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 them leaking 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, 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 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"