Most of the modern diesels of today (2-15 liters or so, where most of my experience lies) have a minimum of ring bypass and create very small amounts of crankcase blow by. That is not to say that the breather on a Cummins engine (let’s say) doesn’t have some “puke” dripping from it on occasion; it means that there are other factors involved besides ring bypass. In the case of the Cummins “B”, this engine is very susceptible to an overfill of the crank case w/ oil and on an “inline drive” the problem gets amplified as the rise of the bow during operation exaggerates the build up of oil in the rear of the oil pan, causing “whipping” of the oil. This has a tendency to push the oil out the rear breather in some cases. The same engine turned around in a V-Drive install may not show this problem. Another factor is the build up of “condensation” (moisture,etc) that leaves the crank case as the engine is running. This is where a “closed” system can offer some value. I personally prefer a “catch” bottle to monitor the “puke” that comes out of an engine. In the case of the “Walker Air Sep”, for a Detroit w/ gobs of crap coming out of the crank case in some cases, this system has proven to be a “godsend” to many. In the case of an “Air Sep” on a modern Cummins B or C, I say NO, as I don’t want the “puke” to dribble back into my engine. The “Racor” and some others, on the other hand, have a collection capability and one can drain the ugly stuff on a regular basis and also monitor its amount.
Generally engine manufactures will be forced to come up w/ a type of closed system over the years for all the diesels. In its simplest of forms, it’s merely a mechanism that enables the crankcase to see a “low pressure area” and allow the “fumes” to be burned during combustion, while separating most of the liquids…Yanmar and some others use a very basic system by putting the crankcase hose near the air inlet and accomplish this “low pressure” thing.. Another problem that has come up with these systems is w/ “sea water aftercooled” diesels; a lot of these “fumes, puke, etc.” don’t make it past the cold aftercooler to the combustion chamber, causing increased maintenance of this accessory. So what is the answer? On a new “B”, a “catch” bottle, or, if you have to have one, go w/ the Racor type and drain it on occasion..Remember that in ALL cases, all of the plumbing used for these systems MUST be designed to drain completely during any type of operation. In particular w/ the “B”, a loop was created in some of the “Air Sep” installs just after the case vent. This created a “liquid trap” (like under your sink), effectively sealing the crankcase from even breathing at all (couch engineering).
In the case of Volvo’s or other manufacturers systems of containing crankcase blow-by, the same basic principles will apply. From my experience with any modern hi-speed, 4-stroke, turbocharged, sea water aftercooled “balls to the wall” horsepower per cubic inch marine diesels, which seem to also have relatively shallow oil pans that cause more than normal turbulence from whipping of the oil, crankcase blow-by is a current problem that is being looked at by all manufacturers. On a GOOD engine without a scuffed piston, stuck ring, or ?????, there is a certain amount of blow-by/crankcase fumes and possible oil that gets “whipped” out the breather hoses. In a fairly recent case (in my area)with a Yanmar 6LY/315, the crankcase was overfilled (not the engine manufacturer’s fault) and upon using the boat the crank whipped and threw so much oil out the breather tube (which happened to be attached to the inlet of the turbo) that the engine decided that this was all the fuel it needed. It went to (probably) 5000+ rpm before it self-destructed. The point I’m trying to make here is that a breather system needs to come from a “QUIET” area of the engine. The placement and design of this system needs to be looked at from more than one perspective……case in point — CUMMINS: their couch engineers seem to think that all engines are installed in boats in the same direction and all boats operate at the same angles….whatever. With the supposedly “tried and proven” crankcase breather location on a Cummins B (which is on the left side/rear of the block,) they’ve actually chosen a lousy location for the higher HP/RPM engines in in-line drive applications. The oil pan is so shallow that it causes excessive whipping of the oil in many applications, and leads to excessive crankcase blow-by.
So, what am I really trying to say here? First, all crankcase ventilation systems need to address the operating characteristics of the installation. In-line drives are usually “nose up” and therefore, it should be common sense to find an area towards the forward high part of the engine (valve cover, timing cover) to attach a breather hose. This hose should be routed to a device to allow liquid separation before the gases are sucked into the intake. As for V-drive installations, they are typically “nose down” during most operating conditions and need to find a high quiet area for crankcase breathing near the back of the engine (bell housing end.) The low pressure area of the air inlet can be used effectively to burn / recirculate gases from normal crankcase blow-by, but needs more individual engineering to adapt to all applications.
So, what might be this perfect “closed” crank case ventilation system in a boat?? In my opinion it would be one that could ingest all of the normal(and a little extra) crank case bypass from a modern “state of the art” marine diesel, separate all of the gases from the “puky liquids” that will fowl after-coolers, turbos, etc., let the liquids be captured and drained in a convenient way to discarded, and that by burning these crank case by-products, that we don’t create some other high maintenance problem.
As for the Volvo TAMD 40++ series question, which prompted this article, these are “state of the art” marine diesels operating at levels un-approachable just a few yrs ago. These operating conditions will dictate that more control w/ be necessary over crankcase bypass, exhaust system design, prop loading , etc. etc. etc..These operating conditions/parameters are not unique to Volvo, but are common to all of these competitive marine diesels that are giving us a new feel for diesel technology.