Know About a BOV or a Blow Off Valve

A BOV or blow off valve is found inside a turbocharged engine. BOV prevents the occurrence of a phenomenon called compressor surge and reduces the probability of engine wear and tear. These one-way valves release the compressed air into the atmosphere above a specific set pressure.

Identification of a BOV or blow off valve

While releasing the compressed air into the atmosphere, a BOV produces a distinct sound. This feature of a blow off valve is appreciated by many buyers and is the reason for trumpet-shaped BOVs. This shape of a BOV improves the hissing sound, which is similar to many turbocharged sports cars.

Caution

The blow off valve or BOV sound is not always beneficial. Vehicles that have the BOV positioned immediately after the mass airflow sensor can confuse the engine's control unit leading to a stalling or halting. This confusion can occur during the closing of the throttle. In many instances, the higher boost pressure aggravates the situation making it only worse. Such instances can spoil the foul spark plugs and the catalytic converters. This can alsolead to the combustion of the unburned fuel and melting of the converter.

Function of a BOV or blow off valve

A blow off valve or BOV is responsible for controlling the compressor surge. A compressor surge or compressor stall is the uncharacteristic airflow that results from lifting the throttle from a turbocharged vehicle, which lacks a bypass valve or has a defective one.

Operation of a blow off valve or BOV

A vacuum hose connects a BOV to the intake manifold and when the throttle closes, it reduces the relative pressure of the intake manifold below the zero bar. This difference in the low and high pressure runs the piston of the blow off valve or BOV.  Next, the excess pressure of the turbo charger is let out into the atmosphere or redirected to the intake upstream of the compressor inlet.

Operation of BOV

A BOV or a blow off valve is connected to the intake manifold by a vacuum hose. When the throttle closes, the relative pressure of the intake manifold falls below the zero bar.  It is this difference in the low and high pressure or the differential pressure, as it is known, that operates the piston of the blow off valve. Then the excess pressure of the turbocharger is let out into the atmosphere or redirected to the intake upstream of the inlet of the compressor.

Explanation of the terms used

Bar: Bar is a pressure unit of 100 kilopascals and equals Earth's atmospheric pressure at sea level. 

Intake manifold: The key feature of the intake manifold is either the equal distribution of air for direct injection engine, or directing the combustion mixture to every port of the intake manifold in the cylinder folds. The distribution or allocation function is very significant in enhancing or optimizing the performance and adeptness of the combustion engine. The intake manifold can also be used as a carburetor mount, fuel injectors, throttle body, and other engine components. A partial vacuum is said to be present in the intake manifold because of the hindrance created by the throttle valve and the downward movement of the pistons.  

This manifold or partial vacuum can also be used for driving auxiliary systems. Apart from that, it can also be used to suck the exhaust gases and unburned fuel that enters the crankcase of an engine. 

The intake manifold was mainly manufactured from cast iron or aluminum, but nowadays composite plastic materials are also being used.In the picture you see the BOV and the intake manifold.

Disadvantages of BOV

The advantage of a BOV or a  blow off valve is that it prevents a phenomenon known as a compressor surge. Compressor surge kicks in when the throttle of an unvented turbocharged engine closes. By closing the throttle plate of the turbo engine, the throttle traps the pressure (which is of quite a high scale) in the intake system and forces the pressure wave back to the compressor. This occurrence slows down the compressor wheels, even bringing it to a complete halt. It finally creates a hissing or fluttering sound that is easily heard by the driver.

Disadvantages of a BOV or  blow off valve

A sensor known as a mass air flow sensor is used to measure the air mass that enters an internal combustion engine. The air mass is significant for the control unit of the engine or the ECU (Engine Control Unit) so that the correct mass of fuel can be balanced or delivered to the engine. But when a mass airflow engine is used on the  blow off valve or BOV, the engine control unit displays more fuel than is actually used. This is because it doesn't take away the measurement of the intake charge. The engine then starts working for a brief period on a fuel-rich mixture.

This fuel-rich mixture can stall or thwart the movement of the engine. It happens when the throttle is closed. The situation is made worse with boost pressure of higher magnitude. Random or occasional events as such can only be a headache, but frequent events of this type can negatively influence the catalytic converter and the spark plugs.