A variable geometry turbocharger, or VGT, helps an engine make more power by using exhaust gas energy to compress incoming air. More compressed air lets the engine burn more fuel efficiently, which increases torque and power. The key difference from a fixed turbo is that a VGT can change the angle of small vanes around the turbine wheel.
This lets the turbo respond well at low engine speed while still flowing enough exhaust at high engine speed.
The adjustable vanes act like movable nozzles that guide exhaust gas into the turbine wheel. When the vanes close, the exhaust passage becomes narrower, gas speed increases, and the turbine spins up faster for quicker boost. When the vanes open, more exhaust can pass through with less restriction, which prevents excessive boost and reduces exhaust back pressure.
An actuator, controlled by the engine computer, moves the vanes based on engine speed, load, boost pressure, and exhaust conditions.
Key Facts
- Boost pressure increases intake air density so more oxygen enters each cylinder.
- At low rpm, closed vanes increase exhaust gas velocity and improve turbo response.
- At high rpm, open vanes allow greater exhaust flow and help prevent overboost.
- Power from airflow can be estimated by P = pressure difference x volume flow rate.
- Turbo shaft speed depends on turbine power from exhaust gas and compressor load from intake air.
- Pressure ratio = absolute compressor outlet pressure / absolute compressor inlet pressure.
Vocabulary
- Variable geometry turbocharger
- A turbocharger with adjustable turbine vanes that control exhaust flow onto the turbine wheel.
- Turbine wheel
- The wheel driven by hot exhaust gases, connected by a shaft to the compressor wheel.
- Adjustable vanes
- Movable guide blades that change the speed and direction of exhaust gas entering the turbine.
- Actuator
- A mechanical, vacuum, hydraulic, or electric device that moves the turbo vanes to a commanded position.
- Boost pressure
- The pressure above atmospheric pressure produced by the compressor and sent into the engine intake.
Common Mistakes to Avoid
- Thinking closed vanes always mean more power. Closed vanes improve low-speed response, but at high rpm they can create too much back pressure and limit flow.
- Confusing exhaust pressure with boost pressure. Exhaust pressure drives the turbine, while boost pressure is produced by the compressor on the intake side.
- Ignoring absolute pressure in pressure ratio calculations. Pressure ratio uses absolute pressure, so atmospheric pressure must be added to gauge boost pressure.
- Assuming the actuator directly makes boost. The actuator only changes vane position, and boost comes from the compressor spinning faster due to turbine energy.
Practice Questions
- 1 A turbo compressor inlet is at 100 kPa absolute and the outlet is at 180 kPa absolute. What is the compressor pressure ratio?
- 2 A VGT produces 40 kPa of gauge boost at sea level where atmospheric pressure is 101 kPa. What is the absolute intake manifold pressure?
- 3 Explain why a VGT closes its vanes during low engine speed acceleration but opens them during high engine speed operation.