{
  "name": "Methane combustion slip factors for gathering compressor driver engines",
  "url": "https://tetrasoftco.com/tools/compressor-engine-methane-emissions/",
  "factorsReviewed": "2026-08-03",
  "license": "Free to use with attribution to TetraSoft. The underlying figures are transcribed from the cited open-access article.",
  "source": {
    "title": "Methane Exhaust Measurements at Gathering Compressor Stations in the United States",
    "authors": "Vaughn, T. L.; Luck, B.; Williams, L.; Marchese, A. J.; Zimmerle, D.",
    "journal": "Environmental Science & Technology",
    "year": 2021,
    "datePublished": "2021-01-07",
    "volume": 55,
    "issue": 2,
    "pages": "1190-1196",
    "doi": "10.1021/acs.est.0c05492",
    "url": "https://doi.org/10.1021/acs.est.0c05492",
    "access": "Open access under CC BY-NC-ND.",
    "method": "A novel in-stack tracer gas method based on EPA Method ALT-012, coupled with FTIR species measurement, on engines operating as found under normal daily circumstances.",
    "enginesMeasured": 133,
    "stations": 67,
    "states": 11,
    "operators": 9
  },
  "conclusion": {
    "quote": "the GHGRP combustion slip emission factor is inadequate for internal combustion engines and should not be used in the GHGI",
    "attribution": "Vaughn et al. 2021, Results and Discussion"
  },
  "engineTypes": [
    {
      "id": "4slb",
      "abbreviation": "4SLB",
      "label": "Four-stroke lean-burn",
      "measuredInThisStudy": true,
      "unitsTested": 63,
      "observedRangeKgPerHour": {
        "low": 0.3,
        "high": 12.6
      },
      "note": "The workhorse of gathering compression and the class where the study and the GHGRP disagree most sharply."
    },
    {
      "id": "4srb",
      "abbreviation": "4SRB",
      "label": "Four-stroke rich-burn",
      "measuredInThisStudy": true,
      "unitsTested": 39,
      "observedRangeKgPerHour": {
        "low": 0.01,
        "high": 4.5
      },
      "note": "Every 4SRB unit tested carried non-selective catalytic reduction after-treatment. The AP-42 factor is for uncontrolled engines, which is the likeliest reason the measured factor lands at 43 percent of it."
    },
    {
      "id": "2slb",
      "abbreviation": "2SLB",
      "label": "Two-stroke lean-burn",
      "measuredInThisStudy": false,
      "unitsTested": 0,
      "observedRangeKgPerHour": null,
      "note": "Not measured in this study. Only published inventory factors are available, and only on a fuel-input basis."
    },
    {
      "id": "igt",
      "abbreviation": "IGT",
      "label": "Industrial gas turbine",
      "measuredInThisStudy": false,
      "unitsTested": 0,
      "observedRangeKgPerHour": null,
      "note": "Not measured in this study. Turbines emit far less unburned methane than reciprocating engines, so the factors are three orders of magnitude smaller."
    }
  ],
  "perUnit": {
    "units": "kg CH4 per hour per unit",
    "basis": "Average combustion slip per unit while operating, with 95 percent confidence interval.",
    "citation": "Methane Exhaust Measurements at Gathering Compressor Stations in the United States, Table 2",
    "caveat": "Covers four-stroke lean-burn and four-stroke rich-burn only. The study did not measure two-stroke lean-burn engines or gas turbines, so no per-unit value exists for them and none has been substituted.",
    "bases": [
      {
        "id": "study-measured",
        "label": "Study measured",
        "description": "The direct field measurements, weighted per unit. The interval is wide because it carries the full spread of what individual engines actually did.",
        "values": {
          "4srb": {
            "value": 0.4,
            "low": 0.24,
            "high": 0.68
          },
          "4slb": {
            "value": 5.77,
            "low": 5.03,
            "high": 6.5
          }
        }
      },
      {
        "id": "study-ef",
        "label": "Study emission factor",
        "description": "The emission factor developed from those measurements, applied back to the sample the same way an EPA factor would be. This is the number to compare against the others.",
        "values": {
          "4srb": {
            "value": 0.4,
            "low": 0.37,
            "high": 0.42
          },
          "4slb": {
            "value": 5.62,
            "low": 5.15,
            "high": 6.17
          }
        }
      },
      {
        "id": "ap42",
        "label": "AP-42",
        "description": "The EPA compilation of air pollutant emission factors. Closest of the three published factors to what was measured.",
        "values": {
          "4srb": {
            "value": 0.94,
            "low": 0.87,
            "high": 1
          },
          "4slb": {
            "value": 6.13,
            "low": 5.63,
            "high": 6.73
          }
        }
      },
      {
        "id": "ghgi",
        "label": "GHGI",
        "description": "The US Greenhouse Gas Inventory factor, which traces back to a 1996 GRI/EPA report and applies one value to every reciprocating engine class.",
        "values": {
          "4srb": {
            "value": 5.09,
            "low": 4.72,
            "high": 5.41
          },
          "4slb": {
            "value": 7.2,
            "low": 6.55,
            "high": 7.97
          }
        }
      },
      {
        "id": "ghgrp",
        "label": "GHGRP",
        "description": "The Greenhouse Gas Reporting Program Subpart C combustion slip factor, one value for all internal combustion drivers.",
        "values": {
          "4srb": {
            "value": 0.01,
            "low": 0.01,
            "high": 0.01
          },
          "4slb": {
            "value": 0.01,
            "low": 0.01,
            "high": 0.01
          }
        }
      }
    ]
  },
  "fuelInput": {
    "units": "lb CH4 per MMBtu of fuel input",
    "basis": "Unburned fuel emitted as methane, on a fuel energy input basis.",
    "citation": "Methane Exhaust Measurements at Gathering Compressor Stations in the United States, Table 1",
    "note": "The only basis that covers two-stroke lean-burn engines and industrial gas turbines.",
    "bases": [
      {
        "id": "study",
        "label": "Study",
        "values": {
          "4slb": 1.15,
          "4srb": 0.1
        }
      },
      {
        "id": "ap42",
        "label": "AP-42",
        "values": {
          "2slb": 1.45,
          "4slb": 1.25,
          "4srb": 0.23,
          "igt": 0.0086
        }
      },
      {
        "id": "ghgi",
        "label": "GHGI",
        "values": {
          "2slb": 1.27,
          "4slb": 1.27,
          "4srb": 1.27,
          "igt": 0.0044
        }
      },
      {
        "id": "ghgrp",
        "label": "GHGRP",
        "values": {
          "2slb": 0.002,
          "4slb": 0.002,
          "4srb": 0.002,
          "igt": 0.002
        }
      }
    ]
  },
  "national": {
    "scope": "US gathering compressor units.",
    "citation": "Methane Exhaust Measurements at Gathering Compressor Stations in the United States, Table 3",
    "estimates": [
      {
        "id": "study",
        "label": "Study",
        "kgPerHour": 37437,
        "low": 26870,
        "high": 49746,
        "ggPerYear": 328,
        "ggLow": 235,
        "ggHigh": 436
      },
      {
        "id": "ap42",
        "label": "AP-42",
        "kgPerHour": 49367,
        "low": 34919,
        "high": 65711,
        "ggPerYear": 432,
        "ggLow": 306,
        "ggHigh": 576
      },
      {
        "id": "ghgi",
        "label": "GHGI",
        "kgPerHour": 72779,
        "low": 50990,
        "high": 97059,
        "ggPerYear": 638,
        "ggLow": 447,
        "ggHigh": 850
      },
      {
        "id": "subpart-c",
        "label": "GHGRP Subpart C",
        "kgPerHour": 125,
        "low": 88,
        "high": 166,
        "ggPerYear": 1,
        "ggLow": 1,
        "ggHigh": 1
      }
    ],
    "share": {
      "ofGatheringAndBoosting": 24,
      "ofGatheringAndBoostingLow": 17,
      "ofGatheringAndBoostingHigh": 31,
      "gatheringAndBoostingTotalGg": 1391,
      "ofNaturalGasSystems": 6,
      "naturalGasSystemsTotalGg": 5598,
      "inventoryYear": 2020,
      "reportedCombustionSlipGg": 374,
      "note": "Combustion slip reported in the 2020 GHGI for gathering and boosting, 374 Gg, falls within the uncertainty of the study model."
    }
  },
  "exclusions": [
    "Crankcase ventilation, which the authors note may add 3 to 20 percent on lean-burn engines.",
    "Rod packing, seal vents and any other non-exhaust source.",
    "Carbon dioxide from the fuel that did combust."
  ]
}