ENGINE LUBRICANTS

2.13.1 The need for lubrication

The internal-combustion engine has many sliding and rotating components, which are in constant contact with one another. If a lubricant were not used these components, which are made from various materials, would soon seize, and in some extreme cases even melt, due to the friction and heat generated by the components rubbing against each other. To prevent seizure and reduce the friction between components an oil lubrication system is used.

Friction and heat

When two surfaces are in contact, there is an opposition to relative movement between them, which is called friction. If the surfaces are clean and dry, the force needed to overcome friction depends on:

1 The materials from which the surfaces are made.

2 The surface finish, i.e. whether rough, smooth or polished.

3 The load pressing the surfaces together.

For any one pair of clean dry surfaces it can be shown by simple experiments that the ratio::

is a constant number. It is called the ‘coefficient of friction’ for those surfaces.

When friction is overcome and movement between two surfaces occurs, work is done against friction and an equivalent amount of heat is generated at the surfaces. In a continuously running bearing this heat must be dissipated or moved elsewhere to keep the bearing temperature within reasonable limits.

Another result of movement between dry surfaces is wear of the surfaces. The rate of wear depends on the materials and also varies with the load and speed, but in a high-speed machine it is likely to be so rapid as to render the mechanism useless within a very short time.

If the surfaces can be kept totally apart neither friction nor wear can occur, and the primary function of a lubricant is to separate the moving surfaces.

Lubricants may be solid, liquid or gaseous, but liquids Resistance to movement

Load pressing surfaces together

the converter reduces the pollutants. It is typical to fit a single catalytic converter to four- or five-cylinder engines (especially in-line engines). Engines with a greater number of cylinders normally require two or more catalytic converters. This is due to the design of the exhaust layout and also to prevent the catalyst restricting the flow of exhaust gases. Further details on catalytic converter operation are given in section 2.30.

higher the SAE number, the higher the viscosity, or to use a more common expression, ‘the thicker the oil’.

Whereas engines in the past used a comparatively thick oil, such as SAE 50, the need to reduce both fuel consumption and cold-cranking loads has brought about the common use of thin oils, e.g. SAE 10. An even better economy can be achieved with the very thin oil, SAE 5, but this can only be used on engines having extra-close fitting bearings. Suitability can only be determined by seeking manufacturers’ advice.

Viscosity index

The viscosity of lubricants decreases with increasing temperature; the extent of this change is measured by the viscosity index. A high index indicates a relatively small change in viscosity while a low index indicates a large change. The lubricant used should have a suitable viscosity at its normal operating temperature in the engine. This means that when the engine is cold, the viscosity will be unnecessarily high, leading to poor circulation of the lubricant and excessive friction (‘oil drag’), possibly even to the extent of making the engine difficult to start.

Multi-grade oils

In the past the high viscosity oil used in engines during the summer made engine cranking difficult in winter, so different grades were specified for the two seasons.

Nowadays special additives that reduce the change in an oil’s viscosity with temperature are often used, and this has meant that the same grade can be used throughout the year. These oils are called multi-grade or cross-grade, or given trade names that suggest the viscosity remains constant. They can be recognized by the special SAE rating; this has two numbers separated by the letter ‘W’. A typical oil is ‘SAE 5W 40’. In this case the oil is equivalent to SAE 5 when tested at a sub-zero temperature, (the ‘W’

indicates winter conditions) and has a viscosity of SAE 40 at the normal rated temperature.

Oiliness

This property is the ability of an oil to ‘cling’ or be attracted to a metal surface. The effect of this property is seen when a spot of oil is applied to a clean piece of metal; the oil film spreads out over the surface and resists being removed when wiped with a cloth.

Degree of oiliness varies with the type of oil.

Vegetable-based oils are excellent in this respect.

2.13.3 Oils

The lubricant used for motor vehicle engines – and most other components – is oil. Oils are obtained from three main sources:

1 Animal: purified and suitably treated animal fats, such as tallow and whale oil, are used for certain purposes, but decompose too readily to be suitable lubricants in modern motor vehicle engines.

2 Vegetable: these also decompose too readily to be satisfactory, though one example, castor oi1, was used quite extensively at one time. Its chief merit is ability to lubricate under arduous conditions, but after a fairly short time treacle-like deposits are formed, so today it is seldom used.

3 Mineral: oils of this type are refined from natural crude oil and are far more stable than other types.

They form the basis of practically all modern lubricants, and though by no means perfect, they can be improved by the addition of certain chemicals known generally as additives.

Synthetic oils

Although standard mineral oils have been in general use for motor vehicle engines for many years, synthetic oils have increasingly been specified since the 1990s.

Synthetic oils are based on natural crude oil (as are conventional mineral oils) but they have been ‘chemically engineered’ to improve performance. In simple terms, non-synthetic mineral oils are produced using natural crude oil as the raw material or base product, from which suitable fractions are extracted and refined to produce the modified mineral oil. With synthetic oils, the same base product is used but the extracted fractions are modified or changed chemically. Additives are also used (see below) as is the case with conventional mineral oils, with the end result that synthetic oils have benefits over conventional mineral oils.

Synthetic oils generally have improved performance over a wider temperature range compared with mineral oils, including a generally lower viscosity. Importantly, synthetic oils do not ‘break down’ as readily as mineral oils at high temperatures. Because modern engines usually operate at higher temperatures than older designs, synthetic oils assist in ensuring better lubrication and the anti-wear properties of synthetic oils extend engine and component life.

It is important to note that not all engines will benefit from the use of synthetic oil. This is especially true of engines that were designed before synthetic oils were in general use. It is always advisable to refer to a vehicle manufacturers specification for engine oil, or to consult an oil manufacturer prior to using synthetic oil in an engine that was originally designed for conventional mineral oils.

Additives

Additives are used in most oils to improve oil performance. Different additives have different properties and characteristics. Amongst the most important additives for use in engines are:

a Oxidation inhibitors

At high temperatures mineral oils tend to oxidize (combine with oxygen), forming hard deposits on the hottest parts with which they come in contact (e.g. the underside of the piston crown,) and varnish-like deposits on parts that are not quite so hot (e.g. the

piston skirt). Other products of oxidation may be carried in the oil and deposited in other parts of the engine. If they settle in oil passages, they may eventually reduce the oil flow to a dangerous extent.

Nowadays, oxidation inhibitors (or anti-oxidants) are added to the oil to reduce oxidation.

b Detergents

In general use, oil becomes contaminated with oxidation products and with burnt or partly burnt products of combustion which escape past the pistons.

These usually consist of extremely small and relatively soft particles which will not harm the bearings, but which tend to settle out and block up oil passages.

Around piston rings they become baked hard and restrict the free movement of the rings, eventually sticking them completely in their grooves.

The function of detergent additives is to keep these oxidation products in suspension in the oil so that they are not deposited inside the engine. The oxidation products are then removed from the engine with the dirty oil when the oil is changed.

c Viscosity index improvers

Certain chemicals have the property of reducing the change in viscosity of mineral oil caused by change in temperature.

d Anti-foam agents

Some engines suffer from the formation of foam or froth in the oil, and suitable additives are used to reduce this tendency.

2.13.4 Types of oil-based lubrication

Boundary lubrication

This form of lubrication relies on the oiliness property of an oil to coat the surfaces and fill the cavities of low-speed rubbing components to ensure that metal-to-metal contact and resulting wear are avoided.

Boundary lubrication is used in an engine for all sliding components other than the highly loaded bearings that require a pressure feed. The supply of oil for the boundary lubrication film is provided by splash;

this comes from the oil thrown out from the crankshaft bearings.

Hydrodynamic lubrication

The bearings of a modern engine must withstand great loads and high rubbing speeds. If the surfaces are to be adequately held apart when subjected to these arduous conditions, the quantity of oil supplied must be sufficient both to fill the space between the shaft and bearing and to make up for the oil squeezed out from the bearing as it rotates. To ensure these requirements are met, the bearing is force-fed with oil at a pressure sufficient to maintain the supply at all times the engine is running.

Highly loaded bearings rely on ‘hydrodynamic lubrication’ to separate the surfaces. This is achieved by

using the natural movement of the oil (hence the term hydrodynamic) to create an ‘oil wedge’ to lift and centre the shaft in the bearing.

Figure 2.124 shows the principle of this method of lubrication. The bearing is made larger than the shaft to the extent governed by the type of oil to be used and the expected thermal expansion.

When the engine is stationary the shaft rests on the bottom of the bearing and is supported only by a thin oil film (Figure 2.124a), This boundary film provides lubrication when the engine is started.

Initial rotation causes the boundary layer of oil on the shaft to move around with the shaft owing to its oiliness property. This movement, together with the inherent resistance of the oil particles to shear (viscosity), forces the oil between the surfaces to create a ‘wedge’ (Figure 2.124b). This wedge lifts and centres the shaft in the bearing (Figure 2.124c). This diagram also shows an indication of the pressure variation and its relationship to the oil wedge that is generated by the hydrodynamic oil film.

To achieve effective lubrication in the bearing, an adequate supply of good quality oil at a suitable pressure must be provided to maintain an oil wedge.

Figure 2.124 Bearing lubrication – the oil wedge principle

2.13.5 Oil as a coolant

In modern engines, oil helps to cool such parts as pistons and bearings. In its passage around the engine, the oil picks up heat from the hot parts with which it comes in contact. To prevent excessive oxidation and loss of viscosity the oil itself must be cooled. In most normal types of engine the circulation of air around the engine and the oil reservoir (normally the sump) is sufficient for this purpose. Many vehicles today are fitted with an oil cooler, which helps to reduce the

temperature of the oil when the engine is placed under high load. Vehicles such as high performance cars are normally fitted with oil coolers in the form of a small radiator, which is placed in the air stream. The flow of cool air passes across the radiator fins, and this lowers the temperature of the oil as it passes through the radiator. Many engines are fitted with an oil cooler that simply passes engine coolant around the oil filter housing. The hot engine oil passing through the oil filter housing is cooled by the engine coolant which, although it is not cold, is cooler than the oil.