UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD ANN ARBOR, MICHIGAN 48105

- 2498

MEMORANDUM

SUBJECT: Additional Discussion of Selective Enforcement Audit and Confirmatory Testing

for Aerodynamic Parameters for Combination Tractors and for Trailers

FROM: Charles Moulis, Senior Environmental Engineer

Assessment and Standards Division

TO: Docket Number EPA

- HQ

- OAR

- 20 1 4

- 0827

DATE: February 19, 2015

This memorandum discusses potential approaches to applying the longstanding principles behind

s elective e nforcement a udits (SEAs) and confirmatory testing to aerodynamic measurements. The proposed HD Phase 2 standards for combination tractors, and for trailers, reflect (in part) improvements to the vehicle ’s aerodynamics, and further require that manufacturers of tract ors and trailers conduct testing to generate aerodynamic values to be used as inputs to the GEM simulation model used in certifying the vehicle . EPA proposed to use an improved version of
the Phase 1 test procedure s in 40 CFR 1037.521 for measuring aerodynamic parameters. These
Phase 1 and Phase 2 procedures specify how to measure drag area (CdA) by coast ing down a n actual vehicle . The procedures also allow manufacturers to use scaled wind tunnels and computational fluid dynamic (CFD) simulations, as long as the manufacturers adjust the results to be equivalent to coastdown results.

EPA recognizes that compliance testing relying on coastdowns to evaluate aerodynamic parameters differs fundamentally from traditional compliance testing, in which test

- to

- test variability is expected to be small relative to production variability . With coastdown testing, however, test

- to

- test variability is expected to be large r relative to production varia bility. 1 T his has led to EPA considering variations on the traditional approaches. These potential approaches are discussed separately below for tractors and trailers and separately for SEAs and confirmatory testing

.

T he existing regulations in 40 CFR 1037. 201 and 1037.301 provide for SEAs and confirmatory testing for Phase 1 vehicles, but provide less specific descriptions of how such testing would be
conducted than is presented here

. While t his memorandum discusses SEAs and confirmatory testing primarily in the context of the proposed Phase 2 prog r am , EPA is also considering applying some or all of these approaches to the Phase 1 tractor program.

1 See Chapter 3 of the Draft RIA for the Phase 2 NPRM. OFFICE OF
AIR AND RADIATIONSEAs for Tractors

EPA has long required manufacturers to perform SEAs to verify that actual production engines and vehicles conform to their certificates. In the NPRM, we proposed a conventional approach to conducting SEAs with respect to aerodynamics. See 80 FR at 40156 and proposed section
1037.301

. In response to comments, EPA has developed the attached strawman that would use a
different structure for conducting SEAs to evaluate tractor C d As. This structure reflects a possible revision to the proposed approach that would be more consistent with the following principles:

 Test

- to

- test variability for individual coastdown runs can be high, so compliance determinations should be based on average values from multiple runs.

 Coastdown testing of a single vehicle is expensive and time consuming, so testing should focus more on repeat tests for the same vehicle than on tests for multiple vehicles. However, manufacturers should not be required to conduct more than 100 valid

coas tdown runs on any single vehicle.

 Compliance determinations should be based on whether or not the true value for the C d A falls within the bin to which the vehicle was certified, rather than whether or not the true
value for the C d A exceeds the value measur e for certification.

 Given the limited ability to eliminate uncertainty, compliance determination s should
consider the statistical confidence that a true value lies outside a bin.

We believe the structure described here appropriately balances EPA ’ s need to provide strong incentives for manufacturers to act in good faith with manufacturers’ need to avoid compliance actions based on inaccurate testing. Our current assessment is that

, where a manufacturer acts in good faith for certification and uses good engineering judgment throughout the process, false
failures for individual vehicles would be rare and false failures for a family would not occur.

Confirmatory Testing for Tractors

Confirmatory testing is pre

- production testing conducted by EPA as part of the certification

process to verify a manufacturer’s test data

. In the NPRM, we proposed a conventional approach to conducting c onfirmatory testing of tractors with respect to aerodynamics

. See 80 FR at 40156 and proposed section 1037.235. (Note: The Phase 1 regulations did not specify how EPA would use confirmatory tes t data. ) EPA received comments on this section similar to the comments on
SEA s . In response to these comments , EPA is considering the need to revise the proposed approach for confirmatory testing . Although, EPA has not developed a specific parallel
strawman for con firmatory te sting of tractors

, we believe the SEA principles would generally also apply for confirmatory testing. In particular, we note that:

 Since t est

- to

- test variability for individual coastdown runs can be high, confirmatory test
determinations should be based on average values from multiple runs.

 Due to the very large number of different possible tractor configurations, c onfirmatory testing for aerodynamic s should focus significantly on verifying F alt

- aero .

 Compliance determinations should be based on whe ther or not the F alt

- aero used by
manufacturers would result in the true value s for C d A s to fall within the bin to which thevehicle s are being certified. This principle is consistent with the more general statement s

in the proposed §1037.235 (c)(2): If we measur e emissions (or other parameters, as applicable) from your vehicle or component, the results of that testing become the official emission results for the vehicle or component. Note that changing the official emission result does not necessarily require a change in the declared modeling input value.

 Given the limited ability to eliminate uncertainty, compliance determination s should
consider the statistical confidence that a true value lies outside a bin.

S hould we revise the SEA structure for tractors to be similar to the attached strawman, we would likely make some of these same revisions to our confirmatory testing regulations.

SEAs and Confirmatory Testing for Trailers

In the NPRM, we proposed to gen erally apply the same SEA and confirmatory testing structures to tractors and trailer with respect to aerodynamics

. However, we also proposed to retain the authority to require component manufacturers to perform SEAs where certification relies on their
test data. See, e.g. proposed section 1037.301 (d)(4).

Should we revise the SEA and/or confirmatory testing structure for tractors, we would likely make similar revisions to the trailer program. However, the regulations would likely be different

t o reflect the following principles:

 Since test

- to

- test variability for individual coastdown runs can be high, confirmatory test determinations should be based on average values from multiple runs.

 Due to the smaller number of possible trailer configurations (compared to tractor configurations), it would be more possible for EPA to rely on c onfirmatory testing for trailer aerodynamics.

 Trailer manufacturers and trailer component manufacturers have less financial ability to perform SEAs than do t ractor manufacturers. Nevertheless, EPA should retain the authority to require trailer component manufacturers to perform SEAs, especially where
EPA has reason to believe the vehicles are non

- compliant

.

 Given the limited ability to eliminate uncertainty, compliance determination s should
consider the statistical confidence that a true value lies outside a bin.

We believe the appropriate compliance structure for trailers can be simpler than for tractors and
yet provide the st ro ng incentive for manufacturers to act in good faith.Attachment 1: SEA for a Tractor’s CdA

– Strawman for Discussion

 An SEA may be required for any production model to verify its CdA.

 Manufacturers sh ould be given a few months advance notice of the SE A to schedule test
facilities.

 SEAs would begin by requiring the vehicle manufacturer to coastdown one production vehicle selected by EPA. (EPA may work with manufacturers to minimize disruption for customers.)

 CdA from the coastdown would be corrected based on wind tunnel/CFD yaw curves for

the vehicle to determine the wind

- averaged drag area

.

 Mean CdA values would be calculated including confidence intervals (calculate means at the end of each test day or more frequently)

.

 Confidence intervals would be calculated as:

Confidence Interval = mean ± ( a × σ √ n + b )

Where a is a parameter between 1.0 and 2.0, b is a parameter between 0.00 and 0.05, σ is the standard deviation, and n is the number of runs. ( EPA would specify values fo r these parameters in the regulations if this strawman is adopted. )

 The manufacturer would repeat coastdowns for the same vehicle for a minimum of 16 runs (8 in each direction) and until one of the following has occurred:

o A statistically significant mean estimate (mean ± confidence interval ) is entirely below the upper limit for the bin to which the vehicle was certified

– This is a
PASS.

o A statistically significant mean estimate is entirely above the upper limit for the
bin to which the vehicle was certified

– This is a FAILURE.

o The manufacturer concedes failure

– This is a FAILURE.

o 100 coastdown runs (50 pairs ) are completed without determining a statistically significant mean that is entirely above OR entirely below the upper limit for the
bin to which the vehicle was certified

– This is a PASS.

 If the vehicle fails, the manufacturer must coastdown two additio nal vehicles. We may specify other vehicle configurations within the family , or the manufacturer may test another vehicle of the same configuration

.

o If either additional vehicle fails, the manufacturer must coast down two additional
vehicles for each fail ure.

o Continue adding two vehicles for each failure until one of the following has

occurred:

 More than 50% of the vehicles coasted down have passed.

 The manufacturer concedes failure for the family.

 Six vehicles have failed

 The manufacturer has coasted do wn 11 vehicles.

 Individual vehicle aerodynamic configurations that fail an SEA must be assigned to higher CdA bins for the end of year report.

 When a family fails, the certificate is suspended until the manufacturer specifies a higher

F alt

- aero or makes other methodological changes that we determine would result in compliance.

 Manufacturers may perform additional coastdown testing as part of the SEA, and may

ask us to consider the additional results.E xamples:

4 4.5 5 5.5 6 6.5 7 7.5 8

0 20 40 60 80 100 120 140 160 CdA

n

4 4.5 5 5.5 6 6.5 7 7.5 8

0 20 40 60 80 100 120 140 160 CdA

n Pass

Pass4 4.5 5 5.5 6 6.5 7 7.5 8

0 20 40 60 80 100 120 140 160 CdA

n

4 4.5 5 5.5 6 6.5 7 7.5 8

0 20 40 60 80 100 120 140 160 CdA

n Fail

FailAttachment 2

– Draft Regulatory Text for Tractor SEA Strawman

§ 1037.301 Measurements related to GEM inputs in a selective enforcement audit.

* * * * *

(d) To perform a selective enforcement audit with respect to drag area for tractors , use the reference method specified in § 1037.525; we may instead require you to use the same method you used for certification. For this paragraph (d), all measurements for tractors must include

F alt

- aero and adjustments to account for wind

- averaged dra g as applicable under § 1037.525. The following provisions apply instead of 40 CFR 1068.420 for a selective enforcement audit with
respect to drag area: (1) Determine whether or not a tractor fails to meet standards as follows: (i ) We will select a vehicle configuration for testing. Perform a coastdown measurement with the vehicle in its production configuration. Correct the C d A value to a yaw angle of 4.5° using the yaw curve you established in your certification testing. Perform repeat measurements until you reach a pass or fail decision under this paragraph (d)(1). (ii) Calculate statistical values to characterize cumulative test results at least once per day based on an equal number of coastdown runs in each direction. Determin e the mean C d A value and calculate a confidence interval using the following equation, expressed to at least two decimal places:

Eq. 1037.301

- 1

Where:

a = 1.x. [NOTE: EPA IS CONSIDERING VALUES FOR a IN THE RANGE OF 1.0 TO 2.0]

σ = the standard deviation C d A values from the set of coastdown runs (see 40 CFR 1065.602(c)).

n = the total number of coastdown runs.
b = 0.0x. [NOTE: EPA IS CONSIDERING VALUES FOR b IN THE RANGE OF 0.00 TO 0.05] (iii) The vehicle reaches a pass or fail decision based on one of the following criteria: (A) The vehicle passes if the whole confidence interval is at or below the upper limit for the bin to which you certified the vehicle. (B) The vehicle fails if the whole confidence interval is above the uppe r limit for the bin to which you certified the vehicle. (C) The vehicle passes if you perform 100 coastdown runs without the whole confidence interval being above or below the upper limit for the bin to which you
certified the vehicle. (D) The vehicle fail s if you choose to stop testing before reaching a final determination under this paragraph (d)(1)(iii). (2) If you reach a pass decision on the first test vehicle, the emission family passes the SEA and you may stop testing. If you reach a fail decision on the first test vehicle, repeat the testing described in paragraph (d)(1) of this section for two additional vehicles of the same configuration, or of a different configuration that we specify. Continue testing two additional vehicles for each failing vehi cle until you reach a pass or fail decision for the family based onone of the following criteria:

(i) The emission family passes if at any point more than 50 percent of the vehicles have reached a pass decision. (ii) The emission family fails if six vehic les reach a fail decision. (iii) The emission family passes if you test 11 vehicles with five or fewer vehicles reaching a fail decision. (iv) The emission family fails if you choose to stop testing before reaching a final determination under this paragrap h (d)(2). (3) The following provisions apply if your emission family fails an SEA: (i) The certificate for the emission family is automatically suspended. We may reinstate the certificate if you revise F alt

- aero or make other changes to your testing method ology to properly correlate your testing to the reference method specified in § 1037.525. (ii) We may require you to apply any adjustments and corrections determined under paragraph (d)(3)(i) of this section to your other emission families in any future

a pplication for certification. ( 4 ) You must assign an appropriately higher C d A bin for your compliance demonstration at the end of the model year for all configuration(s) you tested that failed, whether or not the family fails the SEA

. ( 5 ) If we test some of your vehicles in addition to your testing, we may decide not to include your test results as official data for those vehicles if there is substantial disagreement between your testing and our testing. We will reinstate your data as valid if you show us that we made an error and your data are correct. If we perform testing, we may choose to stop testing after any number of tests. ( 6 ) If we rely on our test data instead of yours, we will notify you in writing of our decision and the reasons we believe you r facility is not appropriate for doing the tests we require under this paragraph ( d ). You may request in writing that we consider your test results from the same facility for future testing if you show us that you have made changes to resolve the problem

. ( 7 ) We may allow you to perform additional replicate tests with a given vehicle or to test additional vehicles , consistent with good engineering judgment.

* * * * *