A turbocharger recovers energy that would otherwise leave through the exhaust and uses it to push more air into the cylinders. More air means more fuel burnt for the same displacement: more torque and power from smaller engines. The price is a component working in extreme conditions: car turbos easily exceed 200,000 rpm, with inlet gas above 800 °C on diesels and over 950 °C on petrol engines.
The three parts of a turbo
- Turbine side. Exhaust gas enters the turbine housing and spins the turbine wheel, welded to the shaft. This is the hottest part.
- Compressor side. At the other end of the shaft sits the light-alloy compressor wheel, which draws air from the filter and compresses it towards the intercooler and the intake manifold.
- Centre housing (CHRA). The body supporting the shaft, with bearings, seals and oil passages; on many turbos, especially petrol ones, also a coolant jacket. Shaft, wheels and bearings fitted in the centre housing make up the core assembly.
Bearings and lubrication: everything depends on oil
In most turbos the shaft runs on floating journal bearings: bushes that rotate on a double oil film, inside and out. Axial loads are taken by a thrust bearing, also oil-lubricated. Some turbos use ball bearings, but the principle is the same: without a continuous film of clean oil, the turbo destroys itself in seconds.
Oil comes from the engine circuit through the feed pipe, lubricates, carries heat away and returns to the sump by gravity through the drain pipe. That is why a kinked, blocked or uphill drain causes as much trouble as an insufficient feed.
Shaft seals are not tight gaskets but piston-ring type seals: they work thanks to the pressure difference between the centre housing and the wheel housings. If that difference reverses, oil gets past even on a healthy turbo. That is why many “leaking turbos” are not faulty, as we will see.
How boost pressure is controlled
A free-running turbo would give little boost at low revs and too much at high revs. You need a way to control how much exhaust energy reaches the turbine. There are two main systems.
Waste-gate valve
A valve that, once the target pressure is reached, diverts part of the exhaust gas around the turbine. It is operated by a pneumatic actuator, pressure or vacuum, or by an electronic actuator. It is the typical solution on petrol engines.
Variable geometry (VGT or VNT)
Around the turbine wheel there is a ring of movable vanes. At low revs the vanes close, gas speeds up and the turbine spools up quickly; at high revs they open and reduce exhaust back-pressure. The vanes are moved by a vacuum actuator with a solenoid valve or by an electronic actuator. It is the standard solution on modern diesels.
Its weak point is dirt: in town-only use soot cakes the vanes and the ring that moves them, the geometry sticks, and underboost or overboost faults (P0299, P0234) and limp mode follow. Often the turbo is not broken: it is stuck.
Heat: why not to switch off straight after hard driving
After a demanding run the turbine side is extremely hot. If the engine is switched off immediately, oil stops in the centre housing and residual heat cooks it: carbon deposits form (coking) that over time block passages and score the shaft and bearings. Many modern engines keep coolant circulating after switch-off; on the others, a minute at idle before switching off is a habit that extends turbo life.
What it means for the workshop
- The right oil, changed on time, with a quality filter: the turbo’s first insurance.
- Clear feed and drain: a feed pipe coked on the inside cannot be seen from outside.
- An intact air filter: every foreign object that gets past ends up on the compressor wheel.
- Before condemning the turbo, check geometry and actuator: many boost faults are solved without replacing it.
- A turbo leaking oil must be understood, not just replaced: crankcase breather, drain and air filter are often the real culprits.
Frequently asked questions
How fast does a turbo spin?
It depends on size: small car turbos easily exceed 200,000 rpm, while larger heavy-duty turbos spin more slowly. This is one reason why balancing the rotating assembly must be very precise.
What is the difference between a waste-gate and variable geometry?
A waste-gate diverts part of the gas around the turbine once pressure is sufficient; variable geometry changes the angle of the vanes around the wheel and controls gas speed across the whole operating range. The first is typical of petrol engines, the second of diesels.
Why does the turbo whistle?
A slight whine under acceleration is normal. A loud or new whistle more often points to a leak in the hoses or intercooler, or to a rotating assembly with excessive play or out of balance. Check it straight away, because a leak makes the turbo spin faster than intended.