel tank. • As a rule, cylindrical tanks will withstand higher vacuum test values than square section tanks.

  24. FUEL FILTERS • Diesel fuel injection equipment is manufactured with minute clearances and impurities in fuel which, if not removed by the fuel subsystem, can cause premature failures. • A typical fuel subsystem with a primary circuit and a secondary circuit in most cases uses a two-filter arrangement, one in each circuit. • Figure at right shows the flow routing in some typical filters. • Filters may be of either cartridge (spin on), canister with disposable filter element.

  25. DEFINING CLEAN FUEL • Fuel filters are rated by their ability to entrap particulates. • They function on a sort of fishnet principle whereby particles smaller than a certain size pass through the mesh, and those that are too large get entrapped. • Figure at right shows the sectional size of a human hair in relation to some of the dimensions we reference when talking about clean fuel. • A typical secondary filter is about 5 microns.

  26. DEFINING CLEAN FUEL (Cont.) • Filtering is achieved using media ranging from cotton-threaded fibers, synthetic fiber threads, and resin-impregnated paper. • Figure at right shows a typical spin-on type of primary filter. • Normally rated at 10-16 microns.

  27. SECONDARY FILTERS • Secondary filters represent the second filtration stage in two-stage filtering. • Figure at right shows the secondary filter pad assembly used on newer Volvo engines equipped with a fuel pressure sensor. • Normally rated at 2-5 microns.

  28. SECONDARY FILTERS (Cont.) • Water and Secondary Filters • H2O in its free or emulsified state cannot be pumped through most current secondary fuel filters with nominal ratings of 5 μ or less. • The Caterpillar standard is less than 0.01% water content. • This results in the filter plugging on H2O and shutting down the engine by starving it for fuel.

  29. TECH TIP • The technician should note that most filters function with optimum efficiency just before they are completely plugged—in other words, at the end of their service lives. • Accordingly, a filter testing at the maximum restriction specification is still functioning properly and, while it may not have much service life remaining, it should not produce performance problems.

  30. SECONDARY FILTERS (Cont.) • Testing and Servicing a Secondary Filter • Secondary filters are usually charged by the transfer pump. • Testing charging pressure (the pressure downstream from the charging/transfer pump) is normally performed with an accurate, fluid-filled pressure gauge (shown at right) plumbed in series between the transfer pump and injection pump apparatus.

  31. CAUTION • In many EPA post-2007 and 2010 certified diesel engines, charging pressures may be much higher than their predecessors. • This is because the charge side of the fuel subsystem may be used to supply diesel particulate filter (DPF) dosing injectors, which often require higher pressures. Check specs and exercise caution. • The gauge you use must have a high enough service rating to handle the pressures involved.

  32. Procedure for ServicingSpin-on Filter Cartridges • Much dirt is placed in diesel fuel systems by technicians using improper service techniques. • Filters should be primed with filtered fuel. • After the primary filter(s) has been primed and installed, the secondary filter should be installed dry and primed with a hand primer pump or inline electric primer pump if equipped.

  33. Replacement Procedure • Remove the old filter cartridge from the filter base pad using an appropriately-sized filter wrench. • Drain the fuel to an oil disposal container. • Ensure that the old filter cartridge gasket(s) has been removed.

  34. Replacement Procedure • Remove the new filter cartridge from the shipping wrapping. Fill the filter cartridge with clean, filtered fuel poured carefully into the inlet section. • The fuel oil itself should provide the gasket and/or O-ring and mounting threads with adequate lubricant. • Screw the filter cartridge clockwise (CW): (right-hand threads are used) onto the mounting pad.

  35. TECH TIP • When a hand primer pump is fitted to a fuel subsystem, externally prime only the primary filter, ensuring that all the fuel is poured through the inlet side only. • Install the secondary filter dry and prime using the hand primer pump. • When an electric primer pump is fitted to the circuit, use it.

  36. WARNING • When removing filter cartridges, ensure that the old gasket is removed with the old filter. • A common source of air induction in the fuel subsystem is double gasketing of the primary filter. • Double gasketing will usually produce a leak at the secondary filter.

  37. Water Damages Fuel Systems • Three ways water damages fuel systems. • Water has lower lubricity than fuel. • Water tends to promote corrosion. • Water pumps differently than fuel. • Water is not compressible vs. fuel which is .5% compressible per 1000 psi.

  38. WATER SEPARATORS • Most current diesel engine-powered highway vehicles have fuel subsystems with fairly sophisticated H2O removal devices. • Water appears in diesel fuel in three forms: • Free state • Large globules • Emulsified • Small droplets/suspended in fuel. • Semi-absorbed • Finely dispersed in fuel. • H2O separators have been used in diesel fuel systems for many years. • Often, an H2O separator will combine a primary filter and H2O separating mechanism into a single canister. • H2O separators use combinations of several principles to separate and remove H2O from fuel.

  39. H2O Separator Operating Principle (Cont.) • H2O in its free state or emulsified H2O that has been coalesced into large droplets will, because of its heavier weight, be pulled by gravity to the bottom of a reservoir or sump. • Some H2O separators use a centrifuge to help separate both larger globules of H2O and emulsified H2O from fuel. • The centrifuge subjects fuel passing through it to centrifugal force, throwing the heavier H2O to the sump walls where gravity can pull it into the sump drain. • Figure at right shows an assortment of combination filter/H2O separators used on diesel engines.

  40. TECH TIP • To troubleshoot the source of air admission to the fuel subsystem, a diagnostic sight glass can be used. • It consists of a clear section of tubing with hydraulic hose couplers at either end, and it is fitted in series with the fuel flow. • However, the process of uncoupling the fuel hoses will always admit some air into the fuel subsystem, so the engine should be run for a while before reading the sight glass.

  41. FUEL HEATERS • In recent years it is more common to find trucks equipped with fuel heaters. • There are two types of fuel preheaters in current use: • Electric element type • Engine coolant heat exchanger type • Fuel heaters exist that use both electric heating elements and coolant medium heat exchangers and, furthermore, manage the fuel temperature. • The figure on the next page shows a Fuel Pro assembly: • This combines a filter, H2O separator, and thermostatically-controlled heating element.


  43. WATER-IN-FUEL SENSORS • Most current systems use a water-in-fuel (WIF) sensor to alert the data bus of H2O contamination of fuel. • The figure below shows a typical WIF sensor and its circuit used in a Volvo fuel subsystem.

  44. FUEL CHARGING/TRANSFERPUMPS • Fuel charging or transfer pumps are positive displacement pumps driven directly or indirectly by the engine. • On most current diesel fuel systems two types of transfer pump are used: • Plunger-type pumps • Gear-type pumps • Plunger-type pumps are less commonly used today.

  45. FUEL CHARGING/TRANSFER PUMPS (Cont.) This figure shows a typical plunger-type pump used by Caterpillar on its hydraulically-actuated electronic unit injector (HEUI) fueled engines.

  46. FUEL CHARGING/TRANSFER PUMPS (Cont.) • Gear pumps are the most commonly-used fuel transfer pump today, and they share common operating principles with other external gear pumps, such as oil pumps.

  47. PLUNGER TYPE PUMPS • Plunger-type pumps are often used with port helix metering injection pumps (see Figure 19–17) and some HEUI fuel systems (see Figure 19–14).

  48. PLUNGER TYPE PUMPS (Cont.) Single-acting and double-acting plungers may be used, with the latter type specified in higher output engines requiring more fuel.

  49. GEAR-TYPE PUMPS • Gear pumps are also commonly used as transfer pumps, especially on most electronically-managed engines. • These are normally driven from an engine accessory drive and are located wherever convenient.

  50. PRIMING A FUEL SYSTEM • Recommended Procedure: • Locate a bleed point in the system, and crack open the coupling. On an inline injection pump system, a bleed point will often be at the exit of the charging gallery. • Next, if the system is equipped with a hand primer pump, actuate it until air bubbles cease to exit from the cracked open coupling. • Re-torque the coupling. Crank the engine for 30-second segments with at least 2-minute intervals between cranking cycles until it starts. • In most diesel engine systems, the high-pressure injection circuit will self-prime once the subsystem is primed.