one
Not so long ago, one acquaintance of the semi-oligarch complained about the exorbitant oil appetite of a new toy. Say, I bought a Cayenne Biturbo, and he eats two liters of good expensive synthetics for a thousand kilometers …
The toad seems to have won: the semi-oligarch sold his "little piston." But the question remained: where and why does the oil go? And how to choose one that is not spent so eagerly?
The main reason for leaving the oil is its waste (for details, see the column on the right). It is affected by the design and condition of the engine, operation mode, air temperature overboard. And, of course, the properties of the oil itself.
Not a single parameter directly tells how quickly it will die. But indirectly, two values testify to this: the volatility of the oil and the flash point. If the first parameter does not appear practically anywhere and it is difficult to find out, then the flash temperature is indicated in all specifications. At this temperature, vapors ignite from the surface of the oil film when exposed to open flame (in our case, a flame from fuel combustion). It depends on the composition of the oil: the more light fractions in it, the lower the flash point.
For testing, we took seven oils of different types, but one viscosity group corresponding to the “fortieth” according to SAE classification. LUKOIL-Standard 10W-40 mineral oil has a certified flash point of 217 ° C. It will go as a base: we will compare others with it. Three semi-synthetics from the 5W-40 group are ZIC A + hydrocracking oil with a flash point of 235 ° C, Castrol Magnatec (232 ° C) and RAVENOL (224 ° C). The synthetics with the maximum flash point were presented by our TOTEK-Astra Robot based on polyalphaolefins (PAO), classified by the manufacturer as Full Synthetic (246 ° C), and ester Xenum X1 with a record 247 ° C. Well, to find out whether those who believe that synthetics are less likely to burn less than other oils are right, they took another oil - Neste Oil, also positioned as a full synthetic, but with a relatively low flash point - 228 ° C. The viscosity indices for all oils are close, but the fundamentals are completely different: minerals, simple and advanced hydrocracking semisynthetics, good synthetics based on PAO and even the most advanced ester-based synthetic oils.
Strictly measured 3 l of oil is poured into the bench motor, after which a 30-hour “check-in” at a nominal speed of 120 km / h. The engine is simple, VAZ-21083: for such an almost 4, 000 km run at constant speed - a serious test. After the "check-in" the oil is drained to the drop according to a strictly defined ritual. It remains to compare the residuals.
It is known that the products of oil combustion affect the toxicity of exhaust gases - but is it really strong? To determine this, during the tests in a fixed mode of operation, we measure the content of residual hydrocarbons in the exhaust. Since the fuel is the same, all the differences that go beyond the measurement error can be attributed to the so-called non-fuel SN, generated by the evaporation and combustion of oil in the cylinders.
The result confirms our assumptions: less oil with a higher flash point extends. So, “TOTEK-Astra Robot” showed one of the best results; within the limits of measurement error, the Belgian XENUM X1 appeared next to it. Indeed, their flash point lies beyond 245 ° C. Among all the semisynthetics, the Korean ZIC A + with the declared 235 ° C showed the best burn result. And the worst result is with an ordinary mineral water with its 217 ° C. Measurements of HF also indirectly confirm these results.
One can object: they say, and so it was clear that synthetics are better than all other oils! And here it is not: compare the results of the semi-synthetic ZIC A + and the full Neste Oil synthetics - they are not much, but better with Korean products. It is understandable, the motor of stickers on cans does not read, the properties of a hydrocarbon liquid poured into a pallet are important to it.
So what to look for when choosing oil, based on its minimum consumption? The issue is especially relevant for life-beaten motors, which already lack oil from one shift to another. Lovers ask him to drive fast and far, as well as owners of powerful supercharged engines. It is easiest to navigate by flash point, since it is provided on sites for all oils. The higher the better. As our tests showed, a figure above 230 ° C promises a relatively low waste consumption. And if she climbs at 240 ° C, then it’s very good. However, for all the time working with oils in the “forties” group, only two brands could boast such values: XENUM X1 and “TOTEK-Astra Robot”.
It should be remembered that the flash point is different for oils of different viscosity groups. Viscosity, of course, is primary, so first we select the required oil according to SAE, and only then, within the selected group, we will refine our choice by looking for the highest flash point.
WHY AND HOW OIL STILLS
There is an opinion: all the oil that gets into the cylinder will inevitably and irretrievably burn out. Is it so? Not!
The oil is in the cylinders in the form of a film left by the first piston ring. Its average thickness is 10–20 microns, depending on the mode of operation, engine wear, oil viscosity and a bunch of other parameters. If you take a typical one and a half liter engine, it is easy to calculate that with an oil film thickness of 10 microns in one cycle, approximately a cube of oil gets into the cylinders. Let's estimate: if it all burned out, then at 3000 rpm per minute, 1.5 l of oil would fly out into the pipe! So, for each cycle, not the entire oil film burns, but only a small part of it.
Remember how the oil behaves in the pan when you heat it. At first it spreads over a hot surface, then, as it warms up, it begins to boil and stink. And if you pour cold oil immediately on a hot frying pan, you risk splashing your face. Now about the same, but scientifically. When the oil is warmed below the boiling point, it evaporates slowly from the heated surface into the atmosphere. When it boils, evaporation increases sharply. And even at very high temperatures, microexplosions drop oil droplets from the pan.
Everything is similar in the engine cylinder. According to our estimates, the first mode of oil evaporation should prevail, when it does not reach its volume boiling. It would seem that at huge temperatures of fuel combustion in the cylinders, oil should at least twist! But the fact of the matter is that it lies with a thin film on the relatively cold surface of a cylinder cooled by antifreeze, and therefore it does not warm up so much. Only when the pedal is heated to the floor, the surface layers of the oil film begin to boil. Therefore, when driving fast, you have to add oil more often.
WHERE OIL GOES
If there are no oil droplets on the asphalt under the machine, that is, all the seals are intact, it can be argued that the oil is consumed mainly on waste. In turbocharged engines, it is also spent on lubricating turbochargers, so the overall oil loss is greater there. Further - oil leaks through valve stem seals. This consumption can become major if they are completely worn out or completely dry. Something goes off as oil vapor through the crankcase ventilation system.
By the way, besides the fact that money flies with oil, its high consumption is fraught with other problems. This is an increased rate of contamination of the internal surfaces of the engine, because the oil burns poorly and dirty. This is a decrease in the resource of neutralizers that are not able to digest the products of incomplete combustion of heavy oil hydrocarbons. This is an increase in the toxicity of exhaust gases: it is not for nothing that the price in them is divided into fuel and non-fuel, that is, oil.
ON OIL VAPORIZATION
The rate of evaporation of the oil should depend on the temperature at which it began to boil, the fractional composition and thickness of the oil film formed by the first piston ring on the cylinder wall, which, in turn, depends on the high temperature viscosity of the oil. All this is good, but descriptions of oils of such parameters usually do not contain. True, there is the so-called NOACK oil volatility - the lower it is, the less the tendency of oils to burn. The principle for determining this parameter is simple: the oil is heated for one hour at a temperature of 250 ° C, after which the weight loss is estimated. Minerals during this torture lose up to 22–25%, good modern synthetics - less than 8–10%. The higher the class of base oil, the lower the loss of oil by evaporation. Unfortunately, most firms do not indicate this parameter in the descriptions of their oils.
In a real engine, everything is much more complicated. There, at sharply varying temperatures and pressures, a thin film of oil evaporates, which you cannot measure with any model installation. Hence the possible errors: from the method it follows that the volatility of more viscous oils is lower, and in practice, its consumption increases with increasing viscosity of the oil. The reason is simple: the thickness of the oil layer on the walls of the cylinder, and therefore its passage to the heating and evaporation zone, increases sharply with increasing viscosity.
