Bypass Solenoid Valves (Compressor Recirculation Valves)

WARNING: This page is about a different car, the 2017 Cadillac ATS. However, it is still accessible from the selected car via links, so may be relevant.

The turbocharger bypass valves are mounted between the intercooler inlet air tubes and the turbocharger air inlet adapters. They are used to prevent the turbos from exceeding the surge limit of the compressors at low flow and high pressure. This occurs when the engine is running with a high load and the throttle suddenly closes. In this case, flow is almost null and pressure is very high. This not only is damaging to the turbocharger, but also generates undesirable noise and vibration and rapidly decelerates turbine speed. During normal operation the recirculation valves are held closed by both a light spring and high pressure exerted on the back side of the valve, which allows the spring to do minimal work. The bypass solenoids provide either high pressure (intercooler pressure) or vacuum to the valve. When vacuum is applied the change in pressure across the valve becomes large because the front side of the valve sees compressor out pressure while the rear sees vacuum. The spring is overpowered in this situation and the valve opens. The bypass valves are opened when the throttle is closed. This means little to no flow is entering the intercooler. Rather all the air is flowing in a circular path through the compressor (see diagram). The bypass valves are generally open for 1-2 seconds until the boost level drops to the new requested level.

Accelerator Pedal Depressed

The bypass valves are closed. The force of the return spring along with intercooler pressure forces the valves against their seats in the valve housings. Airflow passes through the compressors, across the charge air coolers and through the throttle body to provide maximum boost.

Accelerator Pedal Released

The bypass valves are opened. The force of the return spring is overpowered by vacuum on the rear side of the valve and compressor output pressure on the front side. Most of the airflow is directed through the bypass tubes and flows directly across the charge air coolers and through the throttle body. The turbine speed is kept relatively high and the turbochargers are prevented from exceeding the surge limit of the compressors. Under some conditions only one bypass valve may open, as each compressor is controlled independently.