WIND TUNNEL METHOD 23.09.2015 Christoph Lueginger (BMW), Céline Vallaude (UTAC), Folko Rohde (VW)...

WIND TUNNEL METHOD

23.09.2015Christoph Lueginger (BMW), Céline Vallaude (UTAC),Folko Rohde (VW) on behalf of Annex 4 taskforce

CHASSIS DYNO PRESCRIPTION

WLTP-12-14-rev2e

WIND TUNNEL METHODINTRODUCTION

WLTP-12-xyz rev. 2 - Wind Tunnel Method, 23.09.2015

• Within the Annex 4 Taskforce, the Wind Tunnel Method has been developed as an alternative method for road load determination with

a) wind tunnel + flat beltb) wind tunnel + chassis

dynamometer

• At IWG #8 in Poona, #10 in Stockholm and #11 in Geneva, the method was presented and adopted for case a) using a wind tunnel + flat belt

• For case b) wind tunnel + chassis dynamometer a proposal is now available

• Open Issues:Adoption of GTR proposal for Wind Tunnel + Chassis Dyno

Source: VW

Source: BMW

WIND TUNNEL METHODBACKGROUND

Principle: Measuring cd x A in wind tunnel as defined in Annex 4 paragraph 6.4. Calculate the aerodynamic force (see 6.7.2.) Calculate the total road load by adding the aerodynamic drag to the mechanical drag Calculate the road load coefficients f0, f1, f2 with a least squares regression analysis and

use these parameters as target coefficients.

Road Load Coefficientsf0 [N] 117,3f1 [N/(km/h)] 1,32f2 [N/(km/h)² 0,0224

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

700

800

Fmech [N]

Road Load(Fmech + Fair) [N]

Speed [km/h] F

orc

e [

N]

F Aj=(cd∗ A f ) j∗ρ0

2∗v j

2

3.62

+ Mechanical force (measured on flatbelt/chassis dyno):

= total road load:

Aerodynamic force (with measured cdxA in wind tunnel):


- Measurement cdxA in wind tunnel

- Measurement rolling resistance on chassis dyno

- Correction of rolling resistance due to the influence of the roller radius on rolling behavior of the tire

total road load =aerodynamic drag (wind tunnel)+ mechanical drag (chassis dyno)- correction (calculated)

WIND TUNNEL METHODBACKGROUND

- Measurement cdxA in wind tunnel

- Measurement rolling resistance on flat belt

total road load =aerodynamic drag (wind tunnel)+ mechanical drag (flat belt)

Wind tunnel + flat beltWind tunnel + roller chassis dyno

Velocity v [km/h]

Rollin

g r

esis

tan

ce

roller/

flat

Requirements roller chassis dyno part: Big diameter of roller from at least 1.2 m Formula for correction of measured rolling resistance


WIND TUNNEL METHODCHASSIS DYNO PART6.2. Approval of facilities by the responsible authority

6.3. Vehicle preparation and temperature

6.4. Wind tunnel procedure

6.6. Chassis dynamometer applied for the wind tunnel method

6.6.1. CriteriaIn addition to the descriptions in paragraphs 1. and 2. of Annex 5 the following criteria, described in paragraphs 6.6.1.1. to 6.6.1.6. inclusive, shall apply.

6.6.1.1. Description of the dynamometerThe front and rear axles shall be equipped with a single roller with a diameter of no less than 1.2 metres. The measured forces in the x-direction include the frictional forces in the drivetrain.

6.6.1.2. Vehicle restraint system

6.6.1.2.1. Vehicle position

6.6.1.2.2. Vertical force

No changes, copied from flat belt method(adopted)

Roller chassis dynamometer criteria copied from Annex 5 but with a minimum diameter of at least 1.2 mAs recommendation of Annex 4 taskforce(to be adopted)

Vehicle restraint system according to Annex 7 and Annex 4 flat belt method


WIND TUNNEL METHODCHASSIS DYNO PART6.6.1.3. Accuracy of measured forcesThe accuracy of measured forces shall be as described in paragraph 6.5.1.3. of this Annex apart from the force in the x-direction, which shall be measured with an accuracy as described in paragraph 2.4.1. of Annex 5.

6.6.1.4. Dynamometer speed controlThe roller speeds shall be controlled with an accuracy of ± 0.2 km/h.

6.6.1.5. Roller surfaceThe roller surface shall be as described in paragraph 6.5.1.5 of this Annex.

6.6.1.6. CoolingThe cooling fan shall be as described in paragraph 6.5.1.6. of this Annex.

6.6.2. Dynamometer measurementThe measurement shall be performed as described in paragraph 6.5.2. of this Annex.

No changes, copied from flat belt method and Annex 5

Speed control accuracy a little lower than for flat belt.Roller surface, cooling and dynamometer measurement overtaken from flat belt part


WIND TUNNEL METHODCHASSIS DYNO PART6.6.3.Correction for the dynamometer roller radius

Due to the curvature of the rollers the measured forces on the chassis dynamometer shall be corrected to a reference equivalent to the road (flat surface) and the result shall be referred to as fj.

where:c1 is the tyre rolling resistance fraction of fjDyno;c2 is a chassis dynamometer specific radius correction factor;fjDyno is the force calculated in paragraph 6.5.2.3.3. for each reference speed j, N;RWheel is one-half of the nominal design tyre diameter, m; RDyno is the radius of the chassis dynamometer roller, m.

The manufacturer and responsible authority shall agree on the factors c1 and c2 to be used, based on correlation test evidence provided by the manufacturer for the range of tyre characteristics intended to be tested at the chassis dynamometer.

As an alternative the following conservative formula can be used:

Correction formula based on ISO formula but with empirical factors c1 and c2 to be provided by manufacturer and responsible authority.(to be adopted)


Conservative formula worked out with PFA validation test program within the Annex 4 taskforce as guidance for magnitude of possible correction.Formula is based on ISO formula with smaller correction leading to higher road load.(to be adopted)

WIND TUNNEL METHODCHASSIS DYNO PART

Vehicle 1: Ø Difference (measured): Flat Belt – Roller Dyno: -6,7 N

Ø Correction (calculated): Factor 0,200 (Proposal): -8,0 N Factor 0,250: -9,8 N Factor 1,000 (ISO): -30,9N


Ø Correction (calculated): Factor 0.200 (Proposal): -3,6 N Factor 0.250: -4,5 N Factor 1.000: -15,4 N


Ø Correction (calculated): Factor 0.200 (Proposal): -4,7 N Factor 0.250: -5,8 N Factor 1.000: tbd

Vehicle 2:Ø Difference (measured): Flat Belt – Roller Dyno: -3,7 N

Ø Correction (calculated): Factor 0.200 (Proposal): -7,2 N Factor 0.250: -8,8 N Factor 1.000 (ISO): -28,1 N

ISO formula leads to resultsbeing about 10 to 20 N too low.

Proposed formula with factor 0.2 delivers good results as proved with round robin data from PFA validation exercise.

ISO:

GTR Proposal:


FLAT BELT PLUS WIND TUNNEL METHODCONCLUSION

The GTR text of the method was discussed intensively and developed together within the Annex 4 taskforce and achieved a high acceptance.

The wind tunnel plus flat belt method has already been adopted by IWG.

The wind tunnel plus chassis dynamometer method was developed on the basis of the adopted flat belt part but with a correction of the measured rolling resistance.

The correction formula for the rolling resistance from the chassis dynamometer is based on the validation measurements and leads to much smaller corrections than in the ISO standard and thus to higher road load results.

The wind tunnel plus chassis dynamometer method would give the possibility to use the wind tunnel method to contracting parties who do not have a flat belt.

The Annex 4 taskforce recommends the adoption of the chassis dyno part of the wind tunnel method as an alternative method for measuring road load.

The Annex 4 taskforce suggests a review for phase 2 on the correction of rolling resistance on the chassis dynamometer.


WIND TUNNEL METHOD 23.09.2015 Christoph Lueginger (BMW), Céline Vallaude (UTAC), Folko Rohde (VW)...

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