How a turbocharger works

A turbocharger, or turbo, is an air pump driven by the engine’s own exhaust. Why forced induction makes more power showed what squeezing air does for an engine. This lesson looks at how a turbo does the squeezing, and how it’s kept under control.

A turbo has two fans on one shaft:

• The turbine sits in the exhaust. Hot gas leaving the engine rushes through it and spins it.
• The compressor sits in the intake, on the other end of the same shaft. As it spins, it flings fresh air outwards into its housing, squeezing it, and pushes it on through the intercooler into the engine.

The clever part is where the energy comes from. Around a third of the energy in the fuel goes straight out of the exhaust as heat and pressure. A turbo catches some of that and uses it to pack more air into the engine. It isn’t quite free: the turbine is in the way of the exhaust, so the engine has to push a little harder to get its gas out.

The demo shows one cylinder of the engine above the turbo. Fresh air comes in, through the compressor and up through the intercooler into the engine. Exhaust comes out, down through the turbine and out of the car. The slider is the throttle, as if the car were pulling up through the revs in one gear.

It starts on a light throttle, at 10%. The engine turns at about 1,400 rpm, and the turbo idles at about 15,000 rpm. That sounds fast, but the boost gauge barely moves, because a compressor only squeezes hard when it spins very fast: the boost it makes rises with the square of its speed.

Now press Floor it and watch the readouts:

1. More exhaust. The throttle opens, the engine takes in more air and burns more fuel, and the revs climb. Much more exhaust gas rushes through the turbine.
2. Spooling up. The turbine wheel and shaft take time to speed up. For the first second or so there’s little boost. That wait is turbo lag.
3. Boost builds. As the turbo passes 100,000 rpm the gauge climbs quickly. Boost reaches half its full 1 bar (15 psi) about one and a half seconds after you floor it, and 90% after about three, by when the engine has reached about 4,400 rpm.
4. The wastegate opens. At about 140,000 rpm the turbo makes its full 1 bar (15 psi). The exhaust could spin it much faster, so the wastegate opens and lets some of the gas skip the turbine. By 6,000 rpm about a third of the exhaust goes round it.

Press Lift off and the boost falls away within about a second, as the exhaust flow drops and the turbo slows.

Try holding the throttle at different points too. At half throttle the turbo makes only about 0.26 bar (3.8 psi), and below about a third it makes almost none. That’s why a turbo car can cruise as gently as a small naturally aspirated one.

The model is simplified, but its numbers are typical of a small road-car turbo.

Two different things make a turbo feel slow, and they’re easy to mix up.

• The boost threshold is the engine speed below which there isn’t enough exhaust to spin the turbo up, however long you wait. In the demo it’s about 2,700 rpm: held there with the throttle floored, the turbo just makes full boost; any lower and it can’t.
• Lag is the wait for boost to build once there is enough exhaust, while the turbine speeds up.

Flooring it from low revs, you get both, which is why the demo’s engine is well past its threshold, at about 4,400 rpm, before boost is nearly full.

A bigger turbo can flow more air and make more power at high revs, but it needs more exhaust to spin it, so its threshold is higher, and its heavier wheels take longer to spin up. Choosing a turbo is always a trade between the two.

A road-car turbo is about the size of a melon, and has three main parts.

• The turbine housing is a snail-shaped casting of iron or steel that guides the exhaust inwards onto the turbine wheel. It runs very hot: up to about 950°C (1,742°F) in a petrol engine, enough to glow red on a dyno. The wheel itself is cast from a nickel alloy that stays strong at that heat.
• The compressor housing is another snail-shaped casting, usually in aluminium. The compressor wheel, also aluminium, sucks air in at its centre and flings it outwards, and the widening housing slows the air down, which raises its pressure.
• The centre housing holds the shaft and its bearings between the two. Engine oil is pumped through it, and the shaft floats on a thin film of that oil, much like a crankshaft does. Many turbos run on ball bearings instead, which spin up a little faster. Most modern ones also have coolant passages, so that heat soaking from the turbine after a hard drive doesn’t cook the oil.

Small road-car turbos often spin at well over 150,000 rpm, and the tips of the compressor blades move at around the speed of sound.

Left to itself, a turbo big enough to make good boost at low revs would make far too much at high revs. So every turbo petrol engine has ways to hold boost in check.

• The wastegate is a valve, usually a small flap inside the turbine housing, that lets exhaust bypass the turbine. On simple systems a spring holds it shut, and a canister fed with boost pressure pushes it open when boost reaches the spring’s limit. The ECU can trim this by bleeding off some of that pressure through a solenoid valve, letting boost rise higher. Many modern turbos use an electric motor to work the wastegate instead, so the ECU sets boost directly. Wastegates looks at them in more detail.
• The diverter valve deals with lifting off. When the throttle snaps shut at full boost, the squeezed air has nowhere to go and tries to push back through the compressor, which is still spinning hard. That’s called surge, and it makes a fluttering noise and strains the shaft. The diverter valve opens and lets the air back round to the compressor’s inlet. A blow-off valve does the same job but lets the air out to the atmosphere, which makes the well-known “pssh”. Blow-off valves and diverter valves looks at both.
• The boost sensor in the intake manifold tells the ECU what the turbo is actually doing, so it can open the wastegate and cut the power if boost climbs too high.

Diesels usually work differently. Their turbos have variable geometry: a ring of movable vanes round the turbine. Closed down at low revs, the vanes squeeze the exhaust through a narrow gap, so it hits the wheel faster and spins it up quickly. Opened up at high revs, they let the gas through easily, which does the wastegate’s job too. Variable geometry turbos shows them working.

Engineers have tried many ways to get boost sooner.

• Smaller, lighter turbos. Modern small engines use tiny turbos that spool up almost instantly, and make full torque from 1,500 rpm or so.
• Twin-scroll turbos split the turbine housing into two passages, each fed by cylinders that don’t fire one after another. Each exhaust pulse then hits the turbine cleanly, without getting in the way of the next, which spins it up faster. Twin-scroll turbochargers explains how.
• Variable geometry, as on diesels. It’s rare on petrol engines, because their exhaust is so much hotter, but the Porsche 911 Turbo has used it since 2006.
• Two turbos. On a V engine, one turbo per bank of cylinders keeps each one small. Some engines, such as the one in the 1990s Toyota Supra, used sequential turbos: a small one for low revs, and a second that joins in higher up.
• Turbos inside the V. Many V8s now put the turbos between the cylinder banks, so the exhaust has the shortest path to the turbines.
• Electric help. Some turbos, such as those on recent Mercedes-AMG engines, have an electric motor on the shaft that spins the turbo up before the exhaust can. Formula 1 cars did the same from 2014.

A turbo should last the life of the engine if it’s looked after. When one fails, it’s usually because of its oil.

• Oil starvation and coking. The bearings need clean oil the moment the engine starts. Old, low or wrong oil wears them quickly, and so does a blocked oil feed pipe. Oil left in a very hot turbo after a hard drive can cook into hard carbon, which blocks the passages. Driving gently for the last minute or so before you switch off lets the turbo cool.
• Worn bearings. Once the shaft has play in it, the wheels can touch their housings. Signs are a siren-like whine that rises with boost, a loss of power and blue smoke. Grab the compressor wheel with the engine off: it should spin freely with almost no play.
• Leaking oil seals. Worn seals, or a blocked crankcase breather pushing pressure back through the turbo, let oil into the intake or exhaust. You’ll see blue smoke and oil in the intercooler pipes.
• Damaged wheels. A bit of loose air filter, or a stray nut, sucked into the compressor chips its blades. The turbo whistles, wobbles and soon fails.
• Wastegate trouble. The wastegate’s pivot can wear and rattle, especially when the engine is idling or lifting off: a well-known fault on several VW Group engines. If it sticks shut, boost runs too high (fault code P0234, overboost); if it sticks open, the turbo never makes full boost (P0299, underboost). Either way the ECU usually cuts the power.
• Sticking vanes. On a diesel used mostly for short trips, soot can stick the variable vanes, so it goes into limp mode under hard acceleration. A long run at motorway speeds sometimes frees them.
• Cracks. The turbine housing and exhaust manifold heat up and cool down every journey, and over years they can crack, letting exhaust leak with a ticking noise.

Turbo cars are the easiest to tune, because more boost means more power and the turbo can usually make more than it’s set to.

• A remap raises the boost the ECU asks for, and adjusts the fuel and spark to match. On many modern turbo engines it adds 20 to 30% more power, but it works the turbo, engine and clutch harder, and the turbo spends more of its life at its limit.
• A boost controller on an older car with a simple wastegate does the same job without a new map, by stopping the wastegate canister seeing the full boost. Without changes to the fuel and spark it can make the engine knock.
• A hybrid turbo keeps the original housings but fits bigger wheels, for more power at high revs with only a little more lag. A bigger turbo goes further, but raises the boost threshold, and needs bigger injectors, a new map and usually a stronger clutch.
• A bigger intercooler keeps the air cool when you’re using all that boost.
• A blow-off valve that vents to the atmosphere makes the “pssh” sound, but on a car whose ECU measures the air coming in, the air it lets out has already been counted. The engine then runs rich for a moment after each gear change, and may stutter. A louder diverter valve that recirculates the air avoids this.
• A turbo timer keeps the engine idling for a while after you take the key out, to cool the turbo. Modern water-cooled turbos rarely need one.
• A downpipe, the pipe straight after the turbine, is often swapped for a wider one to let the turbo breathe more easily. It usually contains a catalytic converter, so check the new one has an approved one; removing it is illegal on the road.