 Explain what is meant by viscosity and a viscosity index.

 Explain what is meant by viscosity and a viscosity index.

 State the reason why lubrication is necessary.

 Look back over the previous section and write out a list of the key bullet points here:

ENGINE OIL SPECIFICATIONS

Modern engine oil specifications are based on SAE (Society of Automotive Engineers) viscosity ratings, API (American Petroleum Institute) service ratings and other properties defined by classifications laid down by organisations such as ACEA (Association des Constructeurs Européans d’Automobiles)

Common specifications (SAE is now almost

universal)

and the earlier CCMC (Comité des Constructeurs d’Automobile du Marché Commun) for European vehicles.

API Service Ratings ฀฀ The API service rating classification is based on oil-performance characteristics and consists of two letters. The first letter is either ‘S’ for spark ignition or petrol engines, or ‘C’ for compression ignition or diesel engines. Originally ‘S’ stood for ‘Service’ as in Service Station and ‘C’ for Commercial Vehicles. The increase in diesel-engine usage in light vehicles has brought about the change of meaning. The second letter in the classification denotes the service specification, which has been updated at significant intervals and reflects the greater performance requirements of newer types of engines.

Oil-Grade Classifications The lowest grade of oil is SC/CC, which was suitable for engines produced during the 1960s. As the manufacturing and environmental demands have developed during recent years, improved oil performance has followed. SD and SE classifications cover the 1970s and SF and SG the 1980s. This development will continue with the introduction of newer classifications. As a general rule, a later classification can be used in place of an earlier type, but not the other way round.

Diesel-Engine Oils Diesel-engine requirements are not exactly the same as petrol engines. Separate diesel-engine oils are formulated and marketed and should be used in preference to petrol-engine oils that carry a C classification. Development of C-class oils has been slower than the S class. Turbocharging of diesel engines is now common and these must use the appropriate grade of oil. Recent grades are CD and

CE. ACEA Classifications ฀฀ The ACEA classifications are divided

into three groups. Group A covers petrol engines, group B covers passenger car diesel engines and Group C covers commercial vehicle diesel engines. The development of these classifications was carried out to meet the needs of European vehicles, which have different characteristics than the American engines that are used to set the API standards.

Recommended Oil Grades Most engine and vehicle manufacturers list the SAE, API and other classifications for engine oil for their vehicles. They frequently list oil-producer preferences, which give an indication of the co-operation that has been given by the oil producer in the design and development of the engine. Some manufacturers produce their own oils formulated specifically for their vehicles.

Petrol and Two-Stroke Oils ฀฀ Engine oils are not normally biodegradable and should not

be allowed to enter the environment either as vapour or liquid. Total loss lubrication systems used on small two-stroke engines, such as those on motorbikes and outboard motors, use a 'petroil' mixture of petrol and

a specially-formulated biodegradable oil. Other types of oil should not be used.

 What does SAE stand for and where is it used in relation to oils?

 Look back over the previous section and write out a list of the key bullet points here:

LUBRICATION SYSTEM OPERATION

Oil Feed ฀฀ A good flow of oil from a pump provides a forced feed into the shaft bearings. The large quantity of oil in the bearing generates an oil wedge that maintains separation under the severe conditions during the combustion stroke.

Forced-Feed System ฀฀ The forced-feed system is efficient for the removal of heat and for cleaning by carrying dirt to the filter.

Drillings and Oilways ฀฀ Oil is fed through the engine via drillings and oilways, and returns to the sump to carry heat away. The oil pump takes the oil from the sump and feeds it through the filter where it is cleaned.

Component Lubrication ฀฀ Other components are lubricated by splash from jets of oil, or by the flow of oil from the top of

the engine back to the sump. Full-Flow, Forced-Feed System ฀฀ Modern engines have all

the oil flowing through a filter before entering the oil circuit of the engine. This circuit is known as the full-flow system. The full-flow, forced-feed system provides oil under pressure to critical components.

The majority of engines use a full-flow, forced-feed oil circulation and distribution system. These have a wet sump, and oil is pumped to all parts of the engine by a rotary, positive, displacement pump. The oil is filtered before it enters the main gallery for distribution around the engine.

Dry Sump, Dry-Sump System ฀฀ There are a few exceptions such as some high-performance engines where a dry sump, or dry-sump system, is used.

Forced-Feed Pump ฀฀ The forced feed is provided by a pump driven by the crankshaft or camshaft. A number of different pump designs are used, but all have positive displacement. These have rotating components that sweep past inlet and outlet ports and form chambers that increase in volume, carry oil and then decrease in volume in order to pump and pressurize the oil.

Oil Pressure at Low Engine Speed ฀฀ The oil pressure is controlled so that a sufficient supply is given at low engine speeds. This means that at higher engine speeds there would

be excess pressure and oil flow, but this is relieved by a pressure-relief valve that returns the excess to the sump or to the inlet side of the oil pump.