One of the most common problems with older diesel engines is high oil consumption and high crankcase pressure, resulting in oily fumes in the engine compartment. The fumes and oil mist can permeate the accommodations, making the vessel an unpleasant place to be when the engine is running.
The trouble is caused by glazing of the cylinder bores that allows combustion gases to bypass the piston rings and enter the crankcase. Once the crankcase is pressurized, oil and fumes are forced out of every orifice, making the engine compartment oily and messy, which causes fumes to pass throughout the vessel.
The problem is greatest when the engine is used for hour after hour of motor sailing at modest revs. Another cause of fumes is the 20 minutes of cold running at the start and end of a day sail when the engine is used to maneuver the boat out of and back into the marina berth. A third scenario is where vessels are river-based and need to motor out to sea on the engine at restricted speeds. All of these practices eventually lead to bore glazing. There are ways to minimize the problem, by giving the engine bursts of full throttle for 10 minutes at a time or, when motor sailing, to alter the engine revs every half an hour or so, sometimes running faster and sometimes slower.
The effect of the glazing process is that the walls of the cylinder bores become polished, removing the fine, crisscross grinding pattern that occurs when the bores are finished following reboring.
The resulting high oil consumption means increased running costs and, in extreme cases where the crankcase breather is located right next to the air filter, the oily fumes contaminate the air filter element after only a few hours running, making it essential to change the element almost every month as it becomes fouled with oil fumes.
Owners with closely situated crankcase breather and air filters have tried various methods of solving the problem from blanking off one side of the breather in an attempt to divert fumes away from the air filter to piping the crankcase fumes directly overboard. Blanking off one side of the breather has no effect, however, as the intake suction still pulls the fumes into the air filter element. Piping the fumes overboard will stop the air filter from becoming clogged, but increases the crankcase pressure, which may then lead to oil leaks. It will also mean oil stains down the side of the hull, and neither case will do anything to improve oil consumption.
There is a solution available that, while not curing the glazed bore problem, does prevent oil fumes from escaping and greatly reduces oil consumption. This is the Racor Crankcase Ventilation (CCV) Filter System that is not only suitable for old engines but will lessen the chance of oil leaks and may reduce oil consumption for modern engines in good condition.
Lowering crankcase pressure
The system works by lowering crankcase pressure using intake suction through a limiting valve. The oily fumes from the crankcase are collected in a sealed container that separates the oil from the fumes in a special filter element. The oil is then returned from the Racor element to the sump via a permanent drain with a non-return valve that prevents oil from being forced up into the bottom of the Racor unit.
The remaining volatile fumes and water vapor are passed to the engine inlet manifold via a connection between the air filter and the cylinders, which ensures that the fumes bypass the filter element and no longer contaminate it. On turbo-charged engines, the fumes and water particles – on their way to the cylinders for burning as part of the normal combustion process – actually help to clean the turbine blades. Incidentally, the water within the fumes actually improves combustion and cannot harm the engine in any way.
Servicing of the CCV unit is minimal. The filter element requires replacement after 750 hours running, on average, but there is a red service indicator built into the top of the unit that should be checked regularly. This indicator operates when the filter is fully blocked and pops up inside the top of the unit under crankcase pressure. The indicator must be reset after replacing the filter element.
The benefits of this installation are often immediate: The air filter element remains clean, along with the rest of the engine compartment, and oily fumes no longer permeate the boat. Oil consumption is probably slightly better and the clean engine compartment and fume-free accommodation has made the job very well worth the time taken.
For a detailed description installing a Racor CCV,Click Here