Some drivers, trying to understand the low-temperature characteristics of motor oils, pay attention on the Curing Temperature Indicator and use it to select an oil for winter operation of their vehicle. In this article, we will look at why this approach is not always correct.
It is already mid-autumn, and as winter approaches, more and more attention is being paid to the low-temperature characteristics of motor oils. This is especially true since it is known that about 70% of engine wear occurs during a cold start, and when the temperature drops, this aspect becomes even more relevant.
At first glance, the choice of oil according to the SAE viscosity class can give a general idea, but a more detailed approach allows you to make the optimal choice. Like the ACEA and API standards, as well as the car manufacturers' tolerances, the SAE classification only sets a general framework.
Now let's move on to the technical details. For an oil with a viscosity of SAE 5W, for example, at a temperature of -30°C, the viscosity should not exceed 6600 mPa.s, and at -35°C - 60000 mPa.s, to ensure normal pumpability and reliable engine start. The actual viscosity of each oil is specified by the manufacturer in the technical documentation, where you can find values for different temperatures, such as 30°C, 40°C, 100°C and 150°C.
The low-temperature properties of the oil are often determined not by a specific viscosity, but, for example, by the solidification temperature. This value indicates the temperature at which the oil loses its ability to flow. The exact data on the freezing point is also indicated in the technical characteristics of a particular product and helps to compare oils of the same viscosity class according to SAE, evaluating their behavior at low temperatures.
The fact is that the solidification temperature is far from the only indicator that reflects the operational characteristics of the oil at low temperatures. First, the solidification temperature is not the rotation temperature (it's about the engine crankshaft). Yes, there is such an indicator. Rollover temperature is usually measured according to ASTM standards and varies depending on the type and composition of the oil, especially its viscosity and the additive package used in the formula.
This parameter is often confused with the pour point, but they are different: the pour point indicates the temperature at which the oil stops flowing and becomes solid, while the cranking temperature is exactly the threshold at which the oil will still support the engine to work optimally. Generally, the cranking temperature is lower than the solidification temperature of the engine oil. This is because the pour point defines the point at which the oil loses its fluidity and turns into a solid phase, while the cranking temperature represents a higher temperature threshold at which the oil is still plastic enough to crank the engine.< /p>
Pour point is an indication of the minimum temperature at which the oil can flow, while cranking temperature is more functional for starting the engine and reflects the starting boundary conditions.
Secondly, even these two pour point values for the oils we are looking at are far from what we need to deal with. That is why it is worth paying attention to a number of other indicators that you will not find on the oil tube. This is the density of the oil at 15°C as well as the viscosity at 40°C. Even if these are not subzero temperatures, they reflect the behavior of the viscosity characteristics of the oil as the temperature changes, which is important, at temperatures far from operating.
How so, you ask again? Why is an oil more liquid at average temperatures but at the same time has a lower freezing point? And here is the answer to the main question - the viscosity index. This engine oil indicator characterizes how much the oil changes its fluidity at different temperatures. Oils with a high viscosity index remain more stable under temperature changes, which provides better properties at low temperatures.
It is this indicator that can be considered one of the most important in determining the viscosity-temperature characteristics of motor oil, since at extremely low temperatures (such as -40 °C and lower) the higher index of viscosity of the oil allows it to maintain fluidity better and reduces the likelihood of difficult rotation of the crankshaft.
The high viscosity index is achieved thanks to the use of higher quality base oils and additive packages. For example, PAO synthetic bases, addition of esters, etc. are often used. This is the viscosity index, which reflects how "flexible" is the engine oil in terms of temperature conditions. That is, sufficiently viscous for optimal protection at high temperatures and at the same time sufficiently fluid at negative temperatures. Or in other words, to what extent the oil is able to maintain the viscosity-temperature characteristics within the limits of the values set by the engineers, regardless of the temperatures.