United States

Environmental Protection

Agency

Air and Radiation EPA420-P-99-029

November 1999

M6.EXH.006

Determination of Methane

Offsets as a Function of

Mileage for Light-Duty

Cars and Trucks

 Printed on Recycled Paper
EPA420-P-99-029

November 1999

Determination of Methane Offsets as a Function of

Mileage for Light-Duty Cars and Trucks

Phil Enns

Assessment and Modeling Division

Office of Mobile Sources

U.S. Environmental Protection Agency

NOTICE

This technical report does not necessarily represent final EPA decisions or positions.

It is intended to present technical analysis of issues using data which are currently available.

The purpose in the release of such reports is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a final EPA decision, position, or regulatory action.
1

Carey, P., P. Enns, E. Glover, and M. Sklar, “Determination of Running Emissions as a

Function of Mileage for 1981-1993 Model Year Light-Duty Cars and Trucks,”

Report Number M6.EXH.001, October 1998.

2

Carey, P. and E. Glover, “Determination of Start Emissions as a Function of Mileage and

Soak Time for 1981-1993 Model Year Light-Duty Vehicles,” Report No. M6.STE.003,

October 1998.

Methane -8- 11/8/99

- Draft -

Determination of Methane Offsets as a Function of Mileage

 for Light-Duty Cars and Trucks

Report Number M6.EXH.006

September 1999

Phil Enns

U.S. EPA Assessment and Modeling Division

1.0 Introduction

Previous reports document how the MOBILE6 emission factor model will allocate

vehicle exhaust emissions between engine start (start emissions) and travel (running

emissions)1,2. This split allows the separate characterization of start and running emissions

for correction factors such as fuel effects and ambient temperature. It also enables a more

precise weighting of these two aspects of exhaust emissions for particular situations such as

morning commute, parking lot and freeway driving.

Because methane does not contribute significantly to ozone formation, MOBILE

attempts to separate it from total hydrocarbons in estimating vehicle emissions. This

document describes methodologies for calculating this methane “offset” based on the

separation of start and running emissions proposed for MOBILE6. The procedure follows

the scheme of modeling the in-use deterioration of emissions as a function of accumulated

mileage. For model year 1981-1993 light-duty cars and trucks, stratified into carbureted

versus fuel-injected, the method parallels that used for total hydrocarbons, carbon monoxide

and oxides of nitrogen. This analysis utilizes actual methane data from emissions tests

conducted on vehicles from those model years.

For pre-1981 model years, data of the type used in the newer vehicles is not

available. Therefore, procedures are described for estimating methane as a function of
3

Brzezinski, D. and P. Enns, “The Determination of Hot Running Emissions from FTP Bag

Emissions”, Report No. M6.STE.002, December, 1997.

Methane -8- 11/8/99 -2-

mileage using existing data in combination with methods that are applied in MOBILE

version 5.

2.0 Data

The data underlying the analysis of 1981-93 light-duty vehicles are drawn from a

subset of the Federal Test Procedure (FTP) tests described in the reports cited above. These

tests were conducted by EPA, the American Automobile Manufacturers Association

(AAMA), and the American Petroleum Institute (API). Most, but not all, of these tests

produced measurements of methane. In particular, the sample sizes from the full data set and

the reduced sets for which methane is recorded are compared in the table below, subdivided

by vehicle type and the model year/technology groups used to determine basic emission rates

of total HC, CO and NOx.

 CARS

 ----------------------------------

 | | DATA SET |

 | |-------------------|

 | | REDUCED | FULL |

 | |---------+---------|

 | | N | N |

 |------------+---------+---------|

 |GROUP | | |

 |------------| | |

 |81-82 CARB | 580| 1166|

 |------------+---------+---------|

 |81-82 FI | 88| 126|

 |------------+---------+---------|

 |83-85 CARB | 203| 253|

 |------------+---------+---------|

 |83-87 FI | 688| 726|

 |------------+---------+---------|

 |86-93 CARB | 93| 96|

 |------------+---------+---------|

 |88-93 PFI | 1361| 1605|

 |------------+---------+---------|

 |88-93 TBI | 437| 444|

 ----------------------------------

 TRUCKS

 ----------------------------------

 | | DATA SET |

 | |-------------------|

 | | REDUCED | FULL |

 | |---------+---------|

 | | N | N |

 |------------+---------+---------|

 |GROUP | | |

 |------------| | |

 |81-83 CARB | 72| 180|

 |------------+---------+---------|

 |81-87 FI | 92| 94|

 |------------+---------+---------|

 |84-93 CARB | 125| 134|

 |------------+---------+---------|

 |88-93 PFI | 199| 330|

 |------------+---------+---------|

 |88-93 TBI | 458| 467|

 ----------------------------------

Another key data set is that based on a sample of FTP tests to which were appended a

505-second cycle without an engine start. This cycle is identical to that of Bags 1 and 3, but

contains no emissions associated with the cold start of Bag 1 or warm start of Bag 3. It is

referred to as the Hot Running 505 (HR505). Pure cold and warm start emissions are

estimated by deducting HR505 emissions from the 505 bags that include a start.3

The data from this test program were used to estimate the relation between the
Methane -8- -3- 11/8/99

HR505 and Bags 1 to 3 of an FTP. From this function, the portions of FTP emissions

attributed to start and running are computed. This calculation then was applied to the large

FTP data set described above for which Bags 1 to 3, but not the HR505, are measured.

3.0 Modeling Methane Deterioration in MOBILE6 - 1981-93 Vehicles

To understand the current analysis, it is helpful to review how emissions

deterioration is modeled in MOBILE6. The basic method involves separating start and

running emissions.

3.1 Running Emissions

For the running component, simple linear functions of emissions versus mileage are

fitted by the method of least squares regression. To improve the fit at low mileage, the mean

of emissions is used. This produced piecewise continuous functions in which emission rates

(in grams per mile) are constant at low mileage and increase when mileage accumulation

exceeds approximately 20,000 miles. The higher mileage portion of the function has

constant slope in most cases, but under certain conditions the slope may change, adding a

second “corner” point to the graph of emissions as a function of mileage. There was a

concern that the FTP data suffer from sample bias due to the self-selecting nature of the data

collection. In order to account for such bias, adjustments were made to these running

emission lines using data from a large sample of inspection and maintenance tests conducted

in Dayton, Ohio. (See document (1) for details.) However, methane was not recorded in

these tests, so it was not possible to compute this adjustment for the analysis described in

this report. Instead, we propose using a proportional adjustment for sample bias

corresponding to that used for total HC.

Table 1 reports the deterioration coefficients derived for methane using the reduced

data set. The first slope is always zero, reflecting the use of the low mileage mean (ZML

Emissions). The first corner occurs at the mileage where the emission rate begins to slope

positively. For total HC, only the 1983-87 fuel-injected car category has a second corner.

With methane, several categories have two corners, while the 1983-85 carbureted car group

has zero deterioration at all mileages.

The high emitter adjustment was determined as proportionately the same at a given

mileage as for total HC. These adjustment factors are applied additively to the initial THC

emissions. They raise or lower THC by an amount that is a linear function of mileage and is

zero at mileage zero. In order to apply this factor to methane it is necessary to first compute

unadjusted and adjusted THC at a given mileage. Their ratio is then multiplied by unadjusted

methane to obtain adjusted methane.

Figure 1 illustrates the unadjusted and adjusted methane deterioration lines for the

case of 1988 to 1993 model year port fuel-injected cars. As described, the adjustment in

methane is proportional to the corresponding HC adjustment, which also is illustrated.

Figure 2 shows the final adjusted methane lines by model year-technology group for cars and

trucks.
Methane -8- -4- 11/8/99

3.2 Start Emissions

Deterioration of start emissions (measured in grams) was modeled somewhat

differently. First, the tested vehicles were stratified into categories of “normal” and “high”

emitters depending on how their FTP values compare to applicable standards. For HC and

CO, the normal emitter emissions were regressed against mileage, while high emitter

emissions were fitted by the mean. At a given mileage, the normal and high emitter estimates

were combined in a weighted average, where the weight reflects the proportion of high

emitters at that mileage. Reference (2) gives a complete description of this procedure.

Table 2 presents coefficients used to compute the start portion of emission rates at a

given mileage. The majority of the groups actually show negative slope estimates for the

normal emitter regression lines. However, in all but one of these, the slope is not statistically

significant when tested against zero. Since negative deterioration is intuitively unexpected, it

was decided to use the normal emitter mean value when the regression estimate is negative.

It also is possible for estimated cold start emissions to be negative. This occurs when the

fitted Hot Running 505 value exceeds the observed Bag 1 value. In the case of 1988-93 TBI

cars, this produced an anomaly in which the mean methane of the high emitters is negative

and the start emission deterioration line has negative slope. For this case, the deterioration

line was assigned a constant value equal to its zero mile level. 

Table 3 gives the fraction of high emitters estimated for THC which is used to

compute the weighted average described above. These are the same values used for the full

sample total HC calculations. Graphs of start emissions versus mileage for cars and trucks

appear in Figure 3.

3.3 FTP Emissions

Final FTP deterioration functions are obtained by combining the start and running

estimates according to their relative importance in that test. The weights used coincide with

those used for HC, CO and NOx. They form the equation

FTP=(7.5*Run + .43*CS + .57*HS)/7.5

 whereRun is emissions in grams per mile from the running LA4 portion; CS is emissions in

grams from cold start; and HS is hot start, computed as simply 0.16*CS. The factor 7.5 is the

driving mileage corresponding to Bags 1 and 2 (the LA4 cycle) in the FTP.

4.0 Other Model Years

4.1 Pre-1981 Model Years

In MOBILE5, start and running emissions are not separated. MOBILE5 contains

deterioration functions for methane for each of the three FTP bags. This enables the
4

Brzezinski, D. and J. Gilmore, “Basic Exhaust Emission Rates of Open Loop Vehicles

for MOBILE6," Report No. M6.EXH.005, May 1999.

Methane -8- 11/8/99

calculation of bag values at selected vehicle mileages. These values can in turn be used to

compute running and start emissions when combined with the regression coefficients

derived from the 77-car test data. The validity of this approach rests on the assumption that

start and running emissions in older vehicles occur in a manner similar to that of late model 

vehicles. For carbureted cars, graphs of running and start emissions are shown in Figure 4.

The graphs include the proposed MOBILE6 functions for 1981-82 and for 1983-85. It is

apparent that the deterioration functions for the two versions of the model are reasonably

compatible. Similar graphs for fuel injected cars and for trucks yield the same impression.

Therefore, in the absence of new data, EPA proposes to separate start and running methane

emissions in pre-1981 light-duty vehicles and trucks using the methodology of MOBILE6

applied to the deterioration functions employed in MOBILE5.

For diesel vehicles and motorcycles, FTP bag values are not provided in MOBILE5.

Thus, it is not possible to compute running and start emissions by the method outlined

above. In these cases, EPA proposes to use fractions described in a separate report4 that are

applied to total hydrocarbon deterioration functions to determine methane function

coefficients.

4.2 Model Years 1994 and Later

For model year 1994 and later vehicles subject to Tier I standards, MOBILE6

predicts nonmethane hydrocarbon (NMHC) emissions directly. In the case of LEVs, the

model predicts nonmethane organic gas (NMOG). No adequate methane data exist for

estimating methane deterioration functions for these model years. Therefore, EPA proposes

developing multiplicative methane offsets from these projections in a manner consistent with

the treatment of NMHC and NMOG. In general, emission rates for those pollutants are

obtained using ratios of standards that apply to the newer vehicles to those of 1993 model

year vehicles. For a given class of vehicles, there exists a ratio between NMHC (or NMOG)

in the two model year ranges. The proposed method would apply the same ratio to compute

methane for newer vehicles.

5.0 Summary

EPA proposes combining available methane data with methods employed for

methane in MOBILE5 and total hydrocarbons in MOBILE6 to develop basic emission rates

for methane in MOBILE6. This results in an eclectic approach that attempts to fully utilize

the data while making reasonable assumptions where insufficient data exists. In general, the

assumptions are consistent with methods applied to total and nonmethane HC.
Methane -8- 11/8/99

Table 1: Methane Running Emission Deterioration Model Coefficients for Light-Duty Vehicles

Cars --------------------------------------------------------------------------------------------

 | | | First | First | Second | Second | Third |

 | |ZML Emission| Slope | Corner | Slope | Corner | Slope |

 | | (gr/m) |(gr/m/1000m)|(1000 miles)|(gr/m/1000m)|(1000 miles)|(gr/m/1000m)|

 |------------+------------+------------+------------+------------+------------+------------|

 |Model Year/ | | | | | | |

 |Technology | | | | | | |

 |------------| | | | | | |

 |88-93 PFI | 0.0167| 0.0000| 15.47| 0.0003| 67.89| 0.0003|

 |------------+------------+------------+------------+------------+------------+------------|

 |88-93 TBI | 0.0240| 0.0000| 32.18| 0.0002| N/A| N/A|

 |------------+------------+------------+------------+------------+------------+------------|

 |83-87 FI | 0.0365| 0.0000| 14.12| 0.0006| 81.29| 0.0004|

 |------------+------------+------------+------------+------------+------------+------------|

 |86-93 CARB | 0.0405| 0.0000| 15.19| 0.0002| 71.91| 0.0001|

 |------------+------------+------------+------------+------------+------------+------------|

 |83-85 CARB | 0.0721| 0.0000| N/A| N/A| N/A| N/A|

 |------------+------------+------------+------------+------------+------------+------------|

 |81-82 FI | 0.0271| 0.0000| 13.92| 0.0005| 265.40| 0.0005|

 |------------+------------+------------+------------+------------+------------+------------|

 |81-82 CARB | 0.0845| 0.0000| 22.11| 0.0005| N/A| N/A|

 --------------------------------------------------------------------------------------------

Trucks --------------------------------------------------------------------------------------------

 | | | First | First | Second | Second | Third |

 | |ZML Emission| Slope | Corner | Slope | Corner | Slope |

 | | (gr/m) |(gr/m/1000m)|(1000 miles)|(gr/m/1000m)|(1000 miles)|(gr/m/1000m)|

 |------------+------------+------------+------------+------------+------------+------------|

 |Model Year/ | | | | | | |

 |Technology | | | | | | |

 |------------| | | | | | |

 |88-93 PFI | 0.0291| 0.0000| 19.18| 0.0005| N/A| N/A|

 |------------+------------+------------+------------+------------+------------+------------|

 |88-93 TBI | 0.0253| 0.0000| 16.25| 0.0004| 54.46| 0.0003|

 |------------+------------+------------+------------+------------+------------+------------|

 |84-93 CARB | 0.1118| 0.0000| 36.51| 0.0008| N/A| N/A|

 |------------+------------+------------+------------+------------+------------+------------|

 |81-87 FI | 0.0594| 0.0000| 29.76| 0.0006| N/A| N/A|

 |------------+------------+------------+------------+------------+------------+------------|

 |81-83 CARB | 0.1033| 0.0000| 12.35| 0.0002| 80.35| 0.0001|

 -------------------------------------------------------------------------------------------- 
Methane -8- 11/8/99

Table 2: Methane Start Emission Deterioration Model Coefficients for Light-Duty Vehicles

Cars -------------------------------------------------------------

 | | | | MEAN | MEAN |

 | | | | METHANE | METHANE |

 | | ZML | SLOPE | HIGHS | NORMALS |

 | | (gr/m) |(gr/m/1000m)| (gr/m) | (gr/m) |

 |-----------+------------+------------+----------+----------|

 |GROUP | | | | |

 |-----------| | | | |

 |88-93 PFI | 0.102| -0.0002| 0.178| 0.095|

 |-----------+------------+------------+----------+----------|

 |88-93 TBI | 0.084| -0.0003| -0.073| 0.071|

 |-----------+------------+------------+----------+----------|

 |86-93 CARB | 0.108| -0.0001| 0.242| 0.105|

 |-----------+------------+------------+----------+----------|

 |83-87 FI | 0.115| -0.0004| 0.151| 0.097|

 |-----------+------------+------------+----------+----------|

 |83-85 CARB | 0.174| -0.0001| 0.601| 0.172|

 |-----------+------------+------------+----------+----------|

 |81-82 FI | 0.077| 0.0008| 0.335| 0.116|

 |-----------+------------+------------+----------+----------|

 |81-82 CARB | 0.177| 0.0007| 0.551| 0.211|

 -------------------------------------------------------------

Trucks -------------------------------------------------------------

 | | | | MEAN | MEAN |

 | | | | METHANE | METHANE |

 | | ZML | SLOPE | HIGHS | NORMALS |

 | | (gr/m) |(gr/m/1000m)| (gr/m) | (gr/m) |

 |-----------+------------+------------+----------+----------|

 |GROUP | | | | |

 |-----------| | | | |

 |88-93 PFI | 0.151| -0.0022| .| 0.050|

 |-----------+------------+------------+----------+----------|

 |88-93 TBI | 0.148| -0.0002| .| 0.140|

 |-----------+------------+------------+----------+----------|

 |84-93 CARB | 0.302| -0.0001| 0.556| 0.297|

 |-----------+------------+------------+----------+----------|

 |81-87 FI | 0.098| -0.0000| 0.183| 0.098|

 |-----------+------------+------------+----------+----------|

 |81-83 CARB | 0.423| -0.0001| 0.686| 0.417|

 -------------------------------------------------------------

 

 
Methane -10- 11/8/99

Table 3: Fraction of High Emitters for Light-Duty Cars

 ----------------------------------------------------------------------------------------

 | | GROUP |

 | |----------------------------------------------------------------------------|

 | |81-82 CARB| 81-82 FI |83-85 CARB| 83-87 FI |86-93 CARB|88-93 PFI |88-93 TBI |

 |---------+----------+----------+----------+----------+----------+----------+----------|

 |MILES | | | | | | | |

 |(x1000) | | | | | | | |

 |2.142 | 0.0282| 0.0203| 0.0232| 0.0223| 0.0052| 0.0184| 0.0239|

 |12.823 | 0.0543| 0.0654| 0.0158| 0.0157| 0.0197| 0.0227| 0.0251|

 |29.335 | 0.1580| 0.1613| 0.0047| 0.0406| 0.0526| 0.0422| 0.0270|

 |50 | 0.2906| 0.2861| 0.0917| 0.1003| 0.1042| 0.0800| 0.0386|

 |60.006 | 0.3560| 0.3485| 0.1348| 0.1298| 0.1296| 0.0987| 0.0458|

 |74.239 | 0.4503| 0.4393| 0.1972| 0.1723| 0.1661| 0.1260| 0.0561|

 |87.786 | 0.5416| 0.5275| 0.2578| 0.2078| 0.2012| 0.1525| 0.0661|

 |100.01 | 0.6253| 0.6094| 0.3135| 0.2346| 0.2334| 0.1770| 0.0753|

 |112.948 | 0.7152| 0.6986| 0.3737| 0.2634| 0.2678| 0.2036| 0.0851|

 |124.625 | 0.7976| 0.7812| 0.4290| 0.2898| 0.2992| 0.2280| 0.0940|

 |135.738 | 0.8772| 0.8620| 0.4826| 0.3153| 0.3295| 0.2518| 0.1026|

 |146.315 | 0.9539| 0.9407| 0.5345| 0.3400| 0.3586| 0.2748| 0.1110|

 |156.38 | 1.0000| 1.0000| 0.5847| 0.3638| 0.3866| 0.2972| 0.1190|

 |165.96 | 1.0000| 1.0000| 0.6332| 0.3868| 0.4135| 0.3189| 0.1267|

 |175.077 | 1.0000| 1.0000| 0.6801| 0.4089| 0.4393| 0.3398| 0.1341|

 |183.753 | 1.0000| 1.0000| 0.7253| 0.4303| 0.4641| 0.3601| 0.1412|

 |192.01 | 1.0000| 1.0000| 0.7690| 0.4508| 0.4879| 0.3798| 0.1480|

 |199.869 | 1.0000| 1.0000| 0.8111| 0.4706| 0.5108| 0.3988| 0.1546|

 |207.349 | 1.0000| 1.0000| 0.8516| 0.4896| 0.5327| 0.4171| 0.1609|

 |214.466 | 1.0000| 1.0000| 0.8907| 0.5079| 0.5537| 0.4348| 0.1669|

 |221.241 | 1.0000| 1.0000| 0.9284| 0.5255| 0.5738| 0.4519| 0.1727|

 |227.688 | 1.0000| 1.0000| 0.9646| 0.5425| 0.5931| 0.4683| 0.1782|

 |233.823 | 1.0000| 1.0000| 1.0000| 0.5587| 0.6116| 0.4842| 0.1836|

 |239.663 | 1.0000| 1.0000| 1.0000| 0.5743| 0.6293| 0.4994| 0.1887|

 |245.22 | 1.0000| 1.0000| 1.0000| 0.5893| 0.6462| 0.5141| 0.1936|

 |250.509 | 1.0000| 1.0000| 1.0000| 0.6036| 0.6624| 0.5283| 0.1982|

 ----------------------------------------------------------------------------------------

(Cont.) 
Methane -10- 11/8/99

Table 3: Fraction of High Emitters for Light-Duty Trucks

 -----------------------------------------------------------------

 | | GROUP |

 | |------------------------------------------------------|

 | |81-83 CARB| 81-87 FI |84-93 CARB|88-93 PFI |88-93 TBI |

 |---------+----------+----------+----------+----------+----------|

 |MILES | | | | | |

 |(x1000) | | | | | |

 |2.142 | 0.0500| 0.0659| 0.0000| 0.0301| 0.0094|

 |12.823 | 0.0405| 0.0515| 0.0000| 0.0294| 0.0066|

 |29.335 | 0.0733| 0.0283| 0.0000| 0.0356| 0.0202|

 |45.05 | 0.1181| 0.0370| 0.0000| 0.0546| 0.0381|

 |60.006 | 0.1637| 0.0883| 0.0164| 0.0734| 0.0555|

 |74.239 | 0.2101| 0.1393| 0.0697| 0.0919| 0.0721|

 |87.786 | 0.2573| 0.1900| 0.1230| 0.1102| 0.0884|

 |100.678 | 0.3052| 0.2405| 0.1761| 0.1281| 0.1043|

 |112.948 | 0.3538| 0.2905| 0.2291| 0.1458| 0.1198|

 |124.625 | 0.4030| 0.3401| 0.2819| 0.1631| 0.1349|

 |135.738 | 0.4528| 0.3891| 0.3343| 0.1801| 0.1497|

 |146.315 | 0.5032| 0.4376| 0.3864| 0.1968| 0.1641|

 |156.38 | 0.5540| 0.4855| 0.4381| 0.2131| 0.1781|

 |165.96 | 0.6052| 0.5328| 0.4892| 0.2291| 0.1918|

 |175.077 | 0.6568| 0.5793| 0.5399| 0.2446| 0.2050|

 |183.753 | 0.7087| 0.6251| 0.5899| 0.2599| 0.2178|

 |192.01 | 0.7608| 0.6702| 0.6393| 0.2747| 0.2303|

 |199.869 | 0.8132| 0.7144| 0.6881| 0.2891| 0.2424|

 |207.349 | 0.8656| 0.7578| 0.7361| 0.3032| 0.2541|

 |214.466 | 0.9181| 0.8004| 0.7833| 0.3169| 0.2655|

 |221.241 | 0.9706| 0.8420| 0.8297| 0.3302| 0.2764|

 |227.688 | 1.0000| 0.8827| 0.8753| 0.3431| 0.2870|

 |233.823 | 1.0000| 0.9225| 0.9200| 0.3556| 0.2973|

 |239.663 | 1.0000| 0.9614| 0.9638| 0.3677| 0.3072|

 |245.22 | 1.0000| 0.9993| 1.0000| 0.3795| 0.3167|

 |250.509 | 1.0000| 1.0000| 1.0000| 0.3909| 0.3259|

 ------------------------------------------------------------------
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