Low-viscosity engine oil
To reduce the friction force of moving parts and the mechanical resistance of oil agitation, the viscosity of lubricating oil refers to the measurement of the internal friction force when the oil flows. The viscosity of lubricating oil is not only the basis for its classification but also affects the engine's friction work, the wear of moving parts, the sealing level between piston rings and cylinder walls, as well as the consumption of lubricating oil and fuel. To save fuel and improve cold-start performance, it is necessary to achieve a low viscosity of the lubricating oil. Of course, the term "low viscosity" here refers to the viscosity reaching the lowest allowable value while ensuring adequate engine lubrication.
The engine friction pair's data under working temperature and load conditions is indicated. A viscosity of no less than 3.5 mm² at 100°C ensures lubrication for all friction pairs. A pure petroleum lubricant with a slightly lower friction power loss has a viscosity of 5.7 mm². In many countries, the general engine oil viscosity at 100°C is 4.19 to 1 mm², with the highest being 9.3 to 12.5 mm². This means that the SAE 5W has a viscosity of 3.8 mm² at 100°C and SAE 10W has a viscosity of 4.1 mm² at 0°C; both are within the usable range, ensuring that the low-viscosity engine oil's fuel-saving effect is extremely significant—reducing the viscosity by 1 mm² at 100°C can save about 1.5% of fuel. Theoretically, reducing one SAE viscosity grade while the engine is running normally can save 0.5% to 2.5% of fuel. Experimental evidence shows that reducing one SAE viscosity grade in the engine oil can save 1.0% to over 6.5% of fuel at low temperatures. The above descriptions indicate that when selecting engine oil, one should reasonably choose the viscosity grade based on the operating conditions and engine structure to achieve the goal of both ensuring engine lubrication and saving fuel.
Gear oil's low viscosity trend
Gear transmission accounts for a significant portion. The lubrication of gear transmission is not only crucial for the normal and reliable operation of vehicles but also for all power transmission aspects. It is also closely related to fuel efficiency. The higher the viscosity of gear oil, the more energy is consumed in stirring the oil during the transmission process, leading to a decrease in the vehicle's fuel economy. The theoretical limit for the low-viscosity of automotive gear oil is currently considered to be the viscosity of SAE 70w gear oil (with a kinematic viscosity of 4.1 mm²/s at 100°C). From the perspective of the amount of additives used in low-viscosity gear oils, when the kinematic viscosity is less than 5 mm²/s at 100°C, the amount of additives is high. As the kinematic viscosity approaches 34 mm²/s at 100°C, the amount of additives can increase exponentially. Moreover, increasing the amount of extreme pressure additives can also lead to a reduction in gear lifespan. Additionally, when ensuring the required amount of additives for passing the gear bench test, there is no practical significance in a kinematic viscosity greater than 8 mm²/s at 100°C. Therefore, the theoretically ideal viscosity range for gear oil should be a kinematic viscosity of 58 mm²/s at 100°C, where the torque loss in gear transmission is proportional to the 0.275th power of the lubricating viscosity. Thus, the viscosity of automotive gear oil at which torque loss is acceptable can be determined based on the vehicle's operating conditions. The viscosity of automotive gear oil in China is generally higher, which is highly beneficial from a fuel-saving perspective.
