奥迪两级增压发动机(英文版)
T
HE NEW 3.0-L TDI BITURBO ENGINE FROM AUDI
PART 1: DESIGN AND ENGINE MECHANICS
W ith the arrival of the V6 tDI biturbo engine, Audi is
a dding a high-performance version with two-stage turbo-charging to its V6 tDI engine line-up. At the heart of the engine lies a new turbocharger system from Honeywell turbo technologies (Htt), capable of boosting power output to 230 kW and delivering 650 Nm maximum torque. By adopting all the efficiency measures from the basic V6 tDI monoturbo engine, a combination of
e xcellent performance and extremely good fuel consump-tion figures has been achieved. In the following design and the mechanics o
f the new engine are described, the topics of thermodynamics and application will be dealt with in a second section in the MtZ 2.
D eVelopMent D IEsEL ENGINEs
26
Description of the engine anD installation in the Vehicle
Alongside the V8 TDI which is used in the Audi A8 and Q7, the new V6 TDI biturbo engine represents the top-of-the-range die-sel engine option for the new Audi A6 and the A7. The objective of the development of this engine was to set new standards in the realm of sporty diesel vehicles, by means of an outstanding, dynamic build-up of torque and extraordinarily free-rev-ving characteristics. The intention was to combine excellent performance with good fuel consumption figures, which has been achieved by adopting the following effi-ciency measures from the basic engine: :thermal management
:frictional optimization measures :weight reduction
:eight-speed automatic transmission :start/stop system.
Other requirements for the engine’s devel-opment were that it should be built on the existing assembly line for the basic engine at the engine plant in Gy?r, and that it should utilize the maximum number of common parts offering the benefits of synergy with the V6 TDI monoturbo [1-4].The 46 kW increase in power output compared with the A8 version of the basic engine was achieved primarily by means of a new turbocharging system combined with optimized charge air cooling, as well as modifications to the fuel injection sys-tem. The heart of the new engine, the tur-bocharger system, is located at the rear of the inner V of the engine, and in the clear-
ance space above the gearbox, which can be seen in the cover figure in the partial section view from the rear [5].
? shows the installation of the V6 TDI engines in the C series. The installation of the V6 TDI monoturbo engine can be seen in ① (left), while ① (right) shows the
biturbo. In both pictures it is possible to see the limiting contours of the plenum cham-ber and the bonnet, as well as the position of the gearbox and exhaust system. In ① (left) can be seen the combined close- oupled oxidation catalytic converter with diesel particulate filter (DPF), which is
p ositioned behind the turbocharger . The exhaust leaves the turbocharger to the left, as seen in the direction of travel; it is turned through 180° and then flows through the DPF to the right-hand side of the gearbox. The exhaust system is not visible; it runs past the gearbox on the right, as seen in the direction of travel, and into the underbody. In the case of the biturbo, the installation space for the DPF has been used for the high-pressure turbocharger . The exhaust system does not cross over the gearbox but runs where it can be seen in ① (right), on the left-hand side of the gearbox. The oxida-tion catalytic converter can be seen; the DPF has been relocated into the underbody. Additionally the vacuum unit for the turbine switching valve and the electric actuator for the variable turbine geometry (VTG) of the small turbocharger can be seen. ① makes very clear the major challenge of fitting a V engine with two-stage sequential turbo-charging into the vehicle’s restricted engine compartment.
Dipl.-ing. richarD BauDer
is Head of Diesel Engine
D evelopment at Audi AG in
N eckarsulm (Germany).Dipl.-ing. Jan helBig
is Head of Diesel Engine Mechanics Development at Audi AG in
N eckarsulm (Germany).Dr.-ing. henning MarckwarDt
is Head of Mechanics Development
for Biturbo Diesel Engines at Audi
AG in Neckarsulm (Germany).
Dipl.-ing. halit genc
is Design Engineer in the Diesel
E ngine Development at Audi AG in
Neckarsulm (Germany).
A u t H o r s
? Engine package V6 tDI monoturbo and biturbo
27
01I2012
Volume 73
? lists the main dimensions and char-
acteristic data of the engine. The main geometrical dimensions match those of the basic engine. In order to deliver the high performance reliably in operation, the cylinder heads and the piston assem-bly including piston cooling have been enhanced. This article looks into these assemblies in more detail.
The oil and water pumps have also been revised. The oil pump has been adapted to meet the engine’s increased demand for oil resulting from the improved splash oil cooling of the pistons and the second turbocharger. As in the case of the basic engine, the pump is a controllable vane pump with its volumetric flow in -creased by widening the rotor by approxi-mately 25 %. As a further measure in response to the increased engine cooling required, a higher-capacity water pump has been fitted. In the case of the V6 TDI biturbo, a closed plastic rotor with a dia-meter of 72 mm and three-dimensionally curved vanes is used. As a result the volu-metric flow has been increased by approxi-mately 30 % at the design point in com-parison with the basic engine, with a simultaneous 7 % improvement in effi-ciency at the same operating point.cylinDer heaD
The cylinder head is subject to dynamic loading while the engine is running due to the cylinder pressure, as well as thermo-
mechanical loading due to temperature variation. The peak pressure has not been increased in comparison with the basic engine, though it is utilized across a wider engine speed range under full load, so increasing the overall loading. The ther-mal loading of the cylinder head rises as
the cylinder power output increases. ? shows the maximum material tempera-ture be
t ween the exhaust valves 1 mm below the surface of the combustion chamber plate. The two columns on the left depict the temperatures of the 150 kW and 184 kW versions of the basic engine under full load with a one-part water chamber in the cylinder head. When this unmodified geometry is used for the V6 TDI biturbo the temperature rises to a
c ritical level – with the increase
d risk of cracking of th
e combustion chamber plate as a result o
f thermo-mechanical fatigue after runnin
g for lengthy periods.
For this reason, a cylinder head with a two-part water chamber has been devel-oped for the high-performance engine, ?. The water chamber is divided into top and bottom sections, each supplied by way of separate feeds from the engine block. This arrangement enables a higher volumetric coolant flow (cooling jet) to be directed through the lower water chamber, which cools the areas between the valves and the injector seat. The upper water chamber is adjusted to allow lower volumetric flow by means of restrictor bores in the cylinder
head gasket. The cooling of the lands
? Main dimensions and characteristic data of the V6 tDI biturbo engine
?
Cylinder head material temperatures at 4500 rpm and 95 °C coolant temperature
DeVelopMent DIEsEL ENGINEs
28
between the cylinders is carried out from the cylinder head, as in the basic engine. The pressure difference between the upper and lower sections of the water chamber is used to propel the coolant. The principle of cross-flow cooling has been retained, as has the separate head-block cooling of the basic engine, controlled by the thermal management system [1, 3]. This solution has enabled the maxi m um temperature to
be lowered by 25 K, ③.
The separation of the two coolant jack-
ets results in an intermediate deck in the
cylinder head, which stiffens the structure
and enhances its strength. In the area of
the injector seat, for example, high assem-
bly and dynamic tensions are overlaid
with high temperatures. Calculations show
an improvement in the safety and security
which have been achieved at this point by
switching to the two-part water chamber
with intermediate deck, despite the higher
stress loads in the V6 TDI biturbo. The
new head concept thus combines high
mechanical strength with very low tem-
peratures for an engine of this perfor-
mance class, and as such also points the
way ahead for future high-performance
design concepts.
pistons
The major increase in power output from
the engine also meant that the pistons
needed to be optimized. The basic engine
in all its versions features a piston with
salt-core cooling ducts and a piston pin
running in aluminium. The compression
ratio is 16.8:1. The compression ratio of
the V6 TDI biturbo has been reduced to
16.0:1 by enlarging the piston bowl, ?.
The position of the cooling duct has been
moved slightly upwards and towards the
first ring groove.
T o improve strength, the V6 TDI biturbo
is fitted with a bushed piston with a DLC-
coated (diamond-like carbon) piston pin.
The DLC layer alleviates the tendency of
the pin to seize and reduces the friction in
this area. By using bushes with moulded
? Cylinder head: water cooling jacket design
? V6 tDI biturbo piston
29 01I2012Volume 73
bores, the pressure distribution between the pin and the piston is evened out and the risk of hub cracking is avoided. These measures enabled the pin diameter of the basic engine to be retained, meaning that the con-rod could also be retained as a shared component. The ring package is frictionally optimized as in the case of the basic engine. The higher positioning of the cooling duct and the optimized splash oil cooling enabled the bowl rim tempera-ture to be significantly reduced relative to the piston of the 184 kW engine, ?. This design offers potential for further power increases.
turBocharging systeM
? presents a schematic view of the com-ponent layout in the two-stage turbo-charging system. On the air side, the fresh air flowing in via the air filter and clean air system is pre-compressed by the low-
pressure compressor across the entire map range. In the high-pressure compres-sor, the pressure of the air-mass flow is increased further. The air is then cooled in the intercooler and routed into the engine via the throttle valve, central swirl flap and intake manifold. A self-regulating compressor bypass valve is installed in parallel to the high-pressure compressor. This valve opens depending on the com-pressor output of the low-pressure turbo-charger and the resultant pressure ratio upstream and downstream of the high-pressure compressor. The compression of the low-pressure stage is then sufficient to set the required charge pressure.
On the exhaust side, the high-pressure and low-pressure turbines are configured in series and both fitted with a bypass or wastegate. The bypass of the high-pressure turbine has a large cross-section, which can be infinitely adjusted by way of a turbine switching valve which is pneumatically actuated with vacuum. When the turbine switching valve is closed, the entire exhaust gas flow is partially relieved by way of the high-pressure turbine and then flows through the low-pressure turbine. The high-pressure turbocharger features VTG with an electric actuator motor. When this reaches its speed limit, the turbine switching valve is opened. In this case only part of the exhaust gas mass flow is then relieved by way of the high-pressure turbine; most is routed via the turbine bypass directly to the larger low-pressure turbine. The low-pres-
sure turbocharger is fitted with a wastegate
which regulates the charge pressure at high
exhaust gas mass flow rates.
? shows the turbocharging system
design as implemented for the V6 TDI
biturbo engine. The low-pressure turbo-
charger is housed in the rear area of the
inner V while the high-pressure turbo-
charger, rotated 90°, is positioned behind
the engine above the gearbox. The key
component of the turbocharging system is
the turbine housing of the high-pressure
turbocharger, via which the exhaust gas
mass flows are distributed within the sys-
tem. It incorporates the flange for connec-? Influence of optimized piston cooling on piston temperatures:
maximum bowl rim temperature at 4000 rpm
? schematic view of the
V6 tDI biturbo turbo-
charging system
DeVelopMent DIEsEL ENGINEs 30
tion of the exhaust manifold by way of a Y-piece as well as the flanges for the high-pressure turbine bypass, the low-pressure turbocharger and the exhaust gas recircu-lation line. The turbine switching valve, including seat and shaft, is housed in the turbine housing of the low-pressure turbocharger.
All the other components are grouped around these key components. On the left as seen in the direction of travel is the large vacuum unit, with position feedback for the turbine switching valve, and the electric actuator for the high-pressure tur-bocharger. On the right are the compressor bypass valve, the vacuum unit to actuate the wastegate and the charge air ducting. The compressor bypass valve is designed so as to widen its cross-section rapidly on non-stationary acceleration and yet still prevent unintentional opening due to engine vibration. The pressure losses occurring at the compressor bypass have been reduced by optimizing the geometry of the valve cone down to a minimum. The center housings of both turbocharg-ers are water-cooled. The water and oil supply is provided via external lines.
The turbine housing of the high-pres-sure turbocharger is the most complex cast component of the turbocharger
assembly. The areas of the component
that are subjected to hot exhaust gases
change depending on the position of the
exhaust flap. This results in inhomogene-
ous temperature distribution and therefore
in thermal stresses in the component. In
the course of design optimizations carried
out on the component, the number of
cores was reduced from 16 to eight and at
the same time the thermal stresses in criti-
cal areas were reduced to a non-critical
level. ? shows the number and layout of
the cores in the casting mould before and
after optimization.
With two-stage turbocharging, the
responsiveness of the engine is dictated
by the tight closure of the turbine switch-
ing valve. Even the tiniest leaks will lead
to significant loss of enthalpy for the high-
pressure turbine. Consequently, special
attention was paid during the develop-
ment process to the seal achieved by the
turbine switching valve. T o evaluate the
seal achieved by the turbine switching
valve, a pressure difference of 2.5 bar is
applied by way of the flap valve on the
component test rig and the resultant volu-
metric flow leakage is determined. In an
early phase of the project, two different
turbine switching valve designs were
compared in with regard to leakage:
: a centrally mounted changeover flap
valve (butterfly design)
: a side-mounted changeover flap valve
(swing valve design).
The tests carried out revealed at an early
stage that, in its new condition, the swing
valve offered significant advantages over
the butterfly design in terms of the seal
achieved, ?. The leakage of the swing
valve design as new is many times less
than that of the butterfly design. It also
proved much better over lengthy running
periods. As the swing valve also offers
major benefits in terms of flow pressure
losses because it is moved fully out of the
bypass duct, Audi chose to develop this
solution for series production. The large
bypass flap in combination with the high
turbine intake pressures do however
require high actuator forces in order to pre-
vent the flap from opening of its own
accord, even under transient operating
? turbocharging system of the V6 tDI biturbo (Htt)
? High pressure turbine
housing: optimization of casting tools
31 01I2012Volume 73
conditions. In order to satisfy these re -quirements a special long-stroke vacuum unit with a large effective cross-section has been developed. The unit has a position feedback feature in the form of a position sensor inside the unit, which has had to be adapted to the long stroke of the unit.
T o assess the influence of the seal achieved by the exhaust flap, acceleration was measured on a vehicle with new com-ponents and with components at the end of endurance testing. The defined maximum permissible leakage quantities at the end of endurance testing guarantee minimal time lag under acceleration in comparison with new components. This is key to the excel-
lent dynamic responsiveness of the engine
throughout the life of the vehicle.
suMMary
With the V6 TDI biturbo, Audi has
launched its most powerful six-cylinder
diesel engine to date. The engine gives the
C-segment cars extraordinarily sporty per-
formance along with low fuel consump-
tion, supplementing the range of Audi
V-engines below the V8 TDI and V12 TDI.
The two-stage turbocharging system has
been implemented in the restricted space
available with no need for compromise in
terms of thermodynamic design and long-
term mechanical durability. The higher
loading on the engine compared with the
basic engine has been taken into account
by means of optimization measures which
open up potential for further increases in
power output for both the biturbo and
monoturbo designs.
references
[1] Bauder, r.; Bach, M.; Fr?hlich, A.; Hatz, W.;
Helbig, J.; Kahrstedt, J.: Die neue Generation des
3.0 tDI Motors von Audi – emissionsarm, leis-
tungsstark, verbrauchsgünstig und leicht [the
new-generation 3.0 l tDI engine from Audi – low
emissions, high performance, good fuel economy
and lightweight design]. 31st International Vienna
Motor symposium, 2010
[2] Bauder, r.; Kahrstedt, J.; Zülch, s.; Fr?hlich,
A.; streng, C.; Eiglmeier, C.; riegger, r.: Der 3.0 l
V6 tDI der zweiten Generation von Audi – konse-
quente Weiterentwicklung eines effizienten An-
triebes [the second-generation 3.0 l V6 tDI from
Audi – consistent further development of an effi-
cient power unit]. 19th Aachen Colloquium Auto-
mobile and Engine technology, 2010
[3] Bauder, r.; Fr?hlich, A.; rossi, D.: Neue
G eneration des 3,0-l-tDI-Motors von Audi, teil 1 –
Konstruktion und Mechanik [New-generation Audi
3.0 l tDI engine, part 1 – design and mechanical
components]. In: MtZ 71 (2010), No. 10
[4] Kahrstedt, J.; Zülch, s.; streng, C.; riegger,
r.: Neue Generation des 3,0-l-tDI-Motors von
A udi, teil 2 – thermodynamik, Applikation und
Abgasnachbehandlung [New-generation Audi 3.0 l
tDI engine, part 2 – thermodynamics, application
and exhaust treatment]. In: MtZ 71 (2010), No. 11
[5] Bauder, r.; Eiglmeier, C.; Eiser, A.; Marck-
wardt, H.: Der neue High Performance Diesel von
Audi, der 3.0 l V6-tDI Biturbo [the new high-
performance diesel from Audi, the 3.0 l V6 tDI
biturbo]. 32nd International Vienna Motor sympo-
sium, 2011
? turbine switching valve leakage behaviour DeVelopMent DIEsEL ENGINEs 32
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