Autocar UK – 31 July 2019

NEWS

3 1 JULY 2019 AUTOCAR.CO.UK 17

FIRE RISK SPARKS VOLVO RECALL

Volvo has recalled 507,000 cars globally, including 69,616 in

the UK, over concerns that a faulty engine component could

cause a fire. The issue affects cars built between 2014 and 2019

and using the firm’s four-cylinder 2.0-litre diesel engine. The

Swedish firm is in the process of contacting affected owners.

BMW X5 is the epitomy

of the aspirational

premium large SUV

WHEN YOU PULL into your local fi lling

station, chances are there will be two types

of petrol on offer: the cheaper premium

grade and pricier super unleaded. So what’s

your poison? Go for the super just because

it sounds like a good idea, or be thrifty and

stick with the premium?

The most signifi cant difference between

the two is the research octane rating (RON)

of the petrol. The octane rating tells you

how resistant the fuel is to detonation,

known as knocking or pinking. In a petrol

engine, petrol is mixed with air, then it’s

compressed and ignited by a spark. When

that happens, the mixture burns outwards

from the point of ignition like a grassland

fi re (but faster). The burn should be

smooth and controlled, but if the mixture

is compressed too much, random pockets

of the mixture spontaneously detonate too

early. It’s audible and can make a sound like

dried lentils being poured into a tin can, or a

diesel-like knocking noise.

A key way of increasing a petrol engine’s

performance is by raising the compression

ratio, or in a turbocharged engine

increasing the boost – or both. Either one

increases pressure inside the combustion

chamber when the fuel ignites. In older

engines, the threshold at which detonation

became a threat had to be carefully

managed by engine designers and tuners,

e s p e c i a l l y w he n t u rb o s c a me a lon g. T he n

back in 1982, Saab’s engine genius, Per

Gillibrand (known as ‘Mr Turbo’), dreamed

up Automatic Performance Control. APC

listened for the onset of knock by using a

microphone attached to the cylinder block

• a knock sensor – and monitoring boost

pressure and engine revs.

Today petrol engines use similar anti-

knock systems, but thanks to much faster

processors in engine computers they can

also use algorithms to predict when knock

will occur. Naturally aspirated engines

delay the point at which combustion is

triggered (retarding the ignition) if knock

threatens, all of which brings us back to the

question of whether you need to fork out the

extra dosh for super unleaded. The answer

is, there’s only one real reason to and that

WORST CASE SCENARIO

is because your

car has a high-

performance

engine or the

handbook explicitly

says you should use

it. Using fuel of a higher

octane than your engine needs or can

benefi t from won’t hurt it, only your wallet.

The difference between premium and

super unleaded these days is a maximum

of two points (97 octane versus 99) and the

chance of a modern engine being damaged

by the lower of the two is nil. However, the

engineers calibrating higher-performance

engines and chasing the best performance

numbers are likely to have done so using the

highest-octane pump fuel available.

The higher octane allows the engine

to use a higher boost pressure and more

aggressive settings to pump out a little more

power. With the lower octane, it may back

off those settings a tad to stay below the

knock threshold. Whether you can notice

the difference subjectively, though, is down

to how attuned you are to your car.

Extreme cases of detonation can badly

damage an engine. This cylinder head from

a very highly tuned competition engine

looks like it has been nibbled by rats. Rest

easy, though, because there’s no chance of

anything like this happening to a production

car by choosing premium petrol over super

unleaded at your local garage.

PERPLEXED AT THE PUMPS?

OCTANE RATINGS EXPLAINED

Here, fuel

and air are

microseconds away

from being compressed and

ignited in a direct-injection

petrol engine. How much

it can be compressed

depends on the octane

rating of the fuel.

UNDER

THE SKIN

JESSE CROSSE

STARTING AT THE TOP

It is no surprise that the big

winner in recent years has

been the premium compact

crossover, as premium

brands originally started at

the top of the SUV tree.

For example, the first

BMW SUV was the X5 and the

Volvo’s first was the XC90,

with the companies then

working down to the X1 and

XC40 respectively. Buyers

on a budget of £30-40,

love the idea of an affordable

crossover with the kudos of

their big brothers.

Non-premium large

crossovers are the only SUV/

crossover segment to have

peaked – or gone ‘ex-growth’

in marketing jargon. There

is no precedent anywhere

in the market for a non-

premium segment to recover

share against premium

competition.

2016

Compact premium SUV (e.g. Volvo XC40) Medium premium

SUV (e.g. Volvo XC60) Large non-premium SUV (e.g. Ford Edge)

2017 2018 2019 Q

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CLASS WAR: HOW COMPACT SUV SALES STACK UP

5 4 3 2 1 0