Free Automotive Guide: Internal Combustion Engine-Lubrication

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pouring oilFor the sake of accuracy, I should say that this third segment in a four part series, will explore the engine lubrication and engine cooling systems. We will examine how oil is pumped from the oil pan to the valve covers, as well as the way that coolant is pumped from the radiator to the intake. Since the majority of production automobiles utilize a wet sump engine oiling system, we will make it the subject of this segment. As two of the most important and vital systems of the engine, the liquid cooling system and wet sump lubrication system share many characteristics. Despite this fact, if they are combined inside of the engine, a minor disaster can occur.

oil panThe lubrication system begins in the oil pan. It acts as a reservoir for the life’s blood of the engine, which the oil pump circulates from the bottom of the engine to the top. Not only does the pump circulate oil through galleys drilled into the engine block, but a thin layer of oil is also used to keep the crankshaft, rods, and camshaft rotating smoothly.


domestic oil pump 2The domestic engine oil pump is typically bolted to the bottom of the engine block, over the largest oil galley, with a thin gasket making a seal between the oil pump housing and the block itself. Some type of strainer is affixed to the end of a pickup tube, which is inserted into the oil pump. The pickup tube is shaped in such a way that the strainer is plunged into the oil pan’s deepest cavern, where the most oil is stored. The oil pump, usually driven by a gear or shaft from the camshaft, pumps oil from the oil pan into the large oil galley.

Often import engines are designed differently. Many have the oil pump integrated into the front cover, which bolts over the end of the crankshaft. As the cover is positioned onto the face of the engine, a notched cog in the oil pump slips onto a notched lip forged on the end of the crankshaft. This provides the drive for the oil pump, which has a pickup tube and screen attached in a similar manner to the domestic engine. The front cover is alive with oil output galleys which provide the engine with oil in a manner similar to the domestic version.

main bearings2Crankshaft main bearing lubrication is the most vital function of the oil pump. The crankshaft rotates in journals which are line bored to yield four perfect circular openings through journals in the center of the engine block, from front to rear lengthwise.  These journals are then cut to create two separate but perfect halves of a circle. The half of the circular opening which is lifted away from the block, and has a one or two bolt holes on each end, is now referred to as a main bearing cap. Once the crankshaft is laid into the engine block, the main bearing caps are bolted back over it to reform a perfect circle and secure it in place. Between the crankshaft and this series of journals (normally there are four) there is a thin bearing which is also designed as two perfectly matched halves of a circle. These are called crankshaft main bearings. The main bearings are machined from a softer metal than either the crankshaft or the block, to prevent damage to the engine components. Similar bearing materials are used as connecting rod and camshaft bearings. The crankshaft is hollow with small holes in each main and rod journal of the crankshaft. Oil is pumped into the main bearing closest to the oil pump and pumped out of the remaining oil galleys where it forms an oil clearance between the crankshaft and the main bearings, in which the crankshaft rotates constantly.

rod bearingThe second set of journals which are manufactured into the crankshaft are for the connecting rods. You may learn more about connecting rods in the article Free Automotive Guide: Internal Combustion Engine-Short Block. The connecting rods are driven by the crankshaft using a similar design to that of the main bearings. Pressurized oil is pumped from small galleys in the crankshaft journals, forming a thin oil clearance between the connecting rods and the crankshaft. Connecting rods not only rotate on the crankshaft but are also pumped up and down, pushing the piston into the corresponding cylinder and withdrawing it, immediately thereafter.

valve trainOil pressure is then pushed up to the camshaft, where it forms an oil clearance between the camshaft bearings (which are pressed into the engine block on an overhead valve engine) and the camshaft. Overhead valve V-designed engines dump oil onto hydraulic lifters. There oil is pumped upwards through the hollow pushrods, exiting the top and showering the rockers with lubricating and cooling oil. It is also pumped into the top of the cylinder heads and into the lifter valley, via small oil galleys. There it washes over critical valve train components (including the camshaft/s and camshaft bearings in overhead cam engines), lubricating and cooling them before draining back into the oil pan. Once the oil reaches the oil pan it is pumped back through the engine.

radiator and fanThe engine cooling system on most modern vehicles uses liquid, or coolant, to cool cast iron, steel, and aluminum engine components that will otherwise reach damaging temperatures. Coolant is air cooled (using ambient air drawn across a finned core by a fan) in a reservoir, called a radiator. The radiator is connected to the engine using thick hoses, which the coolant flows through. One of the two radiator hoses connects the radiator to the water pump. The water pump is driven by the crankshaft using either a serpentine belt, v-belt, or timing belt. Air-cooled coolant is drawn from the lower end of the radiator and into the water pump. It is then pumped from the water pump into coolant passages in the block, intake, and cylinder heads.

water jacketUnbeknownst to the average person, there is a hollow area immediately surrounding the cylinder walls in a liquid cooled engine block. Coolant, drawn from the radiator, using the water pump, is pumped into the engine and through passages in the intake and cylinder heads, into the cooling jackets which totally surround the cylinders on each bank of the engine. This area contains approximately half of the total coolant capacity of the engine. Water is constantly pumped through the engine; removing coolant which has been heated by the engine and replacing it with air cooled coolant from the radiator.

thermostatCoolant, which is consequently heated from flowing through the engine, is returned to the radiator using the upper radiator hose. Coolant flow is restricted, to control engine temperature, using a thermostat. As coolant temperature rises, the thermostat is opened allowing more coolant to flow from the engine. When coolant temperature falls below a predetermined level, the thermostat is gradually closed restricting coolant flow in order to allow the engine to reach the desired temperature.

As coolant is allowed to return to the radiator for cooling it is pumped through a finned core, which air is drawn across. A fan is used to draw ambient air across the fins for cooling purposes. Two primary types of fans are used to cool the coolant in the radiator core. The first type is driven by the engine’s crankshaft using a serpentine or v-belt (for older vehicles). In this type of system, a thermostatically controlled fan clutch is typically used to determine the amount of air which is drawn across the radiator core. The other type of fan is electrically operated using sensors in the engine coolant passages, intake, or cylinder heads. When coolant temperature reaches a predetermined level, an electronic signal is transmitted to some type of relay. The relay is activated providing output voltage which completes an electrical circuit in the fan motor causing it to turn on and cool the radiator core and coolant. Be on the lookout for the final segment in this Midnight Oil Blog four part series on internal combustion engines, entitled Free Automotive Guide: Internal Combustion Engines-Fuel and Air Delivery.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.


  1. What Alemite grease pump does is that it pumps a certain amount of grease into the perfect places where friction is produced because of the movement of machineries and its components; this particular pump not only pushes the right amount of grease, but also makes it show to push the grease at the right time.
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  2. There is no doubt that most of the people don’t know about engine lubrication system of their vehicles. But thanks to your post it’s really helpful to know about engine lubrication system.

  3. Wow man I am searching this type of information it’s really helpful to know about internal combustion and engine-lubrication. Thanks for sharing the informative post.

    1. thanks for the encouraging words.

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