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by Paul Brockington MIMechE

1st May 1970, Page 88
1st May 1970
Page 88
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Page 88, 1st May 1970 — by Paul Brockington MIMechE
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Which of the following most accurately describes the problem?

HIGH ambient temperatures are but one of the factors to be taken into account when considering diesel engine filtration problems in tropical conditions: in many cases vehicles operate in a dust cloud for the greater part of their lives. Overhauls, too, are often performed in a dusty atmosphere, typically the average humidity is high, fuels and lubricants may be of doubtful quality . nor do these factors exhaust the list.

It is appropriate, I think, to concentrate on tropical conditions because these—and sub-tropical—regions of the world include most of the newly developing countries; they cover a far larger area than countries having arctic or near-arctic winters, and the latter areas are generally more sophisticated than the former with regard to driving standards and vehicle servicing.

Good filtration of the fuel, air and lubricant is, of course, normally desirable anywhere; in the tropics it is a critical necessity and it is essential that the operator should know what he is doing and why. If good quality fuels and lubricants are obtainable they may well be contaminated by dirt of some kind when the vehicle is refuelled or the sump is topped up.

Vehicle manufacturers in this country have given a lot of thought to the protection of engines against rapid wear by contaminants in the fuel and oil and have had the full co-operation of filter makers in equipping vehicles destined for overseas

This picture of a Leyland Buffalo was taken in Australia and shows a type of operating condition in which engine wear would be growth, accelerated if the unit were not fitted with an efficient air filter.

markets with appropriate air, fuel and oil filter systems. But they cannot make proper allowance for neglect, carelessness, inadequate servicing and certain "extreme" variations in operating conditions.

Vulnerable The most vulnerable part of piston engines fitted to commercial vehicles is the injector pump of a diesel engine and filtration of diesel fuel oil is normally therefore a more important filtration process than any other. The clearance of the plungers of a diesel injector pump is about 1/2 microns (0.00004 /0.00008in.) and lack of ffitration would normally increase the rate of plunger wear by 10 times in reasonably dust-free ambient conditions and to a much higher rate in a dust laden atmosphere.

As a maker of injection pumps as well as fuel filters, the CAV company has been able to make accurate evaluations of wear rates relative to dust characteristics and so on. Technicians of the company point out that it would be a simple matter to produce a filter that provided low particle transmission but had a short life before it choked or one that did not choke but passed a large amount of abrasive. In practice it is necessary to compromise between particle transmission and filter life.

Having a hardness of 6.0 on the Moh scale, silica is the only constituent of natural dust that is harder than the pump plungers and can score the plunger surfaces. Although the purpose of a diesel fuel filter is to protect the pump against the ingress of silica particles, its useful life in terms of fuel throughput in gallons is generally determined by the percentage of inorganic contaminants, notably asphaltines and waxes, in the fuel. According to CAV, a throughput of 500gal may result in filter blockage if the fuel contains a high percentage of these contaminants, while the same system might be capable of handling up to 5000gal if the contaminant content of the fuel was very small. Before dealing with the job of the filter in trapping silica particles in the element it is appropriate, therefore, to elaborate on the blockage phenomenon.

CAV points out that blockage by inorganic contaminants is proportional to throughput for a given fuel as measured by the pressure drop across the element and that the company's FS bowl-less filter with spiral V-form paper element has an almost limitless silica-retention capacity. And in common with most types of oil filter, filtration efficiency tends to increase with increases in the amount of silica particles in the filter.

Practically every operator in this country will recall the disastrous consequences of the continuing low ambient temperatures of the 1962-63 winter when many thousands of diesel vehicles were immobilized by fuel waxing. Since then the oil companies have marketed a winter grade of fuel with a lower wax content than the summer grade, and in line with this practice fuels supplied to countries with high ambient temperatures normally have a considerably higher wax content than fuels sold in colder regions, which is justified on economic grounds but which, as indicated, may reduce the service life of the element to a fraction of its rated life.

Blockage In practice blockage of the filter is not harmful to the injector pump because it stops the engine before any harm is done, but the typical driver will remove the filter if it is the only way he can get his vehicle moving. And drivers in newly developing countries are probably "more typical" than others.

CAV technicians define blockage as a condition that causes the pressure drop across the paper element to increase to 15 psi which is the limiting operating pressure of the lift pump. Although the particles of wax are extremely small they are highly viscous and tend to stick on the walls of the capillaries of the element and to build up into larger accumulations.

A chart prepared by CAV shows that about 85 per cent of diesel fuels marketed in the UK have an inorganic contaminant content of less than 10 grams per 1000gal and that the content of fuels supplied in African countries is about double the UK average. About 10 per cent of fuels in Africa have a content of 60 grams per 1000gal whereas less than 2 per cent of fuels in this country have a comparable contaminant level. The best UK fuels have a wax content of approximately 0.8 grams per 1000gal and the content of the highest grade African fuel is around 2 grams per 1000gal.

If the size of a silica particle is less than one micron it will pass between the surfaces of the plunger and barrel without doing any harm. And if the particle is more than 20/30 microns (0.0008 /0.0012in.) it is excluded from the plunger area because of its size. Tests by CAV have shown that particles of 10/12 microns cause the most damage and that as little as 5 grams of particles of 5 /20 microns cause rapid wear of the pump. Particles are often wedge shaped and a relatively deep groove is worn in the rubbing surfaces by the thicker part of the wedge after the tip has started the abrasive action. Plungers that are scored to a depth of 2 microns may have a leakage rate of 50 per cent which is sufficient to prevent starting.

Dissolved water is always present in fuel oil and tends to corrode the injectors to cause pitting of highly stressed components and spring breakages and to form interfacial organic material that can result in sticking of lightly loaded moving parts. And a means of extracting moisture from the fuel is particularly important if the fuel is of poor quality and liable to have a higher-than-normal water content or a large amount of moisture is produced by condensation in the storage tank.

In the CAV Filtrap sedimenter /filteragglomerator system, the sedimenter is fitted on the suction side of the feed pump and removes the larger water droplets from the fuel in addition to larger particles of rust, metal, grit and so on. The filteragglomerator is located between the feed pump and the injector pump and also acts as sedimenter, the smaller water droplets that are forced through the pores of the paper element coalescing into larger droplets as the fuel flows downwards through the element.

Deposited by sedimentation, the droplets accumulate in the base of the transparent chamber. Although the cleaning of a storage tank is a costly exercise it is well worth while, according to CAV, and should be done every two to three years. Rust formed in a tank breaks up into a variety of particle sizes and forms a soft paste with a high choking propensity.

When an engine is in good condition and the vehicle is operating on specified routes under close supervision with known variations, the production of contaminants internally and the intake of contaminants from the atmosphere can be pre-assessed with a fair degree of accuracy. Provision can be made in both cases for adequate filtration with due regard to maintenance schedules and the type of lubricant used. If, on the other hand, a vehicle is being operated for the most part a long way from a maintenance depot in constantly varying or extremely arduous conditions by a driver of doubtful technical ability, ensuring that the engine is properly protected by filtration of the air and oil to carry it through to the next overhaul is a problem that is frequently a constant headache to the fleet engineer who seeks to get the best out of his power units. While London Transport can justify operating buses without an oil filter after the first six weeks in service following an overhaul, a similar practice on the part of an operator in typical overseas conditions could be disastrous in terms of accelerated wear and frequent engine failure.

Lubricant specialists of Shell-Mex and BP Ltd point out that the quality of a recommended engine lubricant (including its detergency level) is matched to the characteristics of the power unit including bmep, rpm, peak combustion pressure, ring-belt temperature and the rubbing velocity of the tappet gear. And they emphasize that an oil with a high detergency level is suitable for diesels that can run satisfactorily on an oil of lower detergency level, which is important to the operator who seeks to standardize on one grade of oil to simplify storage and to eliminate the possibility of an engine being filled or topped up with the wrong oil. While a Series 3 high detergency oil (recommended for turbocharger diesels) can be used in any type of diesel, it is not suitable for petrol engines because of the metallic ash that it deposits; and, as will be discussed later, the prevailing detergent level of a lubricant is important with regard to filtration.

Contaminants Of significance in comparing the particle sizes of contaminants in fuel oils and lubricants that cause rapid wear of components, Shell-Mex and BP say that particles of up to 15 microns in a lubricant can do no harm, but that comtaminants should not exceed 2 per cent of the oil by weight. When a new engine is being run-in, debris derived by production or overhaul processes is circulated with the oil and its removal is one of the most essential jobs that the filter has to do in the lifetime of the engine; and if, as is usual, a new filter element is being used filter efficiency is at its worst. Up to the time that it is completely blocked, a filter's greatest efficiency is attained immediately before it is blocked and there would seem to be a good argument in favour of using a partially blocked filter over the running-in period.

When the debris has been removed by the filter, internally produced contaminants mainly comprise those derived from products of combustion, and with moderndetergent lubricants such products are held in suspension in a finely divided and harmless state. If an engine operates continuously at or near its peak output the oil in the hot zone may be oxidized and in effect the detergency level of the oil will be rapidly reduced because the detergent has to cope with additional contaminant. This will also happen if an excess of free carbon is produced as the result of faulty combustion and in the event of serious detergent depletion hot sludge will rapidly build up and tend to block the filter prematurely.

Continuous cold running tends to produce cold sludge if oil detergency is deficient, with the same result.

The oil consumption rate of a diesel may be all-important with regard to the amount of contaminant that an oil filter has to deal with.. If oil consumption is high, depletion of the detergent is reduced or eliminated and

the amount of contaminant in the oil is also reduced, given, that the heavy oil consumption is not symptomatic of a generally poor engine condition. While an operator

who runs his vehicles for a greater mileage between filter changes than advised by the maker may "get away with it" in the case of engines that consume oil at a relatively high rate (particularly if they have a large sump • capacity) he may have serious trouble, resulting from filter blockage, if he neglects to change the filters of engines having a lower oil consumption.

A number of operators of diesels in this country and overseas continue to use a straight mineral oil instead of a detergent type recommended by the oil companies and presumably obtain "satisfactory" results by virtue of a low average load factor and the avoidance of peak-load running for any length of time. In a typical case this could, however, double or treble the rate of contaminant build-up in the filter, and this is confirmed by filter technicians of the Automotive Products Group who say that the rate is at least double compared with that of a filter employed on a similar engine running on a recommended detergent oil.

Although filtering the internally produced contaminants of a diesel is technically a more complex problem than filtering the air, the latter is generally the biggest problem of those who operate vehicles in hot, dusty areas with large and unpredictable variations in atmospheric dust content. As with fuel and oil filtration, changing the filters at appropriate intervals with the necessary care is the key to long engine life. If air filtration is inadequate the dust particles of critical size inhaled with the air will increase the blockage rate of the oil filter or circulate through the by-pass with the oil if the filter is blocked, in addition to accelerating wear of the pistons and cylinders before it is mixed with the sump oil.

British Leyland points out that the lack of space under the bonnet of forward-control vehicles increases the difficulty of providing efficient air filtration, and could favour the use of the smaller oil-bath cleaner because it can be accommodated in the engine compartment in place of the bulkier dry type which clogs more quickly and has to be mounted outside in a dust-vulnerable position.

The oil-bath cleaner is still preferred to a dry type by some operators overseas on the grounds that it is less susceptible to "quick clogging" if servicing is overdue, but quick clogging could be preferable to a loss of efficiency that can go unnoticed and to oil carry-over, which can occur if the cleaner is overfilled or the air velocity is too high. According to Automotive Products a dry paper air filter (of the group's PurOlator Micronic type having a resin-impregnated element) has advantages over an oil-wetted filter with a similar element in terms of dust capacity in the ratio of about 2 to 1 because the dust tends to fall away from a dry surface.

AP produces duplex types of micronic parallel flow fuel and lubrication filters incorporating a lever-operated changeover valve that enables the element of one filter to be changed while the other is in operation and doubles the capacity of the filter assembly. Fuel filter /water separator types are also included in the range together with metal-edge filters for primary fuel filtration, a handwheel fitted to the latter being used to clean the element. Micronic air filters are made in capacities up to 950 cu.ft. /min, the largest models being designed for external mounting and connection to the power unit by trunking. The filters are designed for the removal of particles down to 20 microns.

In line with the opinions given by CAV, Shell-Mex and BP, British Leyland and other companies, AP says that "everything gets out of control" with the type of labour employed in developing countries and replacements of second-rate quality made by backyard concerns are frequently used. In the case of paper filters, it is easy apparently to fabricate a replacement that looks right but has the wrong pore size, leaks in a number of places and deteriorates rapidly.

After an engine has been run in, metallic particles in the oil are mainly derived from wear of the working parts, and although the quantity is normally small some metals can act as a catalytic agent which accelerates oxidation of the oil. The products of oxidation form insoluble gums and resins that tend to block outways and a small quantity of metal can therefore produce a disproportionate increase in oil contamination. A magnetic plug is useful as a means of separating metals from the oil but the capacity of a plug is very limited.

Concentration of transport fleets in developing countries into a smaller number of larger groups promises to promote better training of drivers and mechanics, and the use of more sophisticated vehicles for cross-continental and trunking runs will increasingly justify the fitting of more specialized filter equipment as well as the use of the best fuels and oils. Without competent labour, additional equipment may be classified as "something more to go wrong".

Pre-filtration of the air with a centrifugal type of cleaner is reasonably simple and foolproof and as it greatly reduces the dust-particle loading of the finer main filter in dusty conditions it is virtually a must for some operations. According to G. E. Jones and Sons (Machine Parts) Ltd, Maylands Avenue, Hemel Hempstead, Herts (manufacturers of GUD filters) paper air filters have advantages over the oil-wetted and oil-bath types for intake and breather applications because the efficiency of the latter depends on air velocity, the efficiency being favourable at high rpm and poor at low rpm.

The main problem in the production of paper filters cited by Jones is to provide a realistic element life and the best way of overcoming this problem, according to a spokesman, is to use it in conjunction with a centrifugal pre-cleaner. A GUD pre-cleaner of this type is known as the L95-402K and features fins that are detachable and re-usable. It thus offers a considerable saving to the operator in terms of servicing costs. The company has recently introduced the first of' a new range of GUD air filters, the 62024, L95-402 and L95-402K, designed for applications to heavy trucks, contractors' plant and industrial applications.

In the case of oil filtration, centrifuging the by-pass oil provides continuous auxiliary cleaning of the oil and if the centrifugal unit is properly matched to the lubrication system it can play a major part in promoting efficient filtration and long life of the full-flow filter. The centrifugal by-pass oil filters made by the Glacier Metal Co Ltd, Alperton Way, Wembley, Middx, are operated by the passage of the oil being filtered at bowl speeds of 3000/10,000 rpm and are claimed to produce centrifugal forces up to 1500g and to remove dirt particles down to 1 micron.

Glacier points out that efficient filtration is immediately provided and that this contrasts with a filter having a replaceable element, the pore size of which is relatively large when the element is in a new condition. The filters are said to have five times the dirt capacity of equivalent element type filters, and element replacement is not required.

Additives are not removed with the dirt until they have fulfilled their prescribed function and become attached to other waste products. The filters operate on the Hero turbine principle and are matched to the engine to give the oil sufficient residence time in the bowl for the centrifugal force produced by the rotor to take effect. Because centrifugal force is proportional to the relative density of the dirt particles in the oil, separation of the particles from the oil is selective and the denser metallic and abrasive particles are separated before the remainder of the particles.

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