When I was a teenaged kid working on one of my neighbor's boats, a stern-drive-equipped runabout, I recall my first encounter with the engine's crankcase ventilation system, or CCV. This boat had seen a hard life and the engine, before its eventual rebuild, was tired to be sure. A pair of hoses routed from the top of the valve covers on this 8-cylinder engine to the carburetor air intake emitted a constant stream of "steam" and emulsified oil froth, which ultimately fouled the carburetor and intake manifold. I eventually learned this was a sure sign that the piston rings were worn and the engine was in need of a rebuild.
Recall, however, that this was in an era when the internet was but a distant futuristic communication system and as I remind my children and younger colleagues in the marine industry frequently, in those days technical information, especially accurate technical information, wasn't accessed via Google while sitting at the kitchen table in your pajamas. Instead, it was earned by reading manuals you had to buy, making telephone calls, and even, imagine this, visiting folks in person who knew more about these matters than you did, and talking to them, humbly seeking their wisdom and guidance when you had nothing to offer in return but a smile, a hand shake, and profuse thanks (this is easier to get away with when you are 18 years old). Your only hope of extracting the seemingly priceless knowledge was if the learned one would take a liking to, or maybe pity on, a budding gearhead while standing in a noisy, busy, often disheveled engine dealer's repair shop, which was often plastered with tool distributor ads and calendars (the latter didn't feature tools). These adornments could and often did prove to be a distraction for an 18-year-old, however, they too held educational value; they taught me how to multitask.
Crankcase ventilation systems have taken many forms over the years on both automotive and marine engines. The term "crankcase ventilation" refers to the venting and removal of gases created as a result of a natural process that occurs in almost all internal combustion engines. The combustion chamber, a space located between the top of the piston, often referred to as the crown, and the stationary cylinder head, contains the intense pressure of combusting fuel, flame, and soot as well as compressed air, fuel mist, and exhaust gases. All or most of these components of the internal combustion process are contained within the combustion chamber by a seal created between the piston and the cylinder wall using a series of piston rings. It's remarkable when one stops to think about this, but for a few microns worth of oil that remain on the cylinder walls, the "seal" is achieved entirely with metal-to-metal contact.
Rings are hard, often chrome-plated steel or cast iron, spring-like devices that make up the difference in diameter between the inside of the cylinder and the outside of the piston. Rings maintain the vast majority of gases, soot, fuel, etc., as they move along the cylinder wall at the blinding speed of between 20 and 40 feet per second under intense temperatures that may reach as high as 1,000°F and pressure of about 600psi.
Inevitably, even on an engine that is in good tune and one that is not suffering from extreme wear, some of the gases that are contained within the combustion chamber leak or "blow" by the piston rings. These gases, once they pass by the rings and into the crankcase (the "body" of the engine) are referred to as blow by. Collectively, blow by includes soot or particulate matter, water vapor, unburned fuel and exhaust byproducts such as carbon dioxide, carbon monoxide, and nitrogen oxides to name a few.
Dealing with blow by, even the normal amount, has taken on several different forms over the years. In some cases and even recently it was simply vented to the atmosphere, known as open crankcase ventilation, which, on a boat, means into the engine room. This type of crankcase ventilation hose usually snakes down the side of an engine to the bilge where it ideally emits just a small amount of gas, oil, and water vapor. In most cases it's directed from the crankcase via a hose or hoses to the air intake manifold, closed crankcase ventilation, where it's ingested and "burned" by the engine in a recycling process of sorts. It's somewhat incongruous, however, in that engine air intakes are equipped with efficient and expensive air filters that are designed to keep contaminants out of the engine, while the crankcase ventilation system sends contaminants back into the engine. In general, however, for an engine whose rings are not worn, the level of contamination is relatively minor.
Most crankcase ventilation systems include baffles through which the gases and vapors must pass, capturing some of the oil vapor, returning it to the crankcase. These baffles are far from perfect, however, and depending upon the engine and conditions under which it's operating, oil vapor enters the engine and is burned. This creates excess soot and carbon accumulation in the combustion chamber as well as in the exhaust gases that are left in the vessel's wake.
The quality and effectiveness of crankcase ventilation systems range from primitive, the hose leading into the bilge, to sophisticated, serviceable, monitorable baffled chambers. The goals of the latter closed system are severalfold. One, to "recycle" gases and unburned fuel. Two, to prevent these gases from escaping into the atmosphere/engine room. In addition to making the engine room greasy or sooty, these byproducts are unfriendly to the environment. Three, to reclaim oil by turning oil vapor back into fluid and then diverting it back to the crankcase so it can go on to lubricate the engine once again rather than being burned in the combustion process. Most modern marine diesel engines are equipped with some form of closed crankcase ventilation, some of which are designed specifically by the engine manufacturer for that engine while others are off-the-shelf proprietary closed crankcase ventilation systems. The latter often include more features, greater efficiency and serviceability, as well as more sophisticated oil recycling systems that are particularly good at coalescing oil vapor back into liquid. Some units include a monitor window that alerts the user to a restriction and need for cleaning or replacement of the cartridge or coalesce element.
Although it's not a necessity, most engines and engine rooms both old and new will benefit from the installation of a closed crankcase ventilation system and many engine manufacturers include the proprietary brands as standard equipment. They will reduce carbon buildup within the combustion chambers and reduce oil consumption as well as improving engine efficiency.
It's important to note that a closed crankcase ventilation system is not a cure for excessive blow by. If the engine is worn and blow by is fouling the air filter and intake manifold with oil or frothy, melted, milkshake-like ooze, then it's time for a visit from a mechanic or, possibly, an engine rebuild or replacement. Installing a closed crankcase ventilation system on an engine in this state is tantamount to balancing bald tires.
A final note on closed crankcase ventilation systems: It's critically important that they be installed properly and to the letter of the manufacturer's instructions. An improperly installed system could do more harm than good by, among other things, allowing unfiltered air to enter the engine or in some cases by allowing large amounts of oil to be sucked into the air intake. If your vessel is equipped with a closed crankcase ventilation system, inspect it to make certain it was installed properly, even if it came from the factory, and be certain you fully understand its service needs and indicators.