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  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    48

    A typical commercial flow bench uses manometers across an orifice plate, to measure the

    mass flow rate (usually expressed as CFM @ STP) at a given test pressure. A cylinder head,

    or a number of cylinder heads, once tested, can then be run on a calibrated dynamometer. The

    readings from the dynamometer can then be correlated with mass flow rate results, for a given

    test pressure on a given flow bench.

    Commercial flow bench units may be supplied with a series of charts that attempt to relate the

    mass flow rate through a cylinder head at a given flow bench test pressure, with expected

    dynamometer results.

    6.3 Description Of Flowbench Hardware

    A flowbench was constructed to specifically evaluate the performance of the air filter, throttle

    assembly, and restrictor. The flowbench was constructed with geometry to try and mimic the

    flow from the restrictor into a symmetric plenum. The flowbench uses a stagnation pressure

    probe, and a static pressure probe (wall tapping) to determine the peak velocity,

    downstream, through a 27.5 mm internal diameter pipe. The pipe has a very rough (corroded

    galvanised iron) internal surface finish, and is of sufficient length at ensure fully developed

    turbulent flow at the pressure probes. By assuming fully developed turbulent flow, we can

    also assume a reasonably consistent velocity profile through the pipe over a small range of

    Reynolds numbers.

    Pitot probes were chosen over an orifice plate so as to reduce the required power of the

    vacuum unit, and hence increase the available test pressure. Manometers were favoured over

    differential pressure transducers, to reduce error modes, and so that the device could be used

    again without the need for sourcing additional hardware (except a source of vacuum).

    It must be stressed that this testing device could never accurately measure the true mass flow

    rate through the manifolds tested. It is also important to realize that error analysis for the true

    mass flow rate is impossible to formulate.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    49

    The device does provide a very accurate comparison of mass flow rates at a given

    downstream (plenum) test pressure. The error analysis for the comparison is easily performed

    and yields very encouraging results.

    The available blower limits the choice of downstream test pressure. An industrial vacuum

    cleaner was borrowed to be the source of vacuum. All testing was conducted at 250 mm of

    water, corresponding to absolute plenum pressures near 99.1 kPa.

    The temperature of the flow near the Pitot probes was not measured using a stagnant air

    temperature probe. Instead, the ambient air temperature was used. It is assumed that this

    method did not cause significant error.

    6.4 Restrictor Test Without Throttle Bodies

    The first flow bench test was a comparison between the current restrictor profile and the

    profile used in last years entry. Both profiles exhibit a smooth surface finish. The old profile

    can be seen to display a slight mismatch at the tangency between the radius and exit angle.

    Fittings were used to bring the flow to the restrictors.

    Figure 6-1 Comparison Of 2001 And 2002 Geometry

    Both units displayed some level of unsteady stall. This is obvious due to a fluctuating

    downstream (test) pressure. It can also be heard (and even felt) upstream. The old profile

    displayed a greater level of unsteady stall.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    50

    The new profile, despite displaying a reduced level of unsteady stall, passed a significantly

    lower mass flow rate. (This is difficult to measure, due to the unstable test pressure without

    air filter and throttle body, but a 7% reduction is the ballpark figure)

    6.5 Modified Area Ratio Test

    The new profile was subsequently modified (more correctly a fitting was modified) to match

    the area ratio of the old profile.

    Figure 6-2 Modified Area Ratio Geometries (2002 Device)

    The unsteady stall became more pronounced with this modification, and the mass flow rate

    decreased.

    It seems likely the new profile is affecting a lower mass flow rate than the old profile due to

    geometries upstream of the throat (for tests without air filters and throttle bodies).

    The question remains to be answered as to exactly which geometries upstream of the throat

    are more favourable on the older profile. Possible geometric sources of increased flow rates

    include:

    The parallel tract length

    Parallel tract diameter

    The presence of an inlet angle

    The presence of an inlet angle seems most likely.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    51

    It must be stated that it is somewhat difficult to capture a reading whilst the unsteady stall

    condition is present. The same methodology was used for both units, and the difference in

    mass flow rates is very obvious, although there is significant inaccuracy in the values.

    The modified area ratio test was then performed with the throttle and air cleaner attached. The

    flow was steady with the original area ratio, and a slight fluctuation was noted with the

    increased area ratio. A reduced mass flow rate was recorded with increased area ratio. The

    reduction was 3% +1.9% / -2.4%.

    6.6 Testing With Air Filters And Throttle Bodies

    The new and old profiles were again tested with throttle bodies and air filters (Note: The new

    profile was tested with its original area ratio). Both units display fairly stable flow. The 2002

    unit is very stable, and the 2001 unit fluctuating very slightly. This is a somewhat puzzling

    situation. Obviously the addition of an air filter and throttle body was likely to cause some

    reduction in mass flow rate, and hence lower Reynolds numbers.

    White suggests that separation increases with boundary layer thickness prior to diffusion. The

    addition of an air filter and throttle body might be decreasing the boundary layer thickness.

    The level of swirl might also be having an effect.

    With the addition of air filters and throttle bodies, the new profile has a lower mass flow rate

    at the given test pressure, a reduction of 7% + 1.9% / -2.4%.

    Interestingly, dynamometer results show that this years engine has decreased in maximum

    power (7% 1%).

    A series of modified dynamometer tests was used to show that the power decrease is due to

    components upstream of the plenum. This is explained in detail in section 8.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    52

    7 Dynamometer TestingThe dynamometer testing for this study was performed at Stafford Tune. The dynamometer

    operator was Mr. Paul Masterson. Mr Masterson is a renowned dynamometer operator who

    specialises with engines using Motec engine management systems. The dynamometer at

    Stafford tune is regularly calibrated. Inertia correction, and barometric compensation is also

    available. Stafford tune claim their hardware to be accurate within 1%. An SAE J607

    correction factor was used for test readings.

    The engine was coupled to the dynamometer using a cardan shaft. The inertia corrected power

    figures are the values of power at the shaft. The actual engine exhaust system was in place for

    dynamometer testing.

    The engine systems need be mapped before power readings are taken. This means that the

    parameters of injector pulse width and spark timing are programmed over a range of throttle

    positions and engine speeds.

    Once the engine is mapped, it can be run at WOT over its operating rpm range, and

    horsepower readings taken.

    Figure 7-1 Engine At Dynamometer Testing

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    53

    7.1 Plenum Comparisons

    The first series of comparative tests involved changing between two plenums, whilst using the

    new restrictor, throttle body and air filter. The two plenums are both of symmetric design, and

    both use the same runner lengths. The difference is that the 2002 plenum has significantly less

    volume. The 2001 plenum is 3800 cc whilst the 2001 plenum is 980 cc. The 2002 plenum has

    a smaller runner spacing within the plenum.

    Plenum Comparison

    0.0

    10.0

    20.0

    30.0

    40.0

    50.0

    60.0

    4000 5000 6000 7000 8000 9000 10000 11000 12000

    rpm

    2002 Nm2001 Nm2002 Kw2001 Kw

    Figure 7-2 Plenum Comparison

    It was found that there was little difference in overall peak power levels. The characteristic

    gurgle indicating an early primary pulse can be heard from the engine at 7500 rpm. The

    torque curve dips at 7500 rpm and rises sharply at 8500 rpm, indicating that the primary pulse

    tuning is indeed effective at near 8500 rpm.

    It is interesting to note that the booster from the primary pulse is much more pronounced

    using the plenum with greater volume. These figures are taken without inertia correction.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    54

    7.2 Final Power Readings With Inertia Correction

    It is valid to include inertia correction for our testing. The engine produces very little torque

    and is accelerating a dynamometer with inertia of 0.037 kgm2, at a shaft acceleration of 250

    rpm.s-1.

    2002 Formula SAE

    0.0

    10.0

    20.0

    30.0

    40.0

    50.0

    60.0

    4000 5000 6000 7000 8000 9000 10000 11000 12000

    rpm

    NmKW

    The corrected peak horsepower is 50.4 kW @ 10150 rpm (67.6 hp). As mentioned earlier in

    section 6 of this report, the engine typically operates over a range of 2500 rpm. We can

    clearly see that the integral of power across an rpm range of 2500 rpm is maximised if we

    operate the engine between 9000 rpm and 11500 rpm. The engine produces greater than 46

    kW (62 hp) across this operating range. The rpm limit of the engine should be set slightly

    higher at say 11750 rpm, to encourage the driver to operate the vehicle in the optimum rpm

    range.

    7.3 Removal Of Air Cleaner

    A final test was used to determine the performance of the air filter. The air filter was simply

    removed, and the engine was ramped again. The result was an increase of 0.5 kW. This seems

    to indicate that the air cleaner is indeed adequately sized.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    55

    8 Track TestingAt this stage there has only been one limited track test of the vehicle. The throttle control

    seems to have increased dramatically, although the behaviour is still suited to an experienced

    driver. The lag from quickly opening the throttle plate seems to have decreased

    dramatically.

    Figure 8-1 First Track Testing

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    56

    9 ConclusionsThe new inlet manifold and throttle body will certainly suit an inexperienced driver more than

    the previous years hardware. The design is certainly viable for lean manufacture, is more

    compact, more aesthetically pleasing, and weighs 2.2 kg compared to 6.2 kg for last year. The

    SLS restrictor nozzle has shown no problems with cracking or any other deterioration. The

    hardware enables the engine to be installed with the entire inlet manifold connected. The

    engine can now be installed in under 20 mins.

    The mandatory SAE costing of this hardware is $1600 (AUS) (see Appendix E).

    Unfortunately, the peak horsepower reading is down 7% over last year, 50.4 kW Vs 54 kW

    (68 hp VS 73 hp). The comparison between ramped power curves was not obtained.

    The source of the power loss appears to have occurred due to geometries upstream of the

    restrictor throat. The most probable cause is the geometry between the throat and the throttle

    body.

    The components of the inlet manifold should now be developed experimentally. This will

    certainly be an expensive exercise, but should give pertinent data for future designers, and

    theorists. A recommended experimental evaluation is given in the following section.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    57

    10 RecommendationsThe development of the formula SAE manifold would likely be achieved through five

    separate evaluations. These are:

    Developing the restrictor geometry upstream of the throat.

    Developing the restrictor geometry downstream of the throat.

    Developing the pulse tuning mathematical model.

    Evaluating the performance of symmetric plenums.

    Computational fluid dynamics studies.

    10.1 Developing The Restrictor Geometry Upstream Of The Throat.

    This development is most easily achieved by flow bench testing. A series of flow bench tests

    using a suitably large test plenum, the current throttle body, and air filter, could be used to

    determine optimal conduit geometries upstream of the throat. A fixed downstream geometry

    would be used.

    A suitable downstream geometry might be an exit angle of 5, and an area ratio of 4. The

    upstream geometry might be evaluated for intake angles of 15, 20, and 30. The radius at

    the throat might be evaluated for radii of 30, 40, 50, and 60 mm. Performance curves (the

    measure being mass flow rate) could then be generated. It would be wise to produce

    performance curves for 4 downstream (plenum) pressures. The downstream pressures might

    be 250 mm, 500 mm, 750 mm, and 1000 mm of water.

    The formula SAE flowbench is indeed suitable for this type of evaluation, but a suitably sized

    vacuum source would be required. A suitably large blower, perhaps an ELMO 80H (2.5 kW)

    may cost more than $4000. An alternative would be to use a commercial flow bench at the

    cost of $500 per day. The nozzles would be most easily produced by SLS, and the support

    from The Queensland Manufacturing Institute (QMI) would be required.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    58

    It is imperative that the air filter and throttle body be attached during testing, to affect the

    correct boundary layer thicknesses prior to the restrictor. A reasonably clean environment

    would be required, to ensure consistent performance from the air filter.

    Figure 10-1 Envisioned Performance Curves

    Figure 10-2 Upstream Restrictor Geometries

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    59

    10.2 Developing The Restrictor Geometry Downstream Of The Throat.

    Once an optimal upstream geometry has been found, a second optimisation for downstream

    geometries could be carried out.

    Using the optimal upstream geometry, the downstream geometry might be evaluated for exit

    angles of 3, 5, and 7. The area ratio for each exit angle might be evaluated for a value of

    AR = 2, 4, 6, and 8. It would be wise to produce performance curves again for four

    downstream (plenum) pressures. The downstream pressures should be the same, set at 250

    mm, 500 mm, 750 mm, and 1000 mm of water.

    The performance curves would look similar to figure 10-1.

    Figure 10-3 Downstream Restrictor Geometries

    A dynamometer evaluation of these geometries, for two different plenum designs would be

    advisable. Values of mean plenum pressures, at maximum horsepower, might be achievable.

    The designer might indeed choose a less than optimal downstream geometry to affect a

    practical design.

    10.3 Developing The Pulse Tuning Mathematical Model

    Subsequent to restrictor optimisation, a series of dynamometer tests could be carried out using

    a manifold with variable length pipes. A number of torque curves, for a number of pipe

    lengths may serve to validate a primary pulse-tuning model. It is much easier to build such a

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    60

    manifold using straight pipes. The manifold should use a suitably large volume, perhaps 5

    Litres.

    10.4 Evaluating The Performance Of Symmetric Plenums

    Subsequent to development of both optimised restrictor, and primary pipe lengths, a

    symmetric plenum of suitably small volume might be built and evaluated by a dynamometer.

    10.5 Computational Fluid Dynamics Studies

    Subsequent to all the above studies being performed a computational fluid dynamics study,

    which demonstrates results similar in nature to the experimental studies, might be useful to

    future designers. Great care must be taken with applying boundary conditions. The boundary

    conditions must be accurately determined through a series of experiments, most preferably

    whilst the engine is in operation.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    61

    References

    1. The Society Of Automotive Engineers 2002, Formula SAE Rules 2002, [Online]

    Available at: http://www.sae.org/students/fsaerules.pdf

    2. Automotive Components Limited 1991, ACL Engine Manual, 1st edn., Gregorys

    Scientific Publications, Sydney.

    3. Vizard, D. 1990, How To Build Horsepower, S-A Design Books, California.

    4. Gregorys 1992, EFI and Engine Management Volume 2, Gregorys Scientific

    Publications, Sydney.

    5. Motec Pty Ltd 2002, MoTeC Advanced Engine Management & Data Acquisition

    Systems, [Online] Available at: http://www.motec.com.au

    6. Motec Pty Ltd (?), MoTeC Advanced Engine Management & Data Acquisition Systems

    Training Manual, (?)

    7. Encyclopaedia Britannica Educational Corporation 1966, Flow patterns in venturis

    nozzles and orifices, [U.S.]: Education Development Centre/National Committee for

    Fluid Mechanics Films, videorecording.

    8. Measurement Of Gas Flow By Means Of Critical Flow Venturi Nozzles, International

    Standards Organization, ISO 9300:1995

    9. Miralles, B.T. 2000, Preliminary Considerations In The Use Of Industrial Sonic

    Nozzles, Flow Measurement And Instrumentation, vol.11 no.4, pp.345-350

    10. Runstadler, P.W. 1975, Diffuser Data Book, Creare Inc., Technical Notes 186, Hanover

    11. White, F.M. 1999, Fluid Mechanics, 4th edn., McGraw Hill, Singapore.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    62

    12. Campbell, L.A. 2000, Flow Analysis of Three Different Engine Intake Restrictors,

    undergraduate thesis, Rochester Institute Of Technology, New York

    13. K&N Engineering, Inc. 2002, Home Of High Performance Air Filters, [Online]

    Available at: http://www.knfilters.com/

    14. SuperFlow Corp. 2002, Dynamometers and Flow Benches, [Online] Available at:

    http://www.superflow.com

    15. Measurement Fluid Flows in Closed Conduits; Velocity Area Method Using Pitot Static

    Tubes, International Standards Organization, ISO 3966:1977

    16. Measurement of Fluid Flows in Closed Conduits, Standards Association Of Australia, AS

    2360:1993

    17. Automation Creations Inc. 2002, MatWeb Material Type Search, [Online] Available at:

    http://www.matweb.com/search/searchsubcat.asp

    18. 3D Systems Inc. 2002, 3D Systems-Rapid Prototyping, [Online] Available at:

    http://www.3dsystems.com

    19. Ohata, A. & Ishida, Y. 1982, Dynamic Inlet Pressure and Volumetric Efficiency of Four

    Cycle Four Cylinder Engine, Society Of Automotive Engineers Journal, SAE 820407,

    pp.1637-1648

    20. The Society Of Automotive Engineers 2002, Formula SAE Results, [Online] Available

    at: http://www.sae.org/students/fsaeresu.htm

    21. Braden, P. 1988, Weber Carburettors, HPBooks, Los Angeles.

  • Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation

    Francis Evans 2002

    i

    Appendix A Flowbench Principles

    Figure A- 1 Flow Bench Hardware

    A flowbench was designed with conduit downstream of the restrictor that is of similar internal

    diameter as the plenum used in the engine manifold. A static pressure tap is taken from the

    flowbench plenum. This static pressure is used as the test pressure. Bypass valves are

    adjusted to cause the test pressure to read a certain height at the monometer, indicated P1 in

    figure A-1.

    The flow continues from the test plenum into a suitably long slender pipe.

    The average velocity of flow in the slender pipe is approximately 80 ms-1, which in the 27.5

    mm internal diameter pipe creates Reynolds numbers of Red 1.4 x 106.

    White [11] suggests that for Reynolds numbers in this range, fully developed turbulent flow

    should develop with a turbulent entrance length Le/d 44, for smooth pipes. The pipe used in

    this hardware has a very rough internal surface, but for sake of being conservative the pipe is

    RestrictorP2

    P3

    P1

    Bypass Valves

    Pitot ProbePlenum

    Static Pressure Taps

    Industrial Vacuum Cleaner