Turbocharger Design: Boosting Performance with E-Chargers?



Colin Pawsey
01/13/2016

Driven by fuel efficiency and emission reduction targets, automakers are continually developing new technologies to improve the efficiency and lessen the environmental impact of their vehicles. However, this is no easy task when set against the backdrop of customer expectations of how their cars should drive and what they expect from a manufacturer.

This is evident from the experience of electric and hybrid vehicles, which the public have been slow to warm to due to lack of range and power, among other complaints. One of the areas in which OEM’s have made great strides is the downsizing of engines to improve fuel efficiency, off-setting the reduction in performance by incorporating turbochargers to boost power. This is certainly not a new concept, but the issue that has always been prevalent is turbo lag – the time it takes for the turbo to respond after the driver has pressed the accelerator pedal.

Electrification could spell the end of this problem, and several manufacturers are developing and testing electric turbochargers with a view to offering even higher performance alongside increased fuel efficiency. However, while e-chargers offer certain advantages, there are many challenges to overcome before this kind of technology makes it down to the production lines of mass volume cars.

Advantages of Electric Supercharging

Conventional turbochargers have enabled the downsizing of engines already, with exhaust-driven turbos recycling exhaust gases and improving performance via forced induction to aid combustion. In its crudest form this involves the use of twin fans, the first of which spins up by capturing exhaust gases, which then turns the compressor side of the system forcing air into the engines combustion chambers and increasing engine performance. New designs and multiple turbos have worked towards eliminating turbo lag and enabling further downsizing, but e-chargers could represent the next step in terms of performance.

The key advantage of e-charging tech is that electric motors can respond almost instantly, and therefore completely eliminate turbo lag. Turbo manufacturer and tier one supplier, Valeo, announced at 2015’s Frankfurt Motor Show that it has developed an electric supercharger that responds within 250 milliseconds, reducing lag to practically zero and offering a potential fuel consumption reduction of 10%.

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This response time is a huge advantage over conventional turbos because it supplies boost from idle speed, where exhaust gas driven turbos require high engine speed to provide maximum boost. Superchargers where the compressor is driven from the engine crankshaft have been one solution – as they can be configured to provide boost at low engine speeds – but they also draw power when not in use and can adversely affect fuel economy under normal driving conditions.

Electric superchargers can offer the best of both worlds in the sense that they can eliminate the lag associated with exhaust gas turbochargers, and the physical drag that a supercharger places on the engine. As well as providing that instant boost at low engine speed, e-chargers can also be combined with other electrical components – such as drawing energy from regenerative braking systems – to further reduce power consumption and improve fuel economy.

Design Challenges

While some OEM’s, such as Audi and Ford, are actively investigating and pursuing the technology, others such as BMW are less enamoured, and although they are looking at the technology, they say they are also investigating the alternatives. Some critics have argued that conventional turbos can work just as well, and that when e-boosters are used (aggressively) in real world situations, fuel efficiency and emissions are barely affected.

The two big design challenges are cost and power consumption. Electric superchargers use a great amount of power, and the full benefits may not be viable until vehicles move to 48 volt architecture, whereby the 7-8 kilowatts of power needed to run chargers will be readily available.

This is a huge sticking point because 48 volt architecture is currently considered too expensive and not viable unless there are enough electrical components running on the network for it to be necessary. However, electric vehicles are certain to play a part in a future carbon-zero society, and 48 volt architecture is potentially going to be introduced for advanced stop-start systems. E-chargers now represent another reason to step up to 48 volt subsystems, although at the present time the cost of doing so is prohibitive – particularly in terms of mass produced vehicles.

Development

The auto industry continues to develop and research the technology regardless, and although we may not see the full benefits of e-charging until such a time that 48 volt architecture is the norm, there have been significant strides in recent years.

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Manufacturers

Audi is leading the way among manufacturers, and has already demonstrated the technology in two different concept vehicles over the last two years. In 2014 the automaker showcased an RS5 TDI concept, whereby it added an electric supercharger from Valeo to its twin-turbo 3.0 litre V6 engine. The power compared with the standard engine rose from 313hp to 385hp, and resulted in a coupe that could hit 60mph in approximately four seconds, and posted figures of 47mpg. That made the concept car faster than the standard vehicle, and more than twice as fuel efficient.

Audi demonstrated the technology again in May 2015, this time in a Clubsport TT Turbo Concept, an all-wheel drive car that offers 600hp and 479 lb-ft of torque. The concept vehicle uses two turbos with the 2.5 litre engine – one a traditional exhaust gas driven turbo, and the other an electric supercharger, again from Valeo. The car features a 48 volt subsystem to power the electric turbo, which sends boost to the engine on demand as opposed to waiting for the conventional turbo to spool up. The operating principle is that the conventional turbo is designed for high pressure charges for higher engine power, while the electric turbo assures instant response and power from low engine speeds. The results were again impressive, with the car able to hit 62mph in just 3.6 seconds.

Other manufacturers taking a keen interest in e-charging technology include Ford, Mercedes and Honda, but Audi has taken the box seat with the announcement at their annual press conference in March this year, that all TDI engines will be equipped with an e-charger as standard in the future. No timeframe was established, but the company has confirmed that the new Q7 SUV will be the first production model to be fitted with the technology.

Valeo

Valeo’s collaboration with Audi has put them at the forefront of suppliers in this sector, with Chief Operating Officer Christophe Perillat-Piratoine suggesting that the French company has a 1-2 year head start over its rivals. The supplier showcased its electric supercharger at this year’s Frankfurt Motor Show, and believes that most manufacturers will follow Audi’s lead by incorporating e-charging technology. The Valeo supercharger uses switch reluctance technology to provide the instant (within 250 milliseconds) response which counteracts turbo lag. It can be used with 12 or 48 volt architecture, and can be used alongside or as a replacement for a conventional turbocharger.

According to Valeo, the system can reduce fuel consumption by 10%, and enhance both driving comfort and in-gear acceleration – which improves by 27% without increasing fuel consumption. Furthermore, Valeo suggests that when coupled with its energy recovery system, the supercharger can be used to create a cost-competitive hybrid solution that delivers fuel savings of 15-20%.

Honeywell

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Honeywell is a huge supplier of turbo technology to the automotive and aerospace industries, and it also showcased its developing e-charging technology at the 2015 Frankfurt Motor Show.

While the company doesn’t expect to have its version of the system on the market before 2017, it is already collaborating with Scuderia Ferrari on a version of an e-turbo to be integrated into a hybrid system to improve the performance of their Formula One car. At the same time Honeywell is also partnering with a ‘global manufacturer’ to develop an advanced air compressor to enable fuel cell technology, and says that it sees superchargers playing an immediate role in the electrification of vehicles.

BorgWarner

Another supplier of turbo systems, BorgWarner, has developed an eBOOSTER system, which as a twin-turbo hybrid system is indicative of the technology that we are likely to see implemented in the coming years.

This system uses an electric turbo in tandem with a conventional exhaust gas driven turbo, so that the two work in series. The e-charger uses a flow compressor driven by an electric motor, and is positioned either before or after the conventional turbocharger. With the turbos working in series the pressures of the two units are multiplied, and with perfectly matched flow compressors, the entire system can be optimized to suit the specific purpose and expand its entire power curve. According to BorgWarner, the technology will enable the downsizing of engines while retaining the power associated with large non-supercharged engines of the same output class.

Summary

The electrification of vehicles will continue, driven by the need to improve fuel efficiency and reduce emissions. Electric superchargers have become a key technology in that process, and present a compelling reason to pursue 48 volt architecture.

As ever with development in the auto industry, the bottom line is always cost, and the move to 48 volt subsystems is difficult for this very reason. However, e-chargers are beginning to make their way into production vehicles, and with more manufacturers investigating the possibilities of downsizing their engines, we should quickly see the development of hybrid and twin turbo systems with an electric component in the short term, with full electrification looming on the long-term horizon.

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