A wastegate is a valve that controls the flow of exhaust gases to the turbine wheel in a turbocharged engine system.
Diversion of exhaust gases regulates the turbine speed, which in turn regulates the rotating speed of the compressor. The primary function of the wastegate is to regulate the maximum boost pressure in turbocharger systems, to protect the engine and the turbocharger. One advantage of installing a remote mount wastegate to a free-float (or non-WG) turbo includes allowance for a smaller A/R turbine housing, resulting in less lag time before the turbo begins to spool and create boost.
Wastegate types
External
An external wastegate is a separate self-contained mechanism typically used with turbochargers that do not have internal wastegates. An external wastegate requires a specially constructed turbo manifold with a dedicated runner going to the wastegate. The external wastegate may be part of the exhaust housing itself. External wastegates are commonly used for regulating boost levels more precisely than internal wastegates in high power applications, where high boost levels can be achieved. External wastegates can be much larger since there is no constraint of integrating the valve or spring into the turbocharger and turbine housing. It is possible to use an external wastegate with an internally gated turbocharger. This can be achieved through a specially designed bracket that easily bolts on and restricts the movement of the actuator arm, keeping it from opening. Another route involves welding the internal wastegate shut which permanently keeps it from opening, but failure of the weld can allow it to open again.
External wastegates generally use a valve similar to the poppet valve found in the cylinder head. However they are controlled by pneumatics rather than a camshaft and open in the opposite direction. External wastegates can also use a butterfly valve, though that is far less common.
Internal
An internal wastegate is a built-in bypass valve and passage within the turbocharger housing which allows excess exhaust pressure to bypass the turbine into the downstream exhaust. Control of the internal wastegate valve by a pressure signal from the intake manifold is identical to that of an external wastegate. Advantages include simpler and more compact installation, with no external wastegate piping. Additionally, all waste exhaust gases are automatically routed back into the catalytic converter and exhaust system. Many OEM turbochargers are of this type. Disadvantages in comparison to an external wastegate include a limited ability to bleed off exhaust pressure due to the relatively small diameter of the internal bypass valve, and less efficient performance under boost conditions.
Atmospheric/divorced wastegates
A "divorced" wastegate dumps the gases directly into the atmosphere, instead of returning them with the rest of an engine's exhaust. This is done to prevent turbulence to the exhaust flow and reduce total back pressure in the exhaust system. A Divorced wastegate dumper pipe is commonly referred to as a screamer pipe due to the unmuffled waste exhaust gases and the associated loud noises they produce.
Control
Manual
The simplest control for a wastegate is a mechanical linkage that allows the operator to directly control the wastegate valve position. This manual control is used in some turbo-charged light aircraft.
Pneumatic
The simplest closed-loop control for a wastegate is to supply boost pressure directly from the charge air side to the wastegate actuator. A small hose can connect from the turbocharger compressor outlet, charge pipes, or intake manifold to the nipple on the wastegate actuator. The wastegate will open further as the boost pressure pushes against the force of the spring in the wastegate actuator until equilibrium is obtained. More intelligent control can be added by integrating an electronic boost controller.
Standard wastegates have one port for attaching the boost control line from the charge air supply line or boost control solenoid. Recent advances in internal wastegate actuators bring dual port control.