How a Battery Functions

Engine Oil Introduction

Engine oil performs the following essential functions:

Lubrication

Engine oil forms a film on the cylinder walls, pistons, crankshaft, and other rubbing surfaces. By reducing friction, this oil film minimizes power losses and wear.

Cooling

Heat is generated by friction and combustion during engine operation. As the engine oil circulates throughout the engine, it absorbs this heat and carries it away from individual parts.

Sealing

Engine oil fills the clearances between the piston rings and cylinder walls, thereby improving the seal between them. By preventing combustion gases from escaping into the crankcase, this sealing action prevents power losses.

Corrosion Prevention

Engine oil prevents corrosion on metal surfaces by preventing direct metal-to-air contact.

Cleaning

Engine oil removes carbon and other contamination from moving parts and prevents localized accumulation of such foreign particles.

Engine oil is produced and sold by many companies. Each brand of engine oil contains various additives that are formulated to enhance the oil’s functionality. Ratings are applied to each type of engine oil to indicate its quality and viscosity. For every engine, oil with the appropriate ratings must be selected in accordance with the engine’s characteristics and the ambient temperature at which the engine is used. The use of inappropriate engine oil can cause the following problems:

• Decreased engine performance
• Decreased fuel efficiency
• Difficult starting
• Shortened engine service life

In extreme cases, the use of inappropriate engine oil can cause engine damage. Correct engine oil selection is extremely important.

Engine Oil Classification

Engine oil types are usually indicated by American Petroleum Institute (API) and American Society of Automotive Engineers (SAE) ratings.

API ratings indicate quality/performance and SAE ratings indicate viscosity.

The correct oil for any engine is selected in accordance with these two ratings.

Engine Oil API Ratings

As mentioned, API ratings indicate engine oil quality/performance. Gasoline engine oils are rated such as SE, SF. The first letter stands for service and means that the oil is for service stations for gasoline engines. And the second letter indicates the oil’s rating.

Diesel engine oils are rated like CC or CD. “C” means commercial and indicates that the oil is for diesel engines. The second letter indicates the oil’s rating. To prevent problems with gasoline engines, it is essential to use oil with an API service rating of SG, SH or SJ.

API Engine Service Classification System Symbol

Every oil classified by the API bears a standardized mark (see Fig. 1) on its container. This mark shows the API service rating at the top, the SAE rating in the middle, and the oil’s fuel efficiency attributes at the bottom. The fuel efficiency attributes indicate that the oil provides an improvement in fuel efficiency above a certain level. Engine oils recommended by the API bear the starburst mark shown in Fig. 2.

Engine Oil SAE Ratings

As mentioned, SAE ratings indicate engine oil viscosity. The viscosity and fluidity of oil vary in accordance with the temperature. An increase in temperature causes a decrease in viscosity and an increase in fluidity. Conversely, a decrease in temperature causes an increase in viscosity and a decrease in fluidity.

This principle is highly significant. If, for example, a low-viscosity engine oil is used at high temperatures, its viscosity drops excessively, preventing the formation of an oil film on rubbing surfaces. Thus, the oil ceases to function as a lubricant. If, on the other hand, a high-viscosity oil is used at low temperatures, its viscosity increases excessively. The resulting resistance makes the engine difficult to start, and the oil’s decreased fluidity can prevent correct lubrication. To prevent such problems, it is essential to use engine oil whose viscosity is suitable for the ambient temperature.

Single-grade and Multigrade Engine Oils

Engine oil viscosity is, in its simplest form, indicated by a single rating, for example, SAE 30. However, the use of special additives, enables the formulation of multigrade engine oils, which are capable of meeting the viscosity requirements of more than one SAE rating.

Multigrade oils are classified using double numbers. By way of example, a multigrade oil with an SAE rating of 10W-50 offers SAE 10 viscosity at normal temperatures and provides the lubricating performance of an SAE 50 oil at high temperatures. Thus, a multigrade oil can be used under a wide range of temperatures. (The ‘W’ in ‘10W’ indicates winter use. Oils formulated for winter use are viscosity-tested at a temperature of –18 deg.C (0 deg.F).)

Multigrade oils should be used in regions where the temperature differs greatly between night and day or where large temperature variations occur throughout the year.

Engine Oil Selection Method

The shop manual for each vehicle contains a temperature/viscosity chart of the type shown before. The appropriate oil for the engine’s operating conditions should be selected in accordance with this chart and with any precautions given in the shop manual.