Operating principles – Understanding the vanes, flow area and turbine response
How can a turbocharger change the way exhaust gas flows?
In the previous section, we looked at how a variable-geometry turbocharger can change the way exhaust gas flows through the turbine. But how does it actually do that?
To understand this, we need to look inside the turbine housing.
Unlike a fixed-geometry turbocharger, which has a fixed shape and flow area, a VGT has a set of movable vanes positioned around the turbine wheel.
Think of these vanes as small adjustable guides. By changing their position, they can alter how much space the exhaust gas has to pass through and how the gas is directed onto the turbine wheel.
What happens when the vanes move?
Let’s look at what happens at two different engine speeds.
Low engine speed
Imagine an engine idling or running at low speed. There’s relatively little exhaust gas flowing out of the engine, so there’s less energy available to spin the turbocharger.
With a fixed-geometry turbo, the turbine has to work with whatever exhaust gas flow is available. This is one of the reasons a turbocharger can take time to respond when you accelerate from low engine speeds.
A VGT approaches this differently.
The vanes move closer together, leaving smaller gaps for the exhaust gas to pass through. Because the gas has less space to flow through, it is directed through these gaps at a higher velocity.
Think of putting your thumb over the end of a garden hose. The opening becomes smaller, and the water comes out faster. The principle isn’t exactly the same, but it gives you an idea of how changing the available space can affect flow.
Inside the turbocharger, this change helps direct the exhaust gas onto the turbine wheel in a way that allows it to transfer energy more effectively under these conditions.
The turbine accelerates more quickly, the compressor starts delivering more air, and the engine can build boost sooner.
The vanes haven’t created any extra exhaust energy, they’ve simply helped the turbocharger make better use of the energy that’s already there.
Higher engine speed
Now imagine the engine is working hard at higher speed. It is producing much more exhaust gas, and the turbine needs to deal with that increased flow.
If the vanes stayed in their previous position, the smaller gaps would make it harder for all that exhaust gas to pass through. This could create excessive backpressure, making it harder for the engine to push exhaust gas out of the cylinders.
So the vanes move apart, creating larger gaps for the exhaust gas to pass through the turbine, reducing the restriction caused by keeping the vanes in their more closed position.
The turbocharger can continue supplying the air the engine needs without unnecessarily restricting the exhaust flow.
In simple terms, the vanes move closer together when the turbo needs help responding at low engine speeds, and further apart when the turbine needs to handle greater exhaust flow.
That’s the basic principle behind variable geometry.
Why does this make such a difference?
Remember that the turbine and compressor are connected by a shaft. When exhaust gas spins the turbine, the compressor spins with it, drawing in fresh air and forcing more air into the engine.
The faster the turbocharger can accelerate when the engine needs more air, the sooner it can respond to the driver’s demand for power.
A VGT helps achieve this by changing how exhaust gas reaches the turbine wheel, rather than relying on one fixed arrangement for every engine speed and load.
At low engine speeds, it can help the turbine respond sooner, while at higher engine speeds, it can open up the flow area to accommodate the greater volume of exhaust gas.
This gives the engine a turbocharger that can adapt to changing conditions instead of having to compromise as much between low-speed response and high-speed performance.
It’s not just a case of opening and closing the vanes
There’s one important detail to understand.
Although we’ve described the vanes as moving closer together and further apart, they don’t simply act like a tap that turns boost on and off.
Their position changes the space available for the exhaust gas to flow through, but it also changes the direction in which the gas is guided onto the turbine wheel. Both affect how effectively the exhaust gas spins the turbine.
And there is a limit to how far the vanes can close. Closing them too much can create excessive exhaust backpressure, increasing the work the engine has to do to push exhaust gas out of the cylinders.
The aim isn’t to close the vanes as much as possible or open them as far as possible, it’s to find the position that gives the engine the airflow and turbocharger response it needs under the current operating conditions.
But who decides where the vanes should be positioned, and what physically moves them?
Now comes the important part, the electronics. Next we will take a look at the actuator and Engine ECU’s role in all of this.