Lubricating oils and filters for the engine
Lubrication is a fundamental requirement for the proper functioning of the engine of any self-propelled agricultural machinery. In fact, excessive friction would inevitably lead to a significant reduction in the power generated with a harmful overheating and a simultaneous increase in the wear of moving parts
All endothermic engines, including those of self-propelled agricultural machinery, consist of a large number of moving parts in close contact, often at high speeds of rotation or linear motion. Without adequate lubrication, excessive friction would inevitably lead to significant overheating, with a simultaneous increase in wear and a substantial reduction in the power generated.
Besides lubrication, the oils used for this purpose remove heat from the bearings and coupled parts, ensure adequate compression of combustion gases by sealing around the piston rings and remove any potentially harmful solid residues.
The manufacture of engines intended for agricultural use with ever-increasing performances inevitably entails the adoption of tighter tolerances resulting in higher specific power values and higher working pressures and temperatures.
For tractors specifically, the engine must be able to cope with a wide variety of operating conditions, ensuring optimum performance over wide load ranges. As a result, optimizing lubrication has involved adopting new materials and developing new technical solutions. Meanwhile, to meet the requirements of the latest engines, the lubricants industry has improved the oils' properties.
The engine oils
For tractor engines, 10W-40 and 15W-40 are the SAE classes of lubricating oil most widely used. In detail, the former ensures high performance such as fluidity at low temperatures and adequate viscosity at high temperatures, with simultaneous excellent control of volatility. Moreover, it helps maintain efficient devices for reducing gaseous pollutant emissions, such as the particulate filter (DPF). Class 15W-40 oil shows lower effectiveness at low temperatures. Still, it is particularly suitable for modern high-efficiency and low-emission engines, including those with exhaust gas recirculation (EGR) and after-treatment devices with diesel particulate filters (DPF) and diesel oxidation catalysts (DOC).
As a result, the SAE class of oil to be used is basically a function of the climate of the area where the machine operates while still complying with the manufacturer's instructions. Therefore, since the lubricating properties inevitably degrade with use, the engine oil of a tractor should usually be changed every 500 to 600 operating hours (in any case, after one year). In exceptional cases, and for oils of higher characteristics, it can be up to 750 hours. Note that for low-power tractors (< 60 kW), the replacement interval can be reduced to only 150-300 hours.
The amount of oil introduced into the lubrication circuit of the tractor is a function of engine power. More specifically, the greater the displacement, the greater the quantity of oil needed to ensure adequate lubrication. Generally speaking, this ranges from 8-9 liters for 30-60 kW tractors up to 31-32 liters for those with a power rating of over 200 kW. Together with the need to replace the engine oil when due, these volumes entail an expense that certainly cannot be considered negligible.
The engine oil filter
To carry out its lubricating function in the best possible way, the oil must always be kept at the highest possible level of integrity. As a matter of fact, a significant quantity of suspended particles (especially of metallic nature) in the fluid would lead to premature wear of the engine. This is why at least one filter is always inserted in the engine lubrication circuit in order to continuously purify the oil from the residues generated by the abrasion of the moving parts, as well as from any dust resulting from environmental contamination and carbon residues from combustion.
Filtering, which must necessarily achieve the best compromise between efficiency and minimum resistance to the passage of the lubricating fluid, makes it possible to intercept particles (supposedly spherical) whose diameter is generally between 5 and 60 microns.
The oil flows through the filter, is pressurized by a pump, and then reaches all the parts of the engine that need lubrication. When the filter is clogged, a high-pressure difference is generated between the clean front and the "dirty" front, which therefore tends to clog further. In order to ensure a specific lubricating action, a "by-pass" valve is installed upstream of the filtering element, which opens at a pressure ranging between 0.5 and 3 bar, excluding the filter. In fact, the lubrication (but also cooling) actions ensured with a polluted oil cause less damage than their total absence, which can lead to engine seizure.
Furthermore, the resistance exerted by the filter to the passage of oil (which can lead to the opening of the by-pass valve) can also be caused by the high density of the fluid found at particularly low temperatures. Basically, when the engine is turned on, the cold, thick oil finds it very difficult to pass through the filter element. The resulting pressure increase will consequently trigger the by-pass valve to open. Clearly, even for filters, it is, therefore, necessary to carefully follow the replacement instructions given in the use and maintenance booklet.
Besides the by-pass valve, the filter can be equipped with an anti-drainage gasket located upstream, which prevents the oil from being drained when the engine is switched off and would otherwise return to the sump.
The most commonly used filter on tractors is mechanical, either "immersion" or "spin on". The formers, more straightforward and economical, have the filtering cartridge inserted in a special socket on the motor block. In this case, the by-pass valve is located in a seat directly connected to the filter housing and is never replaced. On the other hand, spin-on filters are made up of metal casings in which both the filtering element and the by-pass valve are located, meaning that both parts must be replaced.
The filtering material
It is usually made of cellulose fibers randomly arranged to form a highly porous structure and is often treated with specific resins to give it more outstanding durability. The particularly absorbent composition ensures a large filtering surface area, excellent resistance to pressure, high rigidity of the folds that are a characteristic feature of the conformation of the filter material, and high resistance to tearing. Nowadays, cellulose fiber is more and more often replaced by synthetic fibers, which have a smaller average diameter of the interstices.
Mixed" filters, made of a combination of cellulose and synthetic fibers, are also very popular: in particular, the latter increase durability, chemical resistance to contaminants and mechanical strength.
Meaning of SAE classification of lubricating oils
The SAE (Society of Automotive Engineers) classification, most recently revised in 2015, categorizes lubricating oils according to their viscosity. According to this ranking, oils are grouped into different classes or grades, each of which is distinguished by a conventional number based on specific viscosity limits reached at high and low ambient temperatures. This classification divides lubricants into two categories according to their viscometrics characteristics, i.e., oils for cold climates or winter seasons and oils for hot climates or summer seasons. Oils belonging to the first category are divided into grades distinguished by the letter "W" (winter) followed by a number. Those in the second category are identified only by a number. Therefore, the first category includes "fluid" oils, while the second has "viscous" oils.
For example, the number 10 in a 10W-40 oil refers to the cold viscosity for winter use. The number 40 takes into account the hot one. The latter, therefore, is the oil's viscosity index at high temperatures. The higher this number is, the higher will be the oil density at high temperatures.