Crankcase ventilation system

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.[3]

In the early 1950s, Professor Arie Jan Haagen-Smit established that pollution from automobile engines was a major cause of the smog crisis being experienced in Los Angeles, California.[4] The California Motor Vehicle Pollution Control Board (a precursor to the California Air Resources Board) was established in 1960 and began researching how to prevent blow-by gases from being released directly into the atmosphere.[5] The PCV system was designed to re-circulate the gases into the air intake so that they could be combined with the fresh air/fuel and more completely combusted. In 1961, California regulations required that all new cars be sold with a PCV system, therefore representing the first implementation of vehicle emissions control device.[6]

By 1964, most new cars sold in the U.S. were so equipped by voluntary industry action so as not to have to make multiple state-specific versions of vehicles. PCV quickly became standard equipment on all vehicles worldwide because of its benefits not only in emissions reduction but also in engine internal cleanliness and oil lifespan.[1][7]

In 1967, several years after its introduction into production, the PCV system became the subject of a U.S. federal grand jury investigation, when it was alleged by some industry critics that the Automobile Manufacturers Association (AMA) was conspiring to keep several such smog reduction devices on the shelf to delay additional smog control. After eighteen months of investigation, the grand jury returned a "no-bill" decision, clearing the AMA, but resulting in a consent decree that all U.S. automobile companies agreed not to work jointly on smog control activities for a period of ten years.[8]

In the decades since, legislation and regulation of vehicular emissions has tightened substantially. Most of today's petrol engines continue to use PCV systems.

In order for the PCV system to sweep fumes out of the crankcase, the crankcase must have a source of fresh air. The source of this fresh air is the "crankcase breather", which is usually ducted from the engine's intake manifold. The breather is usually provided with baffles and filters to prevent oil mist and vapour from fouling the air filter.

Intake manifold vacuum is applied to the crankcase via the PCV valve. The airflow through the crankcase and engine interior sweeps away combustion byproduct gases. This mixture of air and crankcase gases then exits, often via another simple baffle, screen, or mesh to exclude oil droplets, through the PCV valve and into the intake manifold. On some PCV systems, this oil baffling takes place in a discrete replaceable part called the 'oil separator'. Aftermarket products sold to add an external oil baffling system to vehicles, which were not originally installed with them, are commonly known as "oil catch tanks".

The PCV valve controls the flow of crankcase gases entering the intake system. At idle, the manifold vacuum is high, which would draw in a large quantity of crankcase gases, causing the engine to run too lean. The PCV valve closes when the manifold vacuum is high, restricting the quantity of crankcase gases entering the intake system.[9]

When the engine is under load or operating at higher RPM, a higher quantity of blow-by gases are produced. The intake manifold vacuum is lower in these conditions, which causes the PCV valve to open and the crankcase gases flow to the intake system.[10] The greater flow rate of intake air during these conditions means that a greater quantity of blow-by gases can be added to the intake system without compromising the operation of the engine. The opening of the PCV valve during these conditions also compensates for the intake system being less effective at drawing crankcase gases into the intake system in these conditions.

A second function of the PCV valve is to act as a flame arrester and to prevent positive pressure from the intake system from entering the crankcase. This can happen on turbocharged engines or when a backfire takes place, and the positive pressure could damage the crankcase seals and gaskets. The PCV valve therefore closes when positive pressure is present, to prevent it from reaching the crankcase.

The crankcase air outlet, where the PCV valve is located, is generally placed as far as possible from the crankcase breather. For example, the breather and outlet are frequently on opposite valve covers on a V engine, or on opposite ends of the valve cover on an inline engine. The PCV valve is often, but not always, placed at the valve cover; it may be located anywhere between the crankcase air outlet and the intake manifold.

Carbon build-up in the intake manifold may occur when blow-by gases are allowed to permanently contaminate the intake air because of a failing PCV system.[9]

Carbon build-up from blow-by gases on intake valves are usually not a problem in port injected engines. This is due to the fact that the fuel hits the intake valves on the way to the combustion chamber, allowing the detergents in the fuel to keep them clean. However, carbon build-up on intake valves is a problem for engines with direct injection only, as the fuel is injected directly into the combustion chamber. Because of this, fuel system cleaners or fuel additives added to the tank will not help clean these deposits. Methods for cleaning these deposits include spraying cleaner through the intake or direct media blasting of the intake valves.[11]