CARBON FORMATION

Evening Post, Volume CXV, Issue 77, 1 April 1933, Page 15

CARBON FORMATION

Tfte PROPERTIES OF OIL

VOLATILITY FACTOR

Upon Analysis of the carbon deposits in the combustion chambers of a petrol engine it Vis usually found that almost half consists of material other than carbon, suitJi as dust, grit, and metallic particles v.fchieh have entered through the intake, ;, writes W.N.S.B. The fitting of an ;rfr cleaner reduces the presence: for fo.rwign, or non-carbonaceous, material to aY-minimum, and the actual carbon deposits on the piston heads ' and in the combustion chambers are influenced by and due mostly to characteristics? of the lubrication oil used. "SQ.oty" deposits caused by _ ovcrlich mixture w.Uil not be dealt with in this article, for' correct adjustment of the carburettor > prevents the formation of such soot;Vi deposits, and, moreover, abnormal clvvburettion conditions are easily remedicicl by the operator of the vehicle. ■ With regard to vtho lubricating oil, carbon deposition cap be predicted from the carbonisation imlcx value of the particular oil used. The carbonisation index is obtained by' a simple distillation process whereby as-shown what degree of temperature i 9 required before, say, 90 per cent, of .the oil is evaporated, and distilled. ISo that an. oil ■with a 'high carbonisation index figure is known to cause exceiiaivo carbon deposits, and an oil of low carbonisation index value to produce <wily small carbon formation. .Further,!, the volatility of an oil has a marked efitect on carbon* formation, for experiment^ have shown conclusively that carbon (formation is definitely leas with an oil i*f high volatility, and-greater.with an , oil of low volatility. The : Conrads(*ii carbonrcsidue; test value .of. an oal has now been found, after exhaustive experiments, not to indicate the t'riuc extent to which an oil will form cku'boii deposits. 0 Oils of high volatility and ,of low carbonisation index value require only a low temperature to enable iiomplete vaporisation to take place and accordingly produce smaller quantities of carbon deposits. On the other hand, oils of low volatility and of hig'Jv carbonisation index value do not vaporise until (he temperature is very highv and consequently form excessive carbon doposits in an engine as a sufficient; degree of heat is not available to ctiuse , evaporation. : . OIL CONSUMPTION. ■■ Contrary to what may be expected, carbon formation is practically independent of' oil consumption, except iw extreme cases of worn cylinders an*(l injperfect piston-ring sealing.' Tins, surfaces' of.. the';.cpmb,ustion ''eliambera* and piston tops become covered with a| film of oil (in:- cases of exeessivo oil ' consumption1), which is not capable of remaining on those surfaces on Recount of the swirling of the gases and the terrific, reciprocating motion of the pistons, and is therefore carried away by the exhaust gases. It will bo seen that.the rate of carbon formation is relatively independent of oil consumption after oil consumption has reached a certain figure. Of course the presence of an excessive amount of oil affects the nature and quantity of carbon formation under extreme conditions of engine disrepair, but the maintaining of an engine in a, good state of repair for economical operation lies entirely in the hands of the owner. However, under r snnal engine conditions the formation of carbon is independent of oil consumption, and it is only oil volatility that materially affects the deposition of carbon, Experiments have shown that carbon deposits .do 'liof accumulate with time, but the deposits tend to reach a state of equilibrium. The amount of carbon deposited does not increase above a certain stage, but remains constant, This '.state of equilibrium' is reached when there is a sufficiently thick deposit of carbon to so insulate the sur-. faces as to raise the temperature to a point where the oil rapidly and completely evaporates. A low volatility oil or an oil having a high carbonisation index valuo requires a much thicker insulating deposit of carbon than an oil which is more volatile, for when the' temperature is high enough evaporation of tho oil prevents the continuance of. carbon formation. Some oils form, more carbon than others, but all oils cease to cause carbon formation when the thickness of the deposits is such as to raise the temperature to the point where the oil is evaporated. . OIL COOLING. The hardness of carbon deposits is directly proportional to tho quantity of carbon present. Oils of low volatility tend to produce hard deposits of carbon. Modern high compression engines require to bo kept free from carbon deposits if maximum efficiency is to be maintained. Perhaps a suitable carbon "solvent" will be produced to make this task simpler. However, oil coolers aro now being fitted to some cars and commercial vehicles, and consequently it may be expected that oils of lower viscosity!, and therefore of higher volatility, will be used and thus minimise carbon formation. It is interesting to note that the qualities distinguishing a good oil from-fin inferior one are numerous. Such, things as flash point, viscosity, "oiliness," effect of extremes of temperature on viscosity, "gumminess," volatility, .and so on all differ with vanoiiß brands of oil. An oil which is Of high volatility (and so forms little carbon) may perhaps be lacking in "oiline.ss," and the money saved in decarbonising less frequently would be lost in, say, extra wear of cylinders and bearings, for it is well known that "oiliness" in an oil improves greatly the lubrication qualities of that oil.