While the European Commission is currently working towards a new test procedure to determine CO2 emissions via the VECTO simulation model, the US EPA and NHTSA have reviewed and finalized Phase 2 of their comprehensive Heavy-Duty National Program to reduce greenhouse gas emissions (GHG) and increase fuel efficiency for on-road heavy-duty vehicles. Similar to VECTO, also the US have chosen a simulation approach to determine CO2 from heavy-duty vehicles by developing the GEM model (Greenhouse Gas Emissions Model). This was developed by US EPA as a means for determining compliance with EPA’s GHG emissions and NHTSA’s fuel efficiency standards, for Class 7 and 8 combination tractors and Class 2b-8 vocational vehicles.
Both the VECTO and the GEM model use input files that provide data on relevant vehicle, driveline and engine characteristics such as engine efficiency maps, gear ratios, tire rolling resistance, vehicle mass and many more. All of these inputs are defined, and where appropriate, a test procedure is developed to determine them in an accurate and reproducible way. One of the most important inputs in terms of its influence on fuel consumption is the vehicle’s aerodynamic resistance, normally expressed as the product of the drag coefficient Cd and the frontal area A. Since it is rather complicated to measure the aerodynamic resistance, both the US and Europe have included detailed test procedures. In the US this is based on a coastdown test as a reference method, while Europe has chosen for a Constant Speed Test (CST).
