Lubrication Systems for Petrol Engines (Automobile)

11.9.

Lubrication Systems for Petrol Engines

In order to ensure adequate supplies of oil to the engine parts, a reservoir of oil is provided by the sump which is the lower part of the lubrication system and in automobile engines the sump is the oil pan. From the reservoir, oil is distributed throughout the engine either by the splash system or the full pressure system. In case of two-stroke engines, the crankcase cannot be used as an oil reservoir. The lubrica­tion, in this case, is provided by mixing a small proportion of oil with petrol.
In the splash system the oil is maintained in little troughs (Fig. 11.9). There are dippers at the ends of the connecting rods to splash the oil on the various parts like cylinder walls, cam­shafts, gudgeon pins etc. as they travel through the oil troughs towards the bottom of the stroke of the piston. The oil is supplied to the main bearings under pressure due to an oil pump through drilled passages, in the crankcase, called galleries. The oil pump also replenishes the troughs. The system is now practically ob­solete.

Fig. 11.9. Splash lubrication system.

11.9.1.

Full Pressure System

Automobile engines today use ‘forced-feed’ lubrication systems, generally of the wet-sump type in which the sump acts as both an oil-drain return and a storage container. A rotary-type oil-pump provides forced feed. The pump may be driven directly from the crankshaft or indirectly from the camshaft or any auxiliary shaft (Fig. 11.10). Oil from the sump reaches the pump through the submerged gauze strainer and pick-up pipe. The oil is then compressed, which passes through a drilling to the lubrication system.
A pressure-relief valve positioned on the output side of the pump controls the oil pressure. If the oil pressure becomes too high, the relief valve opens and bleeds surplus oil back to the sump. The relief valve may be installed on the filter unit, the crankcase, or the pump housing.
The oil-pump forces the oil through drillings in the crankcase to a cylindrical full-flow filter unit. The oil circulates around the filter bowl, passes through the filter towards its centre, and flows out to the main oil passage, called main oil gallery (Figs. 11.10 and 11.11) which lies parallel to the crankshaft. In most car and commercial vehicle engines, the oil gallery is formed by drilling a hole in the crankcase for full length of the engine and plugging the ends.

Main- and Big-end Bearing Lubrication.

The oil is fed to the crankshaft main journal bearings and in some cases to the camshaft bearings (Figs. 11.10 and 11.11) through various branch cross-drillings in the crankcase. A few heavy commercial engines use a separate pipe located underneath the main-bearing caps and by pedestal brackets. Drillings in these brackets connect the gallery-pipe oil to the main
bearings. By diagonal drillings in the crankshaft a continuous oil is fed to the big-end bearings from the oil grooves around the main-bearings liners. These drillings pass from the main-bear­ing journal to the big-end crankpins (Fig. 11.10) through the crankshaft web.

Fig. 11.10. Forced-feed lubrication system. A. Front sectional view. B. Side sectional view.

Cylinder and Piston Lubrication.

Four separate techniques are used for cylinder and piston lubrication.
(i) Connecting-rod big-end side-clearance oil spray.
(H) Connecting-rod big-end radial-hole oil spray.
(Hi) Connecting-rod small-end radial-hole oil spray.
(iv) Crankcase fixed-jet oil spray.
Nowadays one or a combination of these methods is used to achieve effective cylinder lubrication depending mainly on the operating conditions expected from the engine.
Connecting-rod Big-end Side-clearance Oil Spray. Cylinder and piston lubrication by big-end side-clearance splash (Fig. 11.12) is the simplest and most common method. In this case

Fig. 11.11. Forced-feed lubrication system. A. With push-rod and-rocker valve mechanism. B. With overhead camshaft.
the oil pressure of the lubrication system, the squeezing action between the connecting-rod and the big-end journal, and the amount of side clearance all together cause sufficient oil splash to the cylinder walls and the underside of the piston when the crankshaft throw is near to the TDC.

Fig. 11.12. Connecting-rod big-end side-clearance oil spray.

Connecting rod Big-end Radial-hole Oil Spray.

In this case a small radial drilling hole in each connecting-rod big-end directs a squirt of oil to the thrust side of the cylinder bore once in every revolution of the crankshaft (Fig. 11.13). The diameter of the hole and its angular location is critical in this method of lubricating the cylinder.

Connecting-rod Small-end Radial-hole Oil Spray.

The connecting-rod in high-performance en­gines may have a drilling connecting the big-end to the small-end which causes a positive oil feed to the gudgeon-pin (Figs. 11.14 and 11.17). The big-end bear­ings in heavy-duty diesel engines are grooved so that a
continuous flow of oil is provided through the drilled connecting-rods to the small-end bearings. The small-end eyes have two drillings which may supply jets of cooling oil to the ring-belt areas within the pistons.

Crankcase Fixed-jet Oil Spray.

In turbocharged heavy-duty diesel engines, a jet is positioned in the crankcase which projects upwards to provide a controlled and continuous spray of oil that cools and lubricates the underside of the piston. This system of oil supply is more active in reducing piston and ring temperature than providing additional lubrication for the cylinder-and-piston combination (Fig. 11.15).

Fig. 11.13. Connecting-rod big-end radial-hole oil spray.

Small-end Lubrication

The piston ring scrapes the oil from the cylinder bore on its down-stroke causing pumping action due to which the small-end is positively lubricated. This oil is pushed into the groove behind the lower piston oil-control ring. Subsequently it flows along a drilled passage that

Fig. 11.14. Connecting-rod small-end radial-hole oil spray.

Fig. 11.15. Crankcase fixed-jet oil spray.
intersects the piston gudgeon-pin-boss bores. During pivoting of the small-end of the connect­ing-rod, this oil spreads over the bearings surfaces. Circumferential slots or drillings are made at right angles to the gudgeon-pin bosses, due to which some of the surplus oil is splashed between the piston gudgeon-pin bosses and the small-end of the connecting-rod. This is necessary when the gudgeon-pin is fully floating and there is no oil supply from the connecting-rod. Additionally, there is a limited amount of splash from the big-end side clearance to complete the small-end lubrication (Figs. 11.16 and 11.17).

Fig. 11.16. Semi-floating gudgeon-pin with scraper-ring oil supply.

Fig. 11.17. Fully floating gudgeon-pin with scraper-ring and connecting-rod oil supply.

Camshaft-bearing Lubrication.

There are four basic methods used for supplying oil to camshaft bearings.
(i) Individual cross-drillings in the crankcase directly feed oil from the main oil gallery to each camshaft bearings (Fig. 11.1)
(ii) The drillings in the crankcase, which connect the oil grooves in the crankshaft main bearing to the camshaft bearings (Fig. 11.2A), provide a continuous supply of oil.
(Hi) A single drilling provides oil from the main oil gallery to one of the internally grooved camshaft bearings. This oil then enters a pair of radial cross-drillings into the hollow camshaft. A central axial oil passage in the camshaft supplies oil to the other bearings through single radial cross drillings (Fig. 11.11B).
(iv) A separate camshaft oil passage is drilled into and along the length of the camshaft pedestal block. This drilling has intersecting holes connecting it to the various camshaft bearings (Fig. 11.18) through which oil is fed.

Camshaft-lobe Profile Lubrication.

Methods of lubrication of the camshaft lobes are broadly divided into lubrication for low-mounted camshafts and for high-mounted camshafts. Low-mounted-camshaft lobe lubrication depends mainly on the following :
(a) The big-end side clearance allows oil to be flung out, which splashes the cam lobes each time the crank-throw aligns with the cam shaft, i.e., once every revolution of the crankshaft (Figs. 11.12 A and B).
(b) Draining of oil from the rockers splashes on to the cam profiles (Fig. 11.19).
(c) Oil mist is created by the rotating crankshaft and the rocker and crankcase ventilation system.
High-mounted-camshaft lobe lubrication depends on the type of valve-actuating mechanisms used.
(a) In the direct-acting and centrally pivoted rocker-arm, the cam lobes are provided with a cyclic splash of oil from a drilling in the rocker-arm (Fig. 11.20.)
(b) In the direct-acting and end-pivoted rocker-arm, the cam lobes are lubricated by a spray of oil directed on to the lobes. This is provided by a pipe located between the camshaft-bearing pedestal supports (Figs. 11.21 and 11.26).
(c) In direct-acting cylindrical followers, the cam lobes are lubricated by following three methods (Fig. 11.25.)
• a simple oil-trough splash,
• a radial drilling intersecting the cam base circle,
• an oil spray coming from a drilled passage along the entire length of the cylinder head.

Fig. 11.18. OHCwith directly actuated cylindrical follower.

Poppet-valve Lubrication.

The lubrication of the valve stem and tip is carried out by splash of oil and drainage of surplus oil from the rocker-arm and/or the camshaft lobes. (Figs. 11.22 through 11.26).

Valve Rocker-arm-mechanism Lubrication.

The lubrication of the valve rocker-arm depends on the type of rocker-arm assembly used.
Solid Rocker-arm. These arms are lubricated by an oil drilling or pipe extending from one of the camshaft bearings to a hollow
rocker-shaft which has radial holes aligning with each rocker-arm. The rocker-arm pivot hole may either be bored and used directly over the shaft or be bronze-bushed with internal oil grooves.

Fig. 11.19. High-mounted camshaft with push rod ac­tuated rocker arm.

Fig. 11.20. OHC with centrally pivoted and end-actuated rocker-arm.

Fig. 11.21. OHC with end-pivoted and centrally actuated rocker-arm.
Three ways of feeding the oil to the valve stem-and-springs assembly and to the tappet and push-rod end are shown in Fig. 11.22.
(i) The valve stem and the tappet assembly is lubricated by a single vertical radial drilling in the middle of the rocker-arm (Fig. 11.22A). As the arm rocks, oil is squirted out in both directions. This method can meet the quantity of oil required for small engines, (it) A more controlled lubrication of the tappet and push-rod assembly is achieved by a horizontal drilling between the rocker-arm pivot hole and the tappet end of the arm. The valve-stem end of the rocker-arm has an open grooved channel formed along the top of the rocker-arm through which the surplus oil floods and drains down over the valve and return-springs (Fig. 11.22B). This system is generally adopted on some medium-sized petrol and diesel engines. (Hi) For heavy-duty operation, lubrication is provided by connecting the rocker-shaft feed to a hollow tappet screw due to which oil flows directly into the push-rod bowl-shaped seat and then overflows and drains down the push-rod lubricating the cam follower (Fig. 11.22C). The valve-stem end of the rocker-arm contains a horizontal hole drilled along it so that oil is directly fed to the valve-and-spring assembly. This method, however, may over-lubricate the valve stem if no restriction is imposed on the oil supply to the rockers. This is a problem with this system.
Steel-pressing Rocker-arm with Hollow Push-rod. One of the camshaft bearings supplies oil through oil drilling to the tappet-follower gallery drilling that lies parallel to the camshaft. From this gallery oil flows around an annular groove in each tappet-follower body ensuring positive lubrication. The flow of this oil through the hollow push-rod and to the rocker-arm and the valve is controlled by a valve disc in the tappet (not shown).
Steel-pressing Rocker-arm with Central Hollow Stud. Oil passes through a passage in the first camshaft bearing to the tappet-follower oil gallery drilled alongside the tappets extending the entire length of the cylinder head (Fig. 11.24). From the gallery oil flows around a recess machined on the tappet and then to a short drilling that meets the central rocker-arm

Fig. 11.22. Solid-rocker-arm. oil hole and valve-stem oil-seal arrangements.
pivot-post stud. The stud is hollow and has a radial intersecting hole so that the oil supply from the tappet gallery is connected to the spherical rocker pivot. The oil then splashes and floods the rocker pressing, consequently overspills lubricating both the valve assembly and the top of the tappet follower.

Fig. 11.23. Steel-pressing rocker-arm with hollow-push rod oil supply.

Fig. 11.24. Steel-pressing rocker-arm with central hollow-stud oil supply.

Overhead-camshaft Lubrication.

The method of lubrication of overhead camshafts depends on the type of actuating mechanism used and they are as follows :
(a) With the direct-acting bucket follower camshaft arrangement (Fig. 11.25), the cam­shaft, follower, and valve stem are lubricated either by a drilled hole along the centre of the camshaft axis and intersecting radial holes emerging on the base circle of the cam, or by a drilled hole in the pedestal casing parallel to the camshaft and projecting spray holes directed on to the cam profiles. In either of the cases the follower and the valve stem are lubricated by drainage of oil from the camshaft.
(6) With the indirect end-pivoted rocker-arm arrangement (Fig. 11.26), the camshaft, rocker, and valve stem are lubricated by spraying oil on to the cam faces through an oil pipe attached to the camshaft pedestal housing. The excess oil draining from the camshaft also flows over the rocker-arm and lubricates its pivot joint and the valve tip and stem.

Lubrication of Timing Gears and Chains.

These excessively used components are normally lubricated by a small drilling, which intersects the oil passage running from the main oil gallery to the first main bearing (Fig. 11.10) or the passage from the first main bearing to the first camshaft bearing (Fig. 11.11A). Sometimes
a small pipe from this drilling directs the oil on to the gears or chain. Moreover, in some constructions the sump is shaped to form a timing-gear oil trough, due to which the draining oil submerges the crankshaft gear providing a continuous upward oil splash to the rest of the camshaft drive.

Fig. 11.25. OHC cylindrical direct-acting with a fixed-pedestal spray, a hollow camshaft with a radial oil hole, or simply a trough splash bath.

Fig. 11.26. OHC with end-pivoted rocker-arm and oil-pipe-supply spray.

Crankshaft Oil Passages.

Crankshaft oil passages feed oil from the main-journal bearing to the big-end journal. In its simplest form, the oil passage is a diagonal drilling (Fig. 11.27A) running from the main journal to the big-end journal. Normally the diagonal hole is drilled at an angle to the crank-web centre-line so that, when the crank-pin is in the TDC position and combustion force pushes the connecting rods downwards, some oil still enters between the journal and the bearing. It is because if the exit of the diagonal hole is exactly at the top of the big-end journal, oil can not enter between the bearing and the journal in the TDC position. Additionally the effective projected bearing area is also reduced by chamfered oil hole.
To have an improvement in oil delivery, a cross-drilling (Fig. 11.27B) runs straight through the big-end journal and a diagonal drilling from the main-bearing journal intersects the big-end cross-drilling. Another hole is also drilled diametrically opposite the diagonal-hole’s entry in the main journal, so that when the bearing is loaded at the top or the bottom of the stroke, the other side of the bearing permits oil to enter.
11.9.2.

Oil Pumps

Four basic types of rotary-operating oil-pumps are used in pressure-feed lubrication sys­tems. They are
(i) External-spur-gear pump, (ii) Internal-gear crescent pump, (Hi) Eccentric bi-rotor pump, and (iv) Sliding-vane eccentric pump.
Selection of a pump is usually based on their ease and convenience of being installed and driven. Other considerations are expected pump life, oil-flow rate capacity, priming time, the ability to built up pressure at low speeds, and the ability to deliver oil under higher pressure conditions continuously at high engine speeds.

Fig. 11.27. Crankshaft oil passages.
A. Crankshaft with single oil passage .
B. Crankshaft with diagonal web passage and right-angled cross-drilling in the big-end journal.
The generation of oil pressure by the oil pump depends on the “leaks” in the engine. The “leaks” are the clearances at end points of the lubrication system, such as the edges of bearings, the rocker arms, the connecting rod spit holes, etc. These clearances are introduced for proper operations of the engine. The leakage increases as parts wear and clearance becomes greater. The oil pumps capacity is based on its size, rotating speed, and physical condition. The oil pump capacity is low at engine idling and when the “leaks” are relatively more than the pump capacity. As the engine speeds up, the pump capacity increases and it tries to force more oil out of the “leaks”. This causes the pressure to rise until it reaches the regulated pressure. Engine oil viscosity also takes part in both the pump capacity and the oil leakage. Very low viscosity or thin oil slips past the edges of the pump and flows freely from the “leaks”. Hot oil has low viscosity and, is often accompanied by low oil pressure. Cold oil is more viscous and usually causes high pressures, even with the engine idling. Higher viscosity oil in an engine raises the oil pressure even to the regulated value at a lower engine speed.

External-spur-gear Pump.

This pump (Fig. 11.28) is consisted of two identical meshing spur gearwheels installed in the pump body. The driving gearwheel is rigidly connected to the oil pump drive shaft by shrunk fit or by a keyway. The drive shaft rotates in a bearing bore machined directly in the pump housing, and the driven gear revolves on a bearing post mounted within this housing.
As the gears rotate a low pressure area is produced on the inlet suction side so that oil is drawn in. The oil filling the spaces between the gear teeth is sealed off by the housing walls as the wheels rotate. This trapped oil then moves around the periphery of each gearwheel, in opposite directions in the two gearwheels, to the discharge outlet port. The continuous displace­ment of oil to the outlet port pressurises the oil and increases the rate of oil circulation.
A single stage pump of this type can develop the delivery pressure up to 981 kPa. The discharging capacity of the pump can be determined using the following relation.

The pump components are checked for correct clearance and wear using feeler gauges. The clearance between the gear tooth tips and the pump body should not exceed 0.2 mm. The backlash between the meshing gears should be between 0.1 and 0.2 mm. The end float clearance (between the end-plate and the gears across the open face of the pump casing) from the gear faces should not exceed 0.1 mm. Mechanical and volumetric efficiencies of these pumps are quite high being 95% and 98% respectively.

Internal-gear Crescent Pump.

This pump (Fig.11.29) contains an internal-spur ring gear that runs outside but in mesh

Fig. 11.28. Spur external-gear pump.
with a driving external-spur gear, in such a way that its axis of rotation is eccentric to that of the driving gear. This eccentricity causes a space between the external and internal gears. This space is occupied by a fixed spacer block called the crescent the purpose of which is to separate the inlet and output port areas. The driving gear is driven either by a separate shaft or is keyed to an extension of the front crankshaft main journal. The outer-gear axis of rotation is maintained entirely by the pump casing wall. The housing of the pump around a crankshaft journal permits a very compact pump unit, capable of delivering large flow output at relatively low crankshaft speeds.
The rotation of the gears develops a low-pressure area at the inlet suction end of the crescent so that oil is drawn in. As the gearwheels rotate, oil is trapped between teeth of the inner driver gear and the inside crescent side wall, and between teeth of the outer gear and the outside crescent side wall. This oil is carried round by these teeth to the other end of the crescent, where it is discharged by both sets of teeth into the outlet port chamber. The continuous supply of oil to the outlet side increases oil discharge. Once the space between the gear teeth has been filled with oil, the extra oil squeezed out from the teeth gaps increases the oil pressure. The mean oil pressure and the rate of circulation depend on the amount of oil escaping from the lubrication system’s bearings.
The pump components are checked for correct clearance and wear using feeler gauges. The clearances between the gear tooth tips and the crescent wall for each gear should not exceed 0.3 mm when both gears are is position with the pump body. The backlash between the meshing gears should be between 0.1 and 0.2 mm. The clearance between the outer gear and the body

Fig. 11.29. Internal-gear crescent pump.
should not exceed 0.2 mm. The clearance between gear end-float across the open face of the pump casing and the gear side faces should not exceed 0.2 mm.

Eccentric Bi-rotor Pump.

The pump (Fig. 11.30) uses an inner and an outer rotor installed in the pump body, and the outer rotor is eccentric to the inner. The inner rotor is pressed on to the oil-pump shaft and is
held in position by serrations. This rotor has four lobes which mesh with five segments in the outer rotor. The inner rotor thus revolves the outer rotor, but at a speed which is slower by the ratio of the number of lobes to segments.
First oil is drawn, through the inlet port, into the space between the inner and outer rotors. Due to their eccentricity and difference in size, the gap between the lobes increases and conse­quently oil is filled up in this space. However the space between the rotor lobes moves beyond the

Fig. 11.30. Eccentric bi-rotor pump.
inlet port, thus trapping the oil, which is subsequently carried around between the rotor lobes and segments. With further rotation, the volume of this effective space formed decreases and it is eventually exposed to the delivery port, so that the oil is discharged under pressure to the filter. The pump acts by continuous repetition of this process.
The pump components are checked for correct clearance and wear using feeler gauges. The clearance between the rotor lobe tip and the segment should not exceed 0.2 mm. The clearance between the outer rotor and the body should not exceed 0.25 mm. The end-float clearance (between the end-plate and the rotors) across the open face of the pump casing and the rotors for the internal-gear crescent pump should not exceed 0.2 mm.

Sliding-vane Eccentric Pump.

This pump (Fig. 11.31) contains a rotor installed eccentrically in a cylinder bore machined in the pump body. The rotor is pressed on to the oil-pump shaft and is positively retained by a pin. Four sliding vanes are placed in grooves machined in the periphery of this rotor and are located by centralising rings on each side of the rotor. During the operation of the pump, the vanes are held against the pump-body wall by centrifugal force.
As the rotor moves, the vanes pass over the inlet port formed in the side of the pump body. Due to the eccentricity of the rotor shaft to the casing wall, the space between the vanes in-

Fig. 11.31. Sliding-vane pump.
creases and oil is drawn into the space between the rotor and the pump-body wall. The oil is subsequently carried round between the vanes beyond the inlet port, where the space between the rotor and the pump bore decreases. Consequently the oil is forced out through the discharge port to the oil filter and oil galleries. Due to the displacement of the excessive quantity of oil further, the oil pressure in the engine’s lubrication passages increases.
The pump components are checked for correct clearance and wear using feeler gauges. The clearance between the rotor and the body should not exceed 0.13 mm. The clearance between the vane and the body should not exceed 0.28 mm. The clearance between the vane and the rotor groove should not exceed 0.13 mm. The rotor and vane end-float clearance across the open face of the pump body from the rotor and vanes should not exceed 0.13 mm.
11.9.3.

Oil-pump Drive Arrangements

Skew gear drive for crankshaft oil pump is employed when high mounted camshafts are used. A short shaft with a skew gear meshes with a gear mounted on the front of the crankshaft. This arrangement drives both the ignition distributor and the oil pump with a 2 :1 gear reduction (Fig. 11.32A)
Skew gear drive for camshaft oil-pump is used when a low mounted camshaft is employed. A long shaft with a skew gear meshes with a gear machined directly on the camshaft. This shaft drives both the ignition distributor and the oil-pump (Fig. 11.32B) with a 1: 1 gear ratio.
When a double-stage timing chain is employed with a high-mounted camshaft and an auxiliary shaft (also known as a jack shaft), it is often convenient to have the oil pump drive from this shaft. This also drives both the distributor and the pump (Fig. 11.32C).
For small compact engines with high mounted camshafts, an internal-gear crescent pump is commonly used. This pump is compact, occupies very little space and sits over a keyed external gear on the crankshaft. This serves dual function of driving the pump and generating part of the pumping action (Fig. 11.32D).
For medium-and-large sized commercial engines, driving the pump directly from the crankshaft timing gear (Fig. 11.32E) is preferred. These forms of drive are commonly used with large output pumps.
For some transverse-mounted engines with low-mounted camshafts, it is preferred to drive a pump through a coupling located at the end of the camshaft. This is also compact and dispenses with a separate drive shaft (Fig. 11.32F)

Fig. 11.32. Oil-pump drive arrangements. A. Crankshaft skew-gear drive. B. Camshaft skew-gear drive.
C. Auxiliary-shaft skew drive. D. Crankshaft internal-gear drive.
E. Crankshaft timing-gear drive. F. Camshaft direct end couple drives.

11.9.4.

Pressure Regulator

A pressure regulator or relief valve (Fig. 11.33) limits the maximum pressure in engines with full pressure lubricating systems. If a pressure regulator valve is not used, the engine oil pressure continues to increase as the engine speed increases. Maximum pressure is usually limited to the pressure that delivers an adequate quantity of oil (11 to 22 1pm) to engine lubricating points. After oil leaves the pump, oil films are maintained by hydrodynamic forces. Excessive oil pressure requires more power and also does not provide better lubrication. High oil pressure and consequently the resulting high rates of oil flow may cause erosion to engine bearings in some cases.
The pressure regulator is installed downstream from the other pressure side of the oil pump. It normally contains a spring-loaded piston and in some cases, a spring-loaded ball. When oil pressure reaches regulated pressure (normally between 380 to 414 kPa), it forces the regulator valve back against the calibrated spring, compressing it as the valve is forced back. This allows a controlled “leak” from the pressure system so that the regulated set pressure is maintained. Any change in the regulator valve spring pressure also changes the regulated oil pressure and
higher spring pressure causes higher oil pressure. In most engines, released oil from regulator valve is routed to the inlet side of the pump as shown in Fig. 11.33. The regulator valve is, therefore, usually placed in the oil pump housing or pump cover. This method of oil flow from regulator valve prevents foaming and excessive oil agitation so that the pump receives a solid stream of lubricating oil.

Fig. 11.33. Oil pressure regulator valve.

11.9.5.

Oil Filter

Oil from the pump outlet flows to the oil filter where large particles are trapped, allowing only clean oil to flow into the engine. Very fine particles flow through the filter. These particles are so fine they can get between
engine clearances causing no damage. As the filter traps particles, the holes in the filter become partly plugged so that it traps even smaller particles. This better filtering, however, restricts oil flow which can result in bearing oil starvation. Most oil filters are the spin-on, disposable type. Oil flows from the oil pump into the outside area of the filter (Fig. 11.34) between the filter case and the paper element. The oil flows from the centre of the paper element to the main oil gallery in the engine block. A check valve placed in the top of the oil filter prevents oil from draining back out of the lubrication system through the filter into the crankcase when the engine
is shut off. The oil filter mounting plate contains a bypass valve which allows unfiltered oil to flow from the oil pump directly to the lubrication system if the filter becomes plugged. This bypass valve is set at from 34 to 103 kPa, depending on the engine and normal pressure drop across the filter element.

Fig. 11.34. Oil filter.