Viscosity of engine oils
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The document discusses the key functions and properties of engine oil. It explains that engine oil acts as a lubricant to reduce friction between moving parts, as a coolant to dissipate heat, and to clean the engine by removing dust and carbon buildup. The document also discusses viscosity grades for engine oils, with multi-grade oils rated using two numbers (like 5W40) to indicate their viscosity at different temperatures. Viscosity is identified as the most important property, as it determines how easily the oil flows through the engine. Formulas for Stokes' law and terminal velocity in relation to viscosity are also provided.
![DERIVATION FOR VISCOSITY
Consider a small sphere of radius (r) and density (ρ) falling freely in a viscous medium
(coefficient of viscosity η and density σ).
Total downward force = Weight of sphere – upthrust
F = (4/3) πr3(ρ – σ) g
This is equal to the upward force of viscosity = 6πηrv
∴ 6πηrv = (4/3) π r3 (ρ – σ) g
η= μ= [2𝐫 𝟐 (ρ – σ) g ] / 9v
This is an expression for dynamic viscosity.
Therefore kinematic viscosity is given by,
𝛖 =
𝛍
𝝈](https://image.slidesharecdn.com/viscosityofengineoils-171008073242/85/Viscosity-of-engine-oils-8-320.jpg)
Viscosity of engine oils
- 2. WORKING OF ENGINE OIL Lubrication of moving parts Engines move anywhere from 2000RPM-6000RPM in Street vehicles and up to 19,000RPM +. In Race Engine, Parts would not last with metal to metal contact. You need a barrier between parts to cut down friction, oil is your barrier. Cooling Oil also acts as a heatsink for the engine, and as stated above, you need oil to cut down friction, if there was no oil the heat caused from friction would be enough to destroy any engine. Cleaning Oil acts in another way, it removes dust particles that enter the engine, and some of the carbon build up in the engine. This is one of the reasons it is important to change your oil.
- 3. Engine oils can be made of singular polymers (single grade) or multiple polymers (multi grade) and are rated accordingly. The SAE gives a single grade to engine oils. However, single grade oils cannot withstand the variety of conditions vehicles today are put through. Therefore, multi grade oils are more popular. Multi grade ratings are in the form xWy (eg. 5W40). 'W' is the winter rating which shows that the oil has been tested to work under cold conditions. The 'x' number tells us which single grade polymer the oil behaves like under cold conditions. The lower the number, the less viscous the oil and better the performance in low temperatures. The 'y' number tells us the single grade polymer as much as which the oil thins at high temperatures. The rating is primarily based on the viscosity of the engine oils. Hence, it is evident that viscosity is the single most important characteristic deciding the efficiency of these oils.
- 4. It is the property of a fluid by virtue of which an internal resistance comes into play when the fluid is in motion, and opposes the motion between its different layers. In a vehicle engine, viscosity determines how easily the motor oil flows to different parts of the engine. Units of viscosity: Dynamic: pa-s Kinematic: stokes (St)
- 5. VISCOSITIES OF COMMON FLUIDS (AT 25°C) Fluids Viscosity (Pa·s) Viscosity (cPoise) acetone 3.06 × 10−4 0.306 benzene 6.04 × 10−4 0.604 blood (37 °C) 3–4 × 10−3 3–4 castor oil 0.985 985 corn syrup 1.3806 1380.6 ethanol 1.074 × 10−3 1.074 ethylene glycol 1.61 × 10−2 16.1 glycerol 1.5 1500 HFO-380 2.022 2022 mercury 1.526 × 10−3 1.526 methanol 5.44 × 10−4 0.544 nitrobenzene 1.863 × 10−3 1.863 liquid nitrogen @ 77K 1.58 × 10−4 0.158 propanol 1.945 × 10−3 1.945 olive oil 0.081 81 pitch 2.3 × 108 2.3 × 1011 sulfuric acid 2.42 × 10−2 24.2 water 8.94 × 10−4 0.894
- 6. STOKES LAW AND TERMINAL VELOCITY Stokes law: It was shown by Stokes Law that if a small sphere of radius r be moving with a terminal v through an infinite homogeneous and incompressible fluid of coefficient of viscosity η, it experiences a force F given by, F = 6πηrv (This relation is known as Stokes Law) Terminal velocity: It is the maximum velocity acquired by the body falling freely in a viscous medium.
- 7. WORKING L Engine oil 1.8 cm dia. cylindrical tube (t initial) (t final) Time= t final- t initial Terminal Velocity = L/time
- 8. DERIVATION FOR VISCOSITY Consider a small sphere of radius (r) and density (ρ) falling freely in a viscous medium (coefficient of viscosity η and density σ). Total downward force = Weight of sphere – upthrust F = (4/3) πr3(ρ – σ) g This is equal to the upward force of viscosity = 6πηrv ∴ 6πηrv = (4/3) π r3 (ρ – σ) g η= μ= [2𝐫 𝟐 (ρ – σ) g ] / 9v This is an expression for dynamic viscosity. Therefore kinematic viscosity is given by, 𝛖 = 𝛍 𝝈
- 9. CONCLUSION By performing this experiment, viscosities of different engine oils were calculated and the conclusion was drawn that the engine oil falling in the range of SAE 40 and having minimum viscosity is suitable in colder weather, as it is specified that the car is on its way to a hill station. This oil will ensure it doesn’t get too thick at lower temperatures and will still flow. In our experiment we found that