What is forced induction and how does it work?
The concept of forced induction dates back to 1889, but just how do a turbocharger and supercharger work?
Updated 16 11 2021
IT WASN’T ALL THAT long ago that turbochargers were only used on diesel engines in trucks and buses… and aeroplanes. Then engineers figured they’d be a great way of getting more oomph out of petrol engines for performance cars, and now they feature in everything from small city cars, to big SUVs.
How long have turbochargers been around?
The person credited with the ‘concept’ of the turbocharger was Swiss engineer Dr Alfred Buchi, and his patents date back to 1904. Dr Buchi had a working diesel engine with a turbocharger up and running by 1911, and in 1915 he patented a turbocharger design that wouldn’t be realised until the 1980s.
Dr Buchi was clearly ahead of his time, because turbochargers weren’t used in passenger vehicles until after World War II. Much of the development during the 1950s and ’60s surrounded the use of turbochargers on diesel engines for use in buses and trucks. Plenty of companies during this time began further refinement of turbochargers for use in motorsport.
It wasn’t until 1976 and the release of the Saab 99 (pictured above) that a volume-production road car was released with a turbocharged petrol engine. And from the 1980s onwards most car companies around the world either had prototype or production turbocharged cars in their stable.
Superchargers, which are from the same family as turbochargers (see below), as a concept were thought up by Gottlieb Daimler who suggested pressure charging on his original patent for his ‘car’ in 1889. By 1893, Daimler had built a piston-driven supercharger. Both turbo and supercharges are known as FI, or “forced induction” engines.
How does a car engine work?
A car engine works by drawing air into a cylinder, compressing the air with its piston, adding fuel and then making a controlled explosion which powers the piston down, rotating a crankshaft and ultimately turning the wheels. Basically, fuel + air = power. The ratio of fuel to air is constant, so the more air you have, the more fuel you can add, and the bigger the explosion and the greater the power.
Turbochargers and supercharges compress the air that goes into the engine, so more air fits into the same space in the cylinder. That means you can add more fuel – keeping the fuel/air ratio constant – and therefore get more power.
This is why small turbo engines can produce the same power as larger non-turbo (known as “naturally asipriated”, or NA) engines. The turbo draws in a large amount of air and compresses it into a small space. The NA engine draws in a large amount of air into a larger space but doesn’t really compress it.
The difference between a turbocharger and supercharger is simple. A turbocharger has its compressor driver by a turbine (“fan”) that is spun by the exhaust gases. A supercharged is directly driver by a belt from the engine.
Turbochargers and superchargers are from the same family
The word turbocharger is actually an abbreviation of the word turbo-supercharger. Indeed, a turbocharger is from the same family as a supercharger – it’s all about “forced induction” (FI), or forcing more air into the engine to allow for a bigger, controlled explosion in the cylinders and thus more power. Indeed forced induction, either by turbocharging or supercharging can increase power and torque by around 40% and 30%, respectively.
Unlike a turbocharger which relies on exhaust gasses to power/spin the turbine (a fan), a supercharger is directly connected to the crankshaft, usually by an accessory belt. This belt wraps around a pulley and a drive gear which helps to turn the compressor gear. Once the compressor is spinning (at up to 50,000-plus revolutions per minute) there is air being forced into the engine. Thus more fuel can be injected, thus more power and torque can be produced because of the bigger bang.
How does a turbocharger work?
Like a supercharger, a turbocharger is designed to increase the amount of compressed air into an engine. Think of it like this, a turbocharger is, more or less, like a mouth blowing air onto a fire to get a flame going.
Typically, both a turbocharger and supercharger will pump around eight pounds per square inch (8psi) of compressed air into the engine. This is around twice normal atmospheric air pressure which is 14.7psi. This means, you can generally accept to increase performance of an engine, via turbocharging, or supercharging, by around 40% or a little less (there are efficiency losses during the process).
Unlike a supercharger which uses ‘drive’ from the engine to power its turbine, a turbocharger uses exhaust gases to spin its turbine/fan which can spin at speeds of up to 200,000rpm – the more exhaust gases you pump into the turbine, the faster it will spin. The pressurised and compressed air produced is then forced into the cylinders which, along with more fuel being injected allows more power to be produced because of the bigger explosion.
If you’ve driven an older turbocharged petrol car you’ll have felt something called ‘lag’, basically this describes the time it takes for the turbocharger to begin spinning and forcing air into the engine. It gave you a feeling of pressing the throttle and then nothing, nothing, nothing, and then EVERYTHING. Older turbochargers tended to be larger and thus slower to spin up. Modern units, on the other hand, are smaller, have reduced friction because of new engineering processes and materials used and thus start spinning sooner. Go too small with the turbocharger, though, and it can become its own worst enemy, spinning too fast as engine speeds increase. Some car makers have taken to fitting their engines with two turbochargers (twin charging), a smaller one to provide boost at lower speeds and a larger one that takes over as speed increases.
What about a turbocharged diesel engine?
Both petrol and diesel engines are what we call internal combustion engines. But the combustion is produced in different ways. As diesel has a much higher burning point than petrol, a diesel engine relies on the compression of the fuel and air mixture, inside the cylinder, anywhere from 14 to 20 times its original volume (a petrol engine tends to compress its fuel and air mixture much less than that). This compression causes a spontaneous combustion and forces the piston back down. No spark plug required. That said, older model diesel engines used to feature glow plugs to pre-warm the air in the cylinders. Modern turbo-diesel engines don’t require glow plugs.
Like in a petrol engine, turbocharging a diesel vehicle allows for more air to be pumped into the engine and can often increase output by around 50% compared to a non-turbocharged diesel engine of the same size and technology. Fuel consumption can also be reduced.
Diesel engines are more efficient the hotter they get and so it’s usually only drivers of diesels who do longer distance driving that realise the efficiency benefits. Because of their design and combustion methods, diesel engines tend to produce their power and torque lower in the rev range (below 3000rpm) than an equivalent petrol car which tends to produce peak power and torque higher in the rev range (above 3000rpm). This means diesel-powered cars can actually feel ‘gruntier’ and more fuel efficient (once hot) than their petrol siblings.
What’s the benefit of a turbocharger?
There are widely considered to be three basic benefits to turbocharging an engine. And these are that a relatively small engine, and some makers are turbocharging engines as small as 1.0-litre with three-cylinders, can make much more power than without the turbocharger. As well, using a turbocharger doesn’t add much to the weight of the engine and unless the performance is being used, a turbocharged engine isn’t that much thirstier than a naturally-aspirated (non-turbo) engine of equivalent size and output. And that’s because turbochargers rely on reusing exhaust gasses to spin the fan that aren’t in either naturally aspirated or supercharged engines. It’s also because turbocharging allows for a smaller engine to be used in the first place.
What are the disadvantages of a turbocharger?
Turbochargers often suffer from lag, although this is becoming less of a ‘thing’ with modern turbocharged cars. Turbochargers also tend to operate within a specific RPM band only, and they can get quite hot often requiring extra plumbing to pump engine oil around them to keep them cool; this means turbocharged engines can sometimes be a little heavier on oil than non-turbocharged engines.
What’s a naturally aspirated engine?
Any car without forced induction (turbocharging or supercharging) is referred to as naturally aspirated. And this means, the engine relies on atmospheric pressure and the vacuum caused by the cylinder movement to suck air into the engine.
Are there different types of turbocharger?
Yes, most turbocharged cars use just one turbocharger, known as single turbo. Some engines feature two, twin-turbo (BMW is a particular fan) which uses a smaller turbo lower in the rev range and another for higher in the rev range. Some engines run parallel twin-turbochargers, which is also called bi-turbo.
Then there are twin-scroll turbochargers (most turbochargers are a single scroll) which means the exhaust gases are divided, to allow one set of cylinders to feed one scroll and another set to feed the other scroll, which maintains pressure in the turbocharger regardless of the cylinder firing sequence. The next type of turbocharger is a variable geometry turbocharger, which allows the turbocharger to perform consistently throughout rev range, effectively adapting to more or less pressure. It makes for less lag and smoother power delivery.
The next phase in turbocharger development will be electric turbochargers which allow for an electric motor to be connected to the compressor wheel in the turbocharger to provide power when there’s insufficient exhaust gasses, eliminating lag and allowing a turbocharged engine to produce plenty of low-end torque off idle.