- Jan 29, 2014
- Reaction score
- Montgomery, NY, USA
- VCDS Serial number
A crankcase ventilation system (CVS) is a one way passage for the blow-by gases to escape in a controlled manner from the crankcase of an internal combustion engine.
The blow-by gases are generated when a small but continual amount of gases (air, unburned fuel, combustion gases) leak from the combustion chamber past the piston rings, that is, blow by them, and the piston ring gaps to end up inside the crankcase, causing pressure to build up in there. Additional sources of blow-by that contribute to this effect are gases leaking past the turbocharger shaft, the air compressors (if present) and in some cases the valve stem seals. The blow-by gases, if not ventilated, can condense and combine with the oil vapor present in the crankcase forming sludge or cause the oil to become diluted with unburned fuel, degrading its quality and decreasing its effective life. Additionally, excessive crankcase pressure can lead to engine oil leaks past the crankshaft seals and other engine seals and gaskets. Prolonged period of oil leaks can starve the engine of oil and damage it in a permanent way. Therefore, it becomes imperative that a crankcase ventilation system is used. This allows the blow-by gases, consisting of the combustion products and the oil vapors, to be vented through a PCV (positive crankcase ventilation) valve out of the crankcase. There are three system architectures when the blow-by gas exits the crankcase. It can either enter the air inlet manifold (closed CVS), be vented freely in the atmosphere (open CVS) or be vented in the atmosphere through a filter (filtered open CVS).
[h=2]Early provisions[/h]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.
[h=2]Road draft tube[/h]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.[SUP][/SUP]
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.[SUP][/SUP] The draft tube discharged the crankcase gases, composed largely of unburnt hydrocarbons, directly into the air. This created pollution as well as objectionable odors.[SUP][/SUP]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.[SUP][/SUP]
[h=2]Positive crankcase ventilation (PCV)[/h]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.[SUP][/SUP] 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.[SUP][/SUP][SUP][/SUP]
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.[SUP][/SUP][SUP][/SUP]
In the decades since, legislation and regulation of vehicular emissions has tightened substantially. Today's petrol engines continue to use PCV systems.
[h=2]Components and details[/h]
[h=3]Breather[/h]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 and incompletely burned organic compounds. 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'.
[h=3]PCV valve or orifice[/h]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.
[h=3]Component placement[/h]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.
[h=2]System function and maintenance[/h]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.
[h=2]Alternatives[/h]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.
Crankcase pressure has been a problem inherent to engines ever since the first were built more than a century ago, but it took the intervention of the EPA to finally deal with it. While the crankcase ventilation systems introduced in the 1970s were originally designed specifically to reduce emissions, they had the side benefit of solving an age-old internal-combustion issue.
[h=3]Crankcase Pressure[/h]All engines naturally experience a certain amount of pressure, owing to a number of factors. Cylinder pressures in a typical engine can easily top 150 to 200 psi during the power stroke; the piston rings keep most of this pressure in the cylinder, but their seal against the cylinder wall isn't airtight. Even if the seal were 99.5 percent perfect -- which it isn't -- the crankcase might still pressurize to about 1 psi. This pressure encourages oil leaks through the gaskets and contributes to air pollution by sending a constant stream of untreated oil vapor steaming out of the engine's breather cap.
[h=3]Typical Blow-By[/h]"Blow-by" refers to the amount of gas that makes it past the piston rings and into the crankcase. Generally speaking, a street engine will lose about 1.5 percent of the air that goes through to blow-by; or about 1 cfm per 50 horsepower. So, a 250-horsepower engine would see about 5 cfm of blow-by through the oil breather, and a 500-horsepower engine will get about 10. The same 1.5-percent rule applies roughly to pressure. If your cylinder pressure tops out at 150 psi, you should see about 1 psi of pressure in the crankcase.
[h=3]Cylinder Wear[/h]Worn-out piston rings are half of blow-by equation, since they'll only seal as well as the cylinders themselves. It's common knowledge -- at least among anyone who's ever seen an Engine Restore commercial -- that tiny scratches in the cylinder will allow excess pressure to leak past the rings. But excess cylinder wear does something else too. When the cylinder bore gets larger, the rings extend slightly, losing a bit of their tension and ability to seal the cylinder. This extension also causes the ring gaps to grow slightly, which further encourages cylinder leakage.
[h=3]The PCV System[/h]All new cars come equipped with a positive crankcase ventilation system, which is essentially just a vacuum tube running from the valve cover to the engine's intake. The slight vacuum in the intake tract offsets pressure in the engine, either neutralizing it or creating a slight vacuum. The PCV system uses a valve mounted in the valve cover to keep engine oil from getting sucked through and into the motor; if this valve malfunctions or gets clogged, the PCV system will fail and you're back to a sealed system. In really extreme cases, pressure in the crankcase can actually push the PCV valve out of the valve cover with a pop like a champagne cork.
[h=3]Puffing Pressure[/h]Because of the movement of the pistons and the regularity of the combustion events, pressure will typically come out of the engine in regular puffs of pressure instead of a smooth breeze. These puffs can tell you something about the engine's condition. Ideally, these puffs of pressure should be barely noticeable when you hold your hand over the oil filler cap, manifesting as a slight tremble in pressure. The more powerful the individual puffs, the more pressure is spiking with each event and the more cylinder leakage you have. If the puffs are powerful enough to move your hand, it's about time for a rebuild.
[h=3]Single Cylinder Blow-By[/h]The puffs in pressure should be very regular, with no puff more powerful than another. If you feel a few light puffs followed by a single very strong one, then you know you've got excessive blow-by in one cylinder. A grizzled, old mechanic with the mental reaction time of a ninja and a horse-whisperer-like, intuitive feel for engines can actually give a pretty good prognosis on engine condition just by feeling for the frequency and power of those individual pressure pulses. But for those of us who aren't Yoda with a wrench: regular puffs good, irregular puffs bad.
[h=3]Other Possible Causes[/h]A few things can cause excessive blow-by apart from worn cylinders or rings. Powerful spikes in crankcase pressure are a classic sign of a blown head gasket, or a cracked engine block. This is especially true if the gases coming out of the breather hole carry with them a strong stench of raw gasoline. If you smell raw gas, it's time to hang it up and plan for a rebuild. Leaking exhaust valve seals will also contribute to spikes in crankcase pressure, which are particularly noticeable because the valves are just below the oil filler cap. These puffs will smell more like the exhaust coming out of your tailpipe, with perhaps a slight undertone of additional fuel smell.