Variable geometry turbos

A variable geometry turbo has a ring of small moving vanes round its turbine wheel. By turning the vanes, the ECU changes the size of the gaps the exhaust squeezes through on its way to the wheel. Makers call it a VGT, a VNT (variable nozzle turbine) or VTG (variable turbine geometry).

How a turbocharger works showed the trade-off in choosing a turbo: a small one spins up quickly but chokes the engine at high revs, and a big one flows well but is lazy low down. Variable geometry lets one turbo be both. Nearly every modern turbo diesel car uses one, so if you drive a diesel, yours probably does.

The demo shows a 2.0-litre diesel’s turbine end-on, with the throttle floored. Exhaust comes in at the top, whirls round the housing, then squeezes through the gaps between the Mustard vanes onto the wheel. Watch the inner ring of gas: the narrower the gaps, the faster it whirls. Beside it is the torque the engine makes at each engine speed.

It starts with working vanes at 1,500 rpm. There isn’t much exhaust yet, so the ECU shuts the vanes right down, to 30% open. The gas rushes through the narrow gaps and spins the turbine hard, so the turbo already makes 1.18 bar (17 psi). By about 1,600 rpm it reaches the 1.4 bar (20 psi) the ECU wants, and the engine makes its full 340 Nm (251 lb-ft) from about 1,750 rpm.

Now slide up the revs. There’s more and more exhaust, so the ECU opens the vanes bit by bit: about half open at 2,500 rpm, fully open at 4,500. That holds boost at 1.4 bar (20 psi) without the turbo spinning faster, and lets the exhaust out without choking the engine. The vanes are doing the wastegate’s job, so most diesel turbos don’t have one.

Then try the two faults:

• Stuck part-shut. Soot has stuck the vanes at 45% open. Low down it pulls well, but the ECU can’t open them as the revs rise, so boost overshoots. At about 2,400 rpm it passes the ECU’s limit and the car goes into limp mode.
• Stuck open. The vanes are stuck wide open, so the turbo behaves like a much bigger one. At 2,000 rpm there’s barely 0.11 bar (1.6 psi) of boost, and the car feels flat until well past 3,000 rpm.

Put your thumb over the end of a hosepipe and the water shoots out faster, because the same amount has to get through a smaller gap. The vanes do the same to the exhaust. Shut down, they speed the gas up and aim it at the tips of the turbine’s blades, so a little exhaust spins it hard. Opened up, they let lots of gas through gently, without holding the engine back.

The vanes sit on pins round a ring. A second ring links them all, so they turn together, moved by a lever from an actuator outside the housing. On older diesels that’s a vacuum canister worked by a solenoid; on most modern ones it’s a small electric motor, with a sensor that tells the ECU exactly where the vanes are. The ECU watches the boost sensor and moves the vanes many times a second.

The ECU has other uses for the vanes too. Shutting them a little raises the pressure in the exhaust, which helps push exhaust gas back into the intake through the EGR valve. And on many lorries, shutting them hard makes the engine work against its own exhaust, which helps it brake.

Diesel exhaust is much cooler than petrol exhaust, at most about 800°C (1,472°F) against up to about 950°C (1,742°F). Vanes and pivots that have to keep moving freely in that heat need expensive heat-resistant alloys, so for years petrol engines stuck to fixed turbines with wastegates.

There are exceptions. The Porsche 911 Turbo has used variable geometry since 2006. VW Group’s 1.5 TSI evo in its 130 PS form has one too: it runs on a cycle that keeps its exhaust cooler than usual, which made the vanes affordable on an everyday petrol car.

• Sticking vanes. By far the most common fault. Soot from the exhaust, and oil from a worn turbo or a busy crankcase breather, build up on the vanes and their ring until they stick. It’s worst on diesels used for short trips and gentle town driving, which make lots of soot and rarely open the vanes fully. The classic sign is limp mode under hard acceleration, often with fault code P0234 (overboost) or P0299 (underboost), and sometimes P2563 for the vane position sensor. The power usually comes back after the engine is switched off and on.
• A failed actuator. Electric actuators can wear their gears or lose their position sensor; vacuum ones can split their diaphragm, or lose vacuum through a cracked pipe. Either way the ECU can’t move the vanes, with the same symptoms as sticking.
• The usual turbo faults. Worn bearings, leaking oil seals and damaged wheels happen here too, as covered in How a turbocharger works.

Many diesels sweep their vanes from one end to the other when you start or stop the engine, to keep them free; you may hear a whirr or click from the turbo. A long run with some hard acceleration can free lightly stuck vanes. Cleaning sprays are hit and miss. The reliable fix is to take the turbo off and clean or recondition it.

Looking after the rest of the engine helps the turbo: the right oil, changed on time, and a run at motorway speeds now and then, which also lets the diesel particulate filter clean itself.

• A diesel remap raises the boost target and the fuel. The ECU moves the vanes to suit, which is one reason diesels gain so much from a remap, often 20 to 30% more torque. Pushed too far, the turbo can spin faster than it’s built for, especially high up where the air is thin, and the engine makes black smoke.
• A hybrid turbo keeps the original housing and vanes but fits bigger wheels, for more power at the top with little extra lag.
• Removing the EGR valve or diesel particulate filter often comes with remaps aimed at “curing” soot problems. It’s illegal on a road car.