How VNT™ & VTG™ works
A Garrett® VNT® / BorgWarner® VTG™ turbocharger works by adjusting the gas throat section at the inlet of the turbine wheel in order to optimize turbine power with the required flow velocity.
At low engine speed and small gas flow, the turbocharger reduces the throat section, increasing turbine power and boost pressure. At full engine speed and high gas flow, the VNT™ turbocharger increases the throat section, avoiding turbocharger overspeed and maintaining the booster pressure required by the engine.
The throat section modulation can be controlled directly by the compressor pressure through the use of a pressure actuator, or by the engine management system using a vacuum actuator. To modify the throat section, VNT™ Multivane™ models use a mobile multivane system composed of a number of vanes which rotate relative to the turbine wheel axis.
The benefits of VNT™ technology are:
Improved transient response
Lower fuel consumption
Improved braking power
How VGT™ works:
A Holset VGT™ turbo is an effective, though complex, method of turbocharging using a turbine stage where the swallowing capacity is automatically varied while the engine is running. This permits turbine power to be set to provide just sufficient energy to drive the compressor at the desired boost level wherever the engine is operating.
This is achieved by varying the area of a nozzle, a set of guide vanes that control the flow through the turbine. The Holset VGT™ is unique in that the vanes do not pivot but slide axially. This approach ensures a high level of durability and reliability, essential for commercial diesel applications.
Wastegates - What & Why
Applications that require a good response at low engine speeds benefit from the use of a small turbo.
However, even though they may be designed to spin at up to 250,000 rpm and withstand exhaust gas temperatures of over 1000°C, there is a danger that a small turbine can overspeed and overboost at higher engine speeds.
In order to prevent this from happening, some turbochargers are fitted with a wastegate or turbine bypass: as the pressure reaches the maximum preset level, a valve opens to allow some of the exhaust gas to bypass the turbine and flow straight into the exhaust system.
The simplest form of wastegate control is a pneumatic actuator.
The sensor port on the actuator is connected directly to the compressor outlet and, as pressure rises in the top part of the actuator above the diaphragm, it acts against the pressure of a spring to move a rod, thereby opening the turbine bypass valve (wastegate).