A crankcase
ventilation system is a one way passage for gases to escape in a
controlled manner from the crankcase of an internal combustion
engine.
This is
necessary because internal combustion inevitably involves a small but continual
amount of blow-by, which
occurs when some of the gases from the combustion leak past the piston rings (that is, blow
by them) to end up inside the crankcase, causing pressure to build
up in the crank case. For control of the pressure inside it, a PCV (positive
crankcase ventilation) valve is used to vent the crankcase.
EARLY PROVISIONS
From the late
19th century through the early 20th, blow-by gases from internal combustion
were allowed to find their own way out to the atmosphere past seals and gaskets. It was
considered normal for oil to be found both inside and outside an engine, and
for oil to drip to the ground in small but constant amounts. The latter had
also been true for steam engines and steam locomotives in the decades before. Even bearing and valve designs generally made little to no provision for
keeping oil or waste gases contained. Sealed bearings and valve covers were for
special applications only. Gaskets and shaft seals were meant to limit loss of oil, but they were
usually not expected to entirely prevent it. On internal combustion engines,
the hydrocarbon-rich blow-by gases would diffuse through the oil in the seals
and gaskets into the atmosphere. Engines with high amounts of blow-by (e.g.,
worn out ones, or ones not well built to begin with) would leak profusely via
those routes.
ROAD DRAFT TUBE
The first
refinement in crankcase ventilation was the road draft tube, which is a pipe running from a high location
contiguous to the crankcase (such as the side of the engine block, or the valve
cover on an overhead valve engine) down to an open end
facing down and located in the vehicle's slipstream.
When the
vehicle is moving, airflow across the open end of the tube creates a draft that
pulls gases out of the crankcase. The high location of the engine end of the
pipe minimises liquid oil loss. An air inlet path to the crankcase, called
the breather and often
incorporated into the oil filler cap, meant that when a draft was generated at
the tube, fresh air swept through the crankcase to clear out the blow-by gases.
The road
draft tube, though simple, has shortcomings: it does not function when the
vehicle is moving too slowly to create a draft, so postal and other slow-moving
delivery vehicles tended to suffer rapid buildup of engine sludge due to poor
crankcase ventilation. And non-road vehicles such as boats never generated a draft on the tube, no
matter how fast they were going. The draft tube discharged the crankcase gases,
composed largely of unburnt hydrocarbons, directly into the air. This created
pollution as well as objectionable odors. Moreover, the draft tube could become
clogged with snow or ice, in which case crankcase pressure would build and
cause oil leaks and gasket failure.
POSITIVE CRANKCASE VENTILATION (PCV)
During World War II a different type of crankcase ventilation had to be
invented to allow tank engines to operate during deep fording operations, where the normal draft tube ventilator
would have allowed water to enter the crankcase and destroy the
engine. The PCV system and its control valve were invented to meet this
need, but no need for it on automobiles was recognized.
In 1952,
Professor A. J. Haagen-Smit, of the California
Institute of Technology at Pasadena, postulated that unburned
hydrocarbons were a primary constituent of smog, and that gasoline powered
automobiles were a major source of those hydrocarbons. The GM Research
Laboratory (led by Dr. Lloyd L. Withrow) discovered in 1958 that the road draft
tube was a major source about half of the hydrocarbons coming from the
automobile. The PCV system thus became the first real vehicle emissions
control device.
Positive
crankcase ventilation was first factory-installed on a widespread basis by law
on all new 1961-model cars first sold in California.
The following year, New York required it. 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 (AMA)
was conspiring to keep several such smog reduction devices on the shelf to
delay additional smog control. After eighteen months of investigation by U.S.
Attorney Samuel Flatow, 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. Today's petrol engines continue to use PCV systems.
COMPONENTS AND DETAILS
BREATHER
In order for
the PCV system to sweep fumes out of the crankcase, the crankcase must have a
source of fresh, clean air, called the crankcase breather. To achieve this, the crankcase air inlet is usually
ducted to the engine's air cleaner. 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, drawing fresh
air into the crankcase via the breather. The airflow through the crankcase and
engine interior sweeps away combustion byproduct gases, including a large
amount of water vapour which includes dissolved chemical combustion byproducts.
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'.
PCV VALVE OR ORIFICE
The PCV valve is a variable orifice
that controls the flow of crankcase fumes, admixed with fresh air admitted to
the crankcase by the breather, into the intake tract. With no manifold vacuum,
a restrictor generally a cone or ball is held by a light spring in a position
exposing the full size of the valve's orifice to the intake manifold. With the
engine running, the restrictor is drawn towards the orifice by manifold vacuum,
restricting the opening proportionate to the level of engine vacuum vs. spring
tension. At idle, manifold vacuum is high, but a large amount of extra air would amount
to a vacuum leak, causing the engine to run too lean and/or too fast.
So at high
manifold vacuum, the PCV valve allows only a low flow rate. This is in
accordance with the low volume of crankcase fumes generated at low engine
speeds. At higher engine speeds, with less manifold vacuum, the PCV valve
permits a greater flow rate to keep up with the greater volume of crankcase
fumes; because of the higher engine speed, a greater amount of
"extra" air via the PCV system can be tolerated without upsetting the
engine's running.
At full
throttle, very little manifold vacuum is present, so there is little flow
through the PCV valve. However, this is the condition under which the maximum
volume of crankcase gas is present. Most of it escapes under its own pressure
via the crankcase breather, flowing into the engine's intake tract via the air
cleaner.
A second
function of the PCV valve is to protect the engine in case of a backfire, which causes a sudden high-pressure pulse in the intake
manifold. This forces the PCV valve closed so that the backfire flame can't
reach the crankcase, where it could ignite flammable fumes and cause
damage. Turbocharged engines also experience
periods of high intake manifold pressure during which the PCV valve is closed
and the crankcase fumes are admitted to the engine via the breather and air
cleaner.
Some engines
use a fixed orifice rather than a variable-orifice PCV valve.
COMPONENT PLACEMENT
The crankcase
air outlet, where the PCV valve is located, is generally separated as widely as
practical from the crankcase air inlet. For example, the inlet and outlet are
frequently on opposite valve covers on a V engine, or on opposite ends of the one and only 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.
SYSTEM FUNCTION AND MAINTENANCE
It is
critical that the parts of the PCV system be kept clean and open, otherwise air
flow will be insufficient. A plugged or malfunctioning PCV valve by itself
cannot damage an engine; however, the blow-by gases can instead flow through
the crankcase air inlet and, if there isn't a separate catch can or oil separator at that inlet, the blow-by will
contaminate the air intake manifold. This contamination especially poses a risk
for forced-induction engines. A poorly-maintained engine's PCV system can
eventually contaminate the air intake manifold with oil, and if both the PCV
valve and the crankcase air inlet are blocked, then the crankcase pressure will
build to a level that will damage seals and eventually the engine.
ALTERNATIVES
Not all
petrol engines have PCV valves. Dragsters sometimes use a scavenger system and venturi tube in
the exhaust to draw out combustion gases and maintain a small amount of vacuum
in the crankcase to prevent oil leaks on to the race track. Small two stroke engines use the crankcase to partially compress incoming air;
all crankcase gases are thus burned in the regular flow of air and fuel through
the engine. Many small four-stroke engines such as lawn mower engines and small
gasoline generators simply use a draft tube connected to the intake, between
the air filter and carburetor, to route all blow-by gases back into the intake
mixture. The higher operating
temperature of
these small engines prevents large amounts of water vapor and light
hydrocarbons from condensing in the engine oil.
WORKING