Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems, 3E Chapter 8 Diesel Engine Feedback Assemblies
Introduction The feedback assembly of an engine is the engine s mechanical management apparatus Included components: Timing & accessory gearing The camshaft, tappets, valves & unit injector trains Fuel pumping mechanisms These components are driven by and often timed to the crankshaft
Engine Feedback Assembly Drive Mechanism Heavy duty engines Medium duty engines Gear sets Lighter duty engines Pulleys & belts Chains & sprockets
Timing Gears Location: Front Rear Lubrication: Splash Bearing Spill Timing Lubrication The Gear Gears Location Traditionally, most timing gears were The engine gear ratios gear dictate train is the enclosed rotational in a located at the front of the engine. housing speed & that are responsible permits the for engine maintaining oil to lubricate the Today, timing some the (or rotating fixed common location components. diesel between engines the driven locate their components) timing gear at any sets given at the rear moment of the engine. of engine operation. Some larger engines may locate the timing gear sets in both the front & rear.
Gear Construction Manufacturing Process: Cast Forged Gear Teeth Milled Gear Hardened Heat tempered Surface hardened Flame Nitriding Carburizing Induction process Helical Greater tooth contact area Lower operational noise or Spur Lower thrust loads Gears press fitted to shafts Keys and keyways ensure timing
Inspection, Removal & Installation Visual Installation Removal Timing Inspection When Use Press Cracks a thermostatically fitting gears gears require to shafts, controlled the Pitting use ensure of a oven! puller all essential Heat If the discoloration gear timing to and OEM aspects shaft specified are Tooth considered! removed lipping temperature from the Any engine, of Gear these will gear conditions drop removal over usually shaft may be indicate done correct on a gear a press must temperature be replaced! Ensure the gear is heated evenly! Tempilstick TM may be useful Usually around 300 o F (150 o C)
Timing Geartrain Engines may use several gears in series Idlers & PTO The camshaft may not turn in the same direction as the crankshaft Ensure all gear timing is to OEM procedures!
Checking the Geartrain Timing 1. Locate the crank to TDC on #1 cylinder (compression stroke) 2. In this position, the cam profiles for #1 cylinder will both be on their inner base circle 3. Using the OEM s literature, align the respective timing index marks 4. Some OEMs use offset gear to shaft locating keys to trim engine timing 5. Always check the gear sets for excessive backlash
Timing Overhead Camshafts Camshafts Each camshaft has a different function Valve operations Engine brake Injectors Concept gears used Coaxial springs used to dampen gear train oscillations
Timing OHC Concept Gears 1. Pin the crankshaft 2. Wedge the camshafts 3. Install lower concept gear 4. Install lower gear cover & seal 5. Install vibration damper 6. Install cam gears loosely on each cam nose taper
Timing OHC Concept Gears 7. Place a 0.010 thickness gauge between the teeth of the adjustable idler gear & its mesh point with the injector cam gear 8. Using hand pressure, move the adjustable idler gear into mesh toward the center of the engine 9. Hold the adjustable idler gear & torque the idler fasteners to specification
Timing OHC Concept Gears 10. Remove the thickness gauge 11. Torque the injector concept gear fasteners to specification 12. Remove the camshaft wedges 13. Lubricate the geartrain with whatever lubricant is normally used in the engine
Camshafts Rotates once for ever engine cycle A crankshaft in a four stroke engine turns two revolutions (720 o ) to complete a cycle A crankshaft in a two stroke cycle engine turns one revolution (360 o ) to complete a cycle The camshaft actuates the valve trains The camshaft is supported at its journals with bushings or bearings These bearings are pressure lubricated
Cam Construction & Design Material: Middle alloy steel Hardened journals & cams Wear surfaces finish ground Geometry: Physical shape of the cam Location: Profile outside base circle actuates the trains Convert rotary movement of cam into reciprocating motion Overhead cams are becoming more common Internal to engine block traditional Nitriding or other surface hardening methods are used Camshafts traditionally fail due to lobe wear & are not usually reconditioned
Cam Construction & Design Block mounted camshaft: Camshaft Timing gear Cam bushings Thrust/retaining device Timing cover Auxiliary drive gear
Cam Construction & Design General: Journals Timing gear locating key Lobes Intake Exhaust Sometimes fuel injection
Camshaft Events Creating a valve polar diagram may assist with understanding camshaft events Valve openings Valve closings Injector actuation on engine equipped with: Cummins PT system Mechanical unit injectors (MUI) Electronic unit injectors (EUI) Electronic unit pumps (EUP) Plotted by observing valve activity related to crank rotation
Camshaft Events
Camshaft Terminology Alternatively, This diagram cams illustrates may a be cam designed that is so designed that most so that of their a large periphery percentage is OBC of The The largest smallest circle, radius or of lift the (outer its periphery base circle). (circumference) provided cam, concentric by the cam with is base the cam center line In circle. this case, the train actuated by this cam This is loaded referred for to most as an of IBC the (inner cycle. base The circle) Cummins design. PT injector is an example With this of design, this design the related in use. train mechanism will be unloaded for most of the cycle. For this example, cam rotation
Removing & Installing Camshafts OHC (overhead) design Block mounted More Relatively complex: simple: Tappet Remove assemblies cam caps need to Remove be held cam away from from the cam pedestals Never use force to remove the camshaft It may be necessary to rotate the crankshaft to a specific position before removing camshaft
Camshaft Inspection Visual Cam Lobe Journals Keyways Pitting Scoring Hard surface failure Peeling of lobes Blueing of journals Distortion
Camshaft Inspection Measured Micrometer Dial indicator Thickness gauge Cam profile Journal dimensions Base circle Heel to toe Cam lift Shaft warpage Lobe surface wear
Camshaft Bushings Friction bearings Support camshaft Subject to loading when train rides profile Pressure lubricated Routinely replaced at overhaul Interference fit in block If the bushings are to be reused, they must be measured with a dial bore gauge to ensure they are within OEM spec s
Camshaft Bushings Bushing replacement cylinder block mounted cam Retained by an interference fit in block bores Removed from front to rear (in sequence) Use the correct sized bushing mandrel & slide hammer Use same mandrel sizes to install new bushings Ensure lubrication holes align correctly in the block Ensure correct bushing is driven into each bore!
Camshaft Bushings Bushing replacement -- overhead camshaft Split bearing shells retained by cap crush or lock rings Care should be take to properly align oil holes Thrust washers Camshaft endplay is defined by: Free or captured thrust washers or plates Thrust loads are more extreme with helical gearing More likely to wear on thrust faces
Camshaft Position Sensors CPS On all current engines Signal shaft speed & position to ECM May be either : Pulse generator Hall-effect signaling CPS signals frequency value and irregular tooth @ #1 TDC
Valve & Injector Trains Converts camshaft rotary action to linear action Followers ride the cam profile Effects of cam geometry transmitted to valve & injector assemblies Liner motion is converted to rotary motion at the rocker
Cam Followers Directly ride or actuated by cam profile Also known as: Tappet Lifter Function: To reduce friction Distribute force from cam profile to the train Types Solid lifters Roller cam followers
Pushrods & Tubes Caution! When checking ball & Application: socket integrity, test It does not make economic sense to ever straighten or Cylinder block mounted camshafts them Inspection by dropping Notes: them try to repair a push tube or Checking rod. on a for concrete Wear: floor from a height Transmit force from cam Visually action Pushrod of about to ball rocker 2. & sockets assembly are usually Shock loads increase chemically Subject to shock loads Pushrods deterioration hard proportional with surfaced integrity of hard surfacing to engine issues speed! will The failure will recur it is only Seldom ball always a question fail or ring socket under flat. normal to of tube when! operating integrity Manufactured from alloy conditions steels straightness Usual Next, causes Tubular roll of the Failure: shapes pushrod provide across nearly a flat Often hollow inaccurate Alloyed as high steel a lash section allows settings modulus the pushrod as a The cost of push tube failure can surface if be much it wobbles more sounds Fitted with a ball or socket at flat either engine to solid replace sustain overspeeding cylindrical it! end shock loads & remains while expensive than the cost of replacing keeping relatively it! weight low in to weight a minimum Inspection: Ball & socket integrity Straightness The ball & socket aspect of the pushrod provides the bearing surface, transferring force and allowing the rocker to move through an arc
Rockers Sometimes referred to as tappets Rockers are levers Pivots on rocker shaft Transfer camshaft motion to: Valves Mechanically actuated injectors Used in: Camshaft in block design Overhead camshaft design Pressure lubricated through rocker shaft
Rockers Rocker Ratio Relationship of the rocker to rocker shaft centerline: May be used to amplify cam lift dimension to increase valve or injector travel 1:1 Rocker is centered on rocker shaft Equal cam lift & actuation travel 2:1 Rocker centerline is further from valve or injector Greater actuation travel compared to cam lift
Rocker Inspection Inspect for wear: Push tube end Pivot bore Pallet end If hard surfacing at either end of the rocker shows deterioration, it should be replaced The pivot bushing or an adjusting screw ball can be replaced
Cylinder Head Valves General Cylinder head valves provide a method of: Admitting air to the engine cylinders Releasing air (exhaust) from the engine cylinders The movement of the valves relates to cam profile The location of the valve, the lift and timing can influence cylinder gas dynamics The timing of valve movement depends on: The engine brake (if equipped) If the engine uses variable valve timing (VVT)
Valve Design & Materials Description: Mushroom headed Poppet type Functional Aspects Head Stem Hardened Welded Fillet radius Keeper grooves Margin
Valve Design & Materials Intake Made with middle alloy steels Less temperature concerns than those of exhaust valves Operate at high speeds, need a degree of flexibility 45 o seat angle common 30 o seat angle used by some to increase breathing efficiency Exhaust Made with special ferrous alloys Subject to higher operating temperatures Must sustain high heat through duration of exhaust process Cooled by: Dissipation through valve seat Dissipation through valve guide Intake air at valve overlap
Valve Operation Current diesel engines: Designed to ensure valve seat contact maximized For maximum heat transfer from valve to cylinder head Interference angles seldom machined Rotators used to: Minimize carbon buildup on seats Promote even valve & seat wear Rotators: Use a ratchet principle (or a ball & coaxial spring) Valve rotated fractionally each actuation
Valve Operation Valve Harmonics Cylinder head valves are seated: With a spring, or A pair of springs Valve springs are a critical component of the train Spring force needs to be sufficient to prevent valve float Importance of spring performance increases as RPM increases Real time periods between opening & closing are reduced
Inspecting Springs & Retainers 1. Remove cylinder head 2. Remove valves 3. Check the keepers & valve keeper grooves for wear 4. Inspect the retainers 5. Measure the spring vertical height
Inspecting Springs & Retainers 6. Check valve spring tension 7. Check spring for abrasive wear 8. Check for straightness 9. Check valve spring operation after reinstalled in head
Valve Servicing 1. Before removing cylinder heads: Remove engine brake actuators Remove VVT actuators Remove rocker assembly 2. Remove valves from cylinder head Tag individual valves by location 3. Clean valves On a buffer wheel Glass beading (except on valve stem) May also be used to clean the cylinder head
Valve Inspection 1. After cleaning, visually check for: Dishing Burning Cracks & pits Valve stem straightness Keeper grooves for integrity Any visible wear, nicks or cracks will render the valve scrap! 2. Inspect the valve fillet for cracks & nicks 3. Measure the valve with a micrometer Diameter @ three points through sweep (travel) If any of these conditions are found to exist or the valve is not within specification it must be replaced!
Valve Margin 1. Measure valve margin: Margin is the dimension between the valve seat & the flat face of the mushroom head This specification is critical: After grinding, ensure that this dimension exceeds minimum specified values A valve margin that is lower than specification will result in valve failures caused by overheating
Valve Dressing & Grinding 1. Dress the grinding stone using a diamond dressing tool 2. Adjust the valve grinder chuck to the specified angle 3. Set the carriage stop to ensure the grinding stone will not touch the valve stem 4. Start coolant flow over the valve 5. Make a shallow grinding pass 6. Make the minimum number of passes to produce a face surface free from ridging & pitting 7. Check the valve margin to ensure valve is within spec.
Valve Seat Inserts Used on most current bus & truck diesel engines Cylinder heads are machined Seat inserts are press fitted into the machined recess Advantage: May be manufactured from tough temperature resistant material Easily replaced at cylinder head service Since most of the heat of the valve must be transferred from the valve to the seat, it is essential the contact area of the seat and cylinder head is maximized
Valve Seat Removal & Installation 1. Check valve guides Replace if necessary 2. Remove valve seat 3. Clean seat counterbore 4. Install new insert 5. Check concentricity of the newly installed seat 6. Grind new seat to spec
Valve Seat Grinding The valve seat: Ground with a specific grit emery stone The stone is dressed to the appropriate angle A pilot shaft ensures the concentricity of the grinding The grinding process may affect: Valve head protrusion Recess dimension If the cylinder head has been resurfaced, undersized valve seat inserts are available from most OEMs
Valve Lash Adjustment When the valves are properly adjusted there should be clearance between the pallet end of the rocker and the top of the valve stem This is referred to as lash & is required to accommodate heat expansion Sometimes also referred to as overhead adjustment Valve lash is a factor of: Length of push tubes Materials used in manufacture Application
Valve Adjustment Procedure Sample procedure Always refer to OEM s specifications & procedures Typical four stroke inline six cylinder diesel General information: Firing order: 1-5-3-6-2-4 Companion cylinders: 1-6 at TDC 5-2 at 120 o BTDC 3-4 at 120 o ATDC
Valve Adjustment Procedure View the engine from overhead: Engine position can be identified Observe the valves over a pair of companion cylinders For example, if timing indicator shows #1 & #6 pistons are approaching TDC Valves on #6 are both closed (lash evident) Point at which valves on #1 rock is TDC This is a commonly used method of orienting engine location for valve adjustment Exhaust closing Intake opening Rocker covers removed!
Valve Adjustment Procedure 1. Locate valve lash dimensions 2. Locate the engine timing indicator & cylinder calibration indexes 3. Ensure that the engine will not start by mechanically or electronically no-fueling the engine 4. Bar the engine in its normal direction of rotation
Valve Adjustment Procedure Valve Bridges Also called yokes Adjusted before setting valve lash Setting Valve Bridges 1. Back off rocker arm 2. Loosen yoke locknut 3. Back off yoke adjusting screw 4. Use finger pressure on rocker arm to load pallet end of yoke
Valve Adjustment Procedure 8. Verify the yoke is properly adjusted Insert two equal thickness gauges (.010 or less) between each valve stem & yoke Load yoke with finger pressure on rocker arm Simultaneously withdraw the thickness gauges There should be equal drag on both gauges 9. Adjust yokes in sequence as each valve is adjusted
Setting Valve Lash Always follow OEM instructions! Some OEMs specify an exact setting location The cams may have only a small percentage of base circle Always back out the adjusting screw & reset Use the specified thickness gauge to set lash Always reset jam nut & mark completed rockers Never set a valve too tight! The valve may not close completely when the engine is at operating temperature!
Setting Valve Lash Cautions: Begin valve sets with #1 cylinder Set all valves in sequential firing order Shortcuts: Avoid shortcuts until you know the engine & which shortcuts will involve zero risk Shortcuts applicable to one engine may represent risk if applied to another engine Best shortcuts are outlined by OEMs in service literature
Barring Engines Use OEM Procedure Some specialty tools may assist Ensure the engine is no-fueled! Failure to do so may result in the engine starting with resultant injuries
Variable Valve Timing The means to vary either: The opening of cylinder valves The closing of cylinder valves Variable valve timing was first used for compression engine brakes Initially hydra-mechanically controlled Since 1990 s electro-hydraulically controlled Recently variable intake valve timing introduced Permits retardation of the start of the compression stroke
Other Feedback Assembly Functions Pumping of fuel To injection pressure MUI injectors EUI injectors EUP (electronic unit pumps) High pressure apparatus is actuated by cam profile Push tubes may be larger than those used for valve trains Higher compressional loading Injector trains must be precisely set!
Summary The engine feedback components incorporate: The timing gear train The camshaft Valve & unit injector trains The injection pumping apparatus (in some cases) Camshaft drive gears must be precisely timed with the crankshaft driven engine geartrain to ensure events activated by the feedback assembly are synchronized with those in the engine
Summary The camshaft drive gear is most often interference fit to the camshaft and positioned by a keyway Camshaft gears are heat treated & when fitted to camshaft it is essential that they are heated evenly to a precise temperature for fitting to the camshaft Overheating a heat treated item will result in premature failure Camshaft may rotate either with or opposite the direction of engine rotation Camshafts may use either helical or spur cut gears
Summary The thrust load with helical gears is much higher Gear back lash must be measured using dial indicators or thickness gauges Cam lift on a block located camshaft may be inspected using a dial indicator mounted above the push tube Cam base circle or IBC is that portion of the cam periphery with the smallest radial dimension Cam OBC is that portion of the cam periphery with the largest radial dimension
Summary The critical cam dimension may be checked on an overhead camshaft or and out of the engine camshaft with a micrometer Cam lift can be checked by mounting the cam into a set of V blocks & using a dial indicator A visual inspection of the camshaft should identify most cam failures Cam profile wear may be checked with a straightedge and a thickness gauge
Summary The camshaft may be checked for straightness when mounted in the V blocks Out of the engine camshafts should be supported on V blocks, pedestals or hung vertically to prevent damage Most medium & large bore diesel engine cam followers are of the solid or roller types. Most medium & large bore diesel engines do not use hydraulic lifters
Summary A cam train consists of the series of components it is responsible for actuating Most diesel truck engines, with a block mounted camshaft use trains consisting of a follower assembly, push tubes & a rocker Most valve trains are adjusted with a lash factor to allow for expansion of the material as the engine heats up to operating temperature Injector train settings may have to be precisely set because they help define injection timing
Summary Rocker assemblies provide a means of reversing the direction of linear movement of the push tube or follower. Rockers, in some cases, provide mechanical advantage Cylinder head valves are used to aspirate or breathe the engine cylinders Cylinder head valves are actuated by cam geometry & time the air into and end gases out of the engine cylinders
Summary Exhaust valves are often manufactured out of more highly alloyed steels than intake valves because of the higher temperatures they must sustain The valve margin is a critical refinishing measurement Positive valve rotators are widely used A 45 o cut valve has a higher seating force but lower gas flow than a 30 o cut valve
Summary Valve seats are usually interference fit to the cylinder heat & finish ground concentric to the valve guide bore Most diesel engines do not use an interference angle to seat valves because the seating contact surface area is compromised When setting valve lash, the OEM specifications to engine position for the valve being adjusted should be observed. Cam geometry on some engines is not clearly divisible into IBC & OBC sections
Summary Valve lash should be set using thickness gauges Valve lash is set statically with the engine cold Loosely set valves cause lower cylinder breathing efficiencies & produce top end clatter Loosely set valves may damage cam profiles Valve yokes or bridges do not usually have to be adjusted as part of routine valve adjustment It is important that the bridge is properly supported to prevent its pedestal guide from being damaged
Summary Variable valve timing is commonly used to control exhaust valves for engine braking & also to delay intake valve closing to reduce emissions Creating a valve polar diagram is an effective way; To map valve opening & closing events Map mechanically actuated injectors