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-2014-0827

DATE: February 19, 2015

This memorandum discusses potential approaches to applying the longstanding principles behind 

selective enforcement audits (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 tractors 

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 procedures in 40 CFR 1037.521 for measuring aerodynamic parameters. These 

Phase 1and Phase 2 procedures specify how to measure drag area (CdA) by coasting down an 

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 larger relative to production variability.1 This 

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.

The 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 this memorandum discusses SEAs and confirmatory 

testing primarily in the context of the proposed Phase 2 program, 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 RADIATION
SEAs 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 CdAs. 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 

coastdown runs on any single vehicle.

 Compliance determinations should be based on whether or not the true value for the CdA 

falls within the bin to which the vehicle was certified, rather than whether or not the true 

value for the CdA exceeds the value measure for certification.

 Given the limited ability to eliminate uncertainty, compliance determinations 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 confirmatory 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 test data.) EPA received comments on this section similar to the comments on 

SEAs. 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 confirmatory testing of tractors, we believe the SEA principles would generally 

also apply for confirmatory testing. In particular, we note that:

 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 very large number of different possible tractor configurations, confirmatory 

testing for aerodynamics should focus significantly on verifying Falt-aero.

 Compliance determinations should be based on whether or not the Falt-aero used by 

manufacturers would result in the true values for CdAs to fall within the bin to which the 
vehicles are being certified. This principle is consistent with the more general statements

in the proposed §1037.235(c)(2):

If we measure 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 determinations should 

consider the statistical confidence that a true value lies outside a bin.

Should 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 generally 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 

to 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 confirmatory testing for 

trailer aerodynamics.

 Trailer manufacturers and trailer component manufacturers have less financial ability to 

perform SEAs than do tractor 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 determinations 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 strong 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 should be given a few months advance notice of the SEA 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 for 

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 additional 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 failure. 

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 down 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 

Falt-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.
Examples:

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

Pass
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

Fail

Fail
Attachment 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 

Falt-aero and adjustments to account for wind-averaged drag 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 CdA 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. Determine the mean CdA

value and calculate a confidence interval using the following equation, expressed to at 

least two decimal places:

Confidenceinterval= mean ab

n

  

     

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 CdA 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 upper 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 fails 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 vehicle until you reach a pass or fail decision for the family based on 
one 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 vehicles 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 paragraph (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 Falt-aero or make other changes to your testing methodology 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 

application for certification.

(4) You must assign an appropriately higher CdA 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 your 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. 

* * * * *