BorgWarner eBooster® ElectricallyDriven Compressor –Electric Boosting for Improved Fuel Economy
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Knowledge Library

BorgWarner eBooster® Electrically DrivenCompressor – Electric Boosting for Improved
Fuel Economy
As an addition to conventional turbocharging concepts, BorgWarner’s eBooster®
electrically driven compressor improves boost pressure and transient engine
behaviour at low engine speeds to allow downsizing for improved efficiency orincreased engine output with excellent transient torque.
Dr. Hermann Breitbach, Vice President Global Engineering and Innovation, BorgWarner Turbo Systems

Introduction

Development of more efficient and clean
engines is driven by costumer demand forfuel efficiency and better drivability as well
as challenging regulations. As a result,boosting fuel economy through downsizing
and downspeeding as well as improvementof low-end torque and transient engine

performance has become a major focus of theautomotive industry. To meet the high demands
in terms of fuel efficiency and performance,
OEMs employ different solutions in the fieldof powertrain electrification, such as the introduction of 48-volt electrical systems. This
high-voltage power supply offers significant
improvements in efficiency and facilitates newFigure 1 . BorgWarner’s eBooster® electrically driven compressor supplements a conventionalturbocharger.
BorgWarner Knowledge Library 201 7

1functions such as high-power electric boostingfor enhanced low-end torque and transient
response without any noticeable turbo lag. As
a product leader in clean, energy-efficient propulsion system solutions for combustion, hybrid and electric vehicles, BorgWarner isdedicated to offer the best possible support toits customers. For this reason, the company
further expands its strong portfolio of highperformance boosting technologies for combustion and hybrid engines.

Benefits for both – diesel andgasoline engines

BorgWarner’s eBooster® electrically driven
compressor as shown in Figure 1 was designed to improve boost pressure and transient engine behaviour at low engine speeds
without increasing exhaust back pressureand negatively impacting on the engine gas
exchange since no additional turbine isneeded. This is a great advantage, especiallyfor gasoline engines susceptible to engine
knock. A comparison drawn between the newconcept and a multistage turbocharger solution
shows that the eBooster electrically driven
compressor configuration allows flexible packaging, leaves more exhaust heat for the aftertreatment system and causes less heat flux
into the engine compartment.
The eBooster electrically driven compressor
is preferably installed downstream of the turbo
compressor as seen in Figure 2. Here, withsmallest volume between compressor andengine, the engine response to the eBooster
electrically driven compressor is quickest. With
conventional turbo matching, the eBooster
technology can enhance transient behaviour
while maintaining engine output. In anotherconcept, transient response can be kept constant by utilising a larger turbine with lower
back pressure, which leads to increased engine power and reduced fuel consumption due

Figure 2. The eBooster electrically driven compressor is preferably installed downstream of theturbo compressor.
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to the lower back pressure at high loads andless need for fuel enrichment at full load. Focussing on the emission cycle operating range,
the following analysis demonstrates the potential of a 1 2-volt eBooster electrically driven
compressor with a power of 2 kW. A 2.0-litre
diesel engine with a single-stage variable turbine geometry (VTG) turbocharger was thebasic configuration which was compared totwo equally powerful 1 .6-litre engines, one
featuring a VTG turbocharger with an additionaleBooster electrically driven compressor andone with a regulated two-stage system (R2S®).
Among other results, the 1 .6-litre engine featuring a VTG turbocharger showed a significant
gap in torque without eBooster electricallydriven compressor operation. Due to downsizing to 1 .6 litres, low-end torque is limited.
This can be compensated for using the eBooster
electrically driven compressor. In comparison,
the R2S configuration also meets the torquetargets, but it takes more energy from theexhaust gas flow than the eBooster electrically driven compressor consumes when in
the high-pressure stage. When the electricalenergy is gained from recuperation, theeBooster technology provides an advantageous overall energy balance compared with
the R2S in addition to the main fuel economy
benefits achieved through downsizing.
Before boost pressure builds up, the 2.0-litre
engine offers an initial torque advantage overthe 1 .6-litre engine. However, the torque gradient of the 1 .6-litre VTG engine becomessteeper and achieves full-load torque earlier
than the two other configurations do as soon
as the eBooster electrically driven compressor
starts operating. Nonetheless, the eBooster
configuration can only provide transient boost
in steady-state operation, whereas the R2S
system can maintain a high boost pressure.
According to the FTP-75 drive cycle analysis,
BorgWarner Knowledge Library 201 7the eBooster electrically driven compressor isoperated at 2 kW and a minimum maintained
speed of 6,000 rpm with an average power
consumption of 21 0 W. Since the eBooster
technology provides additional boost pressure,a higher amount of EGR can be used to achieveadvantages in NO X emissions. As a result, fuelefficiency is increased by up to about 4 percent in comparison with the R2S concept.
Compared with the reference engine, advantages with regard to particulate emissions arealso expected as the time of the air-fuel ratioat the smoke limit has decreased by 5 percent.

Advanced design for maximum
efficiency

The eBooster electrically driven compressor
has to meet certain specific design requirements to offer the best possible functionality.
First of all, the inertia of the electrical motor aswell as the electrical and mechanical losses
have to be minimised. Moreover, the motor
should feature a very compact design withintegrated and extremely efficient power electronics. It should also be able to withstandhigh temperatures. Finally, the NVH (noise,vibration, harshness) behaviour has to be considered, and the eBooster technology has tobe modular and available as a 1 2-volt and a48-volt version. Since a brushless permanent magnet DC motor featuring highly heatresistant samarium-cobalt magnets is much
more efficient than asynchronous or switched
reluctance motors, this configuration was selected. The eBooster electrically driven compressor allows a maximum speed of 70,000rpm to be reached from a standstill in 270 ms –virtually in the wink of an eye. This speed
was chosen to realise an on the whole homogenous package with a roughly similar diameter between motor, power electronics andthe compressor side of the eBooster electrically driven compressor.
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Figure 3. The eBooster electrically driven compressor improves transient performance and fueleconomy.

Other substantial design elements of theeBooster electrically driven compressor
include power electronics using parts withminimal resistance specifications and highly
efficient capacitors, a good connection fromelectronic board to housing to provide efficient heat transfer, and a stator optimised forlong ‘on’ times and high duty cycles by utilising a high-density copper filling. Additionally,
since water cooling with good heat transfer tothe stator and the power electronic board wasfeasible for the 48-volt configuration, this option is preferred over air cooling, which onlyworked for the 1 2-volt eBooster electricallydriven compressor. Overall, the 48-volteBooster configuration is preferred because
it achieves a permanent power of around 3
kW under favourable operating conditions
while offering numerous advantages suchas additional benefits with regard to transient
performance and fuel economy as well as
lower currents and higher power for largerdisplacement engines, see Figure 3.
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SummaryBorgWarner’s eBooster electrically driven
compressor supplies boost pressure very
quickly, even at low engine speeds, making it
an excellent and highly efficient addition to
the conventional turbocharger to improve engine fuel economy, transient response andconsequently allow the engine power density
to be increased. In addition, improvements
in fuel efficiency and the optimisation of emissions, particularly in the case of diesel engines,
are facilitated by matching the eBooster electrically driven compressor and turbocharger
as an overall system.

Contact

Email: technology@borgwarner.com
For more information please visit
borgwarner.com
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