Information AboutWastegate |
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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. built into the wastegate. The port on top of this wastegate is a secondary control port. Not visible is the primary control port.]] 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, 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 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. 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. Hydraulic systems for aircraft Most turbocharged aircraft use a hydraulic wastegate control with engine oil as the fluid. Modern 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 acts to maintain an optimum balance between a low turbocharger workload and a quick spool-up time. WASTEGATE CHATTER MYTH There is a common myth in the automotive world about so called "wastegate chatter" or "turbo flutter", a noise created on lifting off the throttle in a turbocharged car. The sound is commonly described as a chipmunk or a rattlesnake and 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. This is also known as Compressor Surge and over time can result in significant damage to the turbocharger. Surge puts an enormous amount of stress on the compressor wheel and thrust bearing which causes them to fatigue and eventually fail. To avoid this damage, turbocharged cars will use a Blowoff Valve (BOV) or Bypass Valve (BPV), which allows the extra pressurized air in the intake out. However, in some cases, i.e. where it 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 BOV as they do not have a throttle. The chatter noise is very noticeable on WRC Rally cars, where no BOV is allowed. SEE ALSO
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