Bypass Air Control

When in deceleration mode, the turbocharger rotates additionally due to the inertial mass of shaft, compressor, and turbine wheel . When the throttle valve quickly closes, the throttle valve upstream condition becomes low discharge rate and high charge rate, resulting in backflow of extra pressure to pressurized air. This places counter-rotating force on compressor wheel . (Brief beat noise and mechanical stress.) To prevent this pumping, ECM opens the divert air switchover valve to release surplus pressure to the upstream of turbocharger.

Courtesy of NISSAN NORTH AMERICA, INC.

When the engine is not running, the divert air switchover valve is closed because the diaphragm is pushed back by the spring built in divert air switchover valve . When in engine load mode, the divert air switchover valve is kept closed by the boost pressure- placed diaphragm. When the throttle valve is closed and ECM detects deceleration mode, ECM controls the divert air switchover valve. The diaphragm is turned down against spring force and boost pressure, and the divert air switchover valve is opened to release surplus pressure.

Courtesy of NISSAN NORTH AMERICA, INC.

When the engine mode is switched from deceleration mode to load mode, the divert air switchover valve is not controlled anymore. The diaphragm is pushed back by the spring and boost pressure, and the bypass switchover valve is closed again.