Lubrication 17 - Crankcase Ventilation
A mechanic from the 1950s or even the 1960s might ask, why one makes such a fuss about a little tube sticking out of the upper part of the crankcase. Sometime ago in the dark past, we became aware of the
problems after some body at Opel mentioned that the actual emission problems were not the exhaust gases but the fumes coming off the (not yet water soluble) paint used in new cars. The problems were brought to
our notice at about the time when they started to encase the car in an airtight plastic film to control and regulate any absorbable gases and liquids.
Thereby, under no circumstances was the engine allowed to drip any oil at all onto the road. Indeed, as little as possible oil in a gaseous or burnt state, should leave the engine. For us as consumers, this is all very
favourable, since modern engines, despite their higher performance, use less oil than ever before. Bit by bit, all the engine-systems that could be responsible for oil in the exhaust gas, have been included. Among
others, e.g., the piston- or cylinder coatings or the honing of the sleeves, where too much roughness, because of the high oil consumption during running in, was forbidden.
Now, how do the legislators do their controlling or measuring? The emission of burnt oil e.g., can be determined together with the Hydro Carbons value. However, to exclude also the direct oil-loss, a certain amount of
vacuum in the crankcase is expected under various (mostly three) operating conditions. Indeed, exactly here, is an important part of the problem. At the time when petrol engines provided the intake suction
themselves, one would simply transfer the vacuum in the inlet manifold to the crankcase. Because the uncharged Diesel, is apparently no longer around, it is much more difficult here, to find a suitable suction point.
That which flows through the compression and particularly the operating pressure, from the combustion chamber and past the piston and its rings into the crankcase, is described as Blow-By-Gas. The seal of the
exhaust-valve shaft may also be damaged and from there, through the oil flow-back pipe, the gas can escape from the cylinder head. There is also such a connection from the charger bearing. Those are three
possibilities of how pressure can build up in the crankcase.
This pressure cannot remain there and, it also may not increase. It could then, e.g., press the oil out of the dipstick or damage the shaft seal. If it is led off, it drags various oil-components with it, from coarse oil
droplets to ultra fine oil-mist. In addition, because when fuel is burnt, a great deal of water develops, this must also be led off, unlike the oil however, under no circumstances back into the oil-sump.
Now, how can this be done? Through increased temperature in the separation element, which leads the water as vapour to the inlet manifold and allows the oil, possibly through an additional line, to flow back again.
Water proves to be dangerous, it can often occur when a lot of short distance driving is done, in winter, if it is not led off, it can freeze, completely blocking the opening to the crankcase. In the worst case, this can lead to
The constructors would be happy, if the blow-by-gases were burnt and were, once and for all, taken care of. However, through the exhaust gas recirculation e.g., they may return again, together with new gases of this
type. The EGR-channels must also remain relatively clean, otherwise the EGR-valve has nothing to regulate. Apart from the legal requirements, this is a further reason, to remove as much oil as possible from the
blow-by-gases, or to mix as much air as possible into the oil components. This means that in addition, to some extent, the scavenging air also flows through.
Due to the fact that one now needs a fairly warm place, possibly near to the cylinder head, to carry out the complicated process of oil separation, one can also regulate the pressure here, since depending on the
operating condition, it can vary a great deal and the crankcase does actually need an even out- or through flow. The result is, that in somewhat more complex systems there is one introduction point before- and one
behind the turbo-charger in the inlet system which has one separator each for the oil-mist and the check-valves.
Oil separation, that used to be a simple spiral in the rising line from the crankcase to the inlet manifold. Later, the metal spiral was replaced by a wire-mesh. However, with this one could not capture oil components
which were smaller than 10 micrometers. On the other hand, a deposition volume of up to 2 liters/h and more, is demanded from this component. After all, there is a lot going on in the crankcase. Not only do the con-
rods and con-rod bearings spray oil all over the place, there are also increasingly more of these oil-spray nozzles, which by the way, are an enormous additional strain on the engine.
This makes it easy to understand, why an even, not too high pressure is desired in the line(s) to the inlet manifold, because of course, already separated oil should not still be dragged along. The position of the
bleeding line in the oil-sump is particularly important for the amount of genuine oil-spray. In this case, perhaps an additional cover against wild oil-spray would be helpful. Perhaps further measures could be taken to
calm the (not too low viscosity) oil or to prevent collisions between rotating parts and the oil from happening in the first place.
As we've said, the oil consumption of our engines is lower that ever before, and that, sometimes even during the running in period. Still, the legal restrictions will continue. In the figure at the top you can see a cyclone-
separator, which is fairly insensitive and nevertheless, quite effective as far as ultra-fine oil is concerned, which nowadays, is taken from the air-flow in the ranges of far less than 10 micrometers. By the way, circulation
separators can also be driven by the gas-flow, indeed, those driven by electric motors have a more stable effect. 06/12