Powerful Turbocharging System for
Passenger Car Diesel Engines

Knowledge LibraryKnowledge LibraryPowerful Turbocharging System for
Passenger Car Diesel Engines
Driven to satisfy escalating demand for increased fuel economy and more performance, BorgWarner engineered a regulated three-turbocharger system (R3S),
consisting of two small parallel high-pressure variable turbine geometry turbochargers integrated with one larger low-pressure stage turbocharger. Developed in collaboration with BMW, the system combines the benefits of two-stage turbocharging
for a high specific power output and parallel sequential turbocharging for maximum
drivability.
By Dr. Frank Schmitt, Senior Manager Customer Application System Performance – Application Engineering Europe,
BorgWarnerMulti-stage Charging PrevailHigh levels of driving dynamics in combination
with low fuel consumption have become features of a wide range of engine models. With theintroduction of two-stage turbocharging forpassenger car diesel engines in 2004 [1 ],BorgWarner was able to develop a turbocharging system which significantly enhances thediesel engine power density. Today’s turbo diesel engines achieve a rated power of up to1 70 kW in the 2.0-l four-cylinder engine and230 kW in the 3.0-l six-cylinder engine which
has been in series production since 2009equipped with a BorgWarner R2S (regulatedtwo-stage) turbocharging system. Two-stage
turbocharger systems have now become thestate of the art.
The R2S turbocharger system uses two turbochargers: a large one in the low-pressure stage
and a small one in the high-pressure (HP) stage [2]. Figure 1 shows a schematic diagram of
two-stage turbocharging as used in commercial
diesel engines. Compared to classical regulaBorgWarner Knowledge Library 201 4

ted two-stage turbocharging, the passenger car
R2S system has an additional bypass valve inthe low-pressure stage turbocharger and acompressor bypass for the high-pressure stagecompressor. The aim of regulated two-stage
turbocharging is primarily to achieve higher
low-speed torque while at the same time increasing nominal engine power output. Even in
the medium speed range, however, a considerable increase in medium pressure is possible.
Only the low-pressure stage turbocharger provides the boost pressure and the full pressure
ratio at the rated power. For this reason, therated power is limited by the maximum pressure ratio, which the compressor of the low-pressure stage can achieve. The compressor of thelow-pressure stage was developed for highpressure ratios and good efficiencies. Typicallyfor R2S, a specific power of up to 85 kW/l canbe realised.
For a higher specific power, single-stage turbochargers such as those used at high engine
1

and Figure 2 show a comparison of thedifferent turbochrager systems. At the ratedpower, the high-pressure stage turbochargerstill has to work and contribute to the boostpressure. A good solution for the high-pressure
stage is a parallel sequential charging system.
At low engine speeds, a small turbocharger
works to provide good low-end torque andtransient behaviour, whereas at high engine
speeds, the exhaust gas mass flow is separated and fed to two turbochargers running inparallel, thus providing the energy required to
run two compressors in parallel in the highpressure stage. The result is a regulated threeturbocharger system as a combination of R2S
and a parallel sequential charging system.
Figure 2 shows the brake mean effective
charging systems. The all-new R3S systemsets a new benchmark with regard to the bmep
curve and poses a challenge for the development of new turbocharger components.
Figure 1 . System layout for R2S charging
systems for commercial diesel engines(permanent two-stage, top), R2S for passenger
cars (middle) and R3S (bottom)

speeds in passenger car R2S applications will
not be sufficient. Even at the rated power, therefore, a two-stage charging system is required. The result is a permanent two-stage
charging system. However, the rated power
can only be achieved successfully if one turbocharger for the low-pressure stage and one
turbocharger for the high-pressure stage run inseries at all times with a valve to regulate theboost pressure – rather like the R2S systemused in many commercial diesel applications.
For superior low-end torque and maximum
transient response, a smaller turbocharger isrequired in the high-pressure stage. Figure 1
BorgWarner Knowledge Library 201 4

Regulated Three Turbocharger System

The major objectives for downsized diesel engines with the new R3S turbocharging system,
see Figure 3, are to match the range of power
output, torque and comfort provided by an engine with more displacement or a higher number of cylinders while equalling the lower fuelconsumption and power/weight ratio typical forexisting diesel engines. The new R3S turbocharging system contains one large turbocharger in the low-pressure stage and twoturbochargers running in parallel in the highpressure stage. The operating principle of theturbochargers and the additional corecomponents of the R3S turbocharging systemae shown in Figure 4 for different operating
modes of the R3S turbocharging system. Theintake air enters the low-pressure stage compressor. Optionally, the compressor of the lowpressure stage can be bypassed as the intake
2Figure 2. Brake mean effective pressure characteristics of R2S and R3S charging systems

air is only throttled and not pre-compressed inthis operating mode. To reduce the charge airtemperature, the intake air passes through anintercooler integrated into the low-pressure
stage compressor housing and is subsequently, depending on the operating mode, compressed further in one high-pressurecompressor stage (operating mode 2 in Figure4) or both high-pressure compressor stages(operating mode 3 in Figure 4). After cooling in
the main intercooler, the charge air is fed into
the combustion chamber via the intake system.
On the exhaust side, at low speeds and loads,
the gas only flows through one of the highpressure stage turbines. This ensures spontaneous charge air pressure build-up and excellent dynamic response characteristics(operating mode 1 in figure 4). At medium engine speed there is enough exhaust gas mass
flow to provide significant boost pressure in thelarger low-pressure stage compressor in addition to the high-pressure stage (operating
mode 2 in figure 4). And at high exhaust gas
BorgWarner Knowledge Library 201 4

through-put rates, a parallel route passing via
the exhaust gas control flap is opened to reduce the exhaust gas backpressure (operating
mode 3 in Figure 4). Subsequently, the exhaustgas is further relaxed in the low-pressure stage
before it enters the exhaust aftertreatment system located close to the engine. To optimise
charge air pressure control, the waste-gate on
the low-pressure stage turbine is activated in
the switchover range from two to three turbochargers and in the rated power range.

Figure 3. R3S turbocharging system with threeexhaust gas turbochargers
3sealing behaviour is used. For improved transient behaviour, turbochargers with Variable
Turbine Geometry (VTG) technology are usedin the high-pressure stage. As one VTG turbocharger in the high-pressure stage does not
rotate permanently, a special sealing system isintroduced.
Another critical aspect is the compressor outlet
temperature. A high compressor outlet temperature may lead to coking of blow-by gases in
the compressor volute. To reduce the temperature downstream of the low-pressure stage turbocharger, a compressor with water cooling isused. In addition, a charge air cooler is required between the compressor of the low-pressure stage and that of the high-pressure stage.

Regulation Valve

The gre at relevance of the regulation valve forengine dynamics causes much attention to be
paid to ensuring that the exhaust gas control
flap has a tight seal. The new so-called spherical design, see Figure 5, consisting of a spherical part in the flap and a cone in the leverleads to a more flexible connection in the valve
Figure 4. Operating modes of the R3Sturbocharging system

New Turbocharger Components

Applying a R3S to diesel engines requires some modifications to the engine. A new injection
system for increased injection mass and a newcylinder head for increased cylinder pressure
are just a few of these [3, 4]. On the turbocharger side, some very important improvements
and developments have to be considered for
R3S. The control systems are very importantfor providing maximum performance. For thisreason, a new regulation valve with maximum
BorgWarner Knowledge Library 201 4

Figure 5. New regulation valve with improved
sealing characteristics
4Figure 6. Compressor for new high-pressure stage turbocharger versus standard compressor

seat. The materials used for the components
were also optimised.

able to take over the delivery of the boostpressure.

Compressor Applications

The compressor for the low-pressure stage
turbocharger is designed for high-pressure ratios. A high efficiency level at a high-pressure
ratio is very important for a minimum compressor outlet temperature because the fresh air iscompressed for a second time in the highpressure stage.

The requirements for the high-pressure stage
turbocharger differ completely from the demands made on a single-stage application. For
this reason, a completely new compressor design is required. In the stationary state, the fullload curve and the part load operating points
require a low-pressure ratio for the compressor. The efficiency level therefore has to be optimised in the relevant area. Figure 6 shows anefficiency increase for the new compressor (redlines) at low-pressure ratios. Nevertheless, during transient response, the compressor has tocover the full speed range up to the maximum
turbocharger speed because, under transientconditions, the high-pressure stage turbocharger has to provide the largest portion of thedeliverable boost pressure until the low-pressure stage turbocharger with its much higher
mass inertia and higher swallowing capacity isBorgWarner Knowledge Library 201 4

Gas Lubricated Face Sealing

The R3S turbocharging system consists of one
low-pressure stage turbocharger and two highpressure stage turbochargers arranged in parallel. In some operating points, only one turbocharger in the high-pressure stage rotates
while the other one does not. This is a major
challenge for the sealing concept.
The turbocharger has an oil inlet and an oiloutlet. Oil is required to lubricate the shaft andwheel assembly of the bearing system. Howe5

Figure 7. Face sealing (in yellow) on the compressor sidever, when the turbocharger is not rotating, thedynamic sealing function of the turbocharger isnot enabled. Because the standard sealing
system is not dynamic, oil will flow out of thecentre section and into the compressor,
causing a high level of oil consumption thatcannot be managed by the engine oil circuit.
A mixture of fresh air and oil will lead to an increase in engine emissions, which is not acceptable either. To prevent oil from entering theair system from the bearing housing of the stationary exhaust turbocharger, a gas lubricatedface sealing for the compressor of the turbocharger was developed [5].
As shown in Figure 7, gas-lubricated face sealing is used for the compressor side. This dynamic sealing concept was specificallydeveloped for use in turbochargers and was
first-to-market in the R3S turbocharging systemapplication. It provides perfect sealing capabiliBorgWarner Knowledge Library 201 4

ty for non-rotating rotors and rotors, which arein operation. At certain low rotor speeds, thesealing switches from a static sealing function
to a dynamic one.
State-of-the-art technology for turbochargers is
one or two piston rings on the compressor andturbine side. With state-of-the-art piston rings,
blow-by achieves problematic high values at increasing boost pressures. Vacuum pressure at
the intake side of the compressor, reinforcedby throttle losses in the intake section and thepump effect on the backside of the compressor
wheel, causes an oil leakage from the bearing
housing to the intake air. Piston rings are dynamic gap seals, which cannot be flooded withoil. Therefore the use of piston rings in a nonrotating turbocharger is not applicable.
Among other components, the face sealing
consists of a slide and a counter-ring. Imme6

diately after rotation starts, the sliding surface
floats on a gas (air) film between the slide andthe counter-ring. This gas film is produced bycrescent-shaped three-dimensional grooves in
the counter-ring, which build up the sealing gap
between slide and counter ring. The face sealing operates without contact. As a result, it isfree from measurable wear and generates very
low friction losses of only a few watts. In nonoperating conditions, the sealing gap is closedby the force of the spring, enabling the slideand counter ring to seal statically.

Water-cooled Compressor Housing

Unlike the R2S turbocharging systems, inwhich only the low-pressure stage turbocharger is used at high engine speeds, there arealways two turbochargers running in series in
the R3S turbocharging system, even at the rated power. The permanent series mode is required to achieve the targeted specific powerwith the choosen downsizing grade. For highspecific power outputs, an extreme boost pressure and pressure ratio are needed. However,a high-pressure ratio also leads to a high compressor outlet temperature. Oil, which is typically in the fresh air mass flow due to engine
and turbocharger blow-by, tends to coke athigh temperatures. The coking layer in thecompressor leads to a reduced power output.
To cope with the high boost pressures, a cooling system, shown in Figure 8, has to be integrated into the low-pressure stage compressor
housing to reduce the compressor outlet temperature and ultimately to prevent the coking ofoil in the compressor volute and the charge aircooler. Figure 8 shows cooling in the contour
area of the compressor housing, can even becombined with a ported shroud compressor.
Alternatively, the compressor can be cooled in
the back wall of the compressor with very similar results. A combination of cooling in theback-wall and cooling in the contour area evenincreases the cooling effects and leads to theminimum compressor outlet temperature.
Water-cooled compressor housings were developed and first-to-market in some diesel R2S
applications. As a result of the good results
achieved in lowering the compressor outlet
temperature, this technology was adapted to
the R3S turbocharging system at the same time.
Compressor cooling is possible at a standard
engine coolant temperature. The reduction of
the compressor outlet temperature can be significantly increased via a lower coolant tem-Figure 8. Water-cooled compressor housing
BorgWarner Knowledge Library 201 47perature. The typical temperature reduction in
the case of water-cooled compressor housing
is shown in Figure 9. The reduction of thecompressor outlet temperature can besignificantly increased via a lower coolant
temperature. However, much lower coolant
temperatures will require a second water-circuit. The integration of a water-cooled compressor housing and an interstage cooler into
the compressor housing, see Figure 1 0, wasfirst-to-market in a BorgWarner application designed to meet extreme package restrictions.

Summary and Outlook

Figure 1 0. Compressor housing with watercooling and interstage cooler

three-turbocharger system (R3S) for ultimate
downsizing to achieve a maximum level of
specific power and a maximum transient response with large improvements for fuel consumption. The first R3S turbocharger
technology debuted with M Performance diesel
engine from BMW, the most powerful six-cylinder passenger car diesel engine in the world[3, 4]. The 3.0-l engine has a maximum output
of 280 kW and a maximum torque of 740 Nm.
Compared with the 3.0-l diesel engine boosted

by a BorgWarner R2S turbocharging system,the 3.0-l diesel engine with BorgWarner’s R3S
turbocharging technology increases power
output by nearly 25 % and improves fuel economy by 8 % while meeting Euro 6 emissions
standards. The diesel engine with the R3S also
improves the transient behaviour, even at abetter fuel consumption which is at least 1 8 %below its eight-cylinder-competitors [4].

BorgWarner developed the new regulated

Figure 9. Compressor map showing thetemperature reduction with a cooled
compressor housing and a typical engine lugline (red dots)
BorgWarner Knowledge Library 201 4For the high engine power and torque output
which can be achieved with the R3S turbocharger system, some modifications of the engine are necessary, for example theoptimisation of the injection system and improvements of the durability to withstand higher
cylinder pressures. For the regulated three-turbocharger system, new developments such asa water-cooled compressor housing, the integration of water cooling and charge air cooler
into the compressor housing, a new regulation
valve, a new compressor for the high-pressure
stage and a new sealing technology were successfully introduced first-to-market.
Due to lower inertia and VTG technology in the8high-pressure stage, especially when used in aparallel sequential charging system, the R3Sprovides a rapid boost pressure in transientconditions. The low-pressure stage turbocharger is designed for the complete mass flow offresh air on the compressor side and the fullexhaust gas mass flow on the turbine side.
This requires large rotors with a high mass inertia; even the rotor includes a high-flow compressor and turbine wheels. During transient
response, the high-pressure stage turbocharger accelerates much more rapidly than thelow-pressure stage turbocharger does.
A first approach to the further improve ment of
the transient response of the R3S turbocharger
system is to use a VTG turbocharger in thelow-pressure stage. The VTG characteristics
allow a better transient response by reducing
the swallowing capacity when closing the VTG
vanes. A measure to reduce the mass inertia
for better transient behaviour of the low-pressure stage is to split the mass flow through one
single low-pressure stage turbocharger andsend it into two parallel smaller turbochargers.
This would approximately half the mass inertia.
Using a different turbine wheel material for asingle low-pressure stage turbocharger, for example titanium aluminide, would allow the rotor
mass inertia to be reduced by approximately
35 %.
R2S turbocharging system from BorgWarnerbegan as a high-performance technology a few
years ago and its market share is increasing
constantly. Similarly, BorgWarner expects tosee growth potential for the R3S technology inthe long term. In-line four-cylinder engines with
R3S will be able to replace six-cylinder engineswith standard charging systems and makea positive impact on fuel economy, emissions
and costs.

BorgWarner Knowledge Library 201 4

References[1 ] Schmitt, F.; Schreiber, G.; Engels, B.: Regulated 2-Stage (R2S) Charging System forHigh Specific Power Engines. 1 3. Aachen
Colloquium Automobile and Engine
Technology, 2004
[2] Schmitt, F.; Schwarz, A.; Schmalzl, H.-P.;
Christmann, R.: The 2-stage Regulated Charging System (R2S) for Passenger Car andCommercial Diesel Engines. In: MTZ 66(2005), No. 1[3] Eidenböck, T.; Mayr, K.; Neuhauser, W.;
Staub, P.: The new BMW 6-cylinder Diesel Engine with Three Turbochargers. In: MTZ 73(201 2), No. 1 0[4] Ardey, N.; Wichtl, R.; Steinmayr, T.; Kaufmann, M.; Hiemesch, D.; Stütz, W.: The all newBMW Top Diesel Engines. 33. Vienna Motor
Symposium, 201 2[5] Simon, C.; Lang, K.; Feigl, P.; Bock, E.: Turbocharger Seal for Zero Oil Consumption andMinimized Blow-by. In: MTZ 71 (201 0), No. 4Contact

Email: technology@borgwarner.com
For more information please visit
borgwarner.com
9