Method

The Energymodule calculates energy and emissions by simulating a given vehicle driving a given route. The energy calculations are physics-based, and depend on the detailed 3D geometry of the route, as well as the physical properties of the vehicle, including the powertrain.

The calculation is separated into four stages. A more detailed description can be found in the SINTEF report "Kjøretøybasert beregning av fart, energi og utslipp. Trondheim: SINTEF 2017 (2017:00031)" (Norwegian).

1. Speed profile

The first stage calculates a target speed, which is the speed the vehicle will try to reach at all points along the route. The target speed calculation uses the 3D geometry and properties of the road (curvature, elevation, width, driving lanes, speed limit) to estimate a target speed for each road segment. The underlying speed models are estimated on the basis of GPS-observations from Norwegian vehicles. The target speed may also be limited by the vehicle's maximum speed (if set), and traffic along the route (if available).

After the initial target speed calculation, the route has been split into short road segments. Each road segment has an individual target speed, and the speeds of each segment is completely independent of the previous segments. In order to avoid unrealistic speed differences between segments, the vehicle's physical properties are used to calculate how much it is physically able to adjust its speed in each segment, resulting in a smooth speed profile.

The charts below show some details of a driving route, and the calculated target speed along the route compared to the speed limit.

2. Energy calculations

The next stage considers each road segment in turn, and calculates how much energy is required to reach the segment target speed, considering elevation changes, horizontal curvature, and how far the driver can see. Road, vehicle and engine properties such as surface friction, vehicle weight and accelerating/braking power will affect the required amount of energy, and may limit the speed change if the vehicle for example does not have the accelerating power to reach the target speed. In such cases, the module will go back and adjust the driver behaviour at an earlier point, to make the trip physically feasible. The result of this step is an adjusted speed profile, as well as calculated engine power needed along the route.

This process simulates driving the entire route, resulting in the actual speed profile and energy demand for the vehicle (as opposed to the target speed profile from the previous step) along the route, as shown in the charts below.

3. Fuel consumption calculations

The third stage uses the energy usage calculated from the previous step to calculate how much fuel is consumed. This step accounts for the fuel to energy conversion efficiency, which varies based on the power demand on the powertrain, and the energy content of the selected fuel. The chart below shows some of the efficiencies used for a battery-electric vehicle driving the example route. Eff_me is the efficiency of the engine (which varies with the energy demand), while eff_gb is the efficiency of the generator (which is constant).

4. Emission calculations

The final stage calculates emissions based on properties of the vehicle's powertrain, including fuel type and fuel parameters and the fuel consumption calculated in the previous stage.

The end result of the full calculation is a detailed estimation of driving speed, energy usage, fuel usage and emissions along the driving route, which can be aggregated to the relevant level.

Limitations

Reliance on detailed input data

Because of the level of detail involved in the speed and energy calculations, input data must be of high quality. For road calculations, the road links must have a detailed 3D geometry, as well as attributes describing the speed limit, the road width, and the number of driving lanes. For rail calculations, the same applies, but the width and number of lanes is irrelevant. Vehicles must also be described in detail, and small errors in the vehicle or road description can cause unexpected results.

When using the Energymodule API with waypoint or stop-id route definitions, the API will use built-in routing functionality to find a route between the waypoints/stops. This routing algorithm is relatively simple, and may not always pick the most practical route. There may also be errors in the road network, which could cause the Energymodule to calculate an unintended route. The road network will be updated regularly, which may cause the results to change if you have calculated on a modified part of the network. Always check your results to ensure they are reasonable. If you need full control over the route, use the links or segments route definitions.

LCA emission factors: assumptions and quality

The lifecycle emission factors for the fuel and vehicle cycles are currently gathered from different sources in literature and environmental product declarations (EPD). The accuracy of these emission factors may vary due to the assumptions made in these publications. Some of these assumptions include:

• system boundaries (what is and is not included in the study)
• geographical origin of fuels and materials
• year of assessment
• characterization of emissions (e.g., assumed CO2-eq value of non-CO2 emission types such as methane; time horizon)
• vehicle lifetime
• vehicle maintenance activities and frequency
• use of primary data for the study
• "closeness" of studied vehicle to reality; this can be especially important for certain vehicle types for which there are few studies available, such as ferries, airplanes, subways and trams
• treatment of biogenic emissions (i.e., biofuels)
• handling of recycled materials

Work is ongoing to harmonize the emission factors to improve the comparability of emissions between different transport modes, as well as improve the realistic representation of the Norwegian vehicle fleets for all modes in the Energymodule API. This is reflected in the default uncertainty grading of "medium" for most of the emission factors. Parameters may be assigned higher uncertainty for the use of proxy values due to lack of data, or the quality of the sources used.

Missing energy usage

The Energymodule calculates energy usage only when the vehicle is moving. That means that energy used while idling or at dock (for ships) is not included in the results.

Although the Energymodule strives to model the physics of the vehicle as detailed as possible, it's still just a model. Real-life driving will always have small deviations from the "perfect" environment we model, such as small detours, lane changes, speed changes due to other traffic, and unmodeled extra energy usage inside the vehicle. This usually means that the Energymodule will underestimate the energy consumption.