Wastegate

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.

Wastegate types

Internal

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. Internal wastegates are often used because they are cheaper to manufacture but are not always ideal when maximizing power. Often, in the case an internal wastegate is too small, this will cause boost creep or boost oscillation which can be seen as a spiky or inconsistent power curve.

External

An external wastegate is a separate self-contained mechanism typically used w ith 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.

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. Divorced wastegate dumper pipes are commonly referred to as "screamer pipes" due to the unmuffled waste exhaust gases 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.

Control

Pneumatic

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.

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 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.

Electric

Some 1940s aircraft engines featured electrically operated wastegates, such as the Wright R-1820 on the B-17 Flying Fortress. General Electric was the biggest manufacturer of these systems. Being before the age of computers, they were entirely analog. Pilots had a cockpit control to select different boost levels. Electric wastegates soon fell out of favor due to design philosophies which mandated the separation of the engine controls from the electrical system.

Hydraulic

Most modern turbocharged aircraft use a hydraulic wastegate control with engine oil as the fluid. Systems from Lycoming and Continental operate on the same principles and use similar parts which differ only in name. Inside the "wastegate actuator", a spring acts to open the wastegate, and oil pressure acts to close the wastegate. On the oil output side of the wastegate actuator sits the "density controller", an air-controlled oil valve which senses upper deck pressure and controls how fast oil can bleed from the wastegate actuator back to the engine. As the aircraft climbs and the air density drops, the density controller slowly closes the valve and traps more oil in the wastegate actuator, closing the wastegate in order to increase the speed of the turbocharger and maintain rated power. Some systems also use a "differential pressure controller" which senses the air pressures on either side of the throttle plate and adjusts the wastegate to maintain a set differential. This maintains an optimum balance between a low turbocharger workload and a quick spool-up time, and also prevents surging caused by a bootstrapping effect.

Wastegate chatter myth

There is confusion in the automotive world about so called "wastegate chatter" or "turbo flutter". A noise created on lifting off the throttle in a turbocharged car, commonly described as a chipmunk or a rattlesnake, is often stated incorrectly as being a result of the turbo's wastegate closing.

The noise is in fact the air compressed by the turbo passing back through the compressor wheel of the turbo after the airflow is abruptly halted by the throttle plate closing, called compressor surge. However, in some cases, i.e. where the throttle plate doesn't open fast enough or is set up to only react to high boost, some chatter will remain. Surge can occur on diesels when the turbo is attempting to pressurize the air at a higher pressure ratio than the compressor wheel can flow at a given speed. Diesels have no use for a Blow off Valve as they do not have a throttle plate.

The chatter noise is very noticeable on World Rally Cars, where anti-lag is used.

A compressor stall like this will NOT cause excess stress or wear on the turbo's shaft or bearings, except in cases where high amounts of boost pressure are being made, above 25psi.

Actual wastegate flutter occurs instead under partial boost conditions such as partial throttle near the boost threshold. It sounds like FftFftFft not ShuShuShu and is caused by the rapid opening and closing of the wastegate at boost levels near the spring pressure. It is commonly heard more clearly and may be more prominent on cars with modified intake silencers, up-pipes, and or downpipes, and is not harmful.

ee also

*Automatic Performance Control
*Dump valve
*EGR or Exhaust Gas Recirculation