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 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

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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 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

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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 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

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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 Plen