overhead camshaft systems
2.14 THE ENGINE LUBRICATION SYSTEM
2.14.1 Main features
The function of the lubrication system of an engine is to distribute lubricant to all the surfaces requiring lubrication. In the earliest motor vehicles, crude lubrication systems were considered quite satisfactory.
As early as 1905 Lanchester used a system in which a pump forced oil under pressure into the crankshaft bearings, and this has developed into the typical system, of which an example is shown in Figure 2.125.
Figure 2.126 shows the same system in the form of a diagram, omitting all components not included in the lubrication system.
The oil is carried in the sump ((16), in Figure 2.125), in which the level must be high enough to cover the pump inlet but not so high that the crankshaft dips into the oil. A dip-stick (6) is a simple means of checking the level of oil in the sump, though other means, such as a float indicator or an electric gauge, may be used. A mechanically driven pump (15) draws oil from the sump and delivers it via the main filter (18) to the main oil gallery (7). A rather coarse strainer (17) is fitted over the pump inlet to protect the pump from small hard objects, which would cause damage.
From the main oil gallery, oil is supplied to the main crankshaft bearings. A feed is also taken from the main oil gallery, through an oilway in the cylinder block and cylinder head (3) to the camshaft bearings. Note that another feed is taken from the camshaft feed across to the rocker spindle (2) to lubricate the rocker shaft bearing. Depending on the design, oil can pass through a drilling in the rocker shaft so that it is able to reach the contact points of the rocker and the valve stem.
Additionally, oil can be fed to the contact point between the rocker and the cam lobe, although on some designs
oil can be pressure fed direct to the cam lobes via drillings in the camshaft.
When the oil has seeped out of the camshaft bearings and the rocker bearings, it is splashed about the camshaft chamber to lubricate valve stems etc. This oil eventually drains back to the sump via tubes or channels cast into the cylinder head and engine block.
Again, depending on the design, restrictors may be used on the feed to the rocker shafts and even to the camshaft bearings, which may not require the same high pressures that are provided for the crankshaft bearings. An excess of oil could result in too much oil passing onto the valve guides and into the combustion chambers, therefore the restrictor can also function as a means of reducing oil flow. It is also usual to have oil seals fitted to inlet-valve stems to prevent oil being drawn into the combustion chambers through the inlet ports.
Holes (10) drilled through the crankshaft convey oil from a groove (12) round the main bearing to the big-end: the groove is supplied from the main gallery (7) via the oil way (11), so that there is an uninterrupted supply to each big-end.
A small hole (9) drilled in a suitable position in the connecting rod allows an intermittent jet of oil to spray onto the cylinder walls, and in some engines a hole (shown in dotted lines) is drilled through the shank of the connecting rod to take an intermittent supply to the small-end bearing.
Oil splashed off the crankshaft lubricates the remaining parts, and eventually drains back to the sump.
The quantity of oil delivered into the system by the pump depends upon the pump capacity and the speed at which it is driven. Exactly the same quantity must
escape from the system, and this can normally happen only through the bearing clearances.
As engine speed increases, the pressure in the system increases in order to force a greater quantity of oil through the constant bearing clearances. The pressure must be sufficient to ensure an adequate flow through the bearings at relatively low speeds. Therefore, at higher speeds, the pump will deliver higher volumes and generate higher pressures, which would be unnecessarily high, and risk bursting joints and causing excessive power loss in driving the pump. The pressure in the system is therefore limited using a pressure relief valve (14).
Figure 2.125 A typical basic lubrication system
in the casing on either side of the meshing point of the gears. When the gears rotate they carry oil from the inlet to outlet ports in the spaces between the teeth. As a tooth of one gear moves out of mesh with a space between two teeth of the other gear, oil flows in through the inlet to fill the void left. On the outlet side, oil is displaced through the outlet port as a tooth of one gear moves into the space between two teeth of the other. Note particularly the direction of the oil flow and the direction of rotation of the gears, shown by arrows.
Eccentric rotor-type pump (Figure 2.128)
The casing (1) has a cylindrical bore in which is fitted the outer rotor (2). The outer surface of this is cylindrical, but a number of lobes are formed on its inner surface. The inner rotor (3) has lobes formed on its outer surface, one fewer than the number on the outer rotor. It is fixed on the driving spindle (4) and mounted eccentrically in the casing so that each of its lobes makes contact with the inner surface of the outer rotor, dividing the space between the rotors into a number of separate compartments of varying size. The size of each compartment varies as the rotors turn. Inlet and outlet ports are cut in the end plate of the pump, and positioned so that the pumping compartments sweep over the inlet port (6) as they increase in size and over the outlet ports (5) as they decrease. The pump is shown assembled at (a) while sketch (b) shows the rotors removed to reveal the ports more clearly.
Figure 2.127 A gear-type oil pump
Figure 2.128 An eccentric rotor-type pump
2.14.2 Oil pumps
Several types of pump are used:
Gear-type pump (Figure 2.127)
For many years this type was almost universal, and it is still in use. Gear-type pumps consist of a pair of gear wheels meshing together in a casing (1), which fits closely around the tips of the teeth and the ends of the gears. One gear (2) is fixed to the driving spindle (3) and drives the other gear (7), which rotates idly on a fixed spindle (6). Inlet (4) and outlet (5) ports are cut
Eccentric vane-type pump (Figure 2.129)
The earlier form of this pump is shown in Figure 2.129a.
The casing (1) has a cylindrical bore in which is fitted the rotor (2) mounted on a shaft eccentric to the casing bore and touching it at one place. Spring-loaded vanes (4) are a close-sliding fit in a slot cut diametrically through the rotor, their outer edges being kept in contact with the casing bore by the spring (3). Oil is carried from the inlet (5) to the outlet (6) as the rotor turns.
A later version of this pump used on some modern engines is shown in Figure 2.129b. This has two one-Figure 2.126 Block diagram of a lubrication system
piece vanes, each with a diametral slot, the two at right angles to one another. Each vane is cut away as shown at (7) and the bore of the casing is not truly cylindrical but shaped so that each vane touches the bore at both ends in all positions.
Rotation of the crankshaft causes both gears to revolve in the same direction. This motion carries oil from the inlet to outlet side in the tooth spaces on both sides of the spacer. Since the teeth mesh together to give a drive, the oil cannot return to the inlet side. As a result, there is a build-up of oil pressure.
As with most pumps, any wear that takes place will allow oil to escape back to the inlet and, as a result, will lower the pumping capacity and prevent the pump from expressing its full pressure. Since the location of an internal/external pump makes it more difficult to remove, it is often recommended that the pump be inspected for wear when other work makes the pump accessible.
Internal/external pumps are more troublesome than eccentric rotor pumps if a car owner uses cheap engine oil that has no anti-foam agents. The air trapped in the system can pass to the hydraulic tappets and make them noisy.
Important note
Pumps submerged in the sump oil are usually self-priming. This means that the oil level causes the oil to flow into the pump so as to wet the gears and bridge the clearances.
However, some pumps are mounted above sump level. This type of pump must be primed by filling it with oil from a convenient external point. This operation is essential if, for any reason, the pump becomes dry, a condition sometimes created when the engine is assembled or when the engine has been allowed to stand unused for a considerable time.
To avoid drain-back of oil from the pump, some systems incorporate a valve between the intake filter screen and the pump.
2.14.3 Oil pressure relief valves
Figure 2.131 illustrates a simple type of pressure relief valve, which consists of a ball held by spring pressure over a hole drilled into the main oil channel leading from the pump to the bearings.
The pressure of oil in this channel exerts a force on the ball (5), tending to lift it off its seat against the load of the spring (3). The spring load is adjustable by screwing the cap (1) in or out, and locking it in the
Figure 2.131 A ball-type oil pressure relief valve Figure 2.129 An eccentric vane-type pump
Figure 2.130 An internal/external-type oil pump Internal/external-type pump (Figure 2.130) This type was primarily introduced on automatic gearboxes. However, its compact size and ease in which it can be driven without the need for a separate drive shaft, has made the pump popular for use on some engines.
Normally two flats on the ‘front’ end of the crankshaft are used to drive the inner gear (1); this meshes with and drives the outer gear (2), which is positioned off-centre. The outer gear is supported in a casing (3) and the wide region between the gears is filled with a crescent-shaped spacer (4) which projects from the casing.
At one end of the pump two ports are formed in the casing to allow oil to flow to and from the pump. On the front face of the pump a flat plate is fitted to blank off the gears and ensure that the only path for the oil is via the gear teeth.
correct position by the lock nut (2). When oil pressure is great enough to lift the ball off its seat, oil is allowed to escape from the main oil channel, thus relieving the pressure and preventing further rise. Oil escaping past the valve returns either to the sump or the pump inlet valve via the passage (4).
Figure 2.132 shows an alternative type of valve in which a plunger (4) replaces the ball, and an alternative method of adjustment is shown. In this case the spring load is adjusted by adding or removing shims (2) above the spring.
Figure 2.132 A plunger-type oil pressure relief valve
Figure 2.133 Position of oil filter in system
2.14.4 Oil filters
The purpose of the filters in a lubrication system is to remove from the oil abrasive particles that would cause rapid wear of the bearings. If a fluid containing solid particles is passed through a porous material (the filter) particles will either be too large to enter the pores, will become lodged in the tortuous passages of the pores, or will pass completely through, depending upon the relative sizes of the pores and the particles. Clearly, the finer the pores the smaller the particles the filter will remove from the fluid, but its resistance to the flow of fluid will be correspondingly greater. Thus a very fine filter will need to have a very large surface area if the flow of oil to the engine bearings is not to be restricted.
A rather coarse wire mesh strainer is usually fitted at the pump intake to protect the pump against the occasional large objects that could enter into the engine, or hard objects large enough to damage the pump. On the pressure side of the system a much finer filter is used.
The modern replaceable filter consists of about four metres of resin-impregnated paper; this is pleated to expose a large external surface area to the oil and is retained in a cylindrical metal canister. The porosity of the paper is designed to trap nearly all particles over 25 microns (0.025 mm), a proportion of smaller particles and any sludge.
The position of the filter in the lubrication ‘circuit’
governs the name used to describe the type; the two common types are:
● full-flow
● by-pass.
A full-flow filter treats all the oil delivered to the bearings provided the filter is clean and the oil is not excessively viscous.
A by-pass filter is fed only a proportion of the oil delivered by the pump. Although this filters out finer particles than the full-flow filter, it only cleans the proportion of oil passing into the filter; the oil passing to the bearings is unfiltered. For this reason most engines today use full-flow filters.
Full-flow filters
In addition to the filter material, this type incorporates a by-pass valve that opens when the pressure drop across the filter exceeds about 1 bar (15 lbf in–2). This valve opens when the filter is clogged, or when the oil is cold, to avoid oil starvation of the bearings.
Although this arrangement gives limited protection to the engine, the continued use of a filter that is not changed at the recommended time will eventually cause damage to the bearings owing to the supply of unfiltered oil.
The replaceable full-flow filter is made in two forms:
● element-type
● cartridge-type.
Element-type
Figure 2.134 shows the construction of this type of filter. The assembly is mounted directly on the side of the crankcase by a pad, which overcomes the need for troublesome external piping. Today most filters use a paper element, but in some cases a felt element, carried on a wire mesh frame and convoluted to give a large surface area, is occasionally fitted to suit other conditions.
The filter should be changed at the recommended service intervals and this is carried out by removing the bolt at the base of the filter casing. This type has a number of seals and these should be renewed when the filter is changed. After renewing the element it takes a few seconds for the engine to fill the filter, so the engine should not be accelerated during this time.
Figure 2.133 shows a schematic layout of the two arrangements:
Cartridge-type
This type, shown in Figure 2.135, is also called a ‘spin-on filter’. It is a throw-away filter designed to simplify the task of removing and refitting.
The canister unit, either screwed directly onto the side of the engine crankcase or onto the oil pump housing, is sealed with a synthetic rubber ring. The cartridge-type oil filter houses a paper filter element and by-pass valve. The cartridge-type oil filter is used on modern engines.
2.14.5 Valve stem sealing
An overhead valve mechanism, using either rockers or an overhead camshaft, needs a good supply of oi1. This requires the valve stems to be fitted with effective seals to stop the oil trickling down the valve guides by gravity and, in the case of the inlet valve, being drawn in by the manifold depression. Although this oil leakage is beneficial as regards stem wear, the oil consumption and exhaust smoke make sealing necessary.
Ineffective valve seals and worn guides can normally be diagnosed by allowing a warm engine to stand for about ten minutes: on restarting, a puff of blue smoke is emitted from the exhaust.
Figure 2.136 shows arrangements for sealing a valve system. In some cases, special care is necessary when removing or refitting the valve to avoid seal damage.
Figure 2.136 Valve oil sealing arrangements Figure 2.135 A cartridge-type filter
Figure 2.134 A full-flow oil filter
2.14.6 Crankcase ventilation
If ideal conditions prevail within the engine, the air spaces in the crankcase, rocker cover and timing cover would be filled with a fine oil mist that coats all working parts. Unfortunately, this is not the case, especially when the vehicle is used for short journeys.
The main problems are:
● Blow-by of the pistons carries unburnt fuel, corrosive gases and steam into the crankcase.
● Steam coming into contact with cold surfaces, such as the camshaft and sump covers exposed to the cold air stream, causes condensation. Water produced in this way mixes with the oil to form an emulsion called cold sludge, a dirty, black, smelly mess that causes corrosion and obstructs oil flow.
The ill effects of these problems can be minimized if the air can be made to circulate around the inside of the
engine. In the past this was achieved by providing a
‘crankcase breather’ for clean air to enter the crankcase and a vent pipe to expel the dirty fumes into the atmosphere. Sometimes the end of the vent pipe was fitted with an airflow detector to lower the pressure and
‘draw out’ the fumes. Systems such as this were effective, but caused air pollution, especially if the engine was worn.
Positive crankcase ventilation (PCV)
PCV is necessary to conform to emission control regulations. The system uses a ‘closed crankcase ventilation’ arrangement to prevent partially burnt fuel and fumes being discharged to the atmosphere. Instead the fumes are returned to the induction system for burning in the combustion chamber.
Figure 2.137 shows the layout of a typical closed ventilation system. In this arrangement the oil filler cap contains a calibrated passage and regulator valve that
Figure 2.137 Closed engine ventilation system – positive crankcase ventilation
allows air to enter the crankcase only when the throttle is less than half open. At full throttle the two hoses connected to the air cleaner and manifold convey the fumes to the induction system for burning in the engine.
To allow the engine to function correctly, especially during slow running, air must not be allowed to enter the system at points other than those designed for entry.
The crankcase should therefore be sealed during the operation of the engine; components such as the oil dip stick and oil filler cap must be effectively sealed to prevent escape of the crankcase fumes.
2.14.7 Pressure indication
An indication of the oil pressure can be signalled to the driver by either a pressure gauge or a low-pressure warning light.
Mechanical type pressure gauge
In the past a pressure gauge system was based on the
In the past a pressure gauge system was based on the