The emissions methodology is based upon emissions factors which describe the rate at which road vehicles emit greenhouse gases according to distance travelled. These emissions factors represent emissions associated with entire vehicles, which can be contrasted with similar transport-related emissions factors which describe emissions on per-passenger or per-unit-of-freight bases. Users can nevertheless choose to 'share' vehicular emissions amongst passengers by specifying the vehicle occupancy.
Emissions - conventionally expressed in terms of mass (e.g. kg) - are calculated by simply multiplying these rates (mass emitted per distance; e.g. kg CO2 per km) by a distance travelled (e.g km).
The rate at which road vehicles emit greenhouse gases varies depending on factors such as the type of fuel used, and the fuel 'efficiency' of the vehicle (i.e. the distance acheived per unit of fuel consumed). Therefore, emissions factors for a broad range of generalised vehicle types are provided.
A total of 36 specific types of road vehicle are represented and are differentiated by their vehicle type (e.g. car, motorbike or van), their fuel type (e.g. petrol, diesel, or several others), and a measure of their size. In the case of petrol and diesel cars and motorbikes, 'size' represents engine size (in litres and cubic centimetres respectively), while vans are differentiated on the basis of their gross vehicle weight. Cars which run on compressed natural gas (CNG), liquefied petroleum gas (LPG), or hybrid or unknown energy sources are differentiated according to qualitative size categories (e.g. 'small', 'medium').
Each vehicle type is represented by six distinct emissions factors which differentiate greenhouse gas emissions into the following types:
- direct CO2 emissions: carbon dioxide emissions produced during vehicle usage, i.e. fuel combustion
- direct CH4 emissions: methane emissions produced during vehicle usage, i.e. fuel combustion
- direct N2O emissions: nitrous oxide emissions produced during vehicle usage, i.e. fuel combustion
- total direct emissions: all direct emissions, i.e. CO2 + CH4 + N2O
- indirect emissions: emissions associated with stages in the fuel production chain such as raw material extraction and fuel delivery
- total or 'life cycle': the total of direct and indirect emissions
Other DEFRA vehicle-based methodologies available represent cars differentiated by market segment and heavy goods vehicles. Methodologies oriented around passenger and freight transport are also available.
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 and size drill down choices.
Activity data required
According to this methodology, greenhouse gas emissions are directly proportionate to distance travelled which therefore must be specified - using the distance profile item value - in order to make an emissions calculation.
In addition, the number of vehicle occupants can be specified by setting the appropriate value using the occupancy profile item value (default = 1).
Users can choose to specify their activity data in a comprehensive range of appropriate units.
Calculation and result
The returned emissions quantities for this methodology are inclusive of CO2, CH4 and N2O. The following discrete amounts are returned:
- CO2: CO2 emissions
- methaneCO2e: CH4 (methane) emissions
- nitrousOxideCO2e: N2O (nitrous oxide) emissions
- totalDirectCO2e: total direct emissions
- indirectCO2e: Indirect emissions
- lifeCycleCO2e: Full life cycle emissions
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.
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