DEFRA transport fuel methodology

The methodology

Emissions model

The emissions methodology is based upon emissions factors which describe the greenhouse gas emissions associated with the specific quantities of transport fuel. These emissions factors can be contrasted with similar transport-related emissions factors which describe emissions on the basis of distance. This methodology is typically more accurate for calculating transport related greenhouse gas emissions since it involves a more direct measure of fuel consumption without the requirement for assumptions about vehicle fuel efficiencies.

Emissions - conventionally expressed in terms of mass (e.g. kg) - are calculated by multiplying these rates (mass emitted per volume; e.g. kg CO₂ per litre) by a volume of fuel (e.g litres) consumed. DEFRA also publish data on fuel densities which can be used to convert volumetric quantities of fuel into their equivalent masses (or vice versa). As such, emissions can additionally be calculated on the basis of mass-based quantities (e.g. kg or tonnes).

Model data

The rate at which fuel combustion produces greenhouse gas emissions varies depending on factors such as the carbon content of the specifc type of fuel burned. Therefore, emissions factors for a number of distinct transport fuels are provided.

Each fuel is represented by six distinct emissions factors which differentiate greenhouse gas emissions into the following types:

• direct CO₂ emissions: carbon dioxide emissions produced during combustion
• direct CH₄ emissions: methane emissions produced during combustion
• direct N₂O emissions: nitrous oxide emissions produced during 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

Similar methodologies

Other DEFRA transport methodologies available representing road vehicles differentiated by size and market segment, heavy goods vehicles, passenger transport and the transport of freight are also available.

Using this methodology

Choosing a specific activity type

To use this methodology, the particular type of fuel must be specified by selecting from of the fuel drill down choice.

Activity data required

According to this methodology, greenhouse gas emissions are directly proportionate to the quantity of fuel consumed. Fuel quantity must be specified in one of two ways in order to make an emissions calculation:

• by volume: specify a volume of fuel using the volume profile item value
• by mass: specify fuel quantity on the basis of mass, using the mass profile item value

Calculation and result

In the case that a mass-based fuel quantity is specified, Discover converts this quantity into a volumetric quantity by using the fuel-specific density values provided in the DEFRA source documentation. 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

These emissions reflect the emissions type specified (i.e. direct, indirect or life cycle) are therefore represent direct emissions by default.

Notes

Default units for CNG

The emissions factors for compressed natural gas (CNG) are presented on the basis of mass in the original DEFRA documentation, i.e. kg [emissions] per kg.

For consistency with other fuels in this category, these values have been converted to volumetric emissions factors (i.e. kg [emissions] per litre) using the density value for CNG (0.175 kg per litre) provided in Annex 11 Fuel Properties of the DEFRA documentation. As described above, emissions for all fuels can nevertheless be calculated on the basis of mass quantities, in which cases Discover references the same fuel property data in order to make the appropriate conversions.

Biofuel blends

Emission factors calculated for petrol supplied at public refuelling stations, factoring in the bioethanol supplied in the UK as a proportion of the total supply of petrol+bioethanol (bio fraction = 3.1% by unit mass, 2.9% by unit volume, 1.9% by unit energy). Emission factors calculated for diesel supplied at public refuelling stations, factoring in the biodiesel supplied in the UK as a proportion of the total supply of diesel+biodiesel (bio fraction = 4.2% by unit mass, 2.9% by unit volume, 1.9% by unit energy). These estimations have been made based on the most recently available reports on the Renewable Transport Fuel Obligation (RTFO).