Abstract

The insides of a car engine get very hot when the engine is running. Motor oil lubricates the moving parts, to keep the engine operating smoothly. What happens to motor oil as the engine temperature goes up?

Objective

The objective of this project is to measure how the viscosity of motor oil changes with temperature.

Introduction

The internal combustion engine that powers your family's car has moving parts that work at high temperatures for billions of cycles over its lifetime. In order to keep the pistons moving smoothly in the cylinders, there is a thin film of motor oil between the piston rings and the cylinder wall. The oil is a lubricant—a slippery liquid that allows the piston to slide freely within the cylinder. Without oil, the piston would not be able to move in the cylinder—the engine would seize up and be ruined.

The primary functions of motor oil are:

• Lubricate engine moving parts to reduce friction and prevent wear.
• Clear engine of contaminants.
• Seal piston and liner for optimum engine efficiency.
• Resist high temperature degradation.
• Promote low temperature lubrication.
• Lubricate over a wide temperature range." (Thibault, 2001)

The oil must be able to keep the piston moving smoothly at cold temperatures (when the engine first starts up) and also at the normally high operating temperature of the cylinders.

One way of measuring an oil's ability to lubricate is to measure its viscosity. The viscosity of a fluid is a measure of the fluid's resistance to flow. You can think of it as fluid friction. Water is an example of a fluid with low viscosity—it pours easily and quickly. Cooking oil has a higher viscosity—it pours more slowly than water. For an engine lubricant, it is important that the viscosity does not change significantly as the temperature increases.

Viscosity of liquids can be measured with a special piece of glassware called a viscometer (see Figure 1). Fluid is drawn up from the cup on the lower left into the tube on the right, using a suction bulb. The suction is removed, and the time it takes for the fluid to drain out is measured. The higher the viscosity, the longer it will take the fluid to drain through the tube. To measure viscosity at different temperatures, the viscometer is placed in a water bath. Because viscometers are expensive (more than $170), you'll use a slightly different technique for this experiment.

Instead of using fancy glassware, you can use a Pyrex beaker or graduated cylinder. You will release a sphere (glass marble or steel ball bearing) at the surface of the liquid and time how long it takes for the sphere to fall to the bottom of the beaker or cylinder. You can make your comparisons using the measured time for the sphere to fall, or you can take your project a little further and actually calculate the viscosity. It's up to you.

In order to calculate the viscosity, you'll have to measure the time it takes the sphere to fall, the distance the sphere falls, and the density of the sphere and the fluid.

The equation (Equation 1) shows you how to calculate the viscosity from your measurements. It may look intimidating at first, because it has some Greek letters in it, but don't let that scare you. The variable commonly used to represent viscosity is the Greek letter "eta" (η). The variable commonly used to represent density is the Greek letter "rho" (ρ). The capital Greek letter "delta" (Δ) is often used as shorthand for taking the difference of something. The other variables in the equation are g, for the accleration due to gravity (980 cm/s²), a for the radius of the sphere (in cm), and v for the average velocity of the sphere as it falls through the fluid (in cm/s). The result is in units of poise (g/cm·s).

So the equation tells you to take the density of the sphere minus the density of the fluid (Δρ), multiply this by 2ga², and then divide the result by 9v. If the sphere falls more quickly (i.e. with greater velocity), v is greater and η, the viscosity, is smaller, as we would expect. Conversely, if the sphere falls more slowly, the viscosity is greater. You would also expect a sphere that has higher density (i.e., is less buoyant) would fall faster than a sphere with lower density (i.e., is more buoyant). The density factor in the equation accounts for this. The amount of friction that the sphere experiences as it falls will be related to its surface area, which is proportional to the square of the sphere's radius.

You can use a water bath to heat or cool the oil to different temperatures in order to see how its viscosity changes with temperature. The Experimental Procedure section has more detailed instructions.

Terms and Concepts

• Viscosity
• Lubricant
• Internal combustion engine

Questions

• What do the SAE ratings on motor oil mean?
• Why is it important for motor oil to maintain its viscosity over a wide range of temperatures?

Materials and Equipment

• Various motor oils with different viscosity ratings, e.g.:
  • 5W-30
  • 10W-40
  • 20W-50
• Small sphere, e.g.:
  • Glass marble or steel ball bearing
• Gram scale for measuring weight of oil and sphere, such as the digital pocket scale from Amazon.com
• Pyrex beaker or graduated cylinder (inside diameter must be larger than sphere),
• Tongs (for retrieving sphere from heated oil)
• Thermometer (0 to 100°C)
• Stop watch
• Large pan
• Hot plate or electric stove
• Water
• Ice

Experimental Procedure

Variations

Careers

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Related Links

• Science Fair Project Guide
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• Materials Science Project Ideas
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