A diverter valve (also sometimes referred to as a compressor bypass valve or blow-off valve) on a turbocharged engine is essentially a pressure relief valve designed to recirculate excess pressure back into the intake system when the throttle closes.
Purpose and Function
When the throttle of a turbocharged engine closes suddenly (such as when shifting gears or decelerating), the turbocharger is still spinning and producing boost. This pressure has nowhere to go, which can lead to:
- Compressor surge: The pressurized air slams against the closed throttle plate, causing the turbocharger compressor wheel to rapidly slow down or even stall. This is inefficient and can damage the turbocharger over time.
- Reduced throttle response: The build-up of pressure needs to be relieved before the throttle can be reopened, which leads to a lag in throttle response.
The diverter valve solves these problems by:
- Providing an alternate path for the pressurized air: When the throttle closes, a signal (typically vacuum) causes the diverter valve to open.
- Recirculating the air: The valve redirects the pressurized air from the turbo outlet back into the intake system, before the turbo inlet. This maintains airflow through the turbo, preventing compressor surge and keeping the turbo spooled up.
Types of Diverter Valves
While the core function remains the same, diverter valves can differ in their design and operation:
- Piston-type: These valves use a piston that slides within a bore to open and close the passage for the air. They are generally reliable and can handle high boost pressures.
- Diaphragm-type: These valves use a diaphragm to control airflow. They can be quicker to respond than piston-type valves but may be less durable under high boost conditions.
Diverter Valve vs. Blow-Off Valve (BOV)
The terms "diverter valve" and "blow-off valve" are often used interchangeably, but there's a key difference:
- Diverter Valve: Recirculates the air back into the intake system. This is typically quieter.
- Blow-Off Valve (BOV): Vents the air directly into the atmosphere. This creates a characteristic "whoosh" sound.
While BOVs are popular for the sound they produce, diverter valves are often preferred, especially on cars with Mass Airflow (MAF) sensors. Vented air is metered by the MAF sensor, and venting it to the atmosphere results in a momentary rich condition because the engine expects to receive that metered air. Recirculating diverter valves avoid this issue.
Advantages of Using a Diverter Valve
- Reduced Compressor Surge: Protects the turbocharger from damage.
- Improved Throttle Response: Keeps the turbo spooled up for quicker acceleration.
- Maintained Airflow: Ensures efficient turbocharger operation.
- Compatibility with MAF Sensors: Prevents fueling issues.
In summary, a diverter valve is a crucial component in a turbocharged engine, promoting efficiency, longevity, and responsiveness by carefully managing pressure fluctuations.
