PublicationAir/Oil Separator with Minimal Space Requirements in the Crankcase Venting System

Dr. Pedro Bastias • Thomas Brückle • Dimitrius Caloghero • Dr. Dieter GraflThorsten Sattler-Lägel • Bernd Spaeth

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Reprint from

MTZ Worldwide

Volume 66, No. 12/December 2005

Vieweg Verlag

GWV Fachverlage GmbH

Wiesbaden

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Air/Oil Separator with Minimal Space Requirements in the Crankcase Venting System

An important function for crankcase ven-ting is the separation of oil mist from blow-by gas. In this area, engine builders place widely varying demands on their development suppliers. Not only must air/oil separator systems be highly efficient, robust, functionally reliable, flexible and compact, they must also be cost-effective. REINZ-Dichtungs-GmbH, a DANA Corpora-tion company, set up a global development team to take on the challenge. The result is one of todays smallest and at the same time most efficient air/oil separator sys-tems – the Multitwister.

1 Introduction

Today and in the future, the demands placed on crankcase venting systems for new internal combustion engines will continue to increase, mainly due to tighter emission legislation, longer service inter-vals, increased power densities (down-sizing combined with turbochargers), direct injection and dethrottling (espe-cially petrol engines). Consequently, the recirculation of coarse and, above all, fine oil particles from the blow-by gas will be-come a central topic for the development suppliers involved, in addition to system sealing and pressure control.The concept and design of an overall

system automatically requires that the following boundary conditions and details are known:- Blow-by map with changes during

service life- Secondary air flow rate in l/min (e.g. by

means of vacuum pumps)- Map of intake manifold vacuum- Quantity and nature of the oil particles

(raw gas)- Target value for the oil content in the

blow-by gas downstream of the separa-tor system in g/h (clean gas) as well as the oil entrainment curve (with diesel applications)

- Permissible crankcase pressure con-ditions

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- Engine-related possibilities for recircu-lating the precipitated oil (drainage)

- Operating point-dependent behaviour of system components such as pressure control valves, throttles, check valves, and the usually multi-stage air/oil separation system itself.

Initially, and depending on the develop-ment stage of the internal combustion engine and its degree of innovation and technology content, less than 10 % of these necessary boundary conditions are known. A further 80 to 85 % of the design criteria for crankcase venting (including the oil mist separation) will be determined during the various prototype construction phases. Usually, reliable parameters are available only shortly before the start of series production, or are subjected to re-configuration after the pilot phase, based on the results of first field tests.The following describes the construc-tion, function, and development of a highly efficient air/oil separator system that meets the requirements in terms of utmost flexibility and simultaneous

minimum reconfiguration efforts, as well as simple and therefore cost-effective as-sembly and joining techniques. Excellent results are achieved if the engine-related development phase is consistently accom-panied by numerical, empirical, and map -based development tools, as well as suitable laboratory and engine test bench testing.The technical and commercial compari-son of Multitwister systems and estab-lished passive applications is intended to provide orientation and guidance. Passive separators are defined as systems in which the oil mist separation is effected without external energy [1], and is based exclusively on the kinetic energy of the blow-by gas and the particle inertia.

2 Multitwister air/oil separator sys-tems

2.1 DesignParallel-connected axial cyclones with two 180-degree guide spirals [2] with opposed senses of rotation are described

as Multitwisters. For design reasons, the Multitwister is built using two identical twister plates, which are assembled in a mirror-inverted manner so that the two pairs of opposed spirals butt against each other, Fig. 1.

2.2 FunctionThe oil-bearing blow-by gas is first accel-erated linearly in the entry duct, followed by rotational acceleration in the area of the first guide spiral. At the transition to the second guide spiral, the gas flow experiences strong turbulence, Fig. 2. Because of the associated high accel-erations and the limited spatial distances, most of the oil droplets impinge on the outer walls. As a result, they form a film on the wall, which is transported from the outlet duct to a plenum chamber by the gas flow. Moreover, the gas flow is par-tially linearised again [3] by the second spiral under recuperation of swirl energy. Depending on the design of the tube end, the wall film is drained as a trickle or in large drops.

Twister plate A Twister plate A

MultitwisterTM- system

180° rotation

Fig. 1: Construction of a Multitwister air/oil separator system with two identical injection-moulded parts.

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2.3 Optimum geometric shapeDue to the highly diversified challenges posed by geometric and physical bound-ary conditions, an experimental approach using the trial-and-error method is not the most efficient one. Therefore, the Multitwister‘s geometric parameters were optimised using numerical and statistical methods (Computational Fluid Dynamics CFD, and Design Of Experiments DOE), with the aim of obtaining a maximum ratio of oil separation to pressure loss.The goal of this multi-stage, statistically-based ratio optimisation by means of DOE is to find the best geometric arrangement of the guide spiral angle, duct diameter and spacing, as well as the entry and outlet geometry of the Twister ducts, and the oil spray control. Furthermore, the geo metric boundary conditions, surface roughness, temperature, viscosities and densities of the flow phases involved must be taken into account. During this process, all variables were always changed within a range that still ensured unproblematic manufacturing using injec-

tion moulding.For the numerical calculation, which fol-lowed a preceding convergence analysis, the oil droplet spectrum itself was taken into account as a separate flow phase, with three different approximate droplet sizes (e.g. 1, 3 and 10 µm). Based on experience, capillary effects, wall friction, and turbulences were described using suitable models [4]. After determining practice-related, variable marginal con ditions for entry and outlet flows, it was possible to calculate the pressure loss, degree of turbulence, flow speed, and friction loss of the blow-by gas, and observe their influence on the separa-tion behaviour of the differently sized oil particles as well as spray control of the outlet flow.Fig. 2 shows examples of the flow paths through a Twister duct. The colours indicate the decrease of static pressure (pressure loss) within the system (pres-sure loss from red to blue). By applying a numerical flow simulation (CFD), the first draft of the Multitwister could be

improved sustainably by reducing geo-metrically avoidable eddy flow losses to a minimum. Furthermore, a mathematical examination of numerous Twister ducts with an arbitrary diameter and operated in parallel showed that there is practi-cally no fluidic interaction between the individual ducts.

2.4 Application reportThe Multitwister design of a standardized injection-moulded component covers a wide range of blow-by volumes. For ex-ample, a standardized 3 mm Multitwister plate with 44 individual ducts for fine separation, as shown in Fig. 1, allows nominal blow-by flow rates of between 2 l/min and about 80 l/min to be cleaned, simply by replacing the spiral guide in-serts with dummy pins in the injection mould. Similarly, an application for 350 l/min for the commercial vehicle sector has recently been implemented with a parallel arrangement of several Twister plates. In fact, any required flow rate is conceivable. Fitted with larger duct

Fig. 2: Numerical calculations of entry and outlet flow conditions are the basis for detailed geometric optimization. Shown here: the pressure loss across a Twister element.

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diameters of 6 to 13 mm, the Multitwister can also be used for pre-separation, Fig. 3. The purpose of pre-separation is to remove surging oil or oil foam upstream of the fine separation stage. These oil quantities can be caused by entrainment in large secondary airflows or by foaming due, e.g. to crankshaft whipping. The Multitwister pre-separator removes the coarse oil particles. Furthermore, the space saved when compared to conven-tional pre-separators can be used as a plenum chamber for the oil.

2.5 Space requirements and mounting positionThe development of air/oil separator sys-tems must also comply with the increas-ing demand for less installation space, plus the need for reduced height in the interests of pedestrian collision protec-tion. With present-day cyclones, in which the axial flow direction in the cyclone is reversed (reverse flow cyclones [5]), and the precipitated oil drains from the cy-clone in the opposite direction to the gas,

the separation efficiency is reduced, if oil drainage is not in the direction of gravity. Our investigations indicate that this is due to the occurrence of pressure pulses in the vicinity of the conical drain exit of the cyclone. Consequently, the angled mount-ing of cyclones to reduce installation height is limited by functional disadvan-tages. In the Multitwister (straight-through cyclone [5]), both the gas and the precipitated oil flow in the same direction at high speed. This forced flow has the advantage that the operation of the Multitwister is independent of its mounting position. Thanks to the simple, two-component plate construction, which can have any geometric shape, and because there is no need to separate oil and gas flows in and behind the air/oil separator, even extremely limited instal-lation spaces are no problem. Simple flow guidance and the flat design of the Multitwister allow it to be integrated eas-ily into a cylinder head cover or a blow-by conducting module, e.g. simply by clip-ping it into place.

2.6 Functional reliabilityIn general, the function of components in the entry and blow-by ducts is endangered by blow-by gas containing condensate and ice crystals, as well as by low-grade resinous oils with a high soot content, and by foamy, mixed derivates (especially petrol engines). However, due to the forced flow and high gas speeds, i.e. high dynamic pressures, deposits in the rel-evant oil separation areas are prevented. Therefore, the Multitwister is extremely resistant to fouling and icing. Worst-case examinations with resinous oil grades and foaming agents, in which possible cold/hot variations of the Multi-twister and air/oil mixtures containing water droplets were tested between -35 °C and +5 °C, showed that system operation remained optimal over time without a notable increase in pressure loss. Even in the cold-start, short-run test, in which oil mist at +5 °C is drawn through the system for a short period, followed by an inoperative cooling period during which the system temperature falls below the

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Fig. 3: Cross section of an application example with Multitwister as a coarse oil separator. Fig. 4: Icing and fouling tests on a test rig and ...

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freezing point, no negative effects were observed.Endurance tests are used during develop-ment to test the engine-specific freezing and fouling behaviour, Fig. 4. For this pur-pose, the blow-by gas is cooled in a heat exchanger, and then drawn through the Multitwister, which is kept below freez-ing point. This enables every conceivable blow-by and oil quality scenario to be tested. In addition, long-term vehicle tests using the company‘s own car pool simulate all-year but non-specific every-day operation.

2.7 Comparison with conventional systemsA comparison table of the Multitwister with other established systems on the market, as shown in Fig. 5, serves to provide orientation and guidance. The table also includes important basic data such as mean separable particle sizes, scalability, space requirement, and appli-cation area. In terms of performance and characteristic curve, the Multitwister is

comparable with the widely used (multi-) cyclone, Fig. 6.In order to examine the features and advantages/disadvantages of both high-performance systems in more detail, the comparison table in Fig. 7 can be used, which not only shows the general system characteristics such as the separation principle, construction, and mounting method, but also the technical perfor-mance, robustness, and adaptability to the requirements of continually changing marginal conditions during the develop-ment stage. The subjects of cost pressure and volume reduction are also covered in the table with the help of typical sys tem-related calculations for cost and installation space.

3 Development environment for new air/oil separator systems

In addition to the increasing demands placed on oil mist separation, the devel-opment environment must also be aimed at and adapted to optimised crankcase

... in the vehicle prove the robustness of the Multitwister air/oil separator system.

Separation Type Volume Labyrinth Multi-Cyclone Multitwister

Separation Inertia Impact Centrifugal force Centrifugal forceprincipleParticle size, ≥ 8 mm ≥ 6 mm ≥ 0,6 mm ≥ 0,6 mmtypicalScaleability 0 + + ++Space requirem. 0 0 + ++ApplicationareaPre-separation x x x

Fine-separation (x) x x

X50 [mm]0 1 2 3 4 5 6 7 8 9 10

28

24

20

16

12

8

4

32

0

Pres

sure

loss

[mba

r]

Labyrinth

Volume/Labyrinth

Multi-Cyclone

Multitwister ø 3 mm

Increasing Blow-by

venting. For a long period, it was suffi-cient to separate the coarse oil particles as completely as possible. But nowadays, the task involves the maximum removal of oil mist from the crankcase venting airflow containing finest particles of <1 µm by utilizing their inertial energy. Two other boundary conditions are the maintenance of a crankcase pressure between 0 mbar and approx. -30 mbar within the range of the engine map, and the reliable separation and recirculation of oil to the crankcase during the engine‘s entire service life and at all temperatures, also in exceptional circumstances such as tilted operation and oil overfill. Additional requirements are measures against oil entrainment, and ensuring maintenance-free system operation.

Fig 5: Comparison of the most important air/oil separation concepts.

Fig. 6: Comparison of the separation and pressure loss behaviour of different air/oil separation concepts with increasing blow-by flow volume.

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3.1 Essential elementsA suitable development environment con-sists primarily of an engine test bench, an aerosol test bench, and numerical flow simulation (CFD), Fig. 8. A special meth-od, »map-based oil separator design«, is applied, in which an algorithm describes and specifies the Multitwister system completely, including pre-separation and fine separation as well as pressure con-trol over the entire operating range.

3.2 Map-based oil separator designThe input variables for the map-based oil separator design are the engine-specific maps of the blow-by, and inlet manifold vacuum, as well as the pressure differ-ence across the throttle flap in the case of a vented crankcase, Fig. 9. The mass flow and pressure-dependent character-istics of the pre-separators and fine sepa-rators, and those of the pressure control valve and other throttles and check valves are taken into account in the calculation. The result is a map of the crankcase pressure and separator characteristics (e.g. X50 value), Fig. 10. Variations of the separator and valve characteristics permit an optimum balance to be found for spec-ified crankcase pressure, oil drainage, and maximum performance of the air/oil separator system.

3.3 Typical procedure for designing a systemEngine test bench measurements provide the basic data, such as speed-dependent and torque-dependent maps of the blow-by and inlet manifold vacuum, as well as the oil quantities at the entry duct and in the uncleaned flow of blow-by gas. Particle size distributions are measured using an optical particle counter. Within a size range extending from 0.2 µm up to 12 µm (typical particle spectrum of an en-gine), the differently sized oil particles are assigned to size classes, thus enabling the size distribution ahead of the air/oil separator system to be determined.On the aerosol test bench, an oil mist is

Separation prinziple

Typical ApplicationAssembly / position

Verbindungs- /Fügetechnik(incl. cover flow chamber)CapabilitiesSeparation effectiveness(Drop size, of which 50 % are separated, at 20,40, 60 L/min Blow-By) . . .with pressure loss

Safety against oil pull through

Cold behaviour

Danger of sooting

Adaptability / scaleability of performanceto system requirements(Development / prototype phase)

Adaptability / scaleability of performanceto operating level

Costs / WeightTypical space requirementfor a systemSystem weightMaintenance required(relative) Costs, estimated(Material, jointing process costs)

passive,Mass inertia

Main / fine separatorSeparation

depending onposition

3 (+1) parts, sealingthrough 3 jointing steps

20: 3.8 mm 2.0 mbar40: 1.5 mm 4.0 mbar60: 1.0 mm 7.0 mbar

bad,pre-separation

necessaryPressure increase,function restricted

through separation-geometry change

sensitivethrough internal drainage

hardly possible,Design only by CFD-

calculation

possible, however onlythrough (dis-) continuouscircuits of multiple flowchambers / openings

0.15 - 0.25 dm3

30 - 70 gno

100 %

passive,Mass inertia

depending on designSeparation relativelyposition independent,

however prefered direction2 (+1) parts, sealing

through 1 jointing step

As pre-separator20: 4.0 mm 1.0 mbar40: 3.1 mm 1.2 mbar60: 2.2 mm 2.2 mbarAs fine-separator20: 2.9 mm 0.6 mbar40: 1.4 mm 3.0 mbar60: 1.0 mm 6.5 mbar

good,Multi-step concept possible

and recommendedmoderate pressure

increase,function okay through

forced flowresistant

through forced flowpossible,

exchange of twister platewith diff. number of

twisters and diameter, CFD-pre-design of advantage

possible,through two-piece system:

adaptive Multitwister

approx. 0.06 dm3

7 - 10 gno

30 %

(Multi-) Cyclone Multitwister

System description

Fig. 7: Detailed technical and commercial comparison of multi-cyclone and Multitwister systems.

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generated with the engine-specific par-ticle sizes. During this process, flow vol-umes of up to 200 l/min can be set, with variable pressure. The heated test bench can be pivoted in all directions up to an angle of 45°. By means of a functional sample produced with Rapid Prototyping, an efficiency curve is determined from the ratio of particle numbers in the size classes of the clean gas (downstream of the oil separator) to particle numbers of the corresponding size classes in the raw gas (upstream of the oil separator), to-gether with the associated pressure loss-es in the system, Fig. 6 and Fig. 10.The functional sample consists of a Multi-twister and control valve system produced with the aid of map-based design, or at least the flow-relevant interior of the

air/oil separator systems or even the en tire cylinder head cover. First, the switching behaviour of the control valve and the oil recirculation are checked. The pre-separator function is then tested with oil quantities of >500 ml/h. A transpar-ent functional sample enables the flow patterns, dead spaces, and possible oc -cur rence of oil lakes (in the aerosol test bench and on the engine) to be easily observed, thus permitting comparisons of the experimental data and that of the CFD calculation to be made. In this way, the system can be further optimised by computation. At the end of the internal development phase, the engine is also the instance for customer-specific evalua-tions of the air/oil separator module. The results always include the residual oil

Blow-by mapTotal oil massin blow-by path

Particle sizeand frequencydistributionbefore separator(raw gas)

/

(Optimized)characteristickurve of separatorefficiency

X

Particle sizeand frequencydistribution afteroptimizedseparator (puregas)

Test bench

Aerosol test bench

Volume distributionafter separator

Cam coverwith separator

Volume distributionbefore separator

CFD + Prototype basedoptimizing cycle

smallparticles

largeparticles

Optical particlecounter

quantities and the crankcase pressures.

3.4 Inclusion of numerical methodsToday, numerical calculations using CFD software play a central role in the design of air/oil separator modules. This enables the virtual design to be improved signifi-cantly. Moreover, it offers the possibility of carrying out quick and meaningful op-timisation cycles at an early development stage in combination with laboratory test benches. In order to use the numerical flow simulations, the flow spaces are derived directly from the 3D-CAD model, and are represented with finite elements. Apart from the key physical input values and boundary conditions, the choice of a suitable calculation model with appro-priately selected coefficients is of the utmost importance. Generally speaking, multi-phase models (taking the gas and liquid phases into account) are best for this purpose. The Discrete Phase Model (DPM) has proved to be highly suitable, as it is able to calculate gas and liquid phases in their dispersed form precisely,with all the important interactive char-acteristics. Calibration is necessary in order to obtain realistic results with the numerical models, because the decisive values for system layout, i.e. pressure loss and degree of turbulence, and the resulting acceleration effects on particles of different size classes, which lead to the formation of a wall film and thus to the required separation effect, depend on many boundary conditions (see Section 1.3). Therefore, a prior comparison of results with suitable functional laboratory samples is recommended. Following a series of DOE-based tests and empiri-cal adaptations of boundary conditions and coefficients, a calibrated numerical calculation method is now available, with which short optimisation cycles are possible, using CAD data from actual applications.Fig. 8: Optimisation cycle comprising engine tests, aerosol test bench, CFD simulation, and prototypes.

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Blow-by X50Pressure inintake pipe

Pressurein crank case

Dp characteristicpre-separation

Dp+X50 characteristicfine-separation

input valuesfunctions determined empiricallyresulting values

Fig. 9: Operational map-based design of air/oil separator systems. Fig. 10: Specific efficiency curves of different air/oil separator systems with exemplary X50 values.

4 SummaryThe Multitwister described in this article represents a high-performance air/oil separator system for the precipitation of fine oil particles from the blow-by gas, and simultaneously combines robustness, cost-optimised design, and high adapt-ability to the most varied engine applica-tions and conditions as well as tightest installation spaces. At present, the com-bination of coarse and fine oil separation with a combined Multitwister represents the smallest available complete system. Apart from labyrinths, (multi-) cyclones are the most widely used methods for oil mist separation, especially for fine oil separation. Disadvantages in terms of robustness, scalability, and cost can be compensated for by the Multitwister sys-tem with the same level of performance.A complete development environment with CFD simulation, map-based devel-opment tools plus laboratory and engine test benches for prototype verification is the pre-requisite for a successful overall system design in the development of new crankcase venting systems.

5 OutlookThanks to the system‘s two-part design, new demands for pressure-dependent adaptation of the system to the engine-related working point, such as the con-trollability of oil separation performance, and integrated emergency systems, can be implemented without great effort and are currently in the prototype stage. Se-ries production of the flexible Multitwister system concept is scheduled for 2006 in Germany and the USA.

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References

[1] Burkholz, A.: Droplet Separation. Weinheim: Wiley-VCH, 1989

[2] Ramachandran G., Raynor P. C., Leith, D.: Collection Efficien-cy and Pressure Drop for a Rotary-Flow Cyclone. In: Filtration & Separation, September/ October 1994, S.631 - 636, Elsevier Science Ltd. 1994

[3] Greif, V.: Reduzierung des Druckverlustes von Zyklon-abscheidern durch Rückgewinnung der Drallenergie sowie Abscheidung bei kleinen und kleinsten Staubbeladungen. Dissertation, Fortschr. Ber. VDI-Reihe 3 Nr. 470, Düsseldorf: VDI-Verlag, 1997

[4] Fiedler, H. E.: Turbulente Strömungen. Vorlesungsskript März 2003, Technische Universität Berlin, 2003

[5] Hoffmann, A. C., Stein, L. E.: Gas Cyclones and Swirl Tubes. Berlin: Springer-Verlag, 2002

[6] Anderson, j. D.: Computational Fluid Dynamics: McGraw-Hill Inc., 1995

[7] Fluent Version 6.2.16 & Gambit Version 2.2.30, Fluent Inc., Fluent User’s Manual , 2005

[8] Bastias, P., et. al.: Air/Oil Separation in Cylinder Head Co-vers. Warrendale: SAE International, 2004

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