Engine Lubrication and Oil Types: A Comprehensive Look
Engine Oil Classification
Classification by Conditions of Service
Oils are classified in laboratories based on their performance under various service conditions.
Classification by Viscosity
The SAE (Society of Automotive Engineers) classification relates oil viscosity to temperature. It consists of 10 grades:
- Six grades from 0 to 25, followed by the letter “W” (Winter), indicate cold-start performance.
- Four grades from 20 to 50 indicate the oil’s viscosity at operating temperature (100 degrees Celsius).
These grades indicate the minimum temperature at which the oil can be safely used.
Monograde Oils
Marked with a single viscosity grade, monograde oils are suitable for environments with minimal temperature fluctuations.
Multigrade Oils
Multigrade oils have a wider operating temperature range. They are identified by two viscosity grades, representing their performance in cold and hot conditions (e.g., 10W-40).
API Classification
The API (American Petroleum Institute) classification categorizes engine oils based on performance standards:
- “S” designates oils for gasoline (Otto) engines (e.g., SJ, SL, SM).
- “C” designates oils for diesel engines (e.g., CF, CF-2, CF-4, CG-4, CH-4, CI-4).
Each subsequent category typically surpasses the performance of the previous one, with newer oils often suitable as replacements for older ones.
ACEA Classification
The ACEA (European Automobile Manufacturers’ Association) classification divides engine oils into three groups:
- “A” for gasoline engines.
- “B” for light-duty diesel engines in passenger cars.
- “E” for heavy-duty diesel engines.
Engine Lubrication System
Oil Pressure
The lubrication system delivers a pressurized flow of oil through passages in the engine block and cylinder head. This oil lubricates bearings, then overflows and returns to the oil sump. The overflowing oil also lubricates other engine components.
Components Lubricated by Oil Pressure
- Crankshaft bearings
- Camshaft bearings
- Rocker arm shafts
- Connecting rod bearings
Oil Impregnation
Overflowing oil, primarily from moving parts, creates an oil mist that coats internal engine components. The oil’s ability to adhere to surfaces (known as “oiliness”) is crucial for this process.
Components Lubricated by Oil Impregnation
- Cylinders
- Piston pins
- Valve guides
- Timing chain or sprockets
- Hydraulic lifters (tappets)
- Other internal timing components
Oil Cooling
The oil in the crankcase is cooled by airflow during vehicle movement. This can be enhanced by aluminum crankcases with cooling fins. However, high-performance engines may require additional cooling.
Oil Cooler Types
Oil-Water Intercooler
This system features two separate chambers: one for lubricating oil and another for engine coolant. Oil from the crankcase passes through the intercooler, then the filter, and finally to the engine.
Air-Oil Exchanger
This is a radiator through which lubricating oil circulates. It is cooled by airflow and the engine’s fan.
Piston Cooling
In direct-injection diesel engines, the combustion chamber formed within the piston requires cooling. Injectors located at the bottom of the cylinder direct a continuous stream of oil to the underside of the piston.
Oil for Mixing (2-Stroke Engines)
In 2-stroke engines, oil is mixed with fuel at a ratio between 2% and 4%. This fuel-oil mixture lubricates the engine before being admitted to the cylinder. The heavier oil adheres to internal components. Clean combustion is essential to prevent residue buildup on spark plugs, exhaust ports, or ducts. The crankshaft bearings rotate on roller bearings in these engines.
Crankcase Ventilation
Gases produced during combustion can enter the crankcase, especially in worn engines. These gases hinder crankshaft rotation and must be recycled back into the intake system after passing through a filter to remove oil.