turbo 1Turbocharging was once synonymous with smoking rubber and race driver wannabes but in recent years it has become a major tool in saving fuel and reducing emissions. Mothers, pensioners and librarians are driving them, frequently with no idea that they’re on turbo boost.

This is clever technology but it’s only recently that turbos have become sophisticated. They should not be confused with superchargers even though the principles are the same. Superchargers became famous with the ‘blower’ Bentleys of the 1920s and during the Second World War when they were fitted to fighter planes, allowing them greater speed at higher altitudes where the air is thin. This is why  British Spitfires, with supercharged Merlin V12s, could dive out of the sun onto German aircraft that couldn’t reach the same altitudes.

Both superchargers and turbochargers pump more air into the cylinders for a bigger bang when the spark plug fires. A bigger bang means more power and more torque, or pulling power. But they go about the job differently. While a supercharger is a pump driven by the engine’s crankshaft, sapping some power, a turbo is driven by escaping exhaust gasses and, thermodynamically, is much more efficient. The bug is that the engine has to be revolving at about 2000 to 2500 rpm before the turbo spins up fast enough to drive extra air into the cylinders.

The experts call the delay in the engine reaching this speed turbo lag. On early turbocharged racecars, most notably the Porsche 917s of the early 1970s, turbo lag could be as long as a second or two, and when the power came on it came with a wallop. Race drivers therefore had to hit the gas for exiting a corner when they were entering it and if they got the timing wrong the results were often spectacular. It took a few years to sort things out, and then the Porsches became unbeatable. Since then turbos have become lighter and respond faster and more gently, especially the twin-scroll turbos fitted in some cars.

Designs are now so efficient that 1.8-litre turbocharged four-cylinder engines are delivering the power and performance of conventional 3-litre six-cylinder motors, and turbo sixes are matching conventional V8s. This is why many current luxury sedans run six and even four-cylinder engines rather than V8s.

What has given turbocharging its real shot in the arm was the announcement by President Obama in 2010 of new American corporate average fuel economy targets for car makers. He decreed that by the 2016 model year (about September 2015) that target would be 35.5 miles per US gallon, or 6.63 litres per 100 km across the range of cars sold in the USA. This led to a wholesale embrace of the technology by US car makers. They realised that adding turbos to their engines would give an approximate 15 per cent improvement in economy.

But while the idea may be new in the USA the Europeans have known about turbocharging for years. Fuel is expensive over there, far more expensive than in the USA or here, and roughly half of all cars and light trucks sold in Europe run diesel engines.

Diesels have heavier internal components than petrol motors and can’t respond as fast or spin up to the same speeds. While petrol motors generally reach 6000 rpm effortlessly diesels are usually at full stretch around 4000. This means they are always a bit on the breathless side. Turbocharging gets them breathing more deeply and modern European turbo diesels now drive more like petrol motors.

Turbocharging is why we’re seeing so many small cars with engines that, even a few years ago would have been regarded as unworkably small. The latest Citroen C4 has a three-cylinder 1.2-litre that’s more powerful than the 1.6-litre four-cylinder it replaces, as well as being quicker to accelerate. But most importantly it uses significantly less fuel; the old one had an official fuel economy figure of 6.9 litres per 100 km, the new one is down to 5.1.

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The first turbo patent was taken out by Swiss inventor Alfred Buchi in 1905 and it was the aircraft industry that gave turbocharging its start, recognising the device’s potential at altitudes where the air is thin. In 1919 American company General Electric successfully added a turbo to an aircraft engine, powering a biplane to a world record altitude of 28,500 feet.

Among car makers Chevrolet started limited use of them in the early 1960s and in 1973 BMW unveiled the 2002 Turbo, but had problems with reliability and fuel usage. Turbos started appearing in Porsche’s road cars in 1975 and in 1978 Saab became the first high volume car manufacturer to make a go of the technology.

Published December 2015

 

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