The amount of engine oil absolutely reduces by running. The passages into which the engine oil flows to lubricate each part of engine consists of the passage to the oil pan and to the combustion chamber or to enter exhaust gas without not returning to the oil pan. The engine oil entered the combustion chamber and exhaust gas is burnt.
The passage in which the oil reduces
• The passage through which the oil lubricating the cylinder wall enters combustion chamber.
• The passage enters the combustion chamber from the gaps between the valve stem and the valve guide bush.
• The passage through which the oil contained in the blow-by gas is sucked into the combustion chamber with the gas.
• The passage through which the oil that lubricating the turbocharger bearing passes from the compressor side to the combustion chamber and from the turbine side to the exhaust gas.
The cause infl uences the amount of oil consumption
• The amount of oil entering the combustion chamber increases due to engine malfunction.
• The engine malfunction occurs due to oil maintenance failure.
• The amount of oil consumption changes depending on the habitual usage and driving method.
Important points to remember when measuring the oil
Immediately after the engine stops, the oil has not completely returned to the oil pan from the cylinder head, therefore, the oil level is lower. Also, when filling oil, it takes time to for it to reach the oil pan. As a result, the oil level is low if the level is checked immediately after the oil has been filled.
When the oil temperature is high, the oil expands, so the oil level becomes high.
When the oil temperature is low due to insufficient warm up, the viscosity is high and the returned oil amount to the oil pan from the cylinder head becomes reduced. As a result, the oil level becomes low.
When parking the vehicle in an inclined place, the oil surface in the oil pan inclines and the oil level cannot be measured accurately.
Measure the Oil Consumption Amount
(1) The oil surface level in the oil pan differs depending on the vehicle inclination, oil temperature and the amount of returned oil from the cylinder head. Inspection should be performed under the same conditions.
Oil level inspection conditions
• A level surface
• Engine warming up (water temperature 80 degrees centigrade)
• Measure the oil level fi ve minutes after the engine stops.
(2) The required period of measurement can be calculated based on the customer’s running distance for a month and the consumption amount.
(3) Measure the fi lled oil accurately using a measuring cylinder. In the customer’s measurement, the difference is large and reliability is low, therefore, accurately measure the consumption amount. It is necessary to judge whether the amount is appropriate or not based on the result.
Oil Degradation and Abrasion Inside the Engine
If the oil and oil filter is not changed periodically, the engine oil degrades. When the oil degrades, not only the consumption amount increases, but also the abrasion inside the engine is accelerated and the amount of oil consumption further increases.
• Oil degradation
If the oxides, abrasion chips, soot, moisture, etc. produced by being burnt mix into the oil, the oil degrades, and the purifi cation or lubrication performance decreases. In a diesel engine, the soot mixes into the oil, the oil viscosity becomes higher, the scrape off performance of the oil lubricating the cylinder becomes worse. As a result, the amount of oil mixed into the combustion chamber increases. Compared to a gasoline engine, diesel fuel contains much more sulfur and acids are found in the burnt gas. Therefore, diesel engine oil is more easily degraded than gasoline engine oil. If the oil change period for a diesel engine is the same as that for a gasoline engine, engine abrasion is abnormally accelerated.
• Abrasion inside the engine
When the oil is degraded, sludge, etc. is produced. Therefore, the abrasion inside the engine is accelerated and the oil loss via the piston rings and the valve guides occur.
(1) Oil loss via the piston rings
If the cylinder wall or the piston rings are worn, its oil scraping off performance worsens. Therefore, the amount of oil consumption increases. If the piston rings and the ring groove are worn, the ring pumping performance increases and the oil amount pumped into the combustion chamber increases. Therefore, the amount of oil consumption increases.
(2) Oil loss via the valve guides
If the valve stem, the valve guide bush and the oil seal are worn, the oil amount entering the combustion chamber increases. Therefore, the amount of oil consumption increases.
The Cause of Increased Oil Consumption
1. Laden conditions
As the number of occupants and items increases, the engine load becomes larger. As a result, the acceleration pedal is depressed more than usual. Therefore, the compression pressure and the combustion pressure becomes higher. Accordingly, as the force that presses down the piston or shock becomes larger, the piston inclines in the cylinder or the shape of the piston ring alters, thus the oil splashing lubrication performance becomes worse. Furthermore, since the cylinder and the piston temperature becomes higher, the engine oil vaporizes and the amount of consumption increases. In a diesel engine, since the fuel injection amount increases when the load is heavy, more soot is produced and the oil degradation is accelerated.
The Cause of Increased Oil Consumption
2. Oil loss when the engine brake is used
When the engine brake is applied, the vacuum pressure is high, therefore, the amount of oil entering the combustion chamber increases and the oil consumption increases. When driving on a highway or uphill, the vehicle speed is often controlled by acceleration operation, so the using frequency of the engine brake becomes high and the amount of oil consumption increases.
3. High engine speed
As the engine speed becomes higher, the piston moves more quickly. Therefore, it is more difficult for the piston ring to splash lubrication the oil on the inside of the cylinder wall. Furthermore, the amount of oil that the crank shaft scrapes off also increases, and the oil supply increases. As a result, the amount of oil consumption increases.
4. In the case of a vehicle with turbocharger
In the case of a vehicle with turbocharger, the oil is exposed to high temperatures (approximately 700 degrees centigrade), so oil degradation is quicker. Further, the turbo charger applies a heavier load, so as a result, it consumes a greater amount of oil.