Selecting car type
To use this category, specify the vehicle fuel type and size via drill down. The fuel drill option contains the following options:
- 'average' (represents the average emissions of all types)
Vehicle size is specified using the size drill option, and corresponds with the following engine sizes:
- small: < 1.4 l
- medium: 1.4 - 2.0 l
- large: > 2.0 l
- small: < 1.7 l
- medium: 1.7 - 2.0 l
- large: > 2.0 l
In the case that US/Canada emissions factors are chosen, the above guidelines for size categories should be increased so that 2.5 litre engines are considered small/medium and 3.5 litre engines medium/large. The data was obtained from the 2008 Fuel Economy Guide published by the US Dept of Energy, available online at www.fueleconomy.gov.
Calculating by fuel consumed
To make calculations according to the quantity of fuel consumed, users need only specify the fuel quantity using the fuelConsumed parameter. The returned value represents CO2 emissions associated with the quantity specified. This is the most accurate method for determining emissions since it makes no assumptions or generalisations regarding the performance, context or efficiency of the vehicle. This calculation method takes precedence over distance-based calculations (see below) - i.e. this calculation will be performed whenever the fuelConsumed parameter is specified.
Calculating by distance
To calculate according to distance, users need to specify at least a distance value, and select between calculations based on either established, generic emissions factors or those derived from fuel consumption data. Occupancy can be specified (see below]) but defaults to a sensible value if not set.
Setting the distance
To specify the distance travelled, set the distance parameter. If users wish to base their emissions calculation on 'typical' car usage, they can alternatively set the useTypicalDistance parameter to 'true', which will return a calculation based on the UK average mileage of 9000 miles per year (equating to 14420 km per year or 1202 km per month).
Using established emissions factors
To use established emissions factors (sourced from the EPA, DEFRA, and the VCA), users need only set the distance parameter (although several further specifications may be made in order to establish more accurate results). If the distance parameter alone is set, the returned amount represents CO2 emissions associated with UK-based car travel according to the distance specified.
Setting the country
Emissions factors relating to both the UK and US/Canada are available, however, and users can switch between each by setting the country parameter to any of the following valid values:
- 'United Kingdom'
- 'United States'
As described above, if no country parameter is specified, the UK emissions factors are used by default. If calculations are stored using profiles, the country can be specified within the profile metadata. Users should note, however, that directly specified country parameters override any set as profile metadata. This is useful, for example, for recording car journey's outside the user's own country.
Deriving emissions factors from fuel consumption
Instead of using pre-established emissions factors, users can opt to give a vehicle's fuel consumption. This can either be a user's own estimate (self-calculated or perhaps obtained from their in-car computer) or one provided by the manufacturer, and results in a more accurate estimate of the kg CO2 emitted. The specified fuel consumption value is used in conjunction with data on the carbon intensity of the respective fuel type taken from The category /home/energy/quantity to obtain a distance-based emissions factor according to:
Factor(mass CO2 per distance) = fuel carbon intensity(mass CO2 per volume) / fuel consumption(volume per distance)
To specify either a manufacturer or self-calculated fuel consumption rate, set either the fuelConsumption or fuelConsumptionOwn parameter. If the former (i.e. manufacturer) is specified, Discover will inflate the value by 15% to account for real world usage.
Emissions factor modifiers
Emissions factors can be further tweaked to provide more accurate calculations by setting several other parameter which represent factors affecting vehicle fuel consumption. These can be accessed as follows:
- set tyresUnderinflated = 'true' to specify under-inflated tyres (increases emissions by 1%)
- set airconFull = 'true' to specify regular use of air conditioning (increases emissions by 20%)
- set airconTypical = 'false' to 'switch off' the typical-air-conditioning-usage assumption (decreases emissions by 5%)
- set ecoDriving = 'true' to specify fuel saving driving practices (e.g. avoiding excessive speed, harsh acceleration and braking; decreases emissions by 10%)
- set regularlyServiced = 'false' to 'switch off' the regular-vehicle-service assumption (increases emissions by 4%)
Note: these are irrelevant and therefore ignored if the user has supplied their "own" estimate of fuel consumption, and only apply if using established, generic emissions factors or an emissions factor derived from manufacturer fuel consumption data.
Users can set multiple occupancy for car travel, which 'shares' the vehicular emissions across all passengers. To specify the number of occupants set the occupants parameter. If no value is set, the calculation will be made on the basis of a single occupant (i.e. the driver) and representing emissions for the entire vehicle journey.