Turbocharging: How Forced Induction Could Induce a Revolution in Sustainable Transportation

by Matt Bryan

People in suits who like spreadsheets and graphs with constant growth have stolen and corrupted a new word for their library of business jargon: turbocharge - to make something more powerful or quick. As a result of this, the general population as well as the corporate world like to slap ‘turbo’ on anything to denote a heightened sense of excitement, but as a petrolhead, and more importantly a relentless pedant, I’d like to present a brief history of the engineering masterpiece that is the automotive turbocharger.

The common or garden internal combustion engine has been around for almost two millennia, and over 130 years coupled to four wheels and a gearbox; simply, it functions through the combustion of fuel vapour in a cylinder to push down a piston which turns the crankshaft and eventually the wheels. A critical part of this cycle is the preparation of the fuel by mixing it with air, and in a naturally aspirated or typical engine, the air rushes in through an air intake, usually in the front grille of the car to funnel in the outside air at the speed the car is moving forwards. When it comes to engine performance and maximum speed and power, airflow is the name of the game. In the past, boy-racers would copy motorsport teams in removing the mesh filters from their air intakes to allow their engines to breathe properly, only to find that they served a purpose in keeping outside pollutants from entering the engine and reducing its lifespan; indeed, many Honda Civics with outrageous rear spoilers have met a grisly end choking to death in metropolitan traffic.

To that extent, the combustion engine had hit its maximum performance and the only way to produce more horsepower was to make a bigger engine that drank more and more fuel; that was until the advent of forced induction. Forced induction is the process of delivering air to the engine at a greater pressure, which has the potential to ‘turbocharge’ (sigh) the power-output of an engine. The first came in the form of superchargers (types of which include centrifugal and twin-screw), an air compressor driven by the rotation of the engine via a belt, but these had pros and cons. Superchargers require some horsepower to run, meaning that they would actually reduce the power output of smaller displacement blocks, meaning that they are only effective on big engines like Ford’s 302 V8, which at five litres, is about three times bigger than a normal European four-cylinder. Because of this, superchargers remained relatively uncommon on production cars, and their huge size meant that retrofitting lead to massive bonnet scoops and a 50s hot-rod look that wasn’t exactly subtle.

A new method of forced induction had been trialled by American aviation engineers in the 1920s, but didn’t make it into cars until the 1960s. These turbochargers were revolutionary; a turbocharger consists of two separate and sealed air chambers connected by a turbine, with engine exhaust gases rushing through one side on their way out of the car and turning a compressor for the intake air. By using the by-products of combustion, a turbo doesn’t reduce engine power like a supercharger, and is much smaller, but comes with its own host of engineering challenges. The turbine can spin up to 250,000 rpm, which is ridiculously fast (something close to 130ms^-1 or 290mph in linear terms for a turbine of radius 5mm) meaning the resulting force on the bearings and other components like oil seals is immense, and that the bearings must be constantly coated by their own supply of oil, otherwise they would just disintegrate. Coupled with the forces, compressing a huge volume of gas particles very quickly causes a rapid increase in temperature (as P∝T), which means that the intake air must be cooled by something more effective than just a car radiator, usually with a separate intercooler, which adds more complicated moving parts.

But even with all the complications, a turbocharger results in a huge increase in engine power, and this has real world advantages, not just for the motorsport niche. Before their mass adoption, consumer diesels engines were terrible; a three-litre diesel would produce around 80bhp, a third of the power of a similar petrol and with three times the fuel consumption. Diesel engines require much higher compression for the heavier fuel fraction to ignite, and so a turbocharger allows these perfect conditions to be reached much more easily, making the engine far more viable. Before turbocharging, diesels engines were reserved for tractors and tanks, but the ability to make them much smaller for the same power allowed their use consumer cars, and now their huge market share.

But diesels are souless; no petrolhead in history has ever got excited over the grumble and cough of liquid coal. They live for high-octane, high-velocity excitement of high performance engines, and the turbocharger helps sate this need in a more environmentally friendly manner. A turbo gives the same performance in a much smaller engine than the naturally aspirated equivalent; for example, the 400bhp threshold used to be the domain of four-litre V8s, but now a two-litre straight-4, an engine literally half the size, can do the same with a turbo and double the miles-per-gallon. The first real mass-market turbocharged car was the Saab 99 Turbo in 1978, producing 143bhp and 40 years later, Alfa Romeo’s Giulia Quadrifoglio set a new standard with 503bhp from a biturbo 2.9-litre V6, and marking the automotive industry’s wholehearted adoption of small-block performance engines. The idea that the turbo will save the planet is fallacy, but conserving ever decreasing oil stocks can only be a good thing.

But not everyone thinks that the humble turbocharger is the future; as they run hotter, they will statistically produce a greater volume of toxic nitrous oxides, and a faulty turbo can lead to fuel igniting too early and lead to catastrophic damage. From a performance perspective, a naturally aspirated engine has a linear torque and power curve, whereas a turbo gives an unnatural gradient, a phenomenon referred to as ‘turbo lag’. At low engine rotation, the turbo is not engaged, and when it does ‘spool up’ between 2,500 and 3,000 rpm, the rapid increase in power can be overwhelming. Turboed engines, especially diesels, feel extremely sluggish at low rpm, and seeing as modern driving style and seemingly fashionable high-ratio transmissions designed for economic motorway-miles favours keeping revs low, most turbos get hardly any use in a real world situation. Most will only ever use theirs accelerating from traffic lights or onto dual-carriageways, times when the benefits of fuel efficiency are thrown out of the window.

However, new technology has tried to remedy the issue of ‘turbo lag’, especially the new “E-Boosting” used by Audi in their SQ7. This system uses a separate 48V circuit with two electric motors to pre-spool the twin-turbos which makes the power and huge amount of engine torque instant, especially through the use of sequential turbocharging too (whereby a smaller turbo is used for low-rpm and then a larger turbo for the usual high-rev power). But these technologies come on a near £100,000 car to fix a problem that has existed for decades and asks the question on everyone’s minds: why are we still using gas-guzzlers? The electric motors used in the turbo pre-boost could be used to instantly spin up the wheels instead in a fully electric vehicle that would have a far higher and more efficient power output than a petrol. Even the more side-lined hydrogen fuel cell technology gives instant gratification on a press of the pedal. But the main issues with these supposed ‘cars of the future’ are price and practicality; EVs are coming down in price, but when they start at the same price as a fully-loaded and flashy German saloon and degradation of the batteries meaning huge depreciation, anyone can see the financial problem. As consumers, we are used to filling up a tank when it gets close to empty, and it being a simple but expensive five-minute affair; but we as a culture find it hard to adapt to the concept of 8 hour charges, especially for longer journeys. High voltage and current ‘super-chargers’ (not to be confused with actual superchargers) promise 45 minute charging, but at the expense of wrecking the lifespan on the batteries, the most expensive part of any electric car. EVs become more practical by the day, but in the meantime, fossil fuels still have their place, and the increasing popularity of the turbocharger can help ease the transition, but it can’t solve a global problem.