What is a turbocharged engine?

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

John Evans
Sep 10, 2018

Ever started a fire? Or seen Bear Grylls start one? 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.

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.

What 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.

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.

How do I recognise a turbocharged engine?

Turbocharged engines go under a variety of names. Diesel turbos are often badged TDI or TDi, short for Turbocharged Direct Injection.

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. Nissan badges its turbo petrols DIG-T, expect to find these in the Juke and Qashqai crossovers.

How much more powerful are turbocharged engines?

Power outputs differ between models, but taking the Ford Fiesta as an example, its least powerful 1.0T EcoBoost turbocharged petrol engine produces 100PS, 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.

The alternative, the Fiesta’s new non-turbocharged 1.1 Ti-VCT petrol engine, produces 85PS and much less torque at far higher engine speeds. The result is that you have to push the accelerator harder and use the gearbox 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 does 42.3mpg. Its turbocharged equivalent, the more powerful 1.0T 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 1.0T 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 T EcoBoost 100PS 3dr costs £15,815 and the 1.1 Ti-VCT 85PS, £15,215, or £600 less. To make back that difference 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.


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