What is a turbocharged engine?

They improve MPG and power compared to normally aspirated models, but what exactly is a turbo engine?

John Evans
Oct 8, 2021

If you’re a certain age you’ll remember the word ‘turbo’ being emblazoned down the sides of some performance cars, and then the phase where the word was put on any old bit of merchandise to make it seem cooler. It might not mean anything when written on a pair of sunglasses, but a turbo is quite useful on an engine.

Think of an engine like it’s a bonfire. Once the embers are going, you’ll notice that a lungful of air is what’s needed to get it burning into a fully fledged blaze - and that’s the effect a turbocharger has on an engine. It increases the volume and speed of the air entering the engine, so that it produces more power. Here’s what you need to know about a turbo.

How does a turbo work?

Not surprisingly, given what it does, it looks a bit like a hairdryer, and it sits under the bonnet. Air is drawn in from the outside at one end, and as it travels along, is compressed by an impeller spinning at very high speeds. The vast quantity of high-speed air that it produces exits the impeller and shoots down the other end of the turbo into the engine, where it mixes with fuel to create a bigger bang and more power.

That’s pretty clever, but cleverer still is the fact that the impeller is powered not by the engine, but by the exhaust gas produced when the fuel and air detonate. Essentially, a turbo turns waste energy into power.

Which engines use a turbo?

For many years, turbocharging was mainly used in petrol engines. However, as diesel engines caught on, it began to be used in these, too.

Today, nearly all diesel engines are turbocharged. Meanwhile, petrol turbocharging is now making a big comeback. Even big SUVs often come with small turbo petrol engines these days, as the engines are now efficient and powerful enough to propel larger vehicles.

Why are car makers offering more petrol turbo engines?

Car makers are under pressure to reduce vehicle emissions and improve fuel consumption. Previously, turbocharging was used to make cars go faster and in the process burn more fuel.

Today, however, it’s used to make small engines more powerful at low engine speeds. As a result, they burn less fuel and are cleaner and more economical, yet feel more powerful at normal driving speeds.

They feel punchier because the power is available sooner. In a non-turbo car the maximum power is usually right near the top of the rev limiter, whereas a turbo car will produce its maximum power much lower down the rev range.

How do I recognise a turbocharged engine?

Turbocharged engines go under a variety of names. Audi, Volkswagen, Seat and Skoda turbodiesels are often badged TDI, short for Turbocharged Direct Injection. Other companies use CRDi, BlueHDi and a simple 'd' suffix after the model name, as with Mercedes and BMW.

One of the best known, small petrol turbos is Ford’s family of EcoBoost engines, found on models including the Fiesta and Focus. Renault’s TCe family of turbo petrol engines, found in the Clio and Captur, is another - you can find these TCe engines in some Dacias too. Nissan badges its turbo petrols DIG-T, expect to find these in the Juke and Qashqai crossovers.

In truth, these days it’s probably easier to say which cars aren’t turbocharged. You won’t find many city cars with turbos, and the cheapest engines in superminis - cars like the Seat Ibiza and Vauxhall Corsa - tend to be naturally aspirated, another way to describe non-turbocharged engines. Mazda’s petrol engines aren’t fitted with a turbo (although these days many feature mild hybrid assistance), and some prestige and performance cars don’t have turbochargers because they have big engines.

Ferrari and other similar companies resisted turbocharging for as long as possible because turbo engines don’t make quite as nice a noise as naturally aspirated ones.

How much more powerful are turbocharged engines?

Power outputs differ between models, but taking the Ford Fiesta as an example, its least powerful 1.0 EcoBoost turbocharged petrol engine produces 100hp, but a lot more pulling power, called torque, which you feel as a shove in the back at very low engine speeds. It makes light work of overtaking and can go from 0-62mph in 10.5 seconds. There’s a version of the same engine, admittedly with a little bit of mild hybrid electrification, that makes over 150hp.

The alternative, the Fiesta’s new non-turbocharged 1.1 Ti-VCT petrol engine, produces 85hp and much less torque at far higher engine speeds. The result is that you have to push the accelerator harder and use the gearbox and revs more to make decent progress. It takes 14.0 seconds to reach 60mph.

What kind of economy gains can I expect from a small turbocharged petrol engine?

Official fuel economy figures for small turbocharged petrol engines tend to flatter them. In the real world, they are only slightly more economical than their equivalents without a turbocharger.

The point is, though, that not only are turbocharged petrol engines more economical but as we’ve seen they are also more powerful, right where you need it in the mid-range when overtaking or cruising at motorway speeds.

According to the Equa Index of real-world fuel economy, Ford’s new non-turbocharged 1.1 Ti-VCT 85PS engine averages 42.3mpg. Its turbocharged equivalent, the more powerful 1.0 EcoBoost 100PS, does 45.2mpg.

What are the road tax gains for turbo engines?

Because they produce lower emissions, small turbo petrol engines cost slightly less to tax, at least in their first year before rates become standardised from year two.

For example, the Fiesta 1.0 EcoBoost 100PS emits 106g/km CO2 and costs £145 to tax in the first year, or £20 less than the Fiesta 1.1 Ti-VCT 85PS.

Do turbocharged engines cost more?

Their economy and performance advantages come at a price, although not as high a one as you might think. The Ford Fiesta Zetec 1.0 EcoBoost 100PS 3dr costs £15,815, £600 more than the equivalent 1.1 Ti-VCT 85PS. To make back that small difference in fuel economy you’d have to drive 67,000 miles in the EcoBoost.

On the other hand, the EcoBoost is likely to be worth more money when you sell it because of its more appealing engine and greater performance. And there's the fact that the more powerful EcoBoost engine should make for a nicer drive when it comes to overtaking and accelerating up to motorway speeds.


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