In A brief history of turbos I gave the turbocharger one minute of explanation and then got on with the story. This is the longer version: what is inside that snail-shaped lump of metal, what each part does, and why it matters when you drive.
There is a British thread running through it too. A large share of the world’s lorry turbochargers trace back to a factory in Huddersfield, and I will come to that near the end.
Open the snapshot: the whole story in five points ▾
- A turbocharger is an air pump that pushes extra air into the engine, so it can burn more fuel and make more power from the same size of engine.
- It is driven by the engine’s own exhaust gases. They spin a turbine, and a shaft joins the turbine to a compressor that squeezes the incoming air.
- The smallest turbos spin at more than 300,000 revolutions per minute and some run at up to 1,050°C. The shaft floats on films of engine oil, which is why the right grade of oil matters.
- An intercooler cools the squeezed air, a wastegate stops the turbo making too much pressure, and turbo lag, the delay before the extra power arrives, is tackled with designs such as variable geometry, ball bearings and electric assistance.
- Holset, founded in Huddersfield in 1952 and now Cummins Turbo Technologies, was reported in 2012 to supply turbochargers for about 40 per cent of the world’s lorries.
In this article
- The whole idea in three sentences
- Two wheels on one shaft
- How fast and how hot
- The part in the middle: bearings and oil
- The intercooler
- The wastegate and the blow-off valve
- Turbo lag, and how engineers fight it
- Why small engines get turbos
- Made in Huddersfield
- Quick questions
- Test yourself
The whole idea in three sentences
An engine makes power by burning fuel with air, and the power it can make depends on how much air and fuel it can get into its cylinders. A turbocharger is an air pump that pushes air into the engine at a higher pressure, which packs more air into the same space. More air means more fuel can be burned, so the same size of engine makes more power.
The clever part is what drives the pump: the hot exhaust gases the engine was about to throw away.
- Exhaust gases leave the engine and spin the turbine.
- The shaft turns the compressor at the same speed.
- The compressor draws in fresh air and squeezes it.
- The intercooler cools the compressed air.
- More air reaches the engine, so it can burn more fuel.
Two wheels on one shaft
Inside a turbo there are two wheels joined by a single shaft, each in its own housing.
- The turbine is the hot side. Exhaust gases leaving the engine blow through it and spin it.
- The compressor is the cold side. It sits on the other end of the same shaft, so it spins with the turbine. It draws in fresh air and squeezes it on its way to the engine.
That is the whole machine. Exhaust gases spin one wheel, and that wheel spins the other.
How fast and how hot
The numbers are hard to picture. Garrett, one of the biggest turbo makers, says its very largest turbo spins at up to 63,000 revolutions per minute and its smallest at more than 300,000. At full speed, a point on the edge of the wheel is travelling at over 1,900 km/h, which is about 1,180 mph.
It is hot work as well. Garrett says some of its turbos run at temperatures of up to 1,050°C.
The part in the middle: bearings and oil
Between the two wheels sits the centre housing, which holds the bearings that the shaft spins in. In the most common design the shaft does not run on metal at all. It floats on thin films of oil.
That oil is the engine’s own, and it has two jobs: it lubricates the bearings and it carries heat away. This is why turbocharged engines are fussy about oil. Modern oil standards include a test for deposits that form at high temperature, mainly to stop turbo bearings seizing. The advice from the engineer who wrote that up is simple: use only the grade of oil in your owner’s handbook.
Heat is still a problem after you switch off. When a hot engine stops, heat soaks from the turbine into the centre housing, and it can bake the oil left inside into hard deposits. Some turbos have a water-cooled centre housing, which uses the engine’s coolant to draw that heat away.
The intercooler
After the compressor, the air usually passes through an intercooler, which engineers also call a charge air cooler. It cools the compressed air. Cooler air is denser, so even more of it fits into the cylinder.
Fitting one can change the shape of a car. When the Porsche 911 Turbo gained an intercooler in 1978, it needed a bigger rear spoiler to make room for it. That spoiler is the tea tray, number three in my ranking of the coolest Porsche spoilers.
The wastegate and the blow-off valve
A turbo has to be kept in check. Two valves do that job.
- The wastegate is on the exhaust side. At a preset level it opens and lets some of the exhaust gases bypass the turbine. That stops the turbo spinning too fast or making more pressure than the engine can take.
- The blow-off valve is on the air side. When the driver lifts off the accelerator and the throttle snaps shut, it releases the pressure that has built up, which protects the compressor.
Turbo lag, and how engineers fight it
Turbo lag is the delay between pressing the accelerator and feeling the extra power. The engine has to produce enough exhaust gases to spin the turbine before the boost arrives. The wheels have weight and take time to speed up, there is little exhaust flow at low engine speeds, and a bigger turbo, which gives more power at high revs, is slower to respond at low revs.
Engineers have found several ways round it:
- Variable geometry. Adjustable vanes inside the turbine change the flow of exhaust gases to suit the engine’s speed and load.
- Twin-scroll. The exhaust gases are kept in two separate streams on their way into the turbine, so they do not interfere with each other.
- Ball bearings. These have less friction than oil-film bearings. Garrett says its ball bearing turbos spin up about 15 per cent faster.
- Electric assistance. Garrett’s E-Turbo has a small, high-speed electric motor on the shaft between the two wheels. It helps the compressor get up to speed sooner, and it can also work as a generator and put electricity back into the car.
Why small engines get turbos
For years a turbo was something added to make a fast car faster, as BMW did with the 2002 Turbo in the history of BMW. Now the more common use is to let a small engine do the work of a big one, which the American Environmental Protection Agency calls “turbo-downsizing”. Its figures show the shift: before 2010 most turbocharged engines in the United States were in large eight-cylinder performance cars, but by the 2013 model year more than 60 per cent of turbochargers were fitted to four-cylinder engines.
Britain has an example of its own in Ford’s 1.0 litre three-cylinder EcoBoost, which was developed in the UK. There is more on that in the history article.
Made in Huddersfield
Garrett, the maker I have quoted most here, is American. Another famous turbo name comes from Yorkshire.
Holset Engineering was founded in Huddersfield on 29 March 1952 by two business partners, Brian Holmes and Paul Croset. It began making turbochargers there in 1958. The American diesel engine maker Cummins bought the company in 1973, and it is now called Cummins Turbo Technologies.
When the company marked its 60th birthday in 2012, the Huddersfield Examiner reported that about 40 per cent of the world’s trucks had a Holset turbocharger, and that around 1,100 people worked at its Huddersfield site.
Quick questions
What does a turbo do? It pumps extra air into the engine so that more fuel can be burned, which makes more power from the same size of engine.
What drives a turbo? The engine’s own exhaust gases. They spin a turbine, which is joined by a shaft to the compressor that pumps the air.
How fast does a turbo spin? Garrett says its smallest turbos spin at more than 300,000 revolutions per minute.
What does an intercooler do? It cools the air after the turbo has compressed it, which makes the air denser.
What is a wastegate? A valve that lets some exhaust gases bypass the turbine, so the turbo cannot spin too fast or make too much pressure.
What is turbo lag? The delay between pressing the accelerator and feeling the extra power, while the exhaust gases spin the turbine up to speed.
What is the difference between a turbo and a supercharger? Both pump extra air into the engine. A supercharger is driven by the engine itself, usually by a belt from the crankshaft, so it responds straight away but uses some of the engine’s power to turn it. A turbo is driven by the exhaust gases, so it uses energy that would otherwise be wasted, but it suffers from turbo lag.
Test yourself
Ten questions on this story. Try it now, then come back in a week and see what stuck.
The British Car Scene
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Sources
Open the sources: all 10 links ▾
- Garrett Motion: How a turbocharger works
- Garrett Motion: Turbo Tech 101
- Garrett Motion: 8 facts about Garrett turbocharged engines
- Garrett Motion: Turbo lag, causes, effects and solutions
- Garrett Motion: the E-Turbo
- Cummins: Turbo componentry
- Dr Edward P. Becker, “The challenges of turbocharging”, Tribology and Lubrication Technology, June 2014
- Huddersfield Examiner: Look back at 60 years of Holset
- Supercharger, Wikipedia
- The pros and cons of turbochargers vs superchargers, Car Throttle
