How it Works: Oil Pump

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How It Works: Oil Pump

Oil Pump
Many highly stressed, rapidly moving components in a modem internal-combustion engine need constant' lubrication if seizure or excessive wear are to be avoided. The function of an oil pump is to supply oil, under pressure, to those parts. of the engine which require this positive lubrication. You don't need us to tell you that the oil pump is one of the most important engine ancillaries. The reliability of an engine depends largely on efficient lubrication, and at the heart of the lubrication system is the oil pump. If the oil pump fails, serious mechanical breakdown will quickly follow. The job of the oil pump is to force oil up out of the sump into the lubrication system at a pressure sufficient to reach all the bearings and contact points.

The oil pump must therefore be, above all, reliable and long-lasting, and for this reason they are comparatively simple devices employing few moving parts and requiring little or no maintenance. Oil pumps may be mounted on the inside or outside of the engine; when the pump is mounted inside the crankcase maintenance is only practical during major engine overhaul. There are three types of oil pump in common use and several different methods of mounting and driving the pump. In all systems, however, the oil pump is mounted between the sump and the oil filter, normally low on the engine block.

The Gear Type Oil Pump

The most common form of oil pump used to be the gear type. It consisted of two meshing gears, which rotate inside a close-fitted housing. As the gears rotated they carried oil around, against the housing. The meshing of the gear teeth forced the oil out into the pump. This gear-type oil pump produced a positive flow of oil to the various parts of the engine to which it was directed. If the flow was blocked, the oil pressure could increase sufficiently to damage the pump. Alternatively, if the engine oil was particularly thick, the pressure required to force the cold oil through the small bearing clearances could cause a similar pressure build-up, damaging the pump. To eliminate these possibilities an oil-pressure relief valve was fitted. This returned oil to the sump, or to the oil tank if a dry-sump system was used, when the pressure created exceeded a predetermined value.

The Rotor Oil Pump

Another form of oil pump, more common than the gear type, is the rotor variety. This consisted of a rotor, with four or five external lobes, which rotated inside an outer ring (termed a stator) having five or six internal lobes. The axis of the inner rotor was offset from the axis of the outer ring. The effect was that, although the outer ring was driven by the inner rotor, the volume between the lobes varied as the two rotated. Oil was thus caught between the lobes and forced out into the pump discharge tube. The action of this type of oil pump can be likened in some ways to the operation of the Wankel or rotary engine. The rotor-type oil pump also required a pressure relief valve.

Eccentric Rotor Oil Pump

The most common type of oil pump in modern cars is the eccentric rotor type, originally made by Hobourn Eaton. The main components are an external and an internal rotor, mounted eccentrically in relation to each other. Each rotor carries a number of lobes, the inner rotor has, say, four lobes mounted on the outside face, while the outer rotor has five lobes facing inwards. The precise configuration differs from pump to pump, but the number of lobes on the outer rotor always exceeds that on the inner rotor by one. Both rotors are mounted within the pump body, the most important features of which are the crescent-shaped inlet and outlet ports. A drive from the camshaft rotates the inner rotor which in turn rotates the outer rotor. Oil is drawn in through the inlet port and trapped between the opposing lobes.

As the outer rotor revolves more quickly the clearance between the lobes decreases and pressure is built up until the outlet port is exposed. As the edge of the rotor passes the outlet port, oil is forced under pressure into the lubrication system. Each space formed by the lobes repeats the process and a continuous flow of oil is created. Of all oil pump types, the eccentric rotor pump is the most efficient and is fitted by nearly all major manufacturers. It is normally long-lasting, but wear will eventually occur between the rotors resulting in increased clearances. Maximum clearance is normally about ,006in. and when this is exceeded both rotors must be changed as a pair. Wear can also occur on the peaks in the pump body, in which case a new pump is required.

The Rotary-Plunger Oil Pump

A less common type of oil pump is the rotary-plunger type of pump. This has only one moving part: a worm gear rotated a rotary plunger. A peg engaged a profiled groove in the rotary plunger to provide the reciprocating movement, so producing the pumping action. A non-return valve was also necessary. The oil flow from this type of pump was intermittent and the pressure produced was generally lower than that produced by the gear or rotor-type oil pumps. It was therefore inherently unsuitable for many engines, but was used in some engines which utilised roller or ball-races as big-end or main bearings, and as such required only minimal lubrication.

In rare instances a plunger oil pump was reciprocated by an eccentric peg on the end of a shaft, which engaged a slot in the plunger. A particular disadvantage of both the gear and rotor-type oil pumps was that after the engine had been stopped, oil could drain back through the system to the sump. Neither of these was therefore entirely suitable for a dry-sump lubrication system which might as an alternative employ a form of plunger pump.

The Sliding-Vane Oil Pump

The sliding vane pump consists of a rotor set eccentrically in a bore machined in the pump body. This rotor carries a number of sliding vanes, normally four. Each vane is located in a groove and, because of the eccentric setting of the rotor, contact forces the vanes to retract. The outer edge of the vane maintains contact with the pump body and therefore the pump is always divided into three chambers of continuously changing volume. As the vanes pass the inlet port oil is sucked into the space and as the size of the chamber decreases the trapped oil becomes pressurized. Therefore, when the vanes pass the outlet port oil is forced into the engine under pressure. This type of pump is highly efficient and has a low leakage rate, but the comparatively large number of moving parts makes it susceptible to wear.

Most modem internal-combustion engines use a wet-sump lubrication system and the engine bearings are usually of the plain white-metal type. These bearings require only a comparatively low-pressure oil supply, of the order of a few pounds per square inch, to provide adequate lubrication. Oil supplied to a bearing at one point is drawn around between the two bearing surfaces by rotation. In this way a wedge of oil is formed which is of sufficient strength to take the full load of the bearings. The high oil pressure produced by a gear, rotor, or vane pump is used to provide a sufficiently fast flow of oil to absorb and carry away heat from the bearing surfaces and other parts of the engine such as the pistons, which would otherwise overheat.

Those engines which use a dry-sump system normally have dual oil pumps. One of these is used to provide oil pressure for lubrication. The second (the 'scavenge' pump) is used to remove the oil from the sump to the oil tank. To prevent a surplus of oil accumulating in the sump, the second pump usually has a larger capacity than the lubricating pump. Oil pumps require no routine maintenance, although if they incorporate a strainer this should be cleaned periodically. The job need only be done infrequently, when the sump is removed, and only when removing sludge from the sump.

Oil pumps are usually driven from the camshaft or crankshaft through suitable gearing. It is this gearing which is likely to be damaged if the oil-pump outlet has been blocked and the pump has suffered from over-pressure. It might be thought that constantly being immersed in oil, oil pumps would have an almost indefinite working life. In fact they have a long working life, but they do eventually wear. As a result the oil pump output will in time become insufficient for 'the requirements of the engine. Most service manuals specify maximum limits for wear between the components of the oil pump. If these wear limits are exceeded, it is normal practice to change the oil pump as an assembly. Such renewal is generally only undertaken as part of a major engine overhaul or reconditioning.

The Double Gearwheel Oil Pump

The double gearwheel type of pump is infrequently employed in modern cars, although the Audi 100 for example still uses it. The principle of this type of pump is similar to that of eccentric rotor type, the difference being that oil is pumped by spaces formed between gear-teeth instead of by lobed rotors. The two gearwheels are meshed together. One is driven from the camshaft and the other, the idler wheel, rotates in turn. These gears are housed in the pump body with only a small clearance between the body and gearwheels. Inlet and delivery ports are provided at opposite sides of the pumps. The gearwheels rotate, forming a depression over the inlet port and drawing oil up into the pump.

Oil is then carried round the pump in the annular space between adjacent gear teeth. The meshing of the gearwheels at the other side of the pump forces oil out of the tooth spaces into the delivery port and into the lubrication system. In some gearwheel pumps the driven gearwheel has one tooth fewer than the idler, to ensure even wear of the gearwheels. This type of pump is simple and reliable, except for a tendency to leak at high pressures, although this can be overcome by mounting the pump low in the crankcase where it is immersed in oil. But it is 25 per cent less efficient than the rotor pump and has therefore largely been superseded.

Dry Sump Oil Pumps

To prevent oil surge during fast driving many race and rally cars employ dry-sump lubrication. In a dry sump system oil is stored in a separate oil tank rather than in the sump and therefore two oil pumps are required. One, the pressure pump, forces oil round the lubrication system, while the other, the scavenge pump, returns the oil to the oil tank from the sump. Dry sump oil pumps are invariably of the eccentric rotor type and normal practice is to combine the two pumps in a single unit so that they can be driven by the same shaft.

The inlet port of the scavenge pump draws oil from the sump and forces it through the oil cooler into the oil tank. Oil is drawn out of the tank by the pressure pump and forced into the lubrication system. Some dry pump systems require two scavenge pumps to return all the oil from the sump. In this case a third pair of rotors is added to the oil pump and the inlet pipe draws from the other end of the sump. The eccentric rotor oil pump is compact and simple and it is possible to drive several sets of rotors off the same shaft. In some specialist oil pumps for racing cars the pressure pump has five sets of rotors. This type of multistage pump uses successive sets of rotors to produce extremely high pressures.

A complete oil pump assembly
A complete oil pump assembly, which features a screw-on oil filter.

A partially dismantled vane-type oil pump
A partially dismantled vane-type oil pump, in which the rotor, which has sliding vanes, spins inside an ovalised chamber; the pressure relief valve is seen with its spring.

A rotor-type oil pump
A rotor-type oil pump.

Maintaining Oil Pressure

The oil pressure at the pump outlet, which is what opens the pressure relief valve, is simply the resistance to flow caused by the bearing clearances and restrictions. The oil pressure gauge, or warning lamp, gives only the pressure at the point where its sender enters that part of the pressurized system – not everywhere, not an average, nor a generalized picture of the systemic pressure. Despite the frequent comparison to hydraulic engineering theory, this is not a “closed system” in which oil pressure is balanced and identical everywhere.

All engines are “open systems”, because the oil returns to the pan by a series of controlled leaks. The bearings farthest from the pump always have the lowest pressure because of the number of leaks between the pump and that bearing. Excess bearing clearance increases the pressure loss between the first and last bearing in a series. Depending on condition, an engine may have acceptable gauge pressure, and still only 5 psi pressure at one connecting rod, which will fail under high load.

The pressure is actually created by the resistance to the flow of the oil around the engine. So, the pressure of the oil may vary during operation, with temperature, engine speed, and wear on the engine. Colder oil temperature can cause higher pressure, as the oil is thicker, while higher engine speeds cause the pump to run faster and push more oil through the engine. Because of variances in temperature and normal higher engine speed upon cold engine start up, it’s normal to see higher oil pressure upon engine start up than at normal operating temperatures, where normal oil pressure usually falls between 30 and 45 psi.

Too much oil pressure can create unnecessary work for the engine and even add air into the system. To ensure that the oil pressure does not exceed the rated maximum, once pressure exceeds a preset limit a spring-loaded pressure relief valve dumps excess pressure either to the suction side of the pump, or directly back to the oil pan or tank. High oil pressure frequently means extremely high pressure on cold start-up, but this is a design flaw rather than an automatic consequence of high pressure. The observation “if you raise the maximum pressure, the cold pressure goes too high” is accurate, but not intentional.

Even the stock pumps (regardless of brand and model) do not have enough relief valve capacity: the relief port is too small to handle the volume of cold oil. This is why there is a significant difference between cold & hot oil, high & low RPM, &c., but it’s typically not a problem with stock engines. A correctly designed relief port (which is not found in production engines) will flow any oil volume the gears will pass, regardless of oil viscosity or temperature, and the gauge reading will only vary slightly.

The oil pressure is monitored by an oil pressure sending unit, usually mounted to the block of the engine. This can either be a spring-loaded pressure sensor or an electronic pressure sensor, depending on the type of sending unit. Problems with the oil pressure sending unit or the connections between it and the driver's display can cause abnormal oil pressure readings when oil pressure is perfectly acceptable.

Low Oil Pressure

Low oil pressure, however, can cause engine damage. Low oil pressure can be caused by many things, such as a faulty oil pump, a clogged oil pickup screen, excessive wear on high mileage engines, or simply low oil volume. Indications of low oil pressure may be that the warning light is on, a low pressure reading on the gauge, or clattering/clinking noises from the engine. Low oil pressure is a problem that must be addressed immediately to prevent serious damage.

The leading cause of low oil pressure in an engine is wear on the engine’s vital parts. Over time, engine bearings and seals suffer from wear and tear. Wear can cause these parts to eventually lose their original dimensions, and this increased clearance allows for a greater volume of oil to flow over time which can greatly reduce oil pressure. For instance, .001 of an inch worn off of the engine’s main bearings can cause up to a 20% loss in oil pressure.

Simply replacing worn bearings may fix this problem, but in older engines with a lot of wear not much can be done besides completely overhauling the engine which is generally not cost effective. Particles in the oil can also cause serious problems with oil pressure. After oil flows through the engine, it returns to the oil pan, and can carry along a lot of debris. The debris can cause problems with the oil pickup screen and the oil pump itself. The holes in the oil pickup screen measure about 0.04 square inches (0.26 cm2).

Holes of this size only pick up bigger pieces of debris and allow a lot of smaller pieces to flow through it. The holes in the screen are so big (relative to debris) because at low temperatures and slow engine speed the oil is very viscous and needs large openings to flow freely. Even with these large holes in the screen, it can still become clogged and cause low oil pressure. A .005-inch-thick (0.13 mm) coating on the screen can reduce hole size to about .03 square inches (0.19 cm2), which in turn reduces the flow of oil by 44 percent.

Even after passing through the oil pickup screen and the oil filter, debris can remain in the oil. It is very important to change the oil and oil filter to minimize the amount of debris flowing through your engine. This harmful debris along with normal engine wear in high mileage engines causes an increase in clearances between bearings and other moving parts. Low oil pressure may be simply because there is not enough oil in the sump, due to burning oil (normally caused by piston ring wear or worn valve seals) or leakage.

The piston rings serve to seal the combustion chamber, as well as remove oil from the internal walls of the cylinder. However, when they wear, their effectiveness drops, which leaves oil on the cylinder walls during combustion. In some engines, burning a small amount of oil is normal and shouldn’t necessarily cause any alarm, where as burning lots of oil is a sign that the engine might be in need of an overhaul.

Drive Systems

In a pushrod engine with a low-set camshaft, the oil pump is normally driven by a system of shafts and gears. The oil pump on an MGB, for example, is internally mounted on the left hand side of the crankcase where it is driven by a short shaft from the skew gear on the camshaft. On a Leyland 1100 the oil pump is driven by a mortise and tenon coupling from the camshaft. In most cars the same shaft is used to drive the distributor. Overhead camshaft engines employ a different drive system, which normally consists of a shaft driven by the crankshaft gears or chain. The oil pump on a Rover 2000 is externally mounted on the right hand side of the block and is driven by a shaft keyed to the intermediate sprocket cluster, which carries the camshaft chains. In some engines the camshaft belt itself is used to drive a special idler shaft, as on the Ford RS1600. Here the chain-driven jackshaft operates the twin oil pumps. This shaft may also be used to drive a mechanical fuel pump.

High Performance Engines

Not all engines have the same oiling needs. High performance engines, for example, place higher stress on the lubricating system. In this case, the lubricating system must be especially robust to prevent engine damage. Most engines in cars on the road today don’t run much past 5,000–6,000 rpm, but that isn’t always the case in performance engines, where engine speeds could reach up to 8000-9000 rpm. In engines like these, it is imperative that the oil circulates quickly enough, or air may become trapped in the oil. Also, to free up power, some engines in performance applications run lower weight oil, which requires less power to run the oil pump. Common oil weights in engines today are usually either 5w30 or 10w30 oil, where as performance engines might use 0w20 oil, which is less viscous.

Also see: Oil Lubrication
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