Intake Throttle and Pre-swirl Device for
Low-pressure EGR Systems

Knowledge LibraryKnowledge Library

Intake Throttle and Pre-swirl Device
for Low-pressure EGR Systems
Low-pressure EGR systems to reduce emissions are state of the art for diesel engines.
They offer efficiency benefits compared to high-pressure EGR systems and will gain
further importance. BorgWarner shows the potential of a so-called Inlet Swirl Throttle tomake use of the losses and turn them into a pre-swirl motion of the intake air entering theturbocharger to improve the aerodynamics of the compressor.
By Urs Hanig, Program Manager for PassCar Systems at BorgWarner and a member of BorgWarner’s Corporate
Advanced R&D Organisation

Technology to meet future Emission
Standards

Low-pressure EGR systems (LP EGR systems), see Figure 1 , for gasoline engines yieldsignificant fuel consumption benefits, they arealso an important technology to meet future
emission standards (e.g. Real Driving Emissions) [1 ]. To achieve the targeted EGR rates inparticular on diesel engines throttling the LP
EGR path is necessary in some areas of theengine operating map. This can be done eitheron the exhaust or the intake side but to throttle
the intake side is the more robust and by far
the more cost efficient solution. In state of theart EGR systems, which means EGR systems
with low pressure drop, the impact of intake
and exhaust throttling on engine efficiency iscomparable.
The IST (Inlet Swirl Throttle) replaces the conventional LP EGR throttle in the intake side before the compressor which is usually a simple
butterfly valve. Throttling always means inducing losses. The approach of the IST is tomake use of the losses and turn them into a
pre-swirl motion of the intake air entering theturbocharger to improve the aerodynamics ofBorgWarner Knowledge Library 201 5

the compressor. Obviously, pre-swirl will have apositive impact on the compressor also in areas where no throttling is required. So the IST
can be used to improve engine efficiency andperformance also in regions where no throttlingor EGR is required.

Approach and Modes of Operation

With IST the throttling effect is achieved by adjustable inlet guide vanes in the fresh air duct.
In other words, IST is an intake throttle designed as a compressor pre-swirl device. Thisapproach is expected to have positive impact
on the combustion engine, like:- higher low-end torque- reduced emissons- lower fuel consumption.
To gain most out of the IST it needs to be operated in different modes depending on the engine operating point, see Figure 2. The angle ofthe inlet guide vanes is adjusted continuouslywith changing engine load and speed and theset point of the vanes is determined by a controls algorithm, also taking the position of theVTG (variable turbine geometry) and the EGR
valves into account.

1Figure 1 . Engine with low-pressure and high-pressure EGR system

In mode 1 some areas of the engine operating
map the target EGR rate can not be met evenwith the EGR valve fully open. In these areas
IST is used to throttle the intake air to drive
EGR so that the desired EGR rate can be reached. As a secondary effect of the throttling apre-swirl with a rotational component with thesame sense of direction as the compressor
wheel (positive pre-swirl) is induced. Because
the operating point in the compressor map isstill away from the surge line, swirl has only
minor effect on compressor aerodynamics. Theswirl helps, however, to improve mixing of EGR
gas and fresh air. This increases compressor
efficiency significantly.
In mode 2, IST improves compressor efficiency. At higher engine loads EGR is faded out byclosing the EGR valve. For high loads and low

Figure 2. Operating modes of the IST
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engine speeds (esp. relevant for downspeeding concepts) the operating point in the compressor map tends towards the surge line as it
moves to higher pressure ratios and lower
mass flows. Close to the surge line compressor
efficiency drops steeply. Here positive pre-swirl
improves compressor efficiency and thus reduces engine pumping losses and the IST angle
is set to make full use of this effect. At maximum load and low engine speed (low-end torque area) the IST induced pre-swirl extends
the compressor map towards lower massflows. This provides extra air allowing forhigher low-end torque.
During mode 3, IST reduces turbo speed. According to the Euler equation negative pre-swirl
reduces the speed of the turbocharger for agiven operating point. This effect can be beneficial because a turbocharger matching isalways a compromise between achievable lowend torque and allowed maximum turbo speed
at rated power (including some margin for highaltitude operation). IST can help to enhance
the quality of the turbo matching by inducing anegative pre-swirl at rated power to reduce turbo speed. This gives an additional degree of
freedom for turbo matching and can ultimately
yield efficiency or performance benefits.
IST is set to fully open (or neutral) mode in theother areas of the engine operating map. In2this mode the optimised aerodynamics of theinlet guide vanes reduces the pressure loss toapproximately 6 mbar at maximum air flow,which was proven to have negligible impact onengine performance.

Design and Realisation

Most important development targets of the IST
were cost efficiency, robustness and low
weight, see Figure 3. This was achieved by amodular design, reduced number of parts, lownumber of seals, especially to the outside, anda careful choice of materials. In addition, theIST had to be compact and easy to integratewith only little necessary modifications to theinterfacing components.
In its final design stage the IST has a size
comparable to a conventional butterfly valve. Itcan either be a stand-alone solution or partiallybe integrated with the LP EGR system and thecompressor housing. And importantly, theaerodynamics of IST can cope with a bend in
the fresh air path upstream of the IST, like often found in real applications.
For further cost reduction the guide vanes areactuated by an off-the-shelve actuator from anEGR valve and a simple mechanism similar toa VTG actuation. The actuator is integrated in
the IST housing. The vanes can completely
close the fresh air duct; they can also be set
into every intermediate position between completely open and closed. In order to induce negative pre-swirl the vanes can also be set tonegative angles. A fail-safe mechanism opens
the vanes completely in case of an actuator
failure.
The aerodynamics of guide vanes and flow
channel was optimised by comprehensive CFD
simulations. Goal was to minimise the losses in
the fully open, but also in the intermediate positions of the vanes. Of great importance wasalso to generate a stable swirl profile at theinlet plane of the compressor wheel with a dedicated radial velocity gradient. Finally, the vanes had to be designed in a way thathigh-cycle fatigue of the compressor wheel
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Figure 3. IST mounted upstream of the LP EGR
junction and the compressor housing

caused by excitations through the intake flow isavoided.

Test Results

To confirm the expected performance of IST
experimental investigations with prototypeswere conducted on an engine dyno and a hot
gas test stand. Some exemplary results areshown in Figure 4.
From literature it is known that pre-swirl does
improve the efficiency of a turbo compressor
operating close to the surge line [2, 3]. The
cause for this improvement are reduced losses
in the intake area of the compressor wheel: apositive pre-swirl changes the direction of thevelocity vector of the intake flow relative to thecompressor wheel making the interaction between flow and wheel more efficient. How IST
works in that respect was investigated on thehot gas test stand to confirm the optimisation
work done by CFD on both the IST guide vanes and the housing geometry. The measurements show that the pre-swirl induced by IST
improved compressor efficiency by up to 2 %for constant flow and pressure ratio, see Figure
4 (top). Moreover, the shift of the surge line toa mass-flow reduced by 50 % was also confirmed (mode 2).
Throttling behaviour, linearity and controllabilityof the IST are key for a successful application
on a real engine (mode 1 ). To investigate this acomprehensive controls approach was needed3

Figure 4. Results from engine dyno and hot gas test stand (modes in the image on the left as shownin Figure 2)

to determine optimal VTG and EGR valve positions taking the impact of IST position andIST operating mode on the air system into account. BorgWarner has a fully functional airsystem controller [4] available as a development tool to which the control of the IST wasadded.
The test engine was a state of the art 2.0-l diesel engine with a rated power of 1 00 kW meeting EPA US07 emission standards. The enginefeatures both HP(high pressure)- and LP EGR
systems. The IST was applied to that engine
and a comprehensive test program was conducted covering the complete engine map. The
results were encouraging: IST showed reproducible and well controllable behaviour. The
reaction of the engine to the IST position waspredictable and potentially critical operation
modes like over speeding the turbocharger oroil leakage from the compressor through lowpressure at the compressor inlet were safely
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prevented. And most important, the desired
EGR rates were also met in steady state andtransient operation.
This first set of investigations was conductedusing the engine’s series turbocharger.
However, this turbo was not able to deliver thehigher turbine power at low engine speeds
which the compressor needed to provide more
air with the shifted surge line. This made it impossible to significantly improve engine lowend torque. In the next project phase it is planned to continue the investigations with a
re-matched turbocharger which will be able to
make use of the extended compressor mapwith IST.
What the engine investigations did show was
the mixing benefit of fresh air and recirculated
exhaust gas from the intake swirl motion. The
improvement of compressor efficiency from themixing could be confirmed in the experiments as4

predicted in CFD: fuel consumption in steadystate was improved by up to 4 % in cycle relevant load and speed points with EGR, see
Figure 4 (middle).
Finally the effect of negative pre-swirl on turbo
speed was quantified (mode 3). At constant
rated power the turbocharger speed was reduced with IST by 3.5 % without compromisingfuel consumption, see Figure 4 (bottom). Thisadditional degree of freedom will be taken into
account during the matching exercise for thenew turbo in the next project phase.

Summary and Outlook

The IST is a new approach to replace thestate-of-the-art butterfly-type intake throttle in a
LP EGR system with an integrated pre-swirl
and throttling device. IST offers remarkable
benefits like reduced fuel consumption and improved engine performance for competitive
cost in a component that is easy to integrate
into current air systems. The ability of IST towiden the compressor map is of particular interest in gasoline engines, which makes IST aninteresting option also as a stand-alonedevice.

References[1 ] Roth, D.; Zhang, L.; Sauerstein, R.; Becker,M.: New Aspects of the Application of Hybrid
EGR Systems to Turbocharged GDI Engines.
1 8th Aachen Colloquium Vehicle and Engine
Technology, 2009[2] Seume, J. et al.: Vorleitradoptimierung. Final report FVV project No. 846, Frankfurt, 2007
[3] Schnorpfeil, J.: Variable Verdichtervordrallregelung für Ottomotoren. Dissertation, RWTH
Aachen, 201 3[4] Shutty, J.; Kienzle, D.; Wenzel, W.; Becker,M.: Air System Management to Improve a Diesel Engine. SAE Technical Paper 2011 -01 , Detroit, 2011

The compact design and the possibility to integrate IST with other components make it a viable solution also in tight package environments. Concerns regarding controllability and oilleakage from the compressor could be overcome by comprehensive engine investigations.
Based on the results achieved so far further
optimisation of the air path with IST is plannedin the next project phase. Most important willbe the new matching of a turbocharger taking
the specific requirements and additional degrees of freedom IST offers into account. This,in conjunction with a more refined controls approach, will help to determine the full benefitsof IST in the FTP test cycle.

Contact

Email: technology@borgwarner.com
For more information please visit
borgwarner.com
BorgWarner Knowledge Library 201 55