Steyr's car has a control mechanism on the exhaust pipe. It is similar to a throttle valve. It is said that the engine speed can be reduced. Is it?

Name: exhaust brake valve. Function: When the car goes down a big slope, exhaust braking is used, and the braking of the whole car is realized by exhaust resistance, so as to reduce the wear of the braking system and improve the safety performance. And because the exhaust brake valve is pneumatic when it is closed and opened, the exhaust brake valve is connected with the gas storage tank. This device is a safety device and has no energy-saving effect.