The most important task of engine oil is, of course, to create an oil film between the surfaces of the mating engine components. In this way, it minimizes friction, limiting it only to energy losses resulting from the sliding of the oil layers.
The engineer of the given engine specifies the appropriate oil viscosity in the vehicle manual. It is not allowed to select an oil of a viscosity different than that recommended by the manufacturer, as this may wear out the engine. Oils for the latest internal combustion engines have lower and lower viscosities, which further minimize the friction between the mating parts. The most important engine components that require lubrication are: timing cams and cam followers, piston rings and cylinder bore, main bearings, crankshaft and camshaft bearings.
Receiving heat
The cooling system is not able to cool all places in the engine. For this reason, the engine oil must dissipate heat from the remaining hot components such as the piston, rings, crankshaft bearings and the camshaft. The cooling properties of the oil are particularly important in the latest “downsizing” combustion engines, which achieve much higher power and torque from a given capacity than the engines of the previous generation.
Sealing rings
Oil sprayed onto cylinder liners and fed under the piston rings seals the combustion chamber. In this way, it prevents exhaust gases from blowing from the combustion chamber into the crankcase, which would result in a reduction in engine power. It is the oil that determines the maintenance of the average useful pressure in the combustion process.
Keeping the engine clean
Keeping the engine clean, flushing out contaminants, dissolving sludge. It all sounds beautiful, but our imaginations are unfortunately far from reality. It is not that clean oil entrains dirt particles, transports them to the oil filter and flows again after the cleaned oil flows. Yes, it is, but only in the case of larger pollutants. The smaller ones circulate in the oil, and the oil’s task is to prevent them from settling, and also … maintain their properties regardless of being “filled” with these impurities. The main pollutant in this case is soot. Good oils have a very high ability to disperse soot, that is, the ability to absorb soot, transport it and prevent it from clumping into larger “lumps” or creating deposits. The oil must also neutralize acid products of combustion (e.g. sulfur) and protect engine components against corrosion.
Impact on performance and fuel consumption
By using low-viscosity oils, we minimize friction between the mating parts of the engine. In this way, we increase power and torque, and reduce fuel consumption, which translates into higher efficiency of the drive unit. However, friction is not enough. A well-chosen oil can actually noticeably improve performance or measurably reduce fuel consumption in cars with extensive equipment controlled by oil pressure. We mean valve timing variators or hydraulic tensioners. If the oil is not able to build up the working pressure in them quickly enough, it will translate into flexibility, acceleration and … a bill at the gas station.
Protection of exhaust gas aftertreatment systems
The Euro 4 standard, in force since 2006, forced diesel producers to use diesel particulate filters. Unfortunately, traditional ingredients commonly used in the oil additives package destroy the filter by irreversibly clogging its porous structure with sulphated ash that cannot be removed. Diesel engines with particle filters (DPF, FAP) therefore require the use of new generation special oils. These are LowSAPS oils, also known in Polish as low ash oils. The name comes from the words Low Sulphated Ash, Phosphorus, Sulfur (low sulfate ash, phosphorus and sulfur content).
Protection against L.S.P.I. in gasoline engines
Despite the amazing performance with small engine dimensions, there are several hazards associated with downsizing that can significantly affect the durability of the engine. In the low speed range (1500-2000 rpm), with high engine load and strong air boost, a phenomenon called LSPI (Low Speed Pre-Ignition) occurs – premature ignition at low engine speed. In other words, it is the phenomenon of ignition of the air / fuel mixture during the compression stroke, occurring before the actual ignition initiated by the spark from the spark plug. Pre-ignition is caused by unburned, glowing oil particles or excess fuel still burning in the combustion chamber. LSPI causes the piston skirt to strike against the cylinder liner, causing severe engine damage. In addition, when the engine is heavily loaded and the power requirement is high, direct injection does not mix the fuel with air perfectly and creates excess fuel zones. Then, the combustion process produces soot. Carbon black causes more “damage” than meets the eye. Soot in the exhaust gas makes it necessary to use GPF systems (gasoline particle filters). Soot from the combustion chamber also seeps into the oil and contaminates it. This in turn makes it necessary to increase the soot dispersibility of the oil in order to limit the agglomeration of contaminants.
Timing chain wear protection
In recent years, it has been found that large amounts of soot in oil negatively affect the durability of timing chains. The soot abrasive on the chain pins, causing it to elongate. As a consequence, it can damage the engine by skipping the timing gear. The correct oil prevents soot build-up on the chain links and therefore protects it. It is worth noting that this applies not only to diesel engines, but also to gasoline engines with direct fuel injection.
The above points are just examples of the real functions of engine oil. In the case of popular, older engines, the use of a newer oil (as long as it complies with the manufacturer’s specification) is of course not mandatory, but it can be good. Unfortunately, such freedom cannot be applied the other way around. The technological advancement of new constructions, as well as the expansion of exhaust gas treatment systems, places specific requirements, disregarding them usually translates into a shortening of the life of the drive unit, exhaust gas treatment systems, and may also affect performance deterioration and increase fuel consumption.