LOSING THE ADVANTAGES OF THE LUBRICANT.
Page 20
Page 21
Page 22
If you've noticed an error in this article please click here to report it so we can fix it.
The Changes Which Take Place in Oil. Why Oil Should be Changed Frequently. Paraffin Unsuitable for Flushing. How to Detect Inefficiency in a Lubricant.
• By E. F. Lowe, Automotive Department, Engineering Division, Vacuum Oil Co., New York City.
ROBABLY the best picture of what happens to the oil can be formed by the following figures, which are the result of .a laboratory analysis of used crankcase oil. The oil used was a high-grade recommended lubricant and the car had been driven 500 miles.
Unlike some other types of power-plant or highly developed pieces of machinery, the automotive engine, except in a feW special instances, uses the oil over and over again. In other words, the lubricating systems are of -the circulating type. If it were not for this fact, there would be no great need to be so much concerned with what changes take place in the oil under service.
The fact that the greatest. percentage of all automotive engines do employ circulating systems, makes it very desirable that the operator know in what way and how the oil is affected in order that efficient and proper engine lubrication may be assured.
Evidence of Fuel Dilution.
It is quite a common occurrence for many motor vehicle operators to report very high mileage figures, and in some instances even the fact that practically no oil at all was consumed.
This is a sure indication of dilution, or the mixing of the fuel with the oil. An engine that delivers power, and does work, requires and consumes lubricating oil. It is a fallacy to think that an engine would not burn up or consume some lubricating oil. In such cases as cited above, the fact which was u36 overlooked was that the oil level gauge . simply indicates the, level of the lubricant in the crankcase reservoir. It does not show whether the lubricating oil is of the same body or fluidity as when it was originally put into the crankease. The continued addition of quantities of fuel to the oil, during engine operation, results in erroneous oil consumption hgures, unless such figures take into consideration this fuel dilution.
Analyses of used crankcase engine oil in 'passenger car operation show some surprising figures of this fuel dilution. .
On a well-known fo-ar-cylinder motorcar in' the lowpriced class, an analysis of the engine oil, after 500 miles, showed a 21 per cent. fuel dilution, whilst, on the other hand, in a light six-cylinder tonring ear, after 100 miles of operation, the dilution was 10.5 per cent. Under abnormal' conditions, it is not uncommon to hear of a 30 per cent. and even a 40 per cent. dilution of the, crankcase engine oil by the fuel.
The rate of dilution is greatest during the' first few hundred miles of operation, while, as the mileage goes up, the rate decreases. Under. normal conditions, between 400 and 1,000 miles, the increase in dilution is not of moment:
Causes of Fuel Dilution.
This is due to the fact that, under engine operating temperatures, there is a tendency for the lighter portions of the fuel to evaporate or distill off from the oil. Experiments show thatthere is a point in which a sort of equilibrium is reached in which the fuel evaporates off from the oil at about the same rate that it is absorbed.
Dilution of the lubricant is caused by 1. Poor vaporization of the fuel charge. 2. Escape of mixture past piston.
Many believe that the repeated heating of an oil causes a reduction of its viscosity. The assertion has bften been made that the oil coming in contact with highly-heated .metallic surfaces in an engine undergoes a sort of "
cracking" process which results in quantities of light fuel fractions being added to the lubricant. Tests made have proved that fins is not true, and while the oil does come in to contact. with the highly-heated p i sto n heads, etc., what actually happens is that the oil becomes heavier in body rather than thinned down by this action. The thinning down of the oil is caused by fuel dilution.
Poor Vaporization,
The decline in the volatility of commercial motor spirit, the improper and frequent use of the carburetter &eke valve, the improper adjustment of the carburetter and poorly -designed intake manifolds
are all causes of incomplete vaporization, which, in turn, results in a dilution of the crankcase oil.
Poor vaporization means that thifuel charge is not completely atomized and the fuel-air mixture enters the combustion chambers wet—due to the minute particles of rate fuel present.
During the suction stroke, these particles of unvaporized fuel are drawn into the cylinders. On both the suction and compression strokes the oil on the cylinder wall is in contact with the air fuel mixture and portions of the unvaporized fuel are absorbed by the oil film. The diluted oil film is then scraped down by the pistons or drips into the crankcase, thus diluting the oil supply.
Dilution is also caused by the escape past the pistons of small quantities of the fuel-air mixture into the crankcase chamber. Uncley the compression stroke the fuel charge is compressed and tends to blow by the pistons. Under conditions of properly fitted pistons and rings and with the use of an oil of proper body and character to maintain an efficient oil seal,' this escape is not of moment. On the other hand, when the cylinder bores become badly worn, when the rings become loose in their grooves, which makes it extremely difficult for the oil to maintain a proper seal, this escape of the mixture materially adds to the rate of the dilution of the oil.
Winter and Summer Dilution. .
There are ethree conditions that result in the dilution being greater in winter than summer,— Low crankcase oil temperatures ; Frequent use of carburetter choke in starting ; Necessity of rich mixtures.
The lower crankcase oil temperatures of winter operation materially reduce the amount of the fuel distilled off, which results in greater percentages of fuel staying in the oil than when the higher operating temperatures of summer conditions are experienced.
The frequent use of the carburetter choke to' facilitate starting and the necessity of rich mixtures during winter operation both increase the amount of raw spirit entering the combustion chamber.
It is for these reasons that the oil should be drained at more frequent intervals during winter than during summer, in order to obtain the most efficient engine lubrication.
After analysing many samples of used engine oil, and considered with respect to engine performance, the following figures were established :
Excessive dilution-20 per cent, or over. Normal dilution—from 10 to 20 per cent. Low dilution-10 per cent. and below.
Under summer operating conditions and using a recommended lubricant, the oil should be drained every 1,000 miles and, during winter' 'every 500 miles.
Paraffin and motor spirit are not lubricants. If either be present in any appreciable amount in the engine oil, the lubricating value of the oil will be greatly decreased.
Engine oil that is diluted 20 per cent. or more by fuel is unfit for engine lubrication. The body or viscosity of the oil is materially decreased, and the fuel content not only decreases the lubricating value, but also decreases the piston ring sealing qualities of the oil, resulting in poor and inefficient engine performance.
Probably, the most serious effect of excessive dilution is that the thinned-down oil might not, he capable of providing a durable film, under all conditions of operation, in the event of which bearing failure or scored pistons and cylinder walls will result.
Dilution is one of the most common troubles encountered in the lubrication of automotive engines and is of great moment in its effects upon efficient and economical operation. Laboratory analyses of fifty samples of used engine oil showed that water was present in all, and varied from a trace to over 50 per cent. Peculiarly enough, due to normal operating conditions, water collects in the engine crankcase chamber.
Water in the Crankcase.
Water vapour is formed from the products of combustion of the fuel charge: Hydrogen and oxygen are both contained in the exhaust gases and -these unite in pertions of two parts of hydrogen to one part of oxygen and form water.
The fact that water vapour is present in the exhaust gases is often evidenced by water dripping from the end of the exhaust pipe of some types of motorcars. Due to differences in design of the exhaust system, the amount of water formed will vary considerably. The water vapour in the exhaust gases coming in contact with the comparatively cool muffler, condenses and forms water.
Poor piston ring seal allows small quantities of the hot and highly compressed gases of the power stroke to blow by the pistons and enter the crankcase chamber. These hot gases coming in contact 'with the lower temperatures of ;le crankcase walls, result in condensation and drops of moisture collect on the walls, gradually increasing, until a sufficient quantity of water is formed to run down into the oil. The continual breathing in and out through the filler or breather pipe also adds to the amount of water formed in. the crankcase. This action is of greatest moment during .winter operation, because of the resulting Iow-crankease temperatures: The seriousness of this water formation is brought forth by one American manufacturer, who, in his instruction book, advises that the water should be drained from the crankcase every day.. Water, being heavier than oil, settles to the bottom and is easily drawn out without draining the supply of the lubricant.
Sediment and Carbon.
All lubricating oils become black in colour in use in an automotive engine. The oil actually becomes burned, for, on each power stroke, the entire film of the oil on the cylinder walls is exposed. to the exceedingly high temperatures resulting from the burning of the fuel charge. These high temperatures, which range from 2,000 to 3,000 degrees Fahrenheit, exert an oxidizing effect on the oil which darkens it in colour. The actual darkening is due to the fact that very minute, fine, particles of carbon are formed which stay in suspension in the oil in a colloidal state.
Carbon particles are also formed on the under side of the highly-heated piston heads and, in time, these deposits accumulate and collect in the crankcase.
Particles of road dust are also drawn in through the breather pipe, and the dust, first collecting in the breather pipe' finally works down to contaminate the engine oil.
The oil also becomes filled with fine particles of worn bearing metal. Even with the most efficient lubricant, there will be some wear on the bearings, cylinders and piston, and the fine metal dust resulting from this wear is carried along by the oil and settles in the crankcase chamber.
This sediment collects on oil screens and, under some conditions, seriously impedes the circulation of the oil through the lubricating system.
The following is a typical example of this sediment as shown in a laboratory analysis of used engine oil : Insoluble material .77 per cent.
Ash .35 per cent.
Ash was composed of : Silica (road dust) 7 per cent.
Iron oxide 70 per cent. Oxides of bearing metals 23 per cent.
Obviously, the amount of sediment and carbon that collects in the engine oil increases with the mileage and this contamination will only result in impairing the lubricating qualities of the oil. Unbelievable as it may seem, even with the best quality, most highly refined and highest priced lubricating oils, after atew.hundred miles of service, practically all oils will show an acid reaction. It is true, however, that in the large majority of cases, and particularly thosein which the best quality oils are used, the amount of acid present is very small, in fact so small that it might be entirely neglected in so far as causing any corrosive effects that are generally associated with acids. However, under other conditions, the formation of acids in the crankcase engine oil is appreciable and must be considered.
All mineral lubricating oils contain weak forms of petroleum acids and, curiously enough, the amount of these acids increases under engine operating eon ditions. Exposure to air and to light and even the heating of some oils, causes the formation of these acids. In general, however, they are of such a weak type that they will not cause any corrosive action.
The formation of mineral acids in the engine. oil is, however, of the greatest moment and can be a source of considerable trouble. Mineral acids originate from the fuel used. In many commercial grades of motor spirit there is a sufficient amount of sulphur or its compounds present to form acids under engine-operating conditions. The dilution of the crankcase oil by the fuel results in these acids getting into the oil and, in analyses of used engine oils, it is shown that, after a, few hundred miles, there is generally a sufficient amount present to give an acid reaction.
Under normal operating conditions, these atids do not form in sufficiently great quantities to do any harm. Still, on the other hand, there have been many instances in which corrosion of the timing• gears, timing chains, and highly polished bearing . surfaces have taken place, due to the formation of mineral acids in the engine oil.
Emulsion Formations.
An emulsion may be defined as a state of matter resulting when two liquids which are not miscible are temporarily mixed by agitation. An emulsion is purely a physical change and, when that agent which causes the emulsion is removed, the liquids will return to their normal condition.
Oil and water will not normally mix. Under violent churning or agitation, however, oil and water can. be made to mix and this mixture is an emulsion. The oil and water are broken up into mintrte particles and the particles of one substance become covered with a film of the other, thus resulting in their apparent mixture.
If this mixing of the oil and water by agitation takes place in the, presence of some finely divided grit, such as carbon particles, the emulsion will be more complete and permanent.
It is not uncommon to have reported that ari oil screen in the lubricating system was clogged up by a black gelatinous substance. This, as a rule, is an emulsion.
What actually occurs is, small quantities of water 038 collect in the crankcase, as well as some finely divided particles of dirt, grit and carbon. The churning up or agitation of the oil by the connecting, rod dippers results in an emulsion of the oil and water. This emulsion, taking place in the presence of these finely divided carbon and dirt particles, as well as quantities of fuel, forms a tenacious black sludge, the extent and character of which depend upon the amount of water, carbon, grit and fuel that is present, as well as the kind of oil used.
Such formations are common and result in the clogging. up of oil screens and even stopping up oil suction pipes. A typical example of a sludge formation is given below. The bronze wire mesh oil screen was completely covered with a slimy black material and, upon analysis, the material proved to he the following : All the foregoing cannot but emphasize the need of frequent and periodical draining of the crankcase oil. " For the purpose of determining the mileage or intervals at which this draining should take place, to assure the most efficient lubrication, as well as economy, many tests and analyses have been made. . With high quality and highly refined lubricating oils and such as are recommended .as suitable for use during summer operation,.the most efficient and safe draining interval is every 1,000 Miles.
Due to the lower engine temperatures, during winter operation, which augment the amount of water and dilution occurring, the oil should be drained every 500 miles.
In draining, do not flush the crankcase with paraffin, as quantities will become trapped in the crankcase. chamber and remain to dilute the fresh supply of oil, thus practically defeating the purpose of draining.
existing sediment or grit.
The dilution of the oil by the fuel, the contamination of the oil by dust particles, grit and fine metal particles, all seriously detract from its lubri eating value. This dilution and contamina tion',,if allowed materially to increase, will result in rapid bearing and cylinder wear and, due to the poor sealing qualities of -the thinor diluted oil, the tendency for escape of moisture, past the pistons will be increased, which, in tarn, reduces power output and engine operating efficiency.
The use of a recommended, high-quality lubricating oil with frequent and periodical draining, as recommended above, are the most important factors in assuring the motOr vehicle operator of the best of engine performance with a minimum of depreciation and wear.