A crankcase ventilation system removes unwanted gases from the crankcase of an internal combustion engine. The system usually consists of a tube, a one-way valve and a vacuum source (such as the inlet manifold).
The unwanted gases, called "blow-by", are gases from the combustion chamber which have leaked past the piston rings. Early engines released these gases to the atmosphere simply by leaking them through the crankcase seals. The first specific crankcase ventilation system was the 'road draught tube', which used a partial vacuum to draw the gases through a tube and release them to the atmosphere. Positive crankcase ventilation (PCV) systems— first used in the Second World War and present on most modern engines— send the crankcase gases back to the combustion chamber, as part of the vehicle emissions control, in order to reduce air pollution.
Two-stroke engines with a crankcase compression design do not need a crankcase ventilation system, because normal operation of the engine involves sending the crankcase gases to the combustion chamber.
Source of crankcase gases
Blow-by, as it is often called, is the result of combustion material from the combustion chamber "blowing" past the piston rings and into the crankcase. These blow-by gases, if not ventilated, inevitably condense and combine with the oil vapor present in the crankcase, forming oil sludge or causing the oil to become diluted with unburned fuel. Excessive crankcase pressure can furthermore lead to engine oil leaks past the crankshaft seals and other engine seals and gaskets. Therefore, it becomes imperative that a crankcase ventilation system be used.
Atmospheric venting
Until the early 20th century, blow-by gases escaped from the crankcase by leaking through seals and gaskets. It was considered normal for oil to leak from an engine and drip onto the ground, as this had also been the case for steam engines in the decades before. Gaskets and shaft seals were intended to limit the leakage of oil, but they were usually not expected to entirely prevent it. The blow-by gases would diffuse through the oil and then leak through the seals and gaskets into the atmosphere, causing air pollution and odors.
The first refinement in crankcase ventilation was the road draught tube. This is a pipe running from the crankcase (or the valve cover on an overhead valve engine) down to a downwards-facing open end located in the vehicle's slipstream. When the vehicle is moving, airflow across the open end of the tube creates suction (a "draught" or draft) that pulls gases out of the crankcase. To prevent too much vacuum being created, the blow-by gases are replaced by fresh air using a device called a breather. The breather is often located in the oil cap. Many breathers had a cup or scoop and were located in the air stream of the engine radiator fan. This type of system is called "Pressure-Suction" type and air is forced into the scoop of the breather and by vacuum is draw out by the road draft tube. Another type used of the pressure suction type was used on VW Porsche air cooled engines whereby the front crankcase pulley has a reverse screw built into it which brings air into the engine and air escapes the crankcase with the road draft tube. This system works very well in getting rid of crankcase vapors which are harmful to the engine. As per the earlier engines, the road draught tube system also created pollution and objectionable odors. The draught tube could become clogged with snow or ice, in which case crankcase pressure would build and cause oil leaks and gasket failure.
On slow-moving delivery vehicles and boats, there was often no suitable air slipstream for the road draught tube. In these situations, the engines used positive pressure at the breather tube to push blow-by gases from the crankcase. Therefore, the breather air intake was often located in the airflow behind the engine's cooling fan. The crankcase gases exited to the atmosphere via a draught tube.
Positive crankcase ventilation (PCV)
History
Although the modern purpose of a positive crankcase ventilation (PCV) system is to reduce air pollution, the original purpose was to allow an engine to operate underwater without the water leaking in. The first PCV systems were built during World War II, to allow tank engines to operate during deep fording operations, where the normal draught tube ventilator would have allowed water to enter the crankcase and destroy the engine.