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Boosting the Future -Concept Layout and Design ofAdvanced Charging Systems

One of the key technologies for future Diesel and gasolineengines will be concepts based on advanced boostingsystems.

Downsized S.I.-engines will be charged, especially in com-bination with direct injection, to combine their fuel savingpotential with driving fun, as the FEV TurboDISI shows.

With regard to Diesel engines, the trend towards higherspecific power outputs lead to an increasing need forcharging systems, which can provide sufficient boost pres-sure over a wide engine speed range at full load. Morestringent emission regulations will lead to increased EGR-rates at higher speeds and loads than today, also requiringnew charging concepts.

FEV has built up extensive knowledge in the field ofcharged engines for more than 20 years. During this time,FEV has developed a large number of turbocharged Dieseland S.I.-engines as well as supercharged engines to seriesproduction, from smaller engines for passenger cars toheavy duty Diesels for railway applications.

Here some tools and methods shall be discussed, focusingon the development of advanced charging systems like

• exhaust manifold integrated T/C,• hybrid charging concepts,• 2-stage T/C concepts.

The first step in the development of a chargedengine is the layout of the overall charging sys-tem. Due to the very stringent package restric-tions of modern engine and vehicle designs

there is a big challenge to find the right compromise bet-ween performance, durability and arrangement in the en-gine compartment. Meanwhile the main challenge for thedesign is to find tailor-made solutions for every en-gine/vehicle concept.

The work tasks must solve very specific questions, whiche.g. include the adaptation of the turbocharger housing tothe exhaust manifold. Overall, the following advantagescan be achieved with those design solutions:

• compact design• elimination of gaskets and bolts• elimination of possible leakage areas• weight optimisation• cost reduction

The layout of the gas exchange system is sup-ported by gas dynamics simulation tools. FEVmainly uses GT-Suite by Gamma-Technologies,supported by its own in-house developed sub-

routines for inter- and extrapolation of turbocharger data. Adatabase of T/C and S/C data helps setting up simulationmodels very quickly.

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Using these tools, different hybrid charging concepts havebeen evaluated in numerous projects. The graph belowshows for example the load response of a gasoline enginewith a combination of T/C and Roots-S/C.

By detailed analysis, the matching of engine and chargingdevice of this system was optimized in advance.

To prove the low-end torque potential derived by simula-tion, the concept was packaged and applied to a FEVdemonstrator vehicle. For deactivation of the superchargerat higher engine speeds, a mechanical clutch was devel-oped, which is self-activating based on a so-called inversecentrifugal principle. Using an ASCET-System, additionalfunctions for control of main throttle and bypass flap were

realized.

The performance prediction was finally confirmed in ademonstrator vehicle.

As described in the introduction, future dieselengine charging technologies will have to bedeveloped towards higher boost pressure levelsand wider operation range. But todays state-of-

the-art VTG turbocharger is a thoroughly developed high-tech component. Further improvements are rather expen-sive and the potential, especially in terms of boost pres-sure ratio, is limited. Therefore, also for diesel engine ap-plications the hybrid charging or 2-stage charging is a cost-efficient way to optimize the overall performance of an en-gine.

For 2-stage charging systems, it is important to note that atleast in case of passenger cars, this does not really meancompressing the air twice, but instead refers to two turbo-chargers of different size e.g. in serial configuration. Atlower speeds, the smaller, high-pressure stage is workingwhile the low-pressure compressor is bypassed. In a tran-sition range, the exhaust flow is partly bypassed aroundthe high-pressure turbine, building up power in the low-pressure stage. At higher engine speeds, the high-pressure stage is completely bypassed. This is illustratedin the next graph, showing the pressure ratios and opera-tion lines of a 2-stage turbocharger system at full load.

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One of the main tasks of the development work is thematching of the two stages in steady-state and transientoperation, focussing especially on a smooth transition be-tween both stages.

The following graph shows the full load torque curves oftwo different diesel engine charging concepts, which werelaid out by FEV, compared to a scatterband of state-of-the-art diesel engines and the BMW 6-Cylinder in-line enginewith Variable Twin-Turbo, which was introduced in 2004.

500 1500 2500 3500 4500 5500Engine Speed rpm

BM

EP

@ F

ull

Lo

ad

60 70 80 kW/ltr

Pass.Car Dieselwith 1-stage VTG-T/C

BMW R6 Variable Twin-TurboFEV Layout S/C + T/CFEV Layout controlled 2-stage T/C

In case of complex configurations like 2-stage chargingsystems, it is important to take the flow situation into ac-count. By means of CFD calculations, the exhaust flowfrom manifold to first stage can be optimized. Also, the flowbetween the stages and through bypasses can be simu-lated, aiming at uniformal flow to the turbine housings, lowpressure losses and reduction of flow disturbance. Thistask still is challenging, for example concerning definitionof boundary conditions for such calculations or predictionof the effect on turbine and compressor efficiency.

Therefore, individual solutions must be investigated foreach customer. An example is shown in the following fig-ure. Moreover, engine testing of prototype systems is to becarried out to validate any simulation work.

0.000.020.040.060.080.100.120.140.160.180.200.220.241.0

1.5

2.0

2.5

3.02 stage turbo: LP turbo

steady-state

transient65 %68 %70 %

55 %

60 %65 %

68 %70 %

72 %73 %

74 %

75 %110000 1/min

90000 1/min

70000 1/min

50000 1/min

30000 1/min

130000 1/min

150000 1/minReference State

pref = 1bar

tref = 293.15K

Pre

ssur

e R

atio

[ ÷

]

corr. Volume Flow [ m3 / s ]

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.081.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.62 stage turbo: HP turbo

steady-state

transient

240000 1/min

90000 1/min

60000 1/min

63 %

63 %

69 %

35 %

55 % 60 %

40 %

45 %

50 %

55 %

65 %

67 % 60 %

190000 1/min

160000 1/min

120000 1/min

220000 1/min

Reference State

pref = 1bar

tref = 293.15K

Pre

ssur

e R

atio

[ ÷

]

corr. Volume Flow [ m3 / s ]

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With the new charging concepts and the result-ing high specific engine power output the ex-haust manifold becomes a high loaded part

caused by exhaust gas temperatures of more than1000°C. The high gas temperatures lead to high materialtemperatures which, in combination with restraint expan-sion capability, lead to the main fatigue mechanism for ex-haust manifolds - TMF (Thermo-Mechanical-Fatigue).

Though a best possible temperature distribution is desir-able for TMF assessment, for existing designs early tem-perature measurement by thermoscan or thermal paintmethods can be performed.

The changing thermal stresses are generated in the ex-haust tube branches as a result of restraining high tem-perature parts against the “cold” cylinder head. Thesestresses can produce premature cracks and high deforma-tion of the manifold. For short development times FEA be-

came one of the most important tools to predict fatiguemechanisms where before heat-cycle endurance tests overa long period of time had to be conducted. Lifetime predic-tion within short time periods now is possible.

In combination with computerised structural optimizationwe earn further potential for reduced development time byeliminating classic “handmade” iteration loops. The shapeoptimization tool controls the closed loop cycle of thermal,mechanical and pos. fatigue analysis and provides an op-timized model for each automatic iteration loop until thedesign corresponding to the supposed optimization targetis reached. Possible optimization targets can be fatiguelife, strain and weight.

Thermal Paint Thermoscan

Temp. [°C]

High

Low

High

Low

Temp. [°C]

2.7 mmBase ModelOptimized Model (Loop 30)

CONTACT: Dipl.-Ing. Oliver LangFEV MotorentechnikNeuenhofstraße 18152078 Aachen, GermanyPhone: (+49)241 5689-652Fax : (+49)241 5689-507E-Mail: Lang_O@fev.dehttp:// www.fev.com