Turbocharging trumps supercharging in the battle for engine downsizing

Naturally aspirated engines have dominated the automotive landscape for decades. However, growing emphasis on the need to improve air quality in recent years has placed significant pressure on global vehicle manufacturers to improve fuel efficiency and ultimately reduce CO2 emissions. Not only have OEMs been subject to growing pressure from consumer groups and environmental activists, but there has also been a stronger push by governments and regulatory bodies to focus on both issues. The power output of a naturally aspirated engine is limited by its displacement and efficiency. Most naturally aspirated engines only achieve a peak volumetric efficiency of 75-80%, which means there is less oxygen available to burn the fuel. As a result, some of the fuel gets wasted, thus the lower fuel efficiency. Performance and efficiency of these engines are further impacted in conditions of low air pressure and density, particularly in elevated places or on a fast highway. Over the years, several technological advancements have helped to improve the volumetric efficiency of a naturally aspirated engine – such as using multiple valves per cylinder, adjusting valve timing by using a camshaft with higher lifts, or through other modifications such as intake manifolds with longer runners, larger carburettors, or air scoop systems. While these advancements have helped, their impact at higher engine speeds, or revolutions per minute (RPM), has been rather limited. “The need of the hour is to continue replacing naturally aspirated engines with more advanced forced induction systems” The need of the hour is to continue replacing naturally aspirated engines with more advanced forced induction systems. Forced induction is simply the process of using a mechanical system to force more air into an engine to make it more efficient. Turbochargers and superchargers are forced induction systems which overcome the limitations of atmospheric pressure by pushing more air into cylinders allowing for more efficient combustion, and thereby, leading to better fuel efficiency. Forced induction systems also help boost engine power. Engines with smaller displacement can match power ratings of larger naturally aspirated engines, and this downsizing also enables vehicle manufacturers to reduce the overall weight of the vehicle, further improving fuel efficiency. When taking a closer look at the profiles of both turbochargers and superchargers, there are clear similarities and differences between their backgrounds, relative strengths and weaknesses, fan following, and credentials to supersede naturally aspirated engines. Similar principles, different execution The turbocharger is a turbine attached to a compressor via a shaft, which draws its power from exhaust gases, and helps draw more air into the engine. It is often accompanied by an intercooler, which cools the compressed air before it is injected into the engine, and a waste gate, which maintains pressure in the exit manifold. Turbochargers, particularly early single turbo models, have a key inherent weakness: turbo lag. Significant efforts have been made to minimize or even eliminate the turbo lag over the years, such as using multiple turbos sequentially or in parallel as a ‘twin turbo’ system. The exhaust can also be divided from different cylinders, using multiple nozzles for improved exhaust scavenging as part of a ‘twin-scroll turbo’. “Turbochargers are the preferred choice of vehicle manufacturers, and are expected to witness a rise in penetration in the near-term” Then there is the use of variable vanes to adjust the air-flow to the turbine, thereby optimising boost at different engine RPMs. This is known as a Variable Geometry Turbo (VGT). An electric or hydraulic motor can also be connected to drive the turbine until it reaches its spooling speed, or in low exhaust gas situations. OEMs have also made an effort to lower rotational inertia within the turbine by using lighter materials, such as titanium-aluminium alloys and silicon nitride ceramics. Bearing systems can also be designed to reduce friction, typically by using foil or ball bearings instead of oil-based bearings. The supercharger could be considered as a half-brother to the turbocharger, similar in principle, by helping the engine draw more air. The primary difference lies in the way they draw their power. Unlike the turbocharger, a supercharger is mounted on top of, or on the side of an engine, and draws its power via a belt directly attached to the engine crankshaft. Being mechanically-driven, superchargers do not suffer from any lag; however, the traditional Roots-style supercharger, with two meshed lobes, suffers from lower efficiency at high boost levels. Superchargers have also witnessed significant technological advancements over the years, however, such as the use of three lobed pumps to improve efficiency, or the use of a centrifugal impeller to push more air into the engine, and improve efficiency at higher boost levels. Some superchargers also use screws instead of meshed lobes to compress and push the air into the engine, known as a Lysholm-type supercharger. Clearly, both forced induction systems have inherent issues with their functionalities. However, given the focus on fuel efficiency and emissions control, turbochargers seem to offer a greater number of benefits. Penetration and consumer preference Traditionally, turbochargers have been more commonly used in diesel engines. According to Honeywell, as of 2015, about 69% of light vehicles in Europe utilised turbochargers, while penetration in the USA, India, China, and Japan was 43%, 23%, 28%, and 22%, respectively. Stringent fuel economy and emissions regulations are expected to drive an increase in penetration, particularly in the USA and China. Superchargers, on the other hand, are more prevalent among high-performance and sports cars, which have high sales in developed economies such as the USA, Canada, Japan, and in Western Europe. North America has the highest penetration of superchargers, accounting for about 40% of all supercharger sales globally. In terms of growth potential, turbochargers are the OEMs’ preferred option, and are expected to witness a rise in penetration in the near-term. Various industry analysts expect the global market for turbocharged vehicles to grow at a CAGR of 7-9% over the next five years, with global penetration rising to beyond 50% of all vehicles by 2022. Demand for better fuel efficiency is likely to drive adoption of superchargers as well, though the acceptance levels are likely be lower than that of turbochargers. Industry research firm Technavio expects the supercharger market to grow at a CAGR of 14% between 2017 and 2022. Turbochargers – the preferred choice European OEMs such as Audi, BMW and Mercedes are pioneers in turbocharger technology, with most of their current portfolios having adopted turbocharged engines. Ford has also successfully leveraged turbochargers with its EcoBoost engines, while General Motors is delving into the mix, using turbocharged engines in the recently announced 2018 GMC Terrain and 2018 Chevrolet Equinox. Japanese companies such as Honda and Toyota are also catching up. “North America has the highest penetration of superchargers, accounting for about 40% of all supercharger sales globally” By comparison, there is less interest in supercharged engines in general. The Volkswagen Group has introduced several supercharged cars in the past, and recently also rolled out a new electrically-supercharged engine for its Passat model range. Twin-charging, which incorporates both a belt-driven supercharger and exhaust-driven turbocharger, is gaining popularity. Mazda recently filed a patent application for a four-cylinder engine that utilises a twin-turbo and an electrically-driven supercharger. The supercharger’s dominance will perhaps continue – and probably end – with high performance and sports cars, at least in the short- to medium-term. Twin-charging, however, could offer superchargers a new lease of life. We could potentially be looking at a new tag-team champion, which could rule the fuel efficiency domain for a long time. Given their effectiveness and lower implementation costs, turbochargers have gained significant prevalence among vehicle manufacturers that are looking to achieve improved fuel efficiency by downsizing their engines without compromising on performance. Advanced development and increased commercialisation of electric turbochargers could be a game-changer, and with the elimination of turbo-lag, turbochargers could reign over its counterparts, given its prowess in achieving better fuel efficiency at much lower costs compared with superchargers.

Turbocharging trumps supercharging in the battle for engine downsizing

Indraneel Bardhan of EOS Intelligence investigates the challenges and opportunities for the automotive industry's key forced induction systems

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