How does a Multigrade Oil help in Car Engine ?

We all have seen engine oil can somewhere, and most commonly at the mechanics shop. How you ever wondered about what’s this 5W-30 written on it ? Well, that’s what we are going to find out through this article

Topics Covered :

Short Introduction to viscosity
Temperature effect on viscosity
Need for Multigrade Oil

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 2. Hence, viscous force on lower layer of 3 acts in such a way that it gets slowed down.
Layer 1 moves at slower speed (v-dv) than 2. Hence, viscous force on 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 which 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 this 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 viscosity of the fluid.

In case of a liquid, the molecules are bonded by a weak chemical bonds.
On increasing 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 Oil

Engine Oil is mostly used in order 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 motor oil ?

So, knowing this, we choose a motor oil with some given viscosity. Now, consider 2 cases :

Case 1 : I use this oil in peak summer days
Case 2 : I use this oil in peak winters

We have already discussed about the effect of temperature on the viscosity. An oil with given viscosity would becomes thinner in peak summer and thicker on peak winters.

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 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 Oil‘.

The speciality of this oil being 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.

Rating :

5W-30 means 5 weight in winter and 30 weight in summer. Hence, viscous nature remains maintained. Now, you know the reason behind 5W-30. Similarly, we also have 10W-40 and many more…

How does a Multigrade Oil help in Car Engine ?

Topics Covered :

1. Short Introduction to viscosity

2. Temperature effect on Viscosity

3. Need for Multigrade Oil

3.1 What happens if oil is too thin or too thick ?

3.2 Effect of Temperature on motor oil ?

Rating :

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