# Methods for Identifying the Quality of Tractor Engine Oil

If the engine is the heart of a tractor, then engine oil is the blood of the engine. When the engine is running, many parts perform relative motion, with contacting friction surfaces sliding at high speed, and operating temperatures can reach 400–600°C. Under such harsh working conditions, engine oil establishes an oil film between two relatively sliding friction surfaces, separating the surfaces of the moving parts to reduce wear. The oil flows between parts, carrying away heat generated during operation to provide cooling; it also removes wear debris from parts and contaminants from part surfaces, providing a cleaning effect; additionally, it carries away water, air, acidic substances, and harmful gases adsorbed on part surfaces, offering anti-rust and anti-corrosion protection. If deteriorated oil continues to be used, it can cause severe damage to machine components. Deteriorated oil forms gum and deposits in the combustion chamber and on the piston crown, creating carbon deposits that lead to abnormal combustion; deteriorated oil adhering to piston rings and valve stems can cause these parts to seize and stick, aggravating wear on various engine components; organic acids produced by oil oxidation increase the acid value of the oil, causing corrosion and rusting of parts (most severe on lead-bronze bearings); sludge formed from oil deterioration, resulting from the mixing of water and oil, congeals in oil lines, not only increasing oil flow resistance but even blocking the oil circuit, causing the diesel engine's lubrication to fail. Therefore, oil quality must be monitored to ensure timely oil changes and quality-based oil changes, preventing mechanical failures. Methods for checking engine oil quality are as follows: 1. Take two glass tubes approximately 10 mm in diameter and 30 mm in length. Fill one with lubricating oil that has been tested and meets quality standards, and fill the other with the oil to be tested, leaving a 5 mm gap at one end, then seal both ends tightly with cork and wax. Invert both tubes simultaneously and measure the time it takes for air bubbles to rise to the top; if bubbles rise quickly, it indicates a decrease in oil viscosity. If the difference between the two exceeds one-quarter, the oil quality is deemed unqualified. 2. Polish a piece of bearing shell into a long strip, fix it at the oil level marking position on the lower end of the oil dipstick, and observe it regularly. If spots or pitting marks appear on the bearing shell, it indicates that the oil has undergone acidification and its quality is unqualified. 3. On clean filter paper, drop one drop each of new oil and the test oil, then observe and compare their oil stains. If the boundary between the central black spot and the surrounding yellow oil stain of the test oil is not very distinct and shows gradual diffusion, it indicates the oil can still be used; if the boundary between the central black spot and the surrounding yellow oil stain is clear with an obvious dividing line, and the black spot diameter is very small, less than one-third of the stain diameter, the oil is semi-aged; if the oil appears completely black, it indicates the oil is fully aged, with its detergent-dispersant additives completely depleted, meaning the oil quality is unqualified. 4. If a large amount of grayish-white oil emulsion with foam is found in the oil by appearance, it indicates that a significant amount of water has mixed into the oil, destroying the additives, reducing viscosity, and rendering it unable to provide lubrication. When a small amount of water enters the oil, it is harder to distinguish: observe the oil film on the dipstick—if the oil film appears yellow or brown and the dipstick markings are clearly visible, the oil quality is good; when the engine is hot, pull out the dipstick—if water bubbles are found in the oil and water droplets are found on the oil filler cap, it indicates water in the oil. You can use the dipstick to drip oil from the oil pan onto clean cotton wadding, move it away from the fuel tank, and ignite the cotton wadding: pure oil ignites easily, while oil containing water produces crackling or popping sounds when ignited, indicating water in the oil. Alternatively, drip oil from the oil pan onto white paper for identification: lubricating oil containing water spreads quickly on the paper, while pure lubricating oil does not spread. 5. When the fuel supply system operates poorly or fuel atomization is inadequate, fuel or an overly rich fuel-air mixture can blow into the crankcase and mix with the oil, diluting it. When oil is diluted by fuel, its viscosity drops noticeably, affecting the formation of the lubricating oil film; additionally, diesel fuel has a high pour point, causing the lubricating oil's pour point to rise and solidify at low temperatures, preventing the engine from pumping oil. When a large amount of fuel mixes into the oil, an increase in the oil level can be observed on the dipstick scale, accompanied by a diesel odor, allowing a judgment that diesel has mixed into the oil; the oil pressure of a diesel engine will also decrease progressively as the diesel content increases. 6. Take two glass cups (other containers may also be used), one filled with the lubricating oil to be checked and the other left empty on the tabletop; raise the cup or container filled with oil to a height of 300–400 mm above the tabletop, tilt the cup rim to let the oil flow slowly into the empty glass cup or container on the tabletop, and observe the oil flow process. Good-quality oil flows in a thin, uniform, and continuous stream; if the flow alternates between fast and slow, with large lumps of liquid falling at times, it indicates the oil has deteriorated. 7. Take clean tissue paper, filter paper, or napkin and drop several drops of the oil to be checked onto it. After the oil has permeated, if black powder appears on the paper surface and there is a dragging sensation when touched by hand, it indicates that the oil contains many impurities, showing that the oil filter's filtration is poor, but it does not necessarily mean the oil has deteriorated; if the central black spot is relatively small and light in color, with a larger surrounding yellow wetting area, and the oil stain boundary is not very distinct with gradual diffusion, it indicates the detergent-dispersant additives in the oil have not been exhausted and it can still be used; if the black spot is larger, the oil appears dark brown and uniform without particles, and the boundary between the black spot and the surrounding yellow oil stain is clear with an obvious dividing line, it indicates the detergent-dispersant additives have failed, meaning the oil has deteriorated and should be replaced promptly. Good oil has no powder, feels dry and smooth to the touch, and shows only a yellow oil stain. 8. Rub the oil between the thumb and index finger repeatedly—good oil feels smooth with no friction; if a rough sensation like sand particles is felt between the fingers, it indicates that the oil contains many impurities and should be replaced with new oil promptly. Alternatively, on a clear day, use a screwdriver to lift some oil, hold it at a 45° angle to the horizontal, and observe the oil droplets against sunlight. Under the light, wear debris in the oil can be clearly seen; if there is excessive debris, the oil should be replaced with new oil. 9. If the oil level in the crankcase rises and the oil becomes thinner, use the nose to smell it; if a diesel or gasoline odor is detected, it indicates that diesel or gasoline has seeped in. In addition to replacing the oil, the source of the fuel leakage should also be eliminated.