Engine Lower End and Lubrication System Theory

© 2012 Delmar, Cengage Learning

Engine Lower End and Lubrication System Theory

Chapter 19


Objectives• Describe the related theory of all of the parts

that make up the lower end• Tell how a cylinder block is made and

understand the functions of its parts• Understand how pistons are constructed and

the reasons behind their various designs


Objectives (cont'd.)• Discuss the various types of piston rings and be

able to make the correct choice when selecting rings for a rebuilt engine

• Understand the differences in the various types of engine bearings

• Describe the parts of the crankshaft and their functions


Introduction• Lower end consists of:

– Crankshaft assembly– Piston– Rod

• This chapter describes:– Lower end parts – Engine block – Lubrication system


Cylinder Block Construction• Cast using cast iron or aluminum

– In a mold called a core• Core is supported around outside of core box• Leaves core holes in finished block

– Molten iron poured into core box• Heat of casting process cooks the sand• Casting cools and sand breaks up• Casting is shaken out• Remaining sand washed way through core holes• Core holes closed off with core plugs



Core Plugs• Usually made of steel

– Brass, rubber, stainless steel, or copper expandable are also used

• Brass and stainless steel are superior in marine environment– Do not rust

• Not used in new cars because of cost• Not needed because coolant prevents rust

• Also known as expansion plugs, welsh plugs, freeze plugs, or soft plugs


Cylinder Bore• Cylinders are bored in the block

– Engines today: little cylinder wall wear• Cylinder bore taper wear

– Forms the ring ridge at the top of ring travel• Causes of cylinder bore taper wear

– High pressure of piston rings against cylinder wall– Top of cylinder receives less lubrication

• Out-of-round wear – Results when piston tilts from one side to the other


Cylinder Sleeves• Aluminum blocks

– Usually have permanently installed iron cylinder sleeves

• Sleeves– Replaceable cylinder bores

• Damaged cylinders can be bored oversize – Accept a pressed-fit dry sleeve

• Wet sleeves – Only contact block at upper and lower ends


Main Bearing Caps• Main bearing bores

– Bored at the factory with bearing caps in place

– Main caps are not interchangeable


Lifter Bores• Bored in the block on engines with camshafts in

the block– Lifters spin in the lifter bores– Very little clearance to the lifters

• Just enough to allow oil to leak below to lubricate camshaft lobes


Crankshaft Design• Journals: polished bearing surfaces• Main bearing journals: support crankshaft as it

turns• Rod bearing journals: offset from main bearing

journal centerline• Counterweight opposite each rod journal:

balances offset rod journals and rod• Crankshafts are cast or forged

– Forged cranks: stronger, but cost more– Cast crankshafts: larger counterweights


Crankshaft End Thrust• Crankshaft is pushed forward by pressure of

end thrust– End thrust is exerted by:

• Torque converter • Release spring pressure of clutch

• Thrust surfaces– Precision bearing surface ground on sides of

crankshaft main bearings• Flanged thrust bearing

– Fits between crankshaft thrust surfaces


Direction of Crankshaft Rotation• Most automotive engine crankshafts rotate

counterclockwise– Except Hondas and Hyundais – Transverse mounted engines follow this standard

• Longitudinal mounted engine – Turns clockwise


Vibration Damper• During combustion: crankshaft twists and

overcorrects in the other direction– Torsional vibration causes crankshaft to break– Timing chain and sprocket wear result– Most vibration occurs at front of the crankshaft

• Vibration damper (i.e., harmonic balancer) – Dampens torsional vibration– Heavy outer inertia ring and inner hub separated

by a synthetic rubber strip


Crankshaft Hardness• Some crankshafts are hardened

– Mostly imports and heavy-duty manufacturers– Must be rehardened if reground– Crankshafts that have not been hardened will

suffer misalignment if rehardened• Most crankshafts tend to work-harden with use

– Used, polished crankshaft will have yellow tint


Bearings• Crankshaft bearings

– Usually two-piece plain bearings with a specially designed surface

• Bearing inserts– Made from many different materials

• Bearing properties– Embeddability, conformability, and fatigue strength

• Inserts are positioned in the bearing bore by a locating lug or dowel


Bearings (cont'd.)• Bearing spread

– Measurement across parting face slightly larger than diameter of bearing bore

• Bearing crush – Bearing extends above parting line of bearing

bore half by about .0005”–.00015”• Bearings come in standard sizes and undersizes

– Undersized: used when crankshaft reground• Cam bearings: often made from seamless steel

tubing with lining bonded to the inside


Connecting Rods• Made from forged or cast steel formed in an I-

beam shape– Forged rods: stronger

• Rod caps are not interchangeable– Oil clearances of bearings vary greatly

• Rod oil holes– Squirt oil on the cylinder wall

• Nearly all engines are left-hand– When a left-hand engine has oil-spit holes they

are to the right when the notches face forward


Pistons• Today’s pistons

– Cast or forged aluminum

– Undergo remarkable stresses


Piston Head and Ring Grooves• Piston head (crown) is round

– Skirt is usually oval• Diameter of head

– Smaller than diameter of skirt• Piston ring grooves

– Top piston ring is positioned as high as possible on piston

– Holes in the oil ring groove allow excess cylinder wall oil to return to the crankcase


Heat Transfer• Piston crown heat

– Transferred through piston rings to water jackets• Some manufacturers use different piston head

shapes to allow compression ratio variation– High compression pistons can only be installed

in one direction in the cylinder


Cast and Forged Pistons• Cast aluminum pistons: most common• Forged pistons: available for heavy-duty or high-

performance use– Dense grain structure– 70% stronger than cast pistons

• Hypereutectic pistons: cannot withstand tensile loads– Better wear characteristics


Piston Skirt• Aluminum

– Expands at twice the rate of cast-iron – To control expansion:

• Taper the piston• Piston skirt is cam ground• Struts of spring-loaded steel cast into them

• Trunk piston – Full-skirt piston used on longer stroke engine

• Slipper-skirt – Designed to clear the crankshaft counterweights


Piston Pin Offset and Piston Pins• Piston pin offset and height

– Different configurations• Piston pins

– Attach piston to connecting rod• Piston pin types

– Pressed-fit in rod– Full-floating


Piston Rings• Most engines use two compression rings and

one oil ring– Top ring: exposed to flame of combustion during

every power stroke• Piston rings:

– Seal combustion pressures– Help cool piston– Control oil consumption


Compression Rings• Forced against cylinder wall by combustion

pressure at top and back of ring– Top ring controls sealing of combustion– Second rings captures pressure that escapes

• Cast in groups – Installed on a mandrel and machined out of

round• Low-tension rings

– Introduced to improve fuel economy


Compression Ring Design


Compression Ring Materials and Coatings

• Most rings: made of plain cast iron– Cast iron rings: used in re-ring jobs– Moly rings: have groove machined on their faces– Chrome rings: last the engine life with no wear

• Premium ring combination:– Moly barrel-faced top ring– Reverse-torsion second ring– Three-piece chrome oil ring


Compression Ring Materials and Coatings (cont'd.)

• High-strength rings– Ductile iron rings: withstand higher temperatures– Steel rings: made from steel wire

• Plasma ceramic rings– Five times as strong as a stock ring

• Resist detonation damage• Cause less cylinder damage• Excellent break-in characteristics

– Cylinder preparation same as for moly rings


Oil Control Rings• Oil rings

– Run at a temperature of 250°F• Oil consumption

– Increases with engine speed– Vacuum during deceleration increases with

compression ratio• Several oil ring designs

– Single-piece cast types– Three-piece type


Engine Balancing• Engine vibration and worn parts

– Results from a lack of engine balance• As engine speed doubles force from imbalance is

multiplied by four– An engine can be balanced to prevent vibration

• Material removed from heavier parts to weigh the same as lighter parts

– Balance shafts• Silent shafts: have counterweights timed to cancel

out engines imbalance


The Lubrication System


Oil Pumps• Gear on the camshaft drives the oil pump

– Types of oil pumps• External gear• Rotor or gerotor• Internal gear or crescent

• Gerotor pumps – Smooth pumping action and less aeration of oil

• Crankshaft-driven oil pumps– Turn twice as fast as camshaft-driven


Pressure Relief Valve• More oil pumped at faster speeds

– Must have a relief valve for excessive pressure• Most relief valves divert excess oil back to inlet

side of pump• Maximum oil pressure is controlled by tension of

the relief valve spring– Too much pressure can burst the oil filter


Oil Pump Screen By-Pass Valve

• Most sump screens have a by-pass valve that opens– Screen is plugged– Oil is too cold or thick to flow freely

• Foreign material will be sucked into the pump


Oil Pressure• Proper lubrication

– Achieved by distribution of clean oil under pressure

– Important: correct amount of bearing clearance • If correct bearing clearances are not maintained: oil

will not reach all areas of engine while idling• Excessive oil clearance near the pump: results in

insufficient oil pressure• Satisfactory oil pressure: around 25 psi

– Indicator lights come on when pressure drop below 10 psi


High-Volume Oil Pumps• Output per revolution

– Depends on diameter and thickness of rotors or gears

• High-volume pumps – Deliver more oil per revolution– Provide more oil to worn engine at idle– May not provide any other advantages to

passenger car engines


Windage Tray and Baffles• At high speeds: revolving crankshaft creates

wind– Causes air pockets around oil pump screen– Causes the pump to lose its prime

• Windage tray– Prevents air pockets

• Baffles – Keep oil from sloshing with car movement

• Check for foreign material trapped under a windage tray or baffle


Dry Sump Lubrication Systems• More complex and cost more to produce

Documents

Engine Lower End and Lubrication System Theory