A crankcase ventilation system removes unwanted gases from the
crankcase
In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block.
Two-stroke engines typically use a crankcase-compression design, resulting in the fuel/a ...
of an
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
. The system usually consists of a tube, a one-way valve and a vacuum source (such as the
inlet manifold
In automotive engineering, an inlet manifold or intake manifold (in American English) is the part of an engine that supplies the fuel/air mixture to the cylinders. The word '' manifold'' comes from the Old English word ''manigfeald'' (from th ...
).
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
A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process.
Interna ...
"blowing" past the
piston ring
A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine.
The main functions of piston rings in engines are:
# Sealing the combustion chamber so that there is mini ...
s and into the crankcase. These blow-by gases, if not ventilated, inevitably condense and combine with the oil vapour present in the crankcase, forming sludge or causing the oil to become diluted with unburnt 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 engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...
s 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
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...
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
A slipstream is a region behind a moving object in which a wake of fluid (typically air or mustard) is moving at velocities comparable to that of the moving fluid, relative to the ambient fluid through which the object is churning. The term slip ...
. 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
A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engin ...
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.
In the early 1950s, Professor
Arie Jan Haagen-Smit established that pollution from automobile engines was a major cause of the
smog
Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and '' fog'' to refer to smoky fog due to its opacity, and odor. The word was then inte ...
crisis being experienced in Los Angeles, California. The California ''Motor Vehicle Pollution Control Board'' (a precursor to the
California Air Resources Board
The California Air Resources Board (CARB or ARB) is the "clean air agency" of the government of California. Established in 1967 when then-governor Ronald Reagan signed the Mulford-Carrell Act, combining the Bureau of Air Sanitation and the Motor ...
) was established in 1960 and began researching how to prevent blow-by gases from being released directly into the atmosphere. 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
Vehicle emissions control is the study of reducing the emissions produced by motor vehicles, especially internal combustion engines.
Types of emissions
Emissions of many air pollutants have been shown to have variety of negative effects on public ...
device.
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.
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
The Automobile Manufacturers Association was a trade group of automobile manufacturers which operated under various names in the United States from 1911 to 1999.
A different group called the Automobile Manufacturers' Association was active in the ...
(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.
In the decades since, legislation and regulation of vehicular emissions has tightened substantially. Most of today's gasoline engines continue to use PCV systems.
Breather
In order for the PCV system to draw fumes out of the crankcase, the system 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 air filter or intake manifold. The breather is usually provided with baffles and filters to prevent oil mist and vapour from fouling the air filter. This phenomenon can be further reduced by installing after-market air oil separators or catch cans, as colloquially known, to pull oil mist out of suspension and collect it in a reservoir before it can reach the intake. A properly designed crankcase breather will also be designed in a manner that promotes the scavenging effect, or the creation of suction within the crankcase breather to further aid in the removal of blow-by gases. It is this effect that keeps the crankcase at slightly negative pressure when a properly functioning PCV system is in place.
PCV valve
![PCV valve](https://upload.wikimedia.org/wikipedia/commons/5/5d/PCV_valve.jpg)
Intake
manifold vacuum
Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.
Manifold vacuum is an effect of a piston's movement on the induction stroke and the ...
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 mist
Oil mist refers to oil droplets suspended in the air in the size range 1~10 μm.
Formation of Oil Mist
Oil mist may form when high pressure fuel oil, lubricating oil, hydraulic oil, or other oil is sprayed through a narrow crack, or when l ...
, 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 tank
An oil catch tank or oil catch can is a device that is fitted into the cam/crankcase ventilation system on a car. Installing an oil catch tank (can) aims to reduce the amount of oil vapors re-circulated into the intake of the engine.
Positive cra ...
s".
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.
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. 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
A flame arrester (also spelled arrestor), deflagration arrester,
or flame trap
are safety devices fitted to openings of enclosures or to pipe work, and are intended to allow flow but prevent flame transmission fuel combustion by extinguishing the ...
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, or even cause a crankcase explosion. 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
A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder b ...
, 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 inlet manifold.
Forced Induction Applications
The PCV valve gains an even more important function in increasingly popular forced induction applications. Excessive crankcase pressure won't only occur due to blow-by gasses escaping past the piston rings it can also be introduced when positive pressure from the intake manifold makes its way into the crankcase. As previously mentioned, in vehicles with forced induction systems such as turbochargers or superchargers, the engine's intake manifold experiences positive pressure under load. This differs from naturally aspirated applications where the intake manifold will remain in vacuum while under load. Thusly, when a forced induction engine is under load the intake manifold can no longer be used to draw blow-by gasses out of the crankcase and will instead begin to exacerbate the problem by increasing crankcase pressure. It is then the job of the PCV valve to isolate the intake manifold and crankcase when the intake manifold is pressurized and allow the flow of blow-by gases out of the crankcase when the intake manifold is under vacuum. In addition to this added role, in boosted applications cylinder pressures are much higher, and consequently, more blow-by gases are pushed into the crankcase thus making a fully functional PCV system all the more important.
Carbon build-up in intake systems
Carbon build-up in the intake manifold will occur when blow-by gases are allowed to permanently contaminate the intake air because of a failing PCV system.
Carbon build-up or
oil sludge
Oil sludge or black sludge is a gel-like or semi-solid deposit inside an internal combustion engine, that can create a catastrophic buildup. It is often the result of contaminated engine oil and occurs when moisture and/or high heat is introduced ...
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.
Alternatives
Two-stroke engines which use
crankcase compression do not require a crankcase ventilation system, since all of the gases within the crankcase are then fed into the combustion chamber.
Many small four-stroke engines such as lawn mower engines and electricity generators simply use a draught tube connected to the intake system. The draught tube routes all blow-by gases back into the intake mixture and is usually located between the air filter and
carburetor
A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meteri ...
.
Dry sump
A dry-sump system is a method to manage the lubricating motor oil in four-stroke and large two-stroke piston driven internal combustion engines. The dry-sump system uses two or more oil pumps and a separate oil reservoir, as opposed to a conve ...
engines in some drag racing cars use scavenging pumps to extract oil and gases from the crankcase. A separator removes the oil, then the gases are fed into the exhaust system via a
venturi tube
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian physicist, Giovanni Battista V ...
.. This system maintains a small amount of vacuum in the crankcase and minimises the amount of oil in the engine that could potentially spill onto the racetrack.
References
{{Automotive engine , expanded
Engine valves
Engine technology
Pollution control technologies