The oil pump in an internal combustion engine circulates engine oil under pressure from the rotating bearings, sliding pistons and camshafts of the engine. This lubricates the bearings , allows the use of high-capacity fluid bearings and also aids in engine cooling .
Along with its primary purpose for lubrication, pressurized oil is increasingly used as a hydraulic fluid to power small actuators . One of the first notable uses of such was for hydraulic tappets in camshaft and valve actuation . Increasingly common recent uses may include tensioners for timing belts or variators for variable valve timing systems .
The type of pump used varies. The gear pump   the trochoid pump  and the vane pump [A] are all commonly used. Plunger pumps have been used in the past, but are now rarely used only for small engines .
To avoid the need for priming , the pump is always mounted downward, either submerged or around the oil level in the sump. A simple wire-mesh strainer with a small pickup pipe hooks up to the bottom of the sump.
For simplicity and reliability, mechanical pumps are used, which are driven from the crankshaft by a mechanical geartrain. It is beneficial to reduce the speed of the pump [b] and therefore it is usual to drive the pump from the cam (if it is mounted in the cylinder block) or the distributor shaft, which rotates at half the engine speed. Laying the oil pump down and down uses a nearly vertical drive shaft, which is driven from the camshaft by a helical oblique gear . Some engines, such as the 1964 Fiat Twin Cam engine , were OHV with a conventional camshaft-driven oil pump in the cylinder block .Started as an engine. When the twin overhead cam engine was developed, the previous oil pump arrangement was retained and the camshaft became a shorter stub shaft. Even when the position of the distributor was moved from the previous block-mount to the mount on the cylinder head camshaft, the oil pump drive remained in the same position, the unused distributor position now covered by a blanking plate .  Smaller engines or scooters may have internal gear pumps mounted directly on their crankshafts.
For reliability, it is rare to use an external drive mechanism, either a separate belt drive or external gear, although camshaft-driven pumps often rely on a single timing belt. Sometimes additional separate belts are used where dry sump pumps have been added to engines during tuning.
Again electric oil pumps are not used for reliability. Some ‘ turbo timer ‘ electric auxiliary oil pumps are sometimes fitted to turbocharged engines. This is a second oil pump that continues even after the engine is turned off, providing cold oil for a few minutes as the turbocharger’s hot bearings cool down. [C] These are supplementary pumps and do not replace the main, mechanical, oil pump.
Electric pumps would again require larger electric motors as the main engine pump and could be cheaper to drive directly from the engine. For example, the oil pump of a BMW S65 engine delivers CA. 45 lpm (litres per minute) oil at 5.5 bar pressure  . A fairly large motor will be required to operate this pump.
The oiling system addresses the need to properly lubricate the engine when it is running. Properly lubricating the engine not only reduces friction between moving parts, but is also the main method by which heat is removed from the pistons, bearings and shafts. Failure to lubricate the engine properly will result in engine failure. The oil pump forces motor oil through passages in the engine to properly distribute the oil to the various engine components. In a typical oiling system, the oil is sifted through a wire mesh sieve from the oil sump (oil pan, in US English) to remove a few large pieces of debris from the oil. The flow carried by the oil pump allows the oil to be distributed around the engine. In this system, oil flows through an oil filter and sometimes an oil cooler, before passing through the oil passage of the engine and into pistons, rings, springs,
The oil pressure generated in most engines should be around 10 psi per 1000 revolutions per minute (rpm), which is around 55–65 psi. 
The local pressure (at the crankshaft journal and bearing) is much greater than 50, 60 psi and C. set by the pump’s relief valve, and will reach hundreds of psi. This high pressure develops from the relative speed in feet per second (rpm or journal size not directly) of the crankshaft journal, balanced against the width of the bearing (up to the nearest pressure leak), oil viscosity and temperature. Bearing clearance (leakage rate).
All the pump pressure does is “fill in the hole” and refresh the oil in the annular space faster than it can leak out. This is why low speed engines have relatively large journals, with only modest pump size and pressure. Low pressure indicates that the leak from the bearings exceeds the delivery rate of the pump.
The oil pressure at the pump outlet, which opens the pressure relief valve, is simply the bearing clearance and resistance to flow due to restrictions.
The oil pressure gauge, or warning lamp, gives pressure only at the point where its sender pressure enters that part of the system—not everywhere, not an average, nor a generalized picture of systemic pressure.
Despite frequent comparisons to hydraulic engineering theory, it is not a “closed system” in which oil pressure is balanced and uniform everywhere. All engines are “open system”, as oil returns to the pan by a series of controlled leaks. The bearings farthest from the pump always have the lowest pressure due to the number of leaks between the pump and that bearing. The additional bearing clearance increases the pressure loss between the first and last bearing in the chain.
Depending on the situation, an engine may have an acceptable gauge pressure, and still only have 5 psi of pressure on a connecting rod, which will fail under high loads.
The pressure is actually created by the resistance to the flow of oil around the engine.  Therefore, oil pressure during operation can vary with temperature, engine speed and engine wear. Colder oil temperatures 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. Due to variations in temperature and the general high engine speed at cold engine starts, it is common to see higher oil pressure at engine start than at normal operating temperature, where normal oil pressure is usually between 30 and 45 psi. . Too high oil pressure can create unnecessary work for the engine and even add air to the system. To ensure that the oil pressure does not exceed the rated maximum, once the pressure exceeds a predetermined limit, a spring-loaded pressure relief valve releases the excess pressure either toward the suction side of the pump, or directly to the oil pan. Or dumps it in the tank.
high oil pressure
The end result of too much oil pressure is that the front or rear main engine seal will be blown out and the oil plug will be blown. In other words, any possible entry into the crankcase could be sealed somehow. High oil pressure often means extremely high pressure at cold start-up, but this is a design flaw rather than an automatic result of high pressure. The observation “if you increase the maximum pressure, the cooling pressure becomes too high” is accurate, but not intentional.
Even stock pumps (regardless of brand and model) do not have sufficient relief valve capacity: the relief port is too small to handle the amount of cold oil. This is why there is a significant difference between cold and hot oil, high and low rpm, &c, but this is usually not a problem with stock engines due to the spring-loaded pressure relief valves mentioned above. A correctly designed relief port (which is not found in production engines) will allow any amount of oil to flow through, regardless of oil viscosity or temperature, and gauge readings will vary only slightly.
Oil pressure is monitored by an oil pressure sending unit, usually mounted on the engine block. This can be either a spring-loaded pressure sensor or an electronic pressure sensor, depending on the type of sending unit. A problem with the oil pressure sending unit or the connection between it and the driver display can cause abnormal oil pressure readings when the oil pressure is perfectly acceptable.
low oil pressure
There are only four reasons for low oil pressure:
- no oil or low oil in the engine;
- Bad or bad oil pump or broken pressure relief valve spring;
- Worn main bearings (large ends have nothing to do with oil pressure as most of this pressure is supplied by centripetal force); And
- Oil gallery fracture or blockage.
Low oil pressure can damage the engine. The first thing to fail will be the cam carrier bearings if the vehicle is OHC because it is fed through a restrictor and the low pressure will starve the top of the engine of lubrication. If the piston has a crown jet (eg, Scania), it can cause a piston/liner nip. Also the crankshaft and connecting rod bearings may be seized. Signs of low oil pressure may be that the warning light is on, a low pressure reading on the gauge, or a clattering/clinking noise from the engine. Low oil pressure is a problem that must be addressed immediately to prevent serious damage.
The major cause of low oil pressure in the engine is wear of vital parts of the engine. Over time, engine bearings and seals become prone to wear and tear. Wear can eventually cause these parts to lose their original dimensions, and this increased clearance allows a greater amount of oil to flow over time which can greatly reduce oil pressure. For example, .001 of an inch worn out of the engine’s main bearings can result in a drop in oil pressure of up to 20%.  Only replacing worn out bearings can fix this problem, but in older engines there is not much that can be done except to replace the engine with a lot of wear.
The particles present in the oil can also cause serious oil pressure problems. After oil flows through the engine, it backs up into the oil pan, and can carry a lot of debris along. Debris can cause problems with the oil pickup screen and the oil pump itself. The holes in the oil pickup screen measure approximately 0.04 square inches (0.26 cm ).  Holes of this size only pick up large pieces of debris and allow many smaller pieces to flow through. The holes in the screen are so large (relative to the debris) because at low temperatures and slow engine speeds the oil is very viscous and requires large openings to flow freely. Even with these large holes in the screen, it can still clog and cause low oil pressure. A .005-inch-thick (0.13 mm) coating on the screen measures the hole size to approximately .03 square inches (0.19 cm) .), which in turn reduces oil flow by up to 44 percent. 
Debris may remain in the oil even after it has passed through the oil pickup screen and oil filter. It is very important to change the oil and oil filter to reduce the amount of debris flowing out of your engine. In high mileage engines with normal engine wear this harmful debris causes increased clearance between bearings and other moving parts.
Low oil pressure can occur simply because there is not enough oil in the sump due to burning oil (usually due to piston ring wear or poor valve seal) or a leak. Piston rings serve to seal the combustion chamber as well as remove oil from the inner walls of the cylinder. However, their effectiveness decreases when they wear out, which leaves oil on the cylinder walls during combustion. In some engines, burning a small amount of oil is normal and not necessarily an alarm, while heavy oil consumption is a sign that the engine may need an overhaul.
high performance engine oil pump
Not all engines require the same oiling. High performance engines, for example, put more strain on the lubrication system. In this case, the lubrication system needs to be particularly strong to prevent engine damage.  Most engines in cars on roads today do not run more than 5,000–6,000 rpm, but this is not always the case in performance engines, where engine speeds can reach as high as 8000–9000 rpm. In engines like this, it is essential that the oil circulates quickly, or air may become trapped in the oil. In addition, in order to free up power, some engines in performance applications run less oil, which requires less power to drive the oil pump. Common oil loads in engines today are usually either 5W-30 or 10W-30 oil, while performance engines may use 0W-20 oil, which is less viscous.
wet and dry sump system
Traditional wet sump engines have an oil pump. It is usually located on the inside of the lower part of the engine, usually on the bottom and/or on one side of the crankshaft. On dry sump engines, at least two oil pumps are required: one is to pressurize and distribute oil around the engine components, and at least one other to ‘scrub’ oil that has accumulated on the bottom of the engine to vacate.  This scavenger pump is sometimes (but not always) located in the ‘sump’ of the engine, and importantly, the flow-rate capacity of this scavenger pump must be greater than that of the pump that operates throughout the engine. Presses on and distributes oil.
Due to the dry sump’s external oil reservoir, excess air can escape the oil before the oil is pumped back through the engine. Dry also because they reduce the amount of spout windage allowing for more power, sloshing up the oil in the rotating assembly, and improves the vacuum ring seal from the scavenge pump.  Dry sumps are more popular in racing applications because of improved power and less oil sloshing due to reduced oil pressure. The disadvantages of a dry drain are increased weight, extra parts, and a greater chance of leaks and problems.