Turbocharging is an excellent way to increase the volumetric efficiency of the internal combustion engine. But the turbocharger must be controlled in order to keep it from spinning at speeds beyond what is desired or what it is capable of. Enter the wastegate.
The wastegate is a valve that diverts a fraction of the exhaust flow around the rotating assembly of the turbocharger. With a constant engine speed, the more exhaust gas that is bypassed, the slower the turbocharger will spin. By finely controlling the amount of diverted exhaust gas, exact boost pressures can be maintained throughout the RPM range.
There are two main wastegate configurations in use today on commercial turbochargers. The first is the internal wastegate, shown in the picture below. This is a small “flapper” valve built into the turbine housing of the turbocharger. It is attached to a shaft that is controlled by pneumatic actuator that normally is mounted to the compressor housing. When the actuator is pressurized, its shaft is extended and the valve in the turbine housing is opened. This open port provides a path for exhaust gasses to bypass the turbocharger internals, and travel directly to the exhaust outlet chamber of the turbocharger. This is the wastegate configuration chosen by almost all manufacturers of factory turbocharged vehicles.
The other configuration is the external wastegate. This valve is separate from the turbocharger, and is normally plumbed directly from the exhaust manifold. It requires a dedicated runner from the exhaust manifold collector to supply the wastegate with exhaust gas flow. When the desired boost level is reached, the wastegate opens and allows exhaust gasses to exit the manifold before entering the turbocharger. These gasses are then dumped to the atmosphere, or routed back into the exhaust system
The popular TiAL External Wastegate
Example of an external wastegate installation
We have many years experience working with both internal and external wastegated turbocharger systems on production and aftermarket vehicles in both low and high horsepower output. Over these years, we have learned firsthand the benefits and drawbacks of each. When designing our Lotus Elise/Exige turbocharger kits, we here at Radium Engineering made a conscious decision to go with internally gated turbochargers. We based this decision on many factors and felt that this was the best solution for us. It can be boiled down to a simple statement: If you can achieve perfect boost control with an internal wastegate, why add the extra cost, complexity, and weight of an external wastegate? External wastegates do have their place, this is most often when a large turbocharger is used to make a relatively small amount of boost and a lot of exhaust gasses must be diverted in order to keep the boost down, or when there is not space for the mechanical parts of an internal gate around the turbocharger. On our turbocharger kits, we have selected turbochargers that are conservatively sized and do not require a large amount of wastegate flow to keep the boost pressures where we need them. We are also striving to keep our prices competitive with other forced induction systems, and the internal wastegate is definitely the most cost effective solution. We utilize electronic boost control with feedback to precisely control our boost levels and to ensure safe and consistent performance from our turbocharger kits. We plan to publish our boost level data with our dyno data.
We hope this has helped shed some light on the design of the exhaust manifold and our turbocharger kit in general.