Calculating Remote Emissions for Electric Vehicles
Background
Emissions for remotes are calculated using engine displacement, or CC, rating. This is because generally the larger the engine rating, the more emissions are produced. More specifically, a vehicle is put into one of the following CC rating groups based on its CC rating:
Less than 1350 cc
1350 cc to less than 1600 cc
1600 cc to less than 2000 cc
2000 cc to less than 3000 cc
Greater than 3000 cc
Each engine rating group is assigned an emissions factor, which varies depending on the fuel type and engine type. This then allows for the calculation of vehicle emissions using the distance travelled by that remote.
Problem
For each remote, we obtain the CC rating from NZTA’s vehicle data. Electric vehicles (plug in non-hybrids, or remotes with no fuel type and electric engine only) used to come with a CC rating of 1, however NZTA has changed their vehicle data so that new EVs have no CC rating. This poses a problem because we are now unable to calculate their emissions since there is now no CC rating.
Solution
Since EVs no longer have CC ratings, we require a method to estimate the CC rating. Such an estimation can be made by attempting to correlate a vehicle’s weight with its CC rating. The task here is to determine using similar remotes in the database if there is a relationship between vehicle weight and CC rating. This was decided based on advice from MfE as a temporary fix until they update the Measuring Emissions Guide.
Results
The following results were based on every remote taken from the database that has an entry in the tvehicledetails table. This amounts to apporximately 14,000 remotes. The data was from a backup of the production database taken on 14/03/2022.
Above is a graph showing vehicle weight vs engine displacement, grouped by body style. Clearly there are several body styles which are heavily skewing the data. As a result, the following body styles were removed to give a better correlation between weight and engine displacement:
Other Truck
Mobile Machine
Back Chassis Only
Flat Deck Truck
Articulated Truck
Tractor
Heavy Bus/Service Coach
Heavy Van
Self Propelled Caravan
Vehicle Weight vs Engine Displacement with less skew and an exponential trendline
The above graph shows the same data with the aforementioned body styles removed, giving a better overall representation between a correlation between weight and engine displacement. The green trendline was added, along with four y-axis lines in red representing the four engine displacement levels (1350, 1600, 2000 and 3000). The intersections between these lines and the green trendline can give good vehicle weight ranges with which to estimate the engine rating, although more accurate weight ranges may be determined using a graph normal distribution of the engine displacement ratings versus vehicle weight.
Above is a graph depicting engine weight versus the normal distribution of engine ratings. The data for the body styles used in the previous graph was collated, and was sorted into each of the five engine rating groups mentioned in the Background. The aim of creating this graph was to find the points of intersection between the bell curves for each group and use these weight levels as the boundaries between the groups. They can be seen here circled in red. These values are 1570kg, 2000kg, 2370kg, and 2940kg.
These levels can be seen plotted on this graph in orange. The first three intersect at the same points as the green trend line, although the final level does not. This confirms that the weight ranges from the bell curves are fairly reasonable levels to take.
Conclusion
In order to estimate the engine rating of electric vehicles without an engine displacement rating, we will sort these vehicles into the following engine rating groups based on these weight ranges:
Less than 1350 cc: Less than 1570kg
1350 cc to less than 1600 cc: 1570kg to less than 2000kg
1600 cc to less than 2000 cc: 2000kg to less than 2370kg
2000 cc to less than 3000 cc: 2370kg to less than 2940 kg
Greater than 3000 cc: 2940kg or greater
This was decided as these weight ranges come from the normal distribution of engine ratings, and these are considered to be the most statistically accurate based on data from remotes in our database.
It should be noted that this is hopefully a temporary fix until we receive better guidance on how to calculate emissions for electric vehicles from the Measuring Emissions Guide. It is currently unknown when this will be.
Limitations
This analysis only includes remotes that are in our database, so may not be representative of all vehicles. However, this should suffice as a solution for the time being in place of something better