The speed and power secrets of car turbochargers
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Turbochargers are so common in today’s cars that it’s rarely worth mentioning them. Except in a few high-performance cases, telling someone that your car has a turbo has about the same effect as talking about the radio or air-conditioning. The lack of these things is more surprising than their presence.
Things weren’t always this way. For a long time there were no turbo cars at all, and later there was a period when turbocharging an engine was just about the most exciting thing you could do to it.
This was all about performance, and that’s still an element today. In recent years, however, manufacturers have been fitting turbochargers principally to improve their cars’ fuel economy and CO2 emissions. What follows is an explanation of how we came to this point.
Supercharging
Internal combustion engines can’t work without air. Air is drawn into the space created by each piston moving down its cylinder when the intake valves are open.
There’s a limit to much air can be introduced without external help. That help is provided by a compressor which pushes air into the cylinder. The process is known as supercharging, and the compressor, of whatever type, is called a supercharger.
The special case of the turbocharger
What we now call superchargers are driven by the engine itself. Turbochargers are driven instead by exhaust gases which are produced regardless of whether the engine is supercharged or not.
The driving force of the exhaust gases therefore amounts to free power, and we all want that.
Another advantage is that the word ‘turbo’ is quite sexy. As an engineer once told us, you’d be much more likely to buy an aftershave named Turbo rather than one called Exhaust Gas-Driven Supercharger.
A problem with turbos
Because they are driven by the engine, superchargers react immediately to a driver’s call for power. Turbos work only when the exhaust gases are going quickly enough, which they won’t be if the engine is either not under load or running at very low rpm.
These are the main reasons for the turbo lag which made early turbocharged cars very difficult to drive. You would press the accelerator expecting power which didn’t actually come in until some time later.
Developments in both turbo and engine technology have almost entirely solved this problem. In most road cars at least, lag is no longer an issue.
Early turbo cars
General Motors was selling turbocharged cars in the US as early as 1962, but they didn’t become popular in Europe until the following decade.
BMW launched its 2002 Turbo in 1973, which was a pity because that was also the year of the global oil crisis. Sales of a very uneconomical car, no matter how quick it was, were bound to suffer.
Saab had better luck with the less powerful 99 Turbo, introduced five years later. It was very quick for its day, though a combination of front-wheel drive and turbo lag, neither of them fully sorted in the late 1970s, made it quite a handful.
Turbos in F1
F1 regulations introduced in 1966 allowed teams to run with 3-litre unsupercharged engines or 1.5-litre supercharged ones. No one gave any thought to the second possibility for more than a decade until Renault, which had already used turbos very successfully in sports car racing, decided to give it a go.
Its first F1 car, the RS01, was very powerful but had horrible turbo lag and produced so much white smoke every time it blew up (which it did a lot) that rivals nicknamed it the Yellow Teapot. Renault persevered, though, and became the first team to win a Grand Prix in a turbo car in 1979.
Soon, almost everyone was running turbos. Power outputs rose alarmingly, and after several attempts to bring them back down to acceptable levels the governing body banned them in 1989. As part of an attempt to make F1 cars more relevant to road-going ones, they were made compulsory in 2014.
Turbocharging transforms rallying
In 1980, Audi began the process of turning rallying upside down by competing for the first time in the quattro. The quattro’s most significant feature was four-wheel drive, but it also had a turbocharged engine which produced around 300bhp in the early days and a great deal more later on.
These were startling figures for the sport at the time, and rival manufacturers had to match them. Unlike in F1, turbocharging was never banned, and it has become an integral part of rallying. Since 1984, every single Drivers’ and Manufacturers’ World Rally title has been won using a turbocharged car.
Turbo diesels
Diesel cars were on sale to the public as early as the 1930s, and were admired for their fuel economy, but as the years wore on they developed a reputation for being very slow.
The answer to this was to turbocharge them. The first production turbo diesel is believed to be the 1978 Mercedes 300SD (a version of the original S-Class) and other manufacturers soon followed.
Turbo diesels made it possible to enjoy considerable performance without using high revs. If a turbo diesel engine and a petrol one have the same maximum power output, the diesel will produce it at much lower rpm, making it easier to drive.
Turbocharging and supercharging
There have been attempts to combine the benefits of turbochargers and mechanical superchargers by fitting both to the same engine. In each case the supercharger provides more power at low revs while the turbo does so at higher ones.
Lancia tried this with the Delta S4, built in just enough numbers as a road car to allow Lancia to go rallying with it. The S4 won its very first event in 1985 and three more the following year before the Group B class in which it ran was banned.
The Volkswagen Group developed a ‘twincharged’ 1.4-litre engine and used it in a large number of Audis, SEATs, Skodas and Volkswagens. It won several categories in the International Engine of the Year awards all the way from 2006 to 2014.
Two or more turbos
Large turbochargers help an engine produce more power than smaller ones but also take longer to get up to speed and therefore create turbo lag.
There are various ways round this. One is to fit two turbos. These may be the same size, but often the manufacturer uses a small one for quick response and a large one for big power outputs.
Several manufacturers have done this. BMW went a step further a few years ago with a three-turbo diesel fitted to the X6 M50d, and has since developed another diesel unit with four turbos, the same number also used for the Bugatti Chiron supercar.
The rise of the small engine
Turbocharging has made possible a reduction in the engine sizes of mainstream cars over the last decade. First Volkswagen and then Fiat started to offer small engines with the same power output as, but better fuel economy and CO2 emissions than, the larger non-turbo ones they had fitted previously.
This was a big deal when CO2 emissions were related to Vehicle Excise Duty charges (as they still are in many countries, though no longer in the UK). The EU test requires very little power to complete, so in cars with turbocharged engines the turbos hardly work at all.
This means they get very good economy and CO2 figures on the test which may not reflect real-world situations. VED payments, where applicable, are therefore lower than they might be.
The most obvious result of this nowadays is the great number of one-litre three-cylinder turbo petrol engines on the market. They’re quite powerful but they give very good official figures. Ford even one uses its EcoBoost version in the Mondeo, a spectacular case of a large car (it’s 16 feet long) with a tiny engine.