Why Do Engines Have Valve Vent Returns to Air Intake?

The PCV system is just one of many little things that make modern engines so much cleaner and more efficient than ever before.

Positive crankcase ventilation systems are fairly humble in terms of sophistication, but they perform a number of important functions in terms of engine dynamics and emissions. An evolution of the simple vent tubes in use from the dawn of the internal combustion engine, the modern and federally mandated PCV system remains a wonderfully simple solution to a multifaceted problem.

Engine Blow-By

The engine's piston rings do a pretty good job of sealing the cylinders, considering the fact that they're constantly sliding along the smooth cylinder walls and must contain between between 150 and 200 psi of pressure in the combustion chamber. In most cases, the piston rings will seal about 98 to 99 percent of the pressure in the chamber, which means that somewhere between 1.5 and 5 psi will leak past the rings and pressurize the engine block. This "blow-by" causes a number of problems.

Blow-By Problems and Early Solutions

Allowing pressure to build up in the crankcase puts a lot of strain on the engine's seals and gaskets; in fact, at one point in time, it was not uncommon for engines to lose a quart of oil every 100 miles or so solely through gasket leaks. Putting a vent tube in the top of the valve cover helped, but the system was passive and didn't eliminate 100 percent of the pressure. Early ventilation systems used a U-shaped tube leading from the valve cover to the bottom of the car to suck blow-by out of the engine using the vacuum of air passing under the car and around the tube.

Positive Crankcase Ventilation

Back in World War II, tank engineers needed to rethink crankcase ventilation so that their tanks could ford deep rivers and streams without water getting into the engine. The initial solution was to use a one-way valve in the tube to allow pressure to leave while keeping water out, but the valve reduced the vacuum extraction effect. So to balance things out, tank builders rerouted the end of the tube to the engine's intake. Engine vacuum provided a much stronger extraction effect than air passing under the tank, dropping pressure in the tube and sucking the valve open so the now-positive crankcase ventilation system worked whether the vehicle was moving or not.

Mandatory PCV Systems

The federal government phased in mandatory PCV systems from the 1960s through the 1970s as a measure to reduce vehicle emissions and oil on the road. The problem with an externally vented PCV system is that a certain amount of the oil will always vaporize in the air inside the engine, and that air is composed of almost 100 percent used exhaust gases. The exhaust gases themselves contributed significantly to the vehicle's overall emissions because they never pass through the catalytic converter, and oil aerosolized in the gas would soak into the road and make it slick when wet.

References

• A Technician's Guide to Automotive Emissions Systems; Larry Carley
• AllData; 1993 Ford Probe PCV System Service, Testing and Replacement