Emission-factor model

Emissions from motor vehicles are a function of the drive cycle, the ambient conditions, and the characteristics of the engine, fuel, and emission-control system. Engines, fuels, or emission-control systems often change from one vehicle model year to the next, sometimes because the emission standards tighten. Moreover, over the life of any particular vehicle, the emissions usually increase as the engine and emission-control system deteriorate or fail with accumulated mileage.

Detailed emissions model, such as the EPA’s MOBILE model, estimate emissions in a target year as a function of zero-mile emissions rates by model year, emission-deterioration rates, average speed, temperature, and other factors. In the emissions models, and in reality, the model year and the accumulated mileage in the target year are critical variables. However, as far as I know, no fuelcycle emissions model properly estimates motor-vehicle emissions as a function of model year and accumulated mileage in a target year. Rather, all of the existing models, including the previous version of this one, simply have estimated or assumed fleet-average emissions in some target year.

In the revised GHG model, emissions in a target year are estimated as a function of the zero-mile emission rate for a base model year, the annual change in the zero-mile emission rate with each new model year, the emissions-deterioration rate for each model year, the accumulated mileage in the target year, and the annual mileage accumulation. Formally:

This model is used to estimate emissions from baseline gasoline and diesel vehicles. The parameter values are shown Table X, and documented in the notes to that table and in the following sections.

Emission-factor parameters for CO, NMOC, and NOx emissions from light-duty gasoline vehicles

In the original version of the model, I estimated the baseline CO, NMOC, and NOx g/mi emission factors by adjusting the output of MOBILE4.1 to account for the effects of the then-new 1990 Clean Air Act Amendments (Tables B.2 and B.3). As noted above, the model now estimates emissions as a function of the zero-mile emission rate for a baseline model year, the annual change in the zero-mile emission rate with each new model year, and the emissions-deterioration rate for each model year. These new emission-factor parameters are specified on the basis of data and estimates in the EPA (1985/1991) and Ross et al. (1995). Ross et al. (1995) perform a detailed analysis of emissions from cars with malfunctioning emission-control equipment and from high-power driving not represented in the standard emissions test (the Federal Test Procedure, or FTP). They add these malfunction and so-called "off-cycle" emissions to "on-cycle" emissions from properly functioning cars to obtain an estimate of real-world emissions from model-year 1993, 2000, and 2010 passenger cars using conventional gasoline.

Note that the estimates of Ross et al. (1995) of lifetime-average emissions from a model-year 2000 vehicle are higher than Wang’s (1996) estimates, based on MOBILE5A, of fleet-average emissions in calendar year 2005.

Because the CEFs for these pollutants are relatively small, the changes in the emission factors have only a minor effect on CO2-equivalent GHG emissions.

Emission-factor parameters for N2O emissions from LDVs

An analysis of new data on N2O emissions (e.g., Dasch, 1992; Battelle, 1995; see Delucchi and Lipman, 1996), along with a re-examination of the original emissions data (Table N.1), show clearly that N2O emissions from LDVs equipped with a 3-way catalyst are a function of the age of the catalyst. Consequently, instead of assuming a constant emission factor of 60 mg/mi for gasoline vehicles (as in the original Table B.2), I now assume that N2O emissions deteriorate at the rate of 1.6 mg/mi per 1000 miles of vehicle age.

This function is used for all gasoline, alcohol, and propane LDVs equipped with 3-way catalyst systems, for all years of the projection. Also, we apply it to all future model years, because we have no basis to believe that N2O emission rates will change significantly in the future. Assuming an average catalyst life of 60,000 miles results in an emission factor of over 100 mg/mi -- significantly higher than the original 60 mg/mi.

The emission factors for HDVs have not been changed.

See Delucchi and Lipman (1996) for further discussion.

Emission-factor parameters for CH4 emissions from LDVs

In light of new emissions data, and a reconsideration of some of the original data (Table M.1), I assume that the zero-mile methane emission rate decreases over time, as emission standards become tighter, but that for any given model year, methane emissions rise slowly with the age of the catalyst. On the basis of the analysis in Delucchi and Lipman (1996), I assume a deterioration rate of 0.0015g/mi per 1000 miles.

Emission factors for AFVs: relative to gasoline and diesel

Previously, one entered emission factors for AFVs directly in grams/mile. Now, one enters for the AFVs a set of emission factors relative to actual g/mi emissions for the baseline gasoline ICEV. Thus, if before one entered 9.0 g/mi CO for the gasoline ICEV, and 4.5 g/mi CH4 for the NGV, one now enters 9.0 g/mi CO for the gasoline ICEV, and relative emissions of 0.50 for the NGV. To the extent that the relative emissions of AFVs are constant over time and technology, this simplifies the process of modeling the effect of a completely different set of emissions standards, or of emissions over time: one need change only the baseline g/mi emissions factors for the gasoline vehicle.

The relative emission factors, along with some supporting data and citations, are shown in Table X. Note that in Appendix B of Volume 2 (DeLuchi, 1993), I assumed that all light-duty ICEVs will emit roughly the same amount of NOx -- in spite of the different NOx emission characteristics of fuels -- on the grounds that all ICEVs will be designed to just meet the relatively stringent new NOx standards. This presumed that auto manufacturers will capitalize low-NOx emissions potential into savings on emission-control equipment. In reality, though, manufacturers might not find it worthwhile to capitalize all of the potential emissions reductions into savings in emission-control equipment, and instead might prefer to meet the emissions standard with a greater margin of safety. This will result in some small variation in NOx emissions across fuel types. Accordingly, I have assumed that alternative-fuel light-duty vehicles, which in emission tests generally emit slightly less NOx than do gasoline vehicles, will have slightly lower NOx emissions on the road.

Virtually all emissions tests of light-duty alternative-fuel vehicles have been performed using the FTP, which as noted above does not represent high-speed, high-power driving. Recently, however, the Auto/Oil Program (1996) tested methanol, ethanol, CNG, and gasoline vehicles over a new high-speed, high-power drive cycle, the REP05, as well as over the FTP. The emissions from AFVs relative to the emissions from gasoline vehicles over the REP05 were different from the relative emissions over the FTP. Emissions from ethanol and methanol relative to emissions from gasoline were lower in the REP05 than in the FTP, but emissions of CO, NMHC, and NOx from CNG relative to emissions from gasoline were higher in the REP05. These findings are provocative, and warrant further investigation. For now, I have folded them into the emissions data base that serves as the basis of my assumptions in Table X.

Recent tests on transit buses (NREL, 1996) and other heavy-duty vehicles (Wang et al., 1993) indicate that the best lean-burn CNG technology virtually eliminates PM and CO emissions, and significantly reduces NOx emissions. HC emissions increase, but most of the HC is methane. Ethanol and methanol significantly reduce PM and NOx, but increase CO and HC (see Table X for details).

Input of heavy-duty vehicle emission factors: g/bhp-hr vs. g/mi

In the original version of the model, emission factors for heavy-duty diesel vehicles (HDDVs) were input directly in grams/mile. However, the emission standards for HDVs actually are in grams/brake-horsepower-hour (g/bhp-hr), not grams/mile. This matters because if all HDVs meet a given g/bhp-hr standard, then the more efficient ones -- the ones that use fewer bhp-hrs per mile -- will emit fewer grams per mile. Therefore, I changed the model so that the input emission factors for HDDVs are in g/bhp-hr, and the gram/mile emissions then are calculated from the input g/bhp-hr data and assumptions about engine energy conversion efficiency. I assume that the baseline diesel engine produces 0.28 BTUs of brake-work for every BTU of fuel consumed, on average, over the drive cycle (Tyson et al., 1992).

Emissions of refrigerant

In Appendix Q of the original report I estimated that three 2.6-lb charges of CFC-12 were emitted over the 108,000-mile life of an LDV. This emission rate of 31.5 mg/mi, multiplied by a GWP of 7,300 (Table 8), resulted in CO2-equivalent emissions of 230 g/mi (Table B.2) -- a sizable fraction of total fuelcycle emissions. However, Ford (Wallington, 1996) has indicated that since 1991 the typical vehicle has had a 2.0-lb charge of refrigerant, and that it is more likely that the equivalent of only one charge is lost over the life of the vehicle, resulting in an emission rate of only 8.4 mg/mi. If the refrigerant is HFC-134a, with a CO2-equivalency factor of 2,000 (Table I), then the result is only 16.8 g/mi CO2-equivalent emissions -- over an order of magnitude lower than the originally estimated emissions.

See Delucchi and Lipman (1996) for further discussion.

Refinery energy required to produce conventional and reformulated gasoline

In Volume 2 (DeLuchi, 1993), I estimated that refineries consumed 0.145 BTUs of process energy to produce 1.0 BTU of conventional gasoline (Table H.6). Recently, Stork and Singh (1995) reported that a linear programming model of a complex refinery estimated that summer conventional gasoline requires 0.155 BTUs/BTU, and winter conventional gasoline 0.141 BTUs/BTU. The simple average, 0.148, is very close to the 0.145 value assumed here. However, Stork and Singh (1995) estimate that reformulated gasoline requires essentially the same amount of energy to produce as does conventional gasoline. It is not clear to me why this should be so.

Refinery energy use: meaning of BTU/BTU measure

In the original version of the model, the measure of refinery energy use, BTU-refinery-energy/BTU-gasoline, was with respect to BTUs of complete gasoline product (including anything produced outside of the refinery, such as MTBE), not to just the refinery-produced hydrocarbon portion of the gasoline. Now, I have changed the meaning of the measure, to BTUs-refinery-energy/BTU-gasoline-HC, where the denominator includes only the refinery-produced hydrocarbon-portion of the gasoline -- not, for example, the energy value of any MTBE produced outside of the gasoline (Table H.6). Conceptually, I am now assuming in effect that any methanol or ethanol (as such, or in MTBE or ETBE) is added to the gasoline outside of the refinery gates.

Projections of refinery energy use

Previously, the user of the model input one set of estimates of the amount of each kind of fuel used by petroleum refineries. This estimate was made on the basis of historical use (e.g., Table H.4 ) and considerations of future trends. Now, the model has a complete set of detailed projections of refinery energy use, from 1995 to 2015. The user specifies the year of analysis, and the model selects the appropriate data series. The projections of the amount of each kind of fuel used by refineries are from the EIA’s Supplement to the Annual Energy Outlook 1996 (1996), available as data tables from the EIA’s FTP site. (I have added projections of the energy content of purchased steam and hydrogen.)

The model also has yearly projections of the overall energy intensity of refinery output, expressed as total BTUs of refinery energy consumed per BTU of product output. For each year, this is calculated as follows:

Allocation of refinery energy to specific products

There is a typesetting error in Table H.5, page H-20 of Volume 2 (DeLuchi, 1993). On the "Desulfurization" line, the values 0.454, 0.302, and 0.070 should be shifted over to the right by one column, so that the 0.454 is under "Gasoline," the 0.302 is under "Dist.", and the 0.070 is under "Residual". There should be a blank (zero) under "Haynes". This is typesetting error only; the values were entered correctly in the model.

Sale or transfer of electricity

According to the EIA’s Manufacturing Energy Consumption Survey 1991 (1994), petroleum refineries on average sell or transfer out about 10% of the amount of the electricity that they purchase. This sold or transferred power should be deducted from electricity purchases, to arrive at a "net purchase" figure for calculating greenhouse gas emissions due to electricity use. In the original report, I did not account for this (Table 4). Now, I multiply the EIA’s projections of refinery electricity use by 0.90.

Crude used as fuel gas or petroleum coke in refineries

The original version of the model did not account for emissions from the production and transport of the portion of the crude oil that ends up being used as fuel gas or petroleum-coke fuel in refineries (Table A.1). This has been corrected. I have added refinery gas and petroleum coke in the appropriate places (Tables 3, 5, and 7).

Emissions of NMOCs, CO, NOx, SO2, and PM from process areas

The original year-2000 emission-factor projections of DeLuchi et al. (1992) have been replaced with year-by-year projections. The projections use base-year emission factors, long-run minimum emission factors, and a rate of approach to the minimum. The base-year (1994) emission factors are calculated from actual emissions in 1994 (EPA, National Air Pollutant Emission Trends 1900-1994, 1995). The long-run minimum potential emission factors are those projected by DeLuchi et al. (1992). The rate of approach is chosen to be consistent with recent trends, and with the projections of DeLuchi et al. for the year 2000.

CH4 and total carbon emissions from process areas

Methane: I have increased the CH4 emission rate from petroleum refineries, from 0.4 g/106 BTU to 2.0 g/106 BTU (Table A.1)., on the basis of a re-examination of original data (which indicate a range of 0.24 to 2.4 g/106 BTU), and an emission factor of 2.4 g/106 BTU cited by the EIA’s Emissions of Greenhouse Gases in the United States 1987-1994 (1995).

Total carbon: Refinery process areas, such as catalytic crackers, emit CH4, CO, VOCs, and CO2. I assume that the carbon in these process-area emissions comes from the petroleum feed. This means that emissions of carbon from refinery process areas constitute lost crude-oil feedstock. The more crude oil lost, the greater the throughput of crude oil required to produce a given amount of gasoline, diesel fuel, etc. The greater the throughput, the greater the use of energy to recovery and transport crude oil, and hence the greater the emissions of greenhouse gases.

The model now accounts for this effect of lost crude oil, by incorporating emissions from the recovery and transportation of the amount of crude that ends up being lost in carbon emissions from process areas. Formally, I estimate grams of CO2-equivalent GHG emissions due to the recovery and transport of crude oil lost at the refinery, per 106 BTU of product out of the refinery:

The effect of this change is quite small, because less than 1% of the crude input is lost.

Note that a similar accounting is not required for the production of alternative fuels, for which the feed-input/fuel-output ratios are assumed to be based on output net of any losses in the plant.

N2O emissions