TAKING CARE OF THE LUBRICATION SYSTEM.
Page 67
Page 68
If you've noticed an error in this article please click here to report it so we can fix it.
• The Fourth of a Series of Articles on Vehicle Lubrication, Dealing with the Maintenance of a System. Dilution by Fuel and Its Minimizing.
THE lubrication requirements of modern commercial vehicles are far more exacting than those of older types. Post-war development has even reached a stage where commercial motor vehicles can travel at speeds approaching those of many modern touring cars. Greater loads, higher average road speeds, and the running of vehicles to strict schedule on daily services are conditions which designers have met by increasing engine speeds to produce a greater power output. This has necessitated the introduction of lighter reciprocating parts and the increased use of aluminium alloy pistons. Forced-feed lubricating systems are frequently used to meet the exacting lubricating conditions and to ensure adequate lubrication at all times. Unless the lubricant used has a high lubricating value, is consistent in quality and has the necessary stability and , character to meet the service demanded, mechanical failure will result.
The hazard of employing inferior grades of oil will be generally recognized, as commercial vehicles are costly to maintain, and only by continuous uninterrupted service on the road will they warrant their expense. A large percentage of mechanical failures may be attributed to faulty and improper lubrication; if it were possible to lubricate all engines perfectly there would be no wear, as no mechanical friction would occur. It is necessary, therefore, to approach the ideal as near as is practically possible by making the best use of high-quality lubricantsof the right type. The oil in the engine sump should be kept in good condition to ensure efficient lubrication, and this may be achieved by giving attention to a few simple details in maintenance.
Oil Supply.—An oil-level indicator or dip-rod is provided in most vehicles for determining the correct level of oil in the engine siimp, but in some instances level testing taps or pet-cocks are relied upon. If the vehicle be in constant use, the oil level in the sump should be checked and, if necessary, filled to the upper level daily, care being taken not to overfill, for this surplus oil if added will not provide better lubrication, but, on the contrary, will cause smoking, oily plugs, undue carbon formation and excessive oil consumption —all, due to an excess of 611 reaching the combustion chambers. Equally important—and this point cannot be overstressed—is the use of clean filling receptacles and proper storage conditions to ensure that the oil added shall be free from contamination. R is surprising how often these simple precautions are overlooked.
Starting up from Cold.—It is important to allow an engine to run light for several minutes after starting up from cold in order to ensure thorough and complete circulation of the oil before the engine and transmission are put under load. Particularly is this precaution necessary in cold weather, and more especially with engines requiring the use of heavy oils and having lubricating systems employing a relatively large amount of oil piping and drilled oil passages. Unequal oil distribution is bound to occur unless such precautions are taken. Storing the vehicle in a heated garage in winter is also desirable, in order to minimize trouble and to facilitate quick starting, Oil Pressures.—Where an oil-pressure gauge is fitted the pressure readings should he noted and periodically checked to ensure that pressure is maintained. If no oil pressure be indicated, reference should be made to the paragraphs in the manufacturers' instruction book dealing with this subject, carefully noting the possible causes. Of the factors which may
cause or contribute towards erratic oil-pressure readings the following may be cited: low oil level; clogged • oil filter ; oil pump not primed (elevated oil pumps) ; broken pump shaft; loose pipe connections; unsuitable oil; defective gauge, broken or clogged gauge tube ; oil pressure relief valve stuck due to grit or foreign matter, and, in the ease of spring-loaded plunger pumps, a broken spring or sluggish plunger. Where a "fluttering" needle is observed on the pressuregauge indicator immediate investigation of the foregoing points should be made, as this phenomenon may be accepted as a warning of impending trouble if not corrected. Steady oil pressure readings are always deSirable, even If they be consistently low. The amount of oil pressure will depend upon circumstances. In splash-circulating oiling systems the pressure is generally only nominal, whereas in force-fed engines having drilled crankshafts the pressure will depend largely on the condition of the main and big-end bearings; fast running engines with closely fitted bearings can usefully employ a higher oil pressure to ensure efficient piston lubrication than engines in a less satisfactory mechanical condition.
To attempt to maintain a high oil pressure in the latter case is a waste of effort and of lubricant, because the result is solely that of over-lubrication. Excessive oil consumption, =Due carbonization and reduced periods between top overhauls usually result, thus involving additional expense. The modern higher speed types of engine sometimes require moderately high oil pressures—up to 20 lb. or 30 lb. per sq. in. when new with the oil hot—but, in the great majority of force-fed engines in good mechanical condition, working oil pressures of 5 lb. to 10 lb. per sq. in. at normal engine speeds are ample.
Fuel Dilution of Crankeaee Oil.—The principal cause of fuel dilution in recent years is the change in character of the fuel in general use. This is usually of heavier gravity and is less volatile than that
sold formerly, and these fuels are more difficult to vaporize in order, with air, to form a proper mixture for complete combustion. Only a small percentage of the fuel vaporizes unless considerable heat be applied.
Volatility means the ease with which a liquid is transformed into a vapour. A highly volatile fluid vaporizes at. a comparatively :low temperature, whilst a less volatile liquid requires a greater amount of heat. If it were possible to use, for commercial vehicle operation, only those portions of the fuel which vaporized at low temperatures, we should have little trouble with the dilution of crankcase oil.
The trend of fuel volatility is well illustrated by the two curves which represent the fuel of 15 years ago with a fuel being used to-day, showing the higher temperatures now required to secure distillation. The curve for paraffin will also be noted-as a comparison.
The temperature required completely to vaporize the heavy fractions of the fuel is referred to as the "end point," and it is these heavier " ends " that fail to vaporize in a cool running engine. These leavier " ends " naturally mix 'with the oil film on the cylinder walls, ultimately to be drawn into the crankcase, thus diluting the oil, A further source of contamination or dilution of the oil in the crankcase is caused by excessive priming and by the use of the carburetter air strangler when starting up from cold. These practices result in a quantity of liquid fuel being drawn into the cylinders via the induction manifold ; this fuel is not entirely consumed by the combustion of the mixture.
Results of Dilution.—When the viscosity or body of the crankcase oil is reduced by thinning, or dilution, the protecting oil film is correspondingly thinner. The abrasive material (such as road dust drawn in through the carburetter, and carbon and metallic particles from the internal parts of the engine) which is suspended in the lubricant is then more apt to come in direct contact with the metal surfaces and excessive wear or perhaps scoring may be caused. The following figures illustrate the effect of mixing the "heavy ends" of the fuel with the crankcase oil:—
TABLE A.
Effect of Dilution of Oil with the Engine not Running Under the Best Conditions.
Unused oil. Used oil.
Viscosity at 1400 P. ... 176 116
Fuel content ... 20.2 p.c.
Foreign material present 0 .34 p.c.
In this ease the vehicle had been used on intermittent service, the engine being operated at sub-normal temperatures. The analysis shows that after running at constantly low temperatures the oil contained 20.2 per cent. of fuel, and it will be seen that this admixture had reduced the body of the oil from 176 seconds to 116 seconds at a temperature of 140 degrees F. This means that a much thinner and weaker off film was present and less protection was afforded to the friction surfaces.
It should be remembered that even with fresh oil the stresses in the engines used for commercial vehicle duties are severe, that a complete lubricating film is very difficult to maintain at all times and dilution reduces considerably the margin of protection against abrasion. The effect is familiar to all in the form of worn bearings, worn piston or piston rings, and scored and worn cylinders. Where aluminium alloy pistons are• employed, piston dryness and dilution must be guarded against at all times, or excessive cylinder wear will result.
Reducing Fuel Dilution—Fuel dilution In cold weather will be minimized if the temperature of the cooling water be maintained by partially covering the radiator arid by running with the fan belt removed. Particularly does this apply in the case of vehicles
038 which operate intermittently with frequent stoppages during winter. Correct adjustment of the carburetter, infrequent priming or use of air strangler, and the use of well-fitting piston rings will also minimize this trouble. Perfect ignition operation is also necessary. Ignition trouble which causes intermittent firing will increase fuel dilution by leaving unburned fuel in the cylinders. Table B gives an analysis of Oil which has been used in summer over an extended period of engine running, the engine always operating at normal temperatures. It will be noted that the fuel content in this case is only 4.8 per cent.
The most appropriate time for draining the crankcase is immediately following a long run, when the oil is hot anti thoroughly agitated. This will carry off most of the sediment, which is all that one can reasonably expect to remove without detaching the oil sump and cleaning out the system. At the same time, oil filters should be removed, if accessible, and cleaned and then carefully and securely replaced.
Paraffin should not be used for flushing out the engine sump ; otherwise, the whole object of draining may be defeated, because of the possibility of some of the paraffin remaining in the sump and diluting the new oil. Reference to the figure will enble the user to note the volatility curve of paraffin. Apart from the dilution feature, if paraffin be admitted into the engine sump it may loosen particles of foreign material that may have adhered to the sides of the case; they will get into the circulation and thus endanger the bearings, owing to partial or complete choking of oil passages. .
In the final article of this series we shall deal with the question of sludge and its constitution, carbon and its effects, the control of carbon formation, the fit of piston rings and of bearings, the provision of piston drainage, and the control of oil consumption; and we shall give a complete lubrication mileage chart showing the average mileage (on a basis of 300 miles running per week) at which attention is necessary for each lubrication point of a vehicle.
When the final article shall have appeared, we should like the subject to be regarded by our readers as being thrown open for discussion, for we feel that there are some points upon which opinions are bound to be divergent.