Wastegate And Blow-Off Valve

On turbocharged engine, the wastegate valve is an overpressure valve that controls the working conditions of the turbocharger.

Following the article that explained how the turbocharger works, in case the engine speed is increasing, i.e. keeping the throttle pedal down, the exhaust gasses flow rate increases itself. Having more exhaust gasses flow rate, the turbine increases its speed and the same the compressor. As a result, the compressors feeds much more clean air into the intake manifold.

This is in continuous, meaning the most the engine speed increases, the most the compressor feeds clean air and the overfeed pressure increases gradually accordingly.

This continuous increasing of turbocharger speed and clean air pressure would not be sustainable by the engine.

To avoid this the wastegate valve opens the exhaust gasses channel bypassing the turbine and let them flowing outside. The rotating speed of the turbine decreases consequently.
In this way the maximum pressure inside the intake manifold is under control and too high pressures are avoided.

The wastegate valve is located on the exhaust manifold and its opening is proportional to the pressure of the clean air inside the intake manifold.

The wastegate valve can be pneumatic or electromechanical. In the oldest applications it is pneumatic. The valve is mechanically made by a spool valve that compresses a spring inside a valve body with one chamber. The pressure inside the intake manifold is addressed to the valve body chamber with a hose. As the pressure is so high to win the spring strength, the spool valve moves opening a by-pass channel. The exhaust gasses flow outside the manifold bypassing the turbocharger and reducing the pressure on the turbine that slows its speed down.

As the pressure inside the intake manifold goes down to lower values, the spool valve moves back closing the by-pass channel. Exhaust gasses are now addressed in full to the turbine again. It’s a continuous mechanical regulation.

On newest cars the wastegate valve is electromechanical with a solenoid. A pressure sensor measures the clean air pressure inside the intake manifold and informs the Engine Electronic Control Unit (ECU). As the pressure is too high the ECU sends a tension to the valve solenoid opening the spool letting the exhaust gasses flow out of the manifold as previously described.
The same as the intake air pressure decreases, the ECU closes the wastegate valve.

In some cases the control system is “dual”. To the mechanical spool valve (through a hose) or to the ECU (from a sensor) are addressed both the pressure of gasses in the exhaust manifold and the pressure of clean air in the intake manifold. With these two signals the regulation is optimal and the engine power can be maximized. This is the best regulation system.

On some high performances engines there is a function called “over boost” that blocks the wastegate valve for a short time in order to get the maximum power from the engine. In the past on some sports cars the “over boost” was controlled manually by the driver to get the maximum power when needed.

Today on some cars this function is invisible to the driver and it is controlled by the ECU that blocks the wastegate valve for a short period when the driver asks high power. It is a very delicate control because a wrong blocking time can damage the engine permanently.

Blow-off valve is another overpressure valve that controls the working conditions of the turbocharger.

As the wastegate valve works during accelerations and throttles pedal down situations, the blow-off valve the opposite works during throttle pedal up situations, when the car is reducing its speed.

The blow-off valve is located on the intake manifold. It works when the turbine and compressor speed is high, i.e. after a strong acceleration, but the air required into the combustion chamber is very low, i.e. when the driver releases the throttle pedal to decrease the speed or to change the gear.

In this particular situation, the throttle body is closed and the clean air compressed by the compressor cannot flow to the combustion chamber.

The high-pressure clean air is pushed back towards the compressor impeller that is pushed back towards the bearing risking to damage it.

The blow-off valve lets the surplus of compressed clean air flow out from the intake manifold.

The blow-off valve is purely mechanical and its opening can be driven by a peak of pressure in the intake manifold or by a pressure difference inside the intake manifold between the areas before and after the throttle body. In both situations the blow-off valve recognizes that there is a surplus of high-pressure clean air compressed by the compressor that is not needed for the combustion and lets it flow away.

From outside the car it’s possible to hear the typical air flowing noise in turbocharged sports cars just before every gear change.

Both wastegate and blow-off valves are simple parts and usually not source of inconveniences. In some cases, when they are not working properly, a cleaning is enough to restore the right working conditions. In case of more severe damages they have to be replaced by service assistance.

Bio: George, founder of Autofella.com, is an enthusiastic blogger and an auto mechanic expert. He has been involved in researching on various topics that are essential to the car owners, especially when it comes to an understanding, maintaining and handling their vehicles.