A wastegate is a valve that diverts exhaust gases away from the turbine wheel in a turbocharged engine system. Diversion of exhaust gases causes the turbine to lose speed, which in turn reduces the rotating speed of the compressor. The primary function of the wastegate is to stabilize boost pressure in turbocharger systems, to protect the engine and the turbocharger. The wastegate is controlled by a wastegate actuator.
An internally gated turbocharger. The internal gate is located to the right of the turbine wheel, but built into the turbine housing. Partially seen at the top is the wastegate actuator arm.
An internal wastegate is an integral part of the turbine housing. The wastegate actuator is commonly attached to the compressor housing with a metal bracket. A flapper valve is generally used by internal wastegates.
On left, an example external wastegate. Partially visible is the shaft of the poppet valve built into the wastegate. The port on top of this wastegate is a secondary control port. Not visible is the primary control port.
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, though this generally involves welding the internal wastegate shut to avoid unwanted leaks.
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 more rare.
A divorced wastegate dumps the gases directly into the atmosphere, instead of returning it 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. 'Divorced' wastegate dumper pipes are commonly referred to as Screamer Pipes due to the unmuffled waste exhaust gasses and the associated loud noises they produce.
Internal gates cannot generally be vented to the atmosphere because the internal wastegate port and turbine exducer are built into the same housing. Occasionally an exhaust downpipe for an internally gated turbocharger will be called 'divorced' because it integrates two tubes where one is intended for the wastegate only. It should be noted that it is difficult to truly separate the exhaust flow of the turbine and wastegate. Thus, internal wastegate turbos will rarely if ever feature an external dump.
The simplest 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.
Wastegate control ports
Standard wastegates have one port for attaching the boost control line from the charge air supply line or boost control solenoid. This is the most common configuration and the only type of configuration found on internal wastegates.
A dual port Tial 44mm wastegate. The side port is the primary port. The top port is the secondary port and not necessary for proper operation.
A dual port wastegate adds a second port on the opposite side of the actuator. Air pressure allowed to enter this second port aids the spring to push harder in the direction of closing the wastegate. This is exactly the opposite of the first port. The ability to help the wastegate remain closed as boost pressure builds can be increased. This also adds further complexity to boost control, requiring more control ports on the solenoid or possibly a complete second boost control system with its own separate solenoid. Use of the second port is not necessary. Secondary ports, unlike primary ports, cannot be simply attached to a boost control line and require electronic control to be useful.
Wastegate Chatter Myth
There is a common myth in the automotive world about so called "wastegate chatter", a noise created on lifting off the throttle in a turbocharged car. The sound is commonly described as a chipmunk or a rattlesnake. This sound is in fact caused by a component called a blowoff valve (BOV) malfunctioning or being incorrectly setup. The BOV releases the excess pressure (from turbocharging/supercharging) between the throttle flap and the turbo when the throttle closes. However when this volume of air is too large the BOV cannot release it all and the pressure is vented back through the compressor turbine. As the air passes back through the turbine the pressure wave is "chopped" by the still spinning turbine creating the fluttering sound that is commonly termed wastegate chatter.