What is a turbocharger?

19 October 2015
 by 
, CarsGuide

Outside of the rarefied air of supercar world, where Lamborghini still insists that naturally aspirated engines remain the most pure and Italian way to produce power, and noise, the days of building cars without turbochargers attached are coming to an end.

It’s impossible, for instance, to get a Volkswagen Golf with a naturally aspirated engine. In the wake of “Dieselgate”, of course, this will hardly matter, because no one wants to play Golf any more.

The fact remains, however, that city cars, family cars, grand tourers and even some super cars are jumping ship in favour of a scuba-powered future. From the Ford Fiesta to the Ferrari 488, the future belongs to forced induction, partly due to emissions laws, but also because the technology has come along in huffs and bounds.

It’s a case of small-engine fuel economy for gentle driving and big-engine power for when you want it.

When it comes to combining higher performance with lower fuel consumption, engineers are almost compelled to design their newest engines around turbocharging technology.

How can a turbo do more with less?

It all comes down to how engines work, so let’s get a tiny bit technical. For petrol-powered engines, an air-to-fuel ratio of 14.7 to 1 ensures a complete burn of everything in the cylinder. Any more juice than that is a waste of fuel.

In a naturally aspirated engine, the partial vacuum created by the descending piston draws air into the cylinder, using the negative pressure inside to suck air through the intake valves. It’s a simple way to do things, but it’s heavily limited in terms of air supply, kind of like someone with sleep apnoea.

In a turbocharged engine, the rulebook is rewritten. Instead of relying on the vacuum effect of the piston, a turbocharged engine uses an air pump to force air into the cylinder, just like a sleep apnoea mask pumps air up your snout.

While it’s possible for turbochargers to compress air at anything up to 5 bar (72.5 PSI) above standard atmospheric pressure, they tend to run at a more sedate 0.5 to 1 bar (7 to 14 PSI) in road cars.

The practical upshot is that with 1 bar of turbo pressure, the engine is getting twice as much air as it would if it were naturally aspirated.

This means the engine-control unit can inject twice as much fuel while still maintaining an ideal air-to-fuel ratio, creating a much bigger bang.

But that’s only half of the turbocharger’s tricks. Let’s compare a 4.0-litre naturally aspirated engine and a 2.0-litre turbocharged engine with 1 bar of boost, assuming that they’re otherwise identical in terms of tech.

The 4.0-litre will use more fuel, even while idling and under small engine load, while a 2.0 uses much less. The difference is that at full throttle, a turbocharged engine will be using as much air and fuel as possible – twice as much as a naturally aspirated engine of the same size or exactly as much as the atmo 4.0-litre.

This means the turbocharged engine can perform at any level from a miserly 2.0-litre all the way up to a thumping four-litre, thanks to forced induction.

So it’s a case of small-engine fuel economy for gentle driving and big-engine power for when you want it.

How clever is that?

As befits an engineering silver bullet, the turbocharger itself is ingenious. As the engine runs, exhaust gases pass through a turbine, causing it to spin at an incredible rate – usually between 75,000 and 150,000 times per minute.

The turbine is bolted to an air compressor, which means that the faster the turbine spins, so too does the compressor, sucking in fresh air and shoving it into the engine.

The turbo works on a sliding scale, depending on how hard you’re leaning on the accelerator. At idle, there’s not enough exhaust gas to make any meaningful speed in the turbine, but as you accelerate, the turbo spools up and delivers boost.

If you bury your right foot, more exhaust gas is being produced, which compresses the maximum amount of fresh air into the cylinders.

So, what’s the catch?

There are, of course, a few reasons why we haven’t all been driving turbocharged cars for years, starting with complexity.

As you can imagine, engineering something that can rotate at 150,000rpm, day after day, for years, without exploding, isn’t easy, and it requires expensive parts.

Turbos also need a dedicated supply of oil and water, increasing the load on the engine’s lubrication and cooling systems.

Because air heats up in the turbocharger, manufacturers have also had to fit intercoolers to reduce the temperature of the air going into the cylinder. Hot air is less dense than cool air, which negates the benefits of the turbocharger, and can also cause damage and pre-detonation of the fuel-air mixture.

The most infamous drawback of turbocharging, of course, is known as lag. As discussed, you need to accelerate and create exhaust to get a turbo to start producing meaningful boost pressure, which meant that early turbo cars were like a delayed light switch - nothing, nothing, nothing, EVERYTHING.

Various improvements in turbo technology have tamed the worst of the laggy nature of early turbo Saabs and Porsches, including variable vanes in the turbine that move according to exhaust pressure, plus lightweight and low-friction components to reduce inertia.

The most exciting step forward for turbocharging is only found – for now, at least – on F1 racers, where a small electric motor keeps the turbo spinning, reducing the amount of time it needs to spool up.

Similarly, in the World Rally Championship, a system known as anti-lag dumps a fuel-air mixture directly into the exhaust, ahead of the turbocharger. The heat of the exhaust manifold makes it explode, even without a spark plug, creating exhaust gases and keeping the turbocharger on the boil.

But what about turbodiesels?

When it comes to turbocharging, diesels are a special breed. It really is a case of hand in glove because, without forced induction, diesel engines would never be as prevalent as they are.

Naturally aspirated diesels can produce a fair hit of low-down torque, but that’s where their talents run out. With forced induction, however, diesels can capitalise on their torquey nature and enjoy the same benefits as their petrol cousins.

Diesel engines are built Tonka-tough to deal with the huge stresses and temperatures contained within, which means that they can bear the added pressures of turbocharging easily.

All diesel engines – naturally aspirated and forced-induction – work by burning fuel in an overabundance of air in what’s known as a lean-burn system.

The only time that naturally aspirated diesel engines get near a ‘perfect’ air-to-fuel mixture is under full acceleration, when the fuel injectors are wide open.

Because diesel is less volatile than petrol, burning it without an abundance of air creates huge amounts of soot, otherwise known as diesel particulates. By flooding the cylinder with air, turbodiesels can avoid this problem.

So, while turbocharging is an amazing improvement for petrol engines, its real coup de grace is saving the diesel engine from becoming a smoky relic. Although Dieselgate might just cause that to happen anyway.

How do you feel about turbochargers finding their way into pretty much anything on four wheels? Tell us in the comments below.

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