Waste Gate in Engines

The waste gate valve is used in turbocharged engines. It is installed to relieve the turbo pressure, when it exceeds the pressure consumed by the engine. For example, during acceleration of the vehicle, the engine RPM is high, the turbo is charging the engine, and the driver decides to change gears. In this moment, the throttle is released, and the engine looses speed while the turbo is still running full. The excess of air pressure is released, to avoid what is called "compressor surge", which is, in a simplified way, something like a pressure wave going back to compressor rotor and messing averything.... This can lead to vibration, noise, rubbing, and severe compressor damage.

It actually releases and regulates the turbo pressure during all operation. But, the most notorious aspect of its operation, is a sound like "pssssshhhhhiiiiiuuuuuu" when the engine throttles back.

Bhrescobar's explanation is pretty good, but the terminology is a little off. The valve in a turbocharged car that makes the distinctive noise is called either the blow-off valve or the compressor bypass valve. Blow-off valves vent intake air pressure to atmosphere (very loud if not muffled), while compressor bypass valves vent to the low pressure side of the compressor.

A waste gate, on the other hand, is used to vent some of the exhaust gas that would otherwise be spinning the turbine, and serves as the primary turbine speed regulator. In the simplest setup, the manifold pressure is piped directly to the waste gate actuator, and is used to open the waste gate. When the car is operating at medium to high output levels, an equilibrium is established, and the waste gate stays partly open (and modulates) to maintain a constant manifold pressure. So the waste gate is in continuous operation (unless the engine is running slowly) and is pretty noiseless. The compressor bypass valve prevents compressor surge and operates when the throttle is suddenly closed. (These days, straight-to-atmosphere blow-off valves are used mainly by those who like to make noise.)

In more sophisticated engines, the waste gate is controlled through electronics, (usually by the same ECU which controls fuel injection and ignition.) In several modern turbocharged engines, the combined controls can produce a perfectly flat torque curve from 1500 rpm on up to near redline.

You said it. We call it a 'dump valve' in the UK, I personally like the group 1 type that make a chattering noise, rather then then the straight hiss you get on a standard type.

A waste gate is found on the turbocharged or supercharged engines. It's purpose is to limit the amount of boost or pressure the turbo or supercharger can apply to the engine. This helps to keep from driving the crankshaft right out thru the oil pan,bearings and all. You can totally destroy a perfectly good engine with too much boost.

A supercharged engine does not have a waste gate. It does have a bypass valve as Ken described very nicely above but SC'd engines do not need a waste gate because the SC is driven by the motor (Crank shaft) not exhaust gases so having a waste gate would actually have the SC pulling air from the waste gate instead of the air intake, no benefit there. On a SC engine I believe they typically open the bypass valve when ever the throttle is released.

Shawn