Crankcase emissions are made up of water, acids, unburned fuel, oil fumes and hydrocarbons. Hydrocarbons are formed by small amounts of unburned fuel. While most of these hydrocarbons are expelled in the exhaust still more hydrocarbons make their way to the crankcase by way of blow-by past the rings.
Since the first engines crankcase emissions were vented to the atmosphere by a road draft tube mounted on the lower side of the engine block. Fresh air came in through an open oil filler cap or breather. The air passed through the crankcase mixing with blow-by gases. The motion of the vehicle and the air blowing past the open end of the road draft tube caused a low pressure area (vacuum) at the end of the tube. This low pressure area draws fresh air into the crankcase and the resulting pressure in the crankcase is expelled through the breather car on the valve cover or oil fill tube.
To control crankcase emissions, the road draft tube was eliminated and replaced by the Positive Crankcase Ventilation (PCV) system. Based on manifold vacuum, the PCV valve opens during times of high vacuum signal and draws the crankcase vapors (and pressure) into the base of the intake manifold where it is mixed with the air/fuel mixture and burned. An inlet vent is used to balance the vacuum being drawn out as to not collapse the oil pan.
Testing, showed the removal of blow-by gases from the crankcase as quickly as possible, was most important to the longevity of the engine. Should large accumulations of blow-by gases remain and condense, dilution of the engine oil would occur to form water, soot, resins, acids and lead salts, resulting in the formation of sludge and varnishes. This condensation of blow-by gases occurs more frequently on vehicles used in numerous starting and stopping conditions, excessive idling and when the engine is not allowed to attain normal operating temperature through short trips.
Evaporative Emissions
Gasoline is a major source of pollution, before and after it is burned in the automobile engine. From the time the fuel is refined, stored, pumped and transported, and again stored until it is pumped into a fuel tank, the gasoline emits unburned hydrocarbons (HC) into the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor was diminished, but not eliminated. However, the automobile still remains the primary source of vaporized, unburned hydrocarbon (HC) emissions.
Before controls were mandated an owner might fill the fuel tank with fuel from an underground storage tank and park the vehicle for some time in a warm area, such as a parking lot. As the fuel would warm it would expand and should no provisions or area be provided for the expansion the fuel would spill out of the filler neck and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition the vehicle manufacturers' added overflow plumbing and/or gasoline tanks with built in expansion areas or domes.
This did not control fuel vapor emissions from the fuel tank. It was determined that most of the fuel evaporation occurred when the vehicle was parked with the engine OFF. Most vehicles carry 10-25 gallons (38-95 liters) of gasoline. Should a large concentration of vehicles be parked in one area such as a large parking lot, excessive fuel vapor emissions would take place increasing as the temperature increased.
To prevent fuel vapor emissions from escaping into the atmosphere, fuel systems were designed to trap the vapors while the vehicle is stationary by sealing the system from the atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if equipped) and the fuel tank when the engine is OFF. When the engine is started the storage system is then purged of the fuel vapors. The vapors are drawn into the engine and burned with the air/fuel mixture.