White paper

Heavy-duty vehicle fuel-efficiency simulation: A comparison of US and EU tools

Vehicle simulation tools can enable a reliable estimation of the fuel efficiency of heavy-duty vehicles while eliminating undesirable sources of variation, such as driver behavior or the influence of ambient conditions. This report discusses the use of simulation tools to certify the whole-vehicle efficiency of heavy-duty vehicles (HDVs), and presents a comparison of the tools used for the regulatory quantification of CO2 emissions from HDVs in the US (GEM v2.0, used for HDV GHG/FE Phase 1 regulation) and the EU (VECTO v2.0.3 beta; not yet used in a regulatory context). The comparison covers both the software implementation and methodological aspects, and is intended to explore the differences between two tools that are intended for similar uses but were developed independently.

Both GEM and VECTO were reviewed based on the available documentation and source code, and the different parameters of the models were aligned to the extent possible. Twelve HDVs were simulated in both models under seven different duty cycles, using both tools. For one of the simulated cycles, a simple “one-at-a-time” sensitivity analysis was performed by controlled variation of selected parameters in both simulation environments.

The results of the simulations show important differences that can be attributed to the model components (e.g., driver model, gearshift strategy) that differed most significantly between the two models. For most parameters, the sensitivity analysis confirms the expected linear behavior of the models within moderate ranges of parameter variation.

Computer simulation offers regulators and OEM manufacturers exciting prospects for the certification of HDV efficiency and CO2 emissions. Whole-vehicle simulation techniques have the potential to reduce vehicle testing efforts worldwide and improve the quality and availability of much-needed HDV efficiency data.