Effects of Gasoline Sulfur Level on

Emissions from the Tier 2 In-Use Fleet

1

Aron Butler, David Choi, Kent Helmer,

Tony Fernandez, Paul Machiele 

(USEPA/OTAQ/ASD)

CRC Real World

April 10, 2013
• Vehicle emissions have long been known to exhibit “NOx 

creep” due to sulfur in the fuel

• Recent gasoline sulfur programs

– Looked at effect shortly after a “cleanout cycle”

– Didn’t attempt to assess impact on emissions from in-use fleet

• This study assesses sensitivity of in-use Tier 2 vehicles to fuel 

sulfur level

– What is the level of reversible catalyst activity loss in the in-use fleet? 

– Do emission benefits of lower sulfur (<10 ppm) continue with mileage 

accumulation?

– What level of overall emission reduction is expected from the in-use fleet?

Background & Objective

2
• Recruited 81 vehicles from owners

– MY 2007-2009 passenger cars and light trucks with 20,000 to 40,000 

odometer miles

– Targeted five vehicles each from make/model/engine “classes” selected for 

EPAct program to be representative of national sales in 2007-8 timeframe

Design Overview - Vehicles

3

Vehicle Make Vehicle Model 

FORD 500, Explorer, F150, Focus 

HONDA Civic, Odyssey 

NISSAN Altima

DODGE Caliber, Caravan 

TOYOTA Corolla, Sienna, Tacoma 

CHEVROLET Cobalt, Impala, Silverado 

JEEP Liberty 

SATURN Outlook
• Two non-ethanol test fuels

– Purchased bulk delivery of typical “Tier 2 cert fuel” with 5 ppm sulfur

– Segregated and adjusted a portion up to 28 ppm

Design Overview - Fuels

Fuel Property ASTM Method Low S Test Fuel

Sulfur D2622 5 ppm

Total Aromatics D5769 31.2 Vol%

Olefins D1319 0.5 Vol %

T50 D86 221°F

T90 D86 317°F

RVP D5191 9.0 psi

4
• 3-bag FTP cycle at 75°F

• Measured gaseous pollutants and PM mass by bag

• High-speed/load “clean-out” consisting of two back-to-back 

US06 cycles

• Focus on three research questions:

– What is “clean-out effect” with 28 ppm test fuel?

• Is sulfur loading on the catalyst reversible? How do emissions from recruited vehicles differ 

before/after a clean-out cycle? 

– What is “clean-out effect” with 5 ppm test fuel?

• Are emissions immediately following the clean-out cycle different at different sulfur levels?

– What is the effect of sulfur level with mileage accumulation?

Design Overview - Procedures

5
Assess effect of reversible sulfur loading in the catalyst immediately after 

vehicle arrives (all 81 vehicles)

Procedures: Clean-out Effect at 28 ppm

6
Assess change in emissions 

as a function of sulfur level 

over mileage accumulation

Subset of one sample of each 

make/model performed 

additional series of repeated 

emission tests covering up to 

180 miles on each sulfur level

Alternated FTP tests with onroad mileage accumulation on 

routes with speeds and loads 

similar to FTP

7

Procedures: Sulfur Level Effect
After mid-term review of available data, the short procedure was extended to include 

additional tests on low-sulfur fuel to provide information about an immediate sulfur 

level effect (23 vehicles including data from this and previous slide)

8

Procedures: Clean-out Effect at 5 ppm
Analysis Approach

• Data analyzed by bag of the FTP (cold start, hot running)

• Log transformation of emission measurements

– Stabilize variance, obtain linearity between dependent variable and fixed 

and random effects, normalize distribution of residual

• Analyzed using linear mixed model

– More robust and flexible in modeling the covariance structure for “repeated 

measures data” 

– Capable of including vehicles with missing data and handling irregularly 

spaced measurements

– Better accounts for within-vehicle mileage dependent interactions

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Analysis Approach

• Used top-down model fitting strategy 

– Start with a saturated model with all candidate fixed effects

– Select a model with most optimal covariance structure

– Reduce the fixed effects portion of the model to fit the final model

• Once the final model was fit, the difference of least squares 

means between the fixed effects of interest were reversetransformed to estimate the percent reduction in emissions

• Analysis of the effect of sulfur level on bag 2 NOx presented 

in greater detail for illustrative purposes

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NOx

(p-value)

THC

(p-value)

CO

(p-value)

NMHC

(p-value)

CH4

(p-value)

PM 

(p-value)

Bag 1 – –

4.7% 

(0.0737) – –

15.4%

(< 0.0001)

Bag 2 31.9% 

(0.0009)

16.5% 

(0.0024) –

17.8% 

(0.0181)

15.3% 

(0.0015) –

Bag 3 38.3% 

(<0.0001)

21.4% 

(<0.0001)

19.5% 

(0.0011)

27.8% 

(<0.0001)

12.0% 

(<0.0001)

24.5% 

(<0.0001)

FTP

Wtd

11.4% 

(<0.0001)

4.1% 

(0.0187)

7.6% 

(0.0008)

3.0% 

(0.0751)

6.9% 

(0.0003)

13.7% 

(<0.0001)

The effect is not significant at α = 0.10 when no reduction estimate is provided.

“Clean-Out Effect” at 28ppm

17 vehicle families; 81 unique vehicles

Number of measurements: n = 479 (242 pre / 237 post)

Percent reduction for FTP set immediately after “clean-out” vs. before
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NOx

(p-value)

THC

(p-value)

CO

(p-value)

NMHC

(p-value)

CH4

(p-value)

PM

Bag 1 10.7%

(0.0033)

8.5%†

(0.0382)

7.5%†

(0.0552)

7.5%

(< 0.0001)

13.9%†

(< 0.0001)

–

Bag 2 59.2%

(< 0.0001)

48.8%

(< 0.0001) –

44.8%†

(0.0260)

49.9%

(< 0.0001)

–

Bag 3 62.1%

(< 0.0001)

40.2%

(< 0.0001)

20.1%

(< 0.0001)

49.9%

(< 0.0001)

29.2%

(< 0.0001)

–

FTP

Wtd

23.0%†

(0.0180)

13.0%†

(0.0027)

11.9%†

(0.0378)

10.6%†

(0.0032)

25.8%†

(< 0.0001)

–

† Model with significant sulfur and mileage interaction term. Where no reduction estimate is 

provided, the effect is not significant at α = 0.10.

Overall Emission Reduction for 5 vs. 28ppm

17 vehicle families; 23 unique vehicles

Number of measurements: n = 228 (114 at 5ppm / 114 at 28ppm)

Percent reduction at 5 vs. 28 ppm at all mileage levels
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NOx

(p-value)

THC

(p-value)

CO

(p-value)

NMHC

(p-value)

CH4

(p-value) PM

Bag 1 5.9%

(0.0896)

5.4%

(0.0118)

7.3%

(0.0023)

4.6%

(0.0465)

11.1%

(<0.0001) –

Bag 2 47.3%

(0.0010)

40.2%

(<0.0001) –

34.4%

(0.0041)

53.6%

(<0.0001) –

Bag 3 51.2%

(<0.0001)

35.0%

(<0.0001)

10.1%

(0.0988)

45.0%

(<0.0001)

25.4%

(<0.0001) –

FTP

Wtd

17.7%

(0.0001)

11.2%

(<0.0001)

8.3%

(0.0003)

8.8%

(0.0003)

21.4%

(<0.0001) –

The effect is not significant at α = 0.10 when no reduction estimate is provided.

Sulfur Effect on “Clean-Out”

17 vehicle families; 23 unique vehicles

Number of measurements: n = 132 (64 at 5ppm / 68 at 28ppm)

Percent reduction at 5 vs. 28ppm for tests <50 miles after cleanout
NOx Bag 2: Data vs. Predicted

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Comparison of actual data (blue) to model prediction (red) for several vehicles
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R-square = 0.72

NOx Bag 2: Data vs. Predicted

Showing high and low sulfur 

data points compared to 

model prediction
Sensitivity Analyses

• Focused on bag 2 NOx

– Low emission rates, large percent reductions

• Examined effects of

– Removing vehicles with very low emission levels (sample < 100 ppb, about half 

of observations)

– Removing tests with zero emission results vs. imputing small values (half the 

minimum valid result, affecting about 8% of observations)

– Removing influential vehicles (restricted likelihood distance > 1.5, three 

vehicles)

• In each case, model predictions remained statistically significant

and within a range of 51-70% reduction (vs. baseline at 59%), 

suggesting robustness of results

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Summary Conclusions

• Catalyst efficiency loss due to sulfur loading is occurring in the Tier 2 inuse fleet

– Measureable effect on emissions of NOxand other pollutants of interest 

– After performing a clean-out cycle on 28 ppm fuel, data shows FTP composite 

NOx was reduced by 11% and NMHC by 3%

• Effectiveness of high speed/load modes in restoring catalyst efficiency are 

limited by fuel sulfur level

– Comparing tests on 5 vs. 28 ppm fuel immediately after a “clean-out” cycle: 

FTP composite NOx emissions were 18% lower, NMHC 9% lower, and CO 8% 

lower

– Suggests the initial clean-out results were limited by 28 ppm sulfur fuel

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• Reducing fuel sulfur levels from 28 to 5 ppm is expected to bring 

significant reductions in NOx, NMHC, and other pollutants of interest in 

the in-use fleet

– Controlling for sulfur effects by mileage where appropriate: FTP composite 

NOx was 23% lower, NMHC was 11% lower, and CO 12% lower

– Emission performance on lower sulfur fuel did not converge with higher 

sulfur as miles were accumulated, but instead showed a constant (or 

increasing) ratio

• The overall findings of significant emission benefits of ≤10 ppm sulfur in 

Tier 2 vehicles are in agreement with other recent studies by EPA and 

automobile and catalyst manufacturers 1,2,3

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Summary Conclusions
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References

1. Chapter 6 of the Regulatory Impact Analysis for the Control of Hazardous Air Pollutants from Mobile Sources Final Rule, 

EPA 420-R-07-002.

2. Ball D., Clark D., Moser D. (2011). Effects of Fuel Sulfur on FTP NOx Emissions from a PZEV 4 Cylinder Application.

SAE 2011 World Congress Paper 2011-01-0300. SAE International: Warrendale, PA.

3. Shapiro, E. (2009). National Clean Gasoline, An Investigation of Costs and Benefits. Published by the Alliance of 

Automobile Manufacturers, Washington, DC.

Further Reading

The study report and data will be available

via the OTAQ website:

http://www.epa.gov/otaq/fuelsmodel.htm