Blowoff valve

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Throttle open, therefore the blowoff valve is shut
Throttle closed, air pushes the blowoff valve open (upwards)

A blowoff valve (also called dump valve or compressor bypass valve) is a pressure release system present in most petrol turbocharged engines. Blowoff valves are used to reduce pressure in the intake system as the throttle is closed, thus preventing compressor surge.[1]

Design

A piston-type blowoff valve; this example releases air into the intake manifold (via the side hose on the left, the small hose on the top provides a reference pressure for the blowoff valve from the inlet manifold).

A key function of blowoff valves is to prevent

turbo flutter and the repeated, high speed cycling will cause a cyclic torque on the compressor and may lead to increased stresses on the bearings and compressor impeller.[2]

A blowoff valve is designed to release pressure in the intake system when the throttle is closed. A "recirculating" type blowoff valve releases the pressurised air back into the non-pressurized section of the intake (i.e. upstream of the turbocharger), while an "atmospheric venting" type blowoff valve dumps the air directly into the atmosphere.

Operation

A vacuum hose connected to the

intake manifold downstream of the throttle
provides a reference pressure for the blowoff valve.

When the throttle is open, the air pressure on both sides of the blowoff valve's piston is equal, therefore the spring holds the valve closed. As the throttle closes, the pressure in the intake manifold drops below the pressure upstream of the throttle, which causes the valve to open. This allows the excess pressure from the turbocharger to be released into either the atmosphere or the intake manifold.

Intake air metering

In the case where an airflow sensor is present upstream from the blowoff valve and the blowoff valve vents to atmosphere, the fuel injection system is unaware that some of the intake air has been vented instead of going into the cylinders. This volume of vented air is no longer relevant to the engine, however it is still included in the calculations for the required amount of fuel, which causes excess fuel to be injected and therefore the engine briefly operates with a fuel-rich mixture after each blowoff valve actuation. Effects of this rich running can include fouling the spark plugs, damaging the catalytic converter, stalling and hesitation when the throttle is opened.

One method to avoid this issue is by locating the airflow sensor downstream of the blowoff valve (known as a blow-through setup, as opposed to the traditional draw-through setup). By using a blow-through method, the MAF won't be affected by the blowoff valve opening, since the pressure is vented before the air reaches the MAF.

Other solutions include using a blowoff valve that recirculates the air back into the intake or using a

manifold absolute pressure
sensor (instead of an airflow sensor) to determine the amount of fuel required.

See also

References

  1. ^ a b "All you ever needed to know about blow-off valves". www.gfb.com.au. Retrieved 1 September 2018.
  2. ^ a b "What is compressor surge?". www.turbobygarrett.com. Retrieved 1 September 2018.
  3. doi:10.1016/j.cja.2018.04.003
    .

Further reading

  • Allard, Alan. Turbocharging and Supercharging. Cambridge, England: Patrick Stevens Limited, 1982.
  • Gorla, Rama, and Khan, Aijaz. Turbomachinery Design and Theory. New York, New York: Marcel Dekker, 2003.
  • Society of Automotive Engineers. Turbochargers and Turbocharged Engines. Warrendale, PA, 1979.
  • Watson, N, and Janota, N. Turbocharging the Internal Combustion Engine. London, England: Macmillan Press Ltd, 1982.


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