US road transport by IPCC

This methodology provides data and a calculation methodology for calculating greenhouse gas emissions associated with US road vehicles - specifically cars, motorcycles, and light and heavy duty vehicles which use 'conventional' fuels (gasoline, diesel). The methodology is sourced from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

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The methodology

Emissions model

Greenhouse gas emissions associated with vehicle transport arise from the combustion of fuels. According to this methodology, CO₂ emissions are directly related to the quantity of carbon which is oxidised during combustion and therefore directly proportionate to the quantity of fuel burned and the concentration of carbon within the particular fuel type. Emissions of CH₄ and N₂O, however, are considered to be related to other factors such as vehicle type, emissions control standards, frequency of starts, and distance travelled.

The IPCC methodology for 'mobile' fuel combustion advises therefore that, for the most accurate greenhouse gas emissions inventory, CO₂ emissions should be calculated on the basis of fuel consumed, whereas those of CH₄ and N₂O should be calculated according to vehicle type, distance travelled and the number of cold starts.

Where only fuel data is available, CO₂ emissions - which represent the majority of vehicular emissions - can be calculated alone. If fuel quantity data is not available a calculation of CO₂ emissions (as well as CH₄ and N₂O) can be made on the basis of distance travelled if fuel economy data (i.e. km per L fuel) can be specified for the vehicle under consideration.

CH₄ and N₂O emissions are multiplied by the appropriate global warming factors in order to express them in the standardised unit of CO₂e.

Emissions calculated using this methodology represent those attributable to an entire vehicle. Per passenger emissions can nevertheless be obtained by specifying the vehicle occupancy.

Model data

The methodology provides emissions factors for converting fuel quantities and distances/engine starts into CO₂, and CH₄ / N₂O emissions respectively for a broad range of road transport scenarios.

CO₂ emissions: According to IPCC methodology, CO₂ emissions are related only to the quantity and type of fuel consumed and not related to the properties of the vehicle used. The IPCC provides the following fuel property data for several types of transport fuel (e.g. petrol/gasoline, diesel):

• CO₂ emissions factor: emissions factors expressed in terms of quantities of CO₂ emitted per unit of energy (kg per TJ)
• Heating value: 'heating' or 'calorific' values (net) describing the energy content of the fuel (TJ per Gg)

These values enable the calculation of CO₂ emissions on the basis of fuel quantities specified in terms of either mass (e.g. kg, tonnes) or energy (e.g. kWh, TJ). If this type of calculation is required, the specific data and associated methodology can be found here. The present methodology uses these values together with values for fuel density in order to derive CO₂ emissions factors for volumetric fuel quantities (e.g. litres, gallons). IPCC does not explicitly provide fuel density information, and therefore this can be specified by the user or otherwise Discover uses density values published by the UK government department DEFRA.

CO₂ emissions factors are calculated thus:

Mass CO₂ per volume fuel = CO₂ per energy quantity_IPCC x energy quantity per mass fuel_IPCC x fuel density_user/DEFRA

Non-CO₂ emissions: CH₄ and N₂O emissions are related primarily to vehicle type, fuel type, emissions control technology, distance travelled and the number of vehicle starts. IPCC methodology provides distinct emissions factors for CH₄ and N₂O for cars, light and heavy duty vehicles and motorcycles differentiated by fuel type (e.g. petrol/gasoline, diesel, CNG, LNG, LPG) and emissions control technology (e.g. low emission vehicle (LEV), advanced three-way catalyst, oxidation catalyst). These data are applicable to US road vehicles. For each vehicle type, CH₄ and N₂O emissions factors are provided in two contexts:

• per distance travelled: mass of emissions associated with each unit of distance travelled (e.g. mg per km)
• per start: mass of emissions associated with each vehicle start (e.g. mg per start)

By multipying these emissions factors by the distance travelled and number of engine starts respectively, an estimate of emissions for each gas is made.

Related methodologies

Other IPCC transport categories are available which represent basic fuel consumption CO₂ and vehicular emissions for European road vehicles and 'alternatively' (cng, lpg, lng, ethanol) fuelled vehicles in the US.

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Using this methodology

Choosing a specific activity type

To use this methodology, the particular type of vehicle which best represents the users emissions scenario must be specified by making selections from each of the type, fuel, technology drill down choices.

Activity data required

Calculations can be made in three ways: (1) by specifying fuel consumption only; (2) by specifying fuel consumed, distance travelled and the number of cold engine starts; or (3) by specifying distance travelled, the number of cold engine starts and vehicular fuel efficiency (CO₂, CH₄ and N₂O).

Specifying fuel used only: Set the volume of fuel consumed using the fuelConsumed profile item value. This method calculates CO₂ emissions only.

Specifying fuel, distance and starts: Set the volume of fuel consumed using the fuelConsumed profile item value, the distance travelled using the distance profile item value, and the number of cold engine starts using the coldStarts profile item value. This method calculates CO₂, CH₄ and N₂O emissions.

Specifying distance and fuel efficiency: Set the distance travelled using the distance profile item value, the number of cold engine starts using the coldStarts profile item value, and the fuel efficiency (or fuel 'economy'; e.g. km per L) using the fuelEfficiency profile item value. This method calculates CO₂, CH₄ and N₂O emissions.

Optional values: A number of optional parameters can also be specified which may reflect the required transport scenario more accurately.

Emissions are implicitly calculated as those attributable to the entire vehicle. If per passenger emissions are required, the occupancy profile item value can be specified, representing the number of passengers for which emissions are accountable. If no occupancy is specified, Discover assumes an occupancy of 1 (equal to total vehicular emissions).

In order to obtain a volumetric CO₂ emissions factor (e.g. mass CO₂ per volume fuel) from the IPCC mass-based emissions factors (i.e. mass CO₂ per mass fuel) a value for fuel density is required. This can be specified on a case-by-case basis using the density profile item value. If left unspecified, the appropriate value is taken from the latest DEFRA fuel properties data.

Calculation and result

The returned emissions quantities for this methodology represent CO₂ only in those cases where only fuel consumption is specified. Otherwise emissions of CO₂, CH₄ and N₂O are returned. The following discrete amounts are returned:

• CO2: CO₂ emissions
• CH4: CH₄ (methane) emissions
• N2O: N₂O (nitrous oxide) emissions
• CO2e: CO₂e emissions (all three gases, converted using these global warming potential)

The individual quantities for CH₄ and N₂O represent absolute quantities rather than CO₂e quantities.

This methodology represents per vehicle emissions and therefore the returned emissions should be considered - by default - to represent those attributable to an entire single vehicle, or its sole occupant depending on the viewpoint of the user (the default value for vehicle occupancy is 1). If multiple vehicle occupancy is specified - by setting the occupancy profile item value to a positive value other than 1 - total vehicular emissions are shared between all occupants and therefore the returned emissions quantity represents the emissions attributable to each occupant.