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, driving style, 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 and distance travelled.
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.
The methodology provides emissions factors for converting fuel quantities and distances 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, CNG, LNG, LPG):
- 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)
CO2 emissions factors are calculated thus:
Mass CO2 per volume fuel = CO2 per energy quantityIPCC x energy quantity per mass fuelIPCC x fuel densityuser or DEFRA
Non-CO2 emissions: CH4 and N2O emissions are related primarily to vehicle type, fuel type, emissions control technology, distance travelled and typical driving style (e.g. urban, rural, highway; hot or cold vehicle starts). IPCC methodology provides distinct distance-based emissions factors for CH4 and N2O (i.e. mg per km) for cars, buses, 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. pre-Euro, Euros 1 ,2, 3, 4). These data are applicable to European road vehicles. For each vehicle type, CH4 and N2O emissions are differentiated according to driving styles:
- urban cold: urban driving which starts with the engine at ambient temperature
- urban hot: urban driving which starts with the engine warmed
- highway: highway driving
- rural: rural driving
Other IPCC transport categories are available which represent basic fuel consumption CO2 and vehicular emissions for 'conventionally' (gasoline, diesel) and 'alternatively' (cng, lpg, lng, ethanol) fuelled vehicles in the US.
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 and distance travelled; or (3) by specifying distance travelled 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 and distance: Set the volume of fuel consumed using the fuelConsumed profile item value and the distance travelled using the distance 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 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, CarbonKit assumes an occupancy of 1 (equal to total vehicular emissions).
Following IPCC guidelines, CH4 and N2O emissions are calculated on the basis of the following 'typical' split of driving styles:
- urban driving, cold engine start: 30%
- urban driving, hot engine start: 10%
- highway driving: 30%
- rural driving: 30%
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)
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.