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PROBLEMS VARY WITH USAGE...

9th September 1977
Page 60
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Page 60, 9th September 1977 — PROBLEMS VARY WITH USAGE...
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Which of the following most accurately describes the problem?

BY FAR the biggest engine lubrication problem facing operators is dilution of the oil by the fuel.

This dilution applies to both petrol, usually as "heavy ends-, i.e. the less volatile components of petrol which do not burn readily, and diesel engines; it is caused by unburnt fuel being allowed to enter the crankcase and mix with the oil.

If there is any sort of problem with the excess fuel controi or fuel return valve, gross contamination will occur. Similarly, weak, dribbly injectors pass too much fuel into the combustion chambers—some of which inevitably finds its way into the oil. Unfortunately, there is no easy test for this contamination.

The effect of the dilution may not become apparent for some time, but it is obvious that it will eventually cause damage to other engine components as the viscosity of the lubricant has been reduced. This dilution is one of the potential dangers of longer drain periods, as a malfunction may occur in the fuel system at any time without becoming immediately obvious. Thus the engine could be run for some time with a very low viscosity oil and wear accelerated. However, even in these circumstances a higher quality crank case lubricant would give a greater degree of protection against fuel dilution.

Both petrol and diesel can also suffer from internal rusting, although this is more noticeable on petrol engines. When the engine is started, condensation takes place producing water at all the cold points of the engine. This water finds its way into the oil causing sludge formation and thus reducing the lubricating effect. In addition, the bright parts of the engine, which may be all right under running conditions, are subject to a rust formation when the engine is closed down.

From this it can be seen that the engine lubricant has a tough job to do and the requirements placed upon it are high. Castrol laid out the following specification for engine oils for protection against corrosion and wear.

7 During short stops or when running at low temperature the oil must protect metallic surfaces against corrosion.

I_ In normal running it must resist degradation (resist oxidation, have good thermal stability). It must counteract the action of fuel and lubricant decomposition products at high temperatures, especially on non-ferrous metals (that is, copper-lead bearings, for example). By intervention in the frictional mechanism it must reduce the consequences of the inevitable metal to metal contact. The oil must resist deposit formations which might affect lubrication (the detergency or dispersive action) and finally contribute to the control of dust and other contaminants.

It can be seen then, that the oil has to meet a large number of requirements.

However, no two service conditions are the same and some bring greater problems than others. Urban delivery, for example, is all stop-start, engine on-engine off work which has a very detrimental effect on both the engine and its oil.

Most diesels used on smaller vehicles are indirect injection and these are very dirty in their operation. Mileage based oil changes are of little value to this type of work, so the oil must be changed on a time basis.

A second problem which faces operators is that of bore glazing. Strangely enough, if the oil is too good during the running-in period the bore sometimes polishes, which is to the detriment of the seal between piston and cylinder wall, thus oil consumption is increased. The oil must be capable, therefore, of allowing proper bedding in. In normal service, it must assist sealing in the piston ring/bore area especially when the pistons are at top dead centre. To do this it must have sufficient viscosity at high temperature coupled with low volatility.

It must be able to limit liner wear and not contribute to the formation of deposits in the piston ring grooves and must fight against such formations.

To sum up, there are quite a number of ways in which an operator can help himself to overcome his lubrication problems. Planned maintenance, covered elsewhere in this supplement, is one of them. It must, however, be tailored to suit his own particular requirements.

Individual vehicles must be checked to ensure that the fuel system, particularly in the case of a diesel, is up to scratch and doing its job properly. Checks to ensure that excessive choke is not used on petrol engines will again reduce the dilution.

If the engine isn't reaching normal temperature or is slow to do so, the thermostat should be investigated and if need be, replaced.

• by Stephen Gray

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