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Design of an Inlet Manifold for a Formula SAE Vehicle, Including Experimental Evaluation
Francis Evans 2002
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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.
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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.
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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