Crankcase ventilation system

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 periods 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).

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.^[1]

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. To remedy this situation manufacturers located the breather air filter in the air stream coming from the engine radiator fan, the manufacturers also modified the breather to incorporate an air scoop to direct the air into the breather filter so that the engine could be ventilated while the car or truck was standing still.^[1] The draft tube discharged the crankcase gases, composed largely of unburnt hydrocarbons, directly into the air. This created pollution as well as objectionable odors.^[1] 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.^[2]

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.^[3] 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.^[1][4]

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.^[5]

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

PCV valve on Ford Taunus V4 engine in a Saab 96, between left valve cover and intermediate flange on intake manifold

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. The venting of crankcase pressure into the intake by the PCV system can cause significant carbon build up on the intake valves and ports of Direct Injected engines. This is unique to Direct Injection engines because the intakes are not cleaned by fuel and fuel additives. This build up commonly leads to intake obstruction, a loss in efficiency and reduction in performance, which cannot be address with a disassembly of the intake manifold. The use of low evaporation synthetic oils and/or oil trapping systems such as a catch can important in such power plants. This problem has also lead to exploration of alternatives to contemporary PCV system that does not vent into the intake and still meet environmental regulations.

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.

References

External links

• Pollution Control System-Electronically Controlled PCV DynoValve USPTO Application#: #20100180872