DEVELOPING TURBOCHARGERS FOR IMO TIER II AND IMO TIER IIIIMO Tier II and IMO Tier III regulations are currently the main drivers in the development of

medium and large sized diesel engines in marine applications, while in the area of gas engines

market demand calls for improved efficiency, combustion stability and increased power density.

Both of these factors dictate parallel development of turbocharging system providing a higher

boost pressure and optimised efficiencies.

40

DEVELOPMENT EMISSIONS

TURBOCHARGERS FOR NEW DEMANDS

With the new ST27 range of radial tur-bine type turbochargers, Kompresso-renbau Bannewitz GmbH (KBB) has addressed and continues to address the challenges of IMO Tier II, the second stage of emissions legislation from the International Maritime Organisation.

Based on the successful HPR range, single stage ST27 turbochargers attain pressure ratios up to 5.5 combined with high overall efficiency. Compared to the HPR range, the ST27 range has been ex -panded by two additional sizes.

The new design features of the ST27 range are described below, highlighting important results from qualification and field tests. Details of the ST27 turbo-chargers’ improved compressor and tur-bine performance are presented. Higher pressure ratios in combination with increased loading on the turbochargers posed a particular challenge with re -gard to bearing design and containment safety. How the ST-series turbochargers ensure stable rotor dynamics behaviour combined with effective rotor damping characteristics is likewise described. Con-tainment safety of the ST turbocharger series is demonstrated by the application of an innovative CAE simu lation concept in combination with hardware tests. A 500 hour endurance test on a medium-speed, heavy fuel oil (HFO) laboratory engine and a field test comprising around 16,000 running hours were carried out. During post-test inspections, all key parts were found to be in good condition.

TWO-STAGE TURBOCHARGING

The development of turbocharging sys-tems for diesel engines and for gas engines in an overall pressure ratio range from 6 to 10 is the next challenge. This can only be achieved using two-stage turbocharging. The development of new low pressure and high pressure ranges of turbochargers has begun. The concept of the two ranges is presented, as are the special design challenges for the high pressure turbochargers. It is emphasised that simulation has becomes more and more important in achieving an effective design process. The first pro-totypes of the new turbochargers will be available for tests on burner test rigs as well as on engine test stands in 2012.

TURBOCHARGERS FOR LOW NOX

Since its launch in 2001, the radial HPR series has been well received by the mar-ket, based on a pressure ratio that is high enough to cover some IMO Tier II appli-cations. More than 5,000 turbochargers from the HPR series have already been delivered to customers.

Higher requirements in terms of exhaust emissions such as those on oxides of NOx in IMO Tiers II and III for ships and EPA Tier 4 for locomotives can – and will be – fulfilled through on-engine measures in the majority of appli-cations. Exploiting charge expansion by the use of the Miller Cycle, charge tem-perature and thus NOx formation can be cut significantly. The reduced filling of the cylinder resulting from a shorter intake time prescribed by stronger Miller Cycles has to be compensated by higher charge pressures. This, in turn, requires a turbocharger system producing higher pressure ratios.

Furthermore, Miller timing and the corresponding reduction in the final tem-perature of the charge under compres-sion hold potential for an increase in the specific output of gas engines, by reduc-ing the tendency to combustion knock.

At the same time, higher efficiencies and enhanced performance remain the main demands made on turbocharging systems, but high reliability, ease of maintenance, operation safety and good response behaviour are no less important.

IMO TIER II

Engine manufactures have implemented the requirements of IMO II in different ways. With pressure ratios up to 4.5, the turbochargers of the HPR series can be used to enable moderate Miller timing. However, pressure ratios of 5 and more are demanded by stronger Miller cycles using inlet valve closure around -30° BBDC. The increase in charge air pres-sures of up to 1 bar at full load for an exemplary engine family with the tran-sition from IMO I to IMO II is shown in ❶. These are the applications for which KBB has developed the ST27 turbo-charger series.

The ST27 represents KBB’s seventh generation of single-stage, high pressure turbochargers. Two sizes have been added compared to the HPR series, ren-dering the ST series suitable for turbo-

AUTHOR

KLAUS BUCHMANNis Engineering Director at

Kompressorenbau Bannewitz GmbH in Bannewitz (Germany).

41 Special Edition MTZ I May 2013

charging engines in an output range from 300 to 4,800 kW.

The ST27 turbochargers have been developed for operation with pressure ratios of up to 5.5 to satisfy the demands of IMO II engines. This development is

characterised by the following essential features: : Same outline dimensions as HPR

(ST3 – ST6) : Compressor with recirculation

for map width enhancement

: Cooling of highly stressed compressor wheels

: Boreless compressor impeller : Bearing with long service life : Non-cooled housing

(optional water-cooled bearing housing for gas engines)

: Jet assist option on the compressor side. The ST3, ST4, ST5 and ST6 frame sizes of the ST27 turbocharger series have been in series production since 2010. In 2011 the ST27 series was extended by the hitherto largest radial turbine turbo-chargers produced by KBB, the ST7 which successfully completed its func-tion and performance tests on the newly-built test rig for large turbochargers in the KBB test centre at Bannewitz.

In addition to thermodynamic map tests, various functional verifications and inves-tigations were carried out in the scope of qualification tests on the ST27. Perfor-mance, shaft motion measurement and proof of containment safety will be treated below by way of examples, whereby the last two of these features, in particular, are decisive for the turbocharger’s reliability in commercial operation. Comprehensive studies were also carried out on compres-sor impeller cooling to keep the opera-tional temperature of the aluminium com-pressor impellers within the permissible component temperature range, as already detailed in previous reports [2], [3].

PERFORMANCE

The increase in pressure ratio from the HPR series (4.2) to the ST27 series (5.2) for the 100 % engine operating point is shown in ❷. At the same time it was also possible to maintain the same level of compressor efficiency. The im pairment of the map width that was to be expected from the increased pressure ratio was compensated by making use of recircula-tion at the compressor inlet. In addition to the compressor, the turbine also had to be dimensioned for the new, higher turbine expansion ratios. As show in ❸ it was possible to significantly improve efficiency compared to the HPR turbine.

ROTOR-DYNAMICS AND SHAFT MOTION MEASUREMENT

The ST27 series called for a basic re-evalu ation of the dynamic properties of the rotor, mainly due to the following design changes:

❷ ST5 turbocharger compressor map

3.5

4.0

4.5

5.0

5.5

20 25 30

Pre

ssur

e ra

tio

BMEP [bar]

IMO II(ST27)

IMO I(HPR)

❶ Comparison between pressure ratios for IMO Tier I and IMO Tier II

DEVELOPMENT EMISSIONS

42

: Reduced bearing spacing : Mass distribution more

towards the turbine : Changed distances of component

centres of gravity relative to bearings due to different compressor impeller fixation.

The ST rotors’ natural frequencies were calculated for different bearing designs. Only rigid body modes of conical and cylindrical shape occur during turbo-

charger operation when the turbine and compressor wheels run synchronously. The two oscillating modes dominate the amplitudes due to rotational bending. The first elastic bending mode occurs above the maximum speed of the turbocharger.

Shaft motion measurements (SMM) were carried out on the two bearing planes and on the compressor for the ST6 size to confirm the rotor bending characteristics of the two rigid body

modes. Other sizes were subjected to a simple check measurement on the com-pressor plane and clearances were opti-mised on the basis of the results from the shaft motion measurements.

Rotor stability was investigated in experiments by varying the rotor imbal-ance and the oil parameters in the bear-ings. As a consequence and following evaluation of the experiments, KBB had to abandon its previous standard design for a compact bearing and change to a radial bearing with squeeze film damp-ing (SFD). The profiled, stationary bush-ings in the squeeze film damper bear-ings showed a number of very good sta-bility characteristics in the experiments.

Due to the use of squeeze film damper bearings, the sub-synchronous vibrations that can cause rotor instability problems (oil whirl/oil whip) were significantly reduced. Speed-synchronous vibrations dominate in the FFT diagram with no conspicuous critical speeds due to higher amplitudes. Absolute instabilities which may result in a total turbocharger failure do not occur, thanks to the use of the squeeze film damper bearings.

The rotor-dynamic investigation of the squeeze film damper bearing is shown in ❹. In these trials, excursions of the compressor impeller were determined in

1 1.5 2 2.5 3 3.5 4 4.5 5

Eff

ecti

ve tu

rbin

e ef

ficie

ncy

Turbine expansion ratio [-]

ST5 turbine

HPR5000 turbine Δη = 2 %

❸ ST5 turbine characteristics

Probe X [%]

-100

-100

-50

-50

0

0

50

50

100

100

Pro

be Y

[%

]P

robe

Y [

%]

Pro

be Y

[%

]

p_oil = 3.0 bar; t_oil = 60 °CImbalance conforming to international standards (DIN ISO)SSV ... subsynchronous vibration, 1 EO ... first exciting order

Max. clearance circle impeller nose

Max. orbit

-100

-50

0

50

100

-100

-50

0

50

100

200 400 600 800Frequency [Hz]

500010000

1500020000

2500030000

35000

Spee

d [rp

m]

0

12

Am

plit

ude

prob

e X

[%] 1 EO

2020

1616

88

44

SSV

11

2

3

11

2

3

❹ Spectrum and orbit of shaft motion measurements

43 Special Edition MTZ I May 2013

experiments by means of shaft motion measurements on the compressor plane. The oil parameters and the balance state of the rotor were varied in numerous fur-ther trials. Even at ten times the permis-sible imbalance, the dynamic radial shaft motion is considerably below the KBB limit.

CONTAINMENT

Proof of the ST27 series’ containment safety on the compressor and turbine sides was provided, based on the require-

ments defined by the Classification So -cieties. In addition to hardware tests, extensive use was made of simulations, as demonstrated in the example of the turbine side tests.

The task was to demonstrate the con-tainment safety of the housings in the event that the turbine wheel bursts at 140 % of its maximum operating speed. The simulation was carried out using the LS-DYNA software. Hardware tests car-ried out at 1.2 times maximum operating speed were used to validate the simula-tion. The speeds were reliably achieved

on the test rig, and produced a sufficiently comparable damage pattern. Turbine wheel strength had to be reduced to induce bursting at the specified speed. The turbocharger on the test rig immedi-ately after the burst test is shown in ❺. The supplementary outer burst protec-tion ensures that fragments escaping from the housing in the event of a pene-tration are safely contained and thus do not endanger people or equipment. The test results demonstrate the design’s safety for bursts up to the test speed. The turbocharger structure completely dissi-pates the energy which is released in the event of a burst.

The test results were then compared with the corresponding simulation model and assessed. It was possible to confirm the plausibility of the simulation, and thus the correct choice of material pa rameters and boundary conditions, by a direct com-parison of the global and local damage.

As shown by the comparison in ❻, the simulation maps reveal not only the global behaviour but also the local over-strain. Following the successful valida-tion of the simulation, it was then used to further assess the required speed (1.4 n max). Apart from the prescribed “3-disc fracture”, various potential frac-ture patterns were also simulated and their resulting structural stress deter-mined so as to achieve a high degree of certainty, as shown in ❼.

The simulation results showed that the left burst pattern, which almost corre-sponds to the natural burst behaviour, led to the highest strain on the components. All critical housing and flange areas were assessed and the design adapt ed to the requirements using an iterative process. Containment safety was demonstrated for the worst case with the introduction of a burst ring and outer burst protection, and

❺ Test rig immediately after the turbocharger containment test run

❼ Simulated cases of turbine burst

❻ Comparison of simulated damage (left) and actual damage (right) in the flange area. The marked e area on the outlet side of the turbine housing is an example of successful validation

DEVELOPMENT EMISSIONS

44

the safety of the ST series’ turbine side was proven for the burst cases a) and b).

FIELD TRIALS

The turbocharger qualification process also includes engine tests. These comprised: a) 500 hour continuous test rig operation

on a six cylinder, 20 cm bore develop-ment engine rated 1,200 kW at 1,000 rpm equipped with an ST4 turbocharger

b) Field tests aboard the Viking Line ferry Viking XPRS on eight cylinder, 20 cm bore engines rated 1,400 kW at 1000 rpm and equipped with ST5 turbochargers: inspections took place at 1,000, 4,400, 8,000, 12,000 and 16,000 operating hours.

Both engines were operated on HFO over the test period. Comparable results were obtained for both turbochargers. Fouling due to combustion residues on the tur-bine side was kept to a normal level through regular turbine washing (at ap -prox. 50 hours), and thus likewise im -pairment of performance due to fouling. Very little bearing wear was found on the thrust and radial bearings on either the compressor or turbine sides. As shown in ❽, the requirement of bearing service lives of at least 25,000 operating hours was thus achieved.

TWO-STAGE TURBOCHARGING FOR IMO TIER III

Different technologies are currently under trial for IMO III engines. These include

extreme Miller cycle timing, exhaust gas recirculation, water injection and exhaust gas aftertreatment (SCR and exhaust gas scrubbers) as well as the use of gas (LNG) in spark ignited or dual-fuel gas engines.

Among the different approaches to ensure compliance with the IMO Tier III exhaust gas legislation in emissions control areas (ECAs), where an 80 % reduction in NOx is specified vis-à-vis IMO Tier I, high pressure turbocharging

with overall pressure ratios of 6 to 10 is the biggest challenge in terms of turbo-charger development. These pressure ratios can only be achieved using two compressor stages. Aside from the use of two turbochargers, this type of turbocharger system also contains an inter mediate cooler and, advisably, a bypass control for the high pressure and low pressure turbines. Should an additional high pressure exhaust gas

❽ Inspection after a 16,000 hour field trial

recir culation (HP EGR) be included, the system be comes very complex and the matching of the turbocharging system to the engine will be clearly more exact-ing than in the past. KBB has accepted this challenge and has further invested in 1D simulation.

A two stage turbocharging group with high pressure EGR and bypass control and its mapping in a simulation model (GT Power) are shown schematically in ❾. High pressures are the special chal-lenge when actually developing the tur-bochargers. A new generation of com-pressors and turbines is needed for the large map widths and optimum efficien-cies required at low pressure ratios. New design solutions need to be pro-duced to ensure oil and gas tightness under the very high absolute pressures.

The targeted map for a high pressure compressor is shown in ❿. From this it becomes obvious why large map widths are required. Both variable valve timing and operation with exhaust gas recircu-lation which has to be switched off non ECA operation (or in a tunnel in rail applications) play a decisive role in this connection.

Com

pres

sor

pres

sure

rat

io (

tota

l-to

tal)

1.0

1.5

2.0

2.5

3.0

3.5

Air flow rate V298 [m3/s]

Locomotive operation with EGR

Locomotive operation with EGR switched off

Variable valve timing

Conventional engine operating line

❿ Compressor map for a high pressure two stage turbocharging system

…

T C EGR-TC

T C

C T

HP-TC

LP-TC

Engine

EGR-TC

HP-stage

LP-stage

❾ Two stage turbocharging system with HP EGR and bypass control

DEVELOPMENT EMISSIONS

46

To realise two stage turbocharging, KBB is working on the development of a low pressure turbocharger series and a high pressure series. The turbochargers in the low pressure series are based on the HPR series and can cover the same throughput ranges. They are complemented by a size 7 axial turbine turbocharger. The turbo-chargers in the high pressure series are also derived from the HPR series and will comprise the sizes 2 to 6. However, external dimensions will change due to the higher ab solute pressures which have to be controlled (see above). The first prototypes of the two series were built in 2012. Engine trials began following tests on the turbo-charger test rig engine trials. For turbocharging engine outputs up to 5 MW, both versions will be available to the market in time for the January 2016 enactment of the IMO Tier III exhaust gas legislation.

SUMMARY AND CONCLUSIONS

The successful launch of the ST27 turbocharger series allows KBB to turbocharge IMO Tier II generation diesel and gas engines using pressure ratios of up to 5.5. Serial deliveries of the ST3 to ST6 frame sizes started in 2010. In 2011 trials also began on the ST7, the series’ largest turbocharger covering engines with outputs up to 4,800 kW.

Aside from achieving the thermodynamic tar-gets, the biggest challenges associated with the development of the new series were to control the higher stresses on components, cope with more demanding rotor-dynamics and provide proof of containment safety.

To provide turbocharging technology for IMO Tier III diesel engines and gas engines in an over-all pressure ratio range from 6 to 10, KBB is deeply involved in the development of low and high pressure turbocharger series for use in two-stage systems. First prototypes have been rig tested and delivered for on-engine testing.

Another interesting field of activity is high pres-sure exhaust gas recirculation, which requires special turbochargers for exhaust gas compres-sion. Simu lation becomes increasingly important for an optimum matching of very complex future turbocharging systems to their engines. Accord-ingly, KBB has carried out fundamental investiga-tions in this field.

REFERENCES [1] Kramer, U.; Buchmann, K.: HPR turbochargers – Field experience and development status of new ST27 turbocharger range, Worldwide Turbocharger Conference 2009 Hamburg[2] Drozdowski, R., Buchmann, K. ST27: A new generation of radial turbine turbochargers for highest pressure ratios, CIMAC Kongress, 2010 Bergen, Paper No. 42 [3] Schulz, O., Buchmann, K., Mostertz, H., Jirschitzka, J.: KBB ST27 Baureihe: Experimentelle Untersuchungen und Betriebs-erfahrungen, 16. Aufladetechnische Konferenz 2011 Dresden

Special Edition MTZ I May 2013

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