We all have seen engine oil somewhere or other, and most commonly at the mechanic’s shop. Have you ever wondered what this 5W-30 is written on? Well, that’s what we are going to find out through this article.
1. Short Introduction to Viscosity
Let’s say you have water flowing over a flat plate. For sure, it’s not going to move freely. But why do we say that? The answer to this is: Viscosity. In simpler words, there is a kind of internal friction among the moving layers of the fluid. This resistance doesn’t let the fluid flow freely over the plate.
You can say that: ‘Friction loves keeping all the layers together’. In technical terms, we say: “Friction opposes relative motion”. Having this understanding, consider layer number 2.
- Layer 3 moves at a faster speed (v+dv) than Layer 2. Hence, the viscous force on the lower layer of 3 acts in such a way that it gets slowed down.
- Layer 1 moves at a slower speed (v-dv) than Layer 2. Hence, the viscous force on the upper layer of 1 acts in such a way that it gets faster.
This fluid property of trying to keep and move the fluid ‘together’ is known as Viscosity.
The thing that separates the low viscous fluid from the highly viscous fluid is the amount of ‘strictness’ that the fluid shows for keeping them together.
- For example, water is less viscous than honey because water doesn’t care much about keeping the layers moving together. While Honey is like a much ‘strict master’ who wants all of these layers to be united/together
Technical Definition for Viscosity –
The measure of resistance to relative motion within the fluid is called viscosity.
2. Temperature effect on Viscosity
Temperature does have an effect on the viscosity of the fluid.
- In the case of a liquid, the molecules are bonded by weak chemical bonds.
- On increasing the temperature, you are actually providing enough thermal energy to the molecules that they break the bonds and become free. This causes the viscosity of fluid to decrease as there is no more dependency of fluid layers on one another because of moving apart. We say the fluid becomes thinner.
- While at much lower temperatures, the fluid tends to become thicker.
3. Need for Multigrade Engine Oil
We all would agree that an engine is nothing but a machine, and we know that in the case of a machine used in machining operations, there are many parts that require grease for proper functioning.
Similarly, Engine Oil is used to ensure that there is proper lubrication among the contact parts, i.e., with engine oil, we ensure there is no wear and tear among the interacting parts.
3.1 What happens if oil is too thin or too thick?
- If the oil you use is too thin, it will just flow out of the surfaces very quickly, and hence won’t be of any help
If the oil is too thick, a lot of power would go into just moving the parts through your ‘thick’ oil
3.2 Effect of Temperature on Engine Oil
So, knowing this, we choose a motor oil with a given viscosity. Now, consider 2 cases :
- Case 1: I use this oil on peak summer days
- Case 2: I use this oil in peak winters
We have already discussed the effect of temperature on the viscosity. An oil with given viscosity would become thinner in peak summer and thicker in peak winter.
SOLUTION :
To overcome this, earlier people used a 30-weight motor oil (already thicker) in summers, so that it thins out and reach required value because of high temperature, while they used a 5-weight motor oil (already thin) in winters, so that it thickens to reach the required value in peak winters due to low temperature.
BUT Now, we have come up with something even better, known as ‘Multigrade Engine Oil‘.
The speciality of this oil is that it can maintain a constant value of viscosity over a wide range of temperatures. Hence, we no longer have to change our oil from season to season.
- The key feature of multigrade oils is their ability to remain fluid when cold and provide adequate viscosity at higher temperatures.
3.3 How to interpret the rating?
5W-30 means 5 weight in winter and 30 weight in summer. Hence, the viscous nature remains maintained. Now, you know the reason behind 5W-30. Similarly, we also have 10W-40 and many more combinations available in the market
FAQ section
