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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

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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 year’s 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

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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

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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 it’s 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 year’s 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

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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 it’s 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

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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

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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

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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

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

the previous year’s 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

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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

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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

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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

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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

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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., Gregory’s

Scientific Publications, Sydney.

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

4. Gregory’s 1992, EFI and Engine Management Volume 2, Gregory’s 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

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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

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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