Establishing a Standard S-Factor: Improving Accuracy and Flexibility in SOC Corrections for HEV Testing

Hybrid-electric vehicle (HEV) fuel economy test procedures require that the net energy change (NEC) of the battery remain small enough to ensure accurate fuel usage results. SAE J1711-2010 required the NEC stay within 1% of fuel energy consumption, assuming that residual changes in state of charge (SOC) would have negligible impact. In practice, however, the asymmetry between fuel and electricity conversion efficiencies means that a 1% SOC imbalance can translate into a fuel consumption error approaching 3%. A standard S-Factor—a dimensionless ratio of marginal fuel change to marginal NEC change—was introduced in J1711-2023 to improve SOC corrections. The method improves accuracy by correcting all results for NEC changes and expands the NEC-to-fuel ratio (NECFR) window, enabling HEVs to use electric propulsion more aggressively and achieve higher fuel economy in testing and real-world usage. Using a standardized value (instead of requiring additional testing to determine the vehicle-specific value) provides a simple, low-burden approach across HEV designs. Empirical data, supported by simulations of HEV powertrains, indicate that there exists a practical range of S-factors common to efficient HEV designs. The standard value represents a defensible best-guess within this range, close enough to provide corrected results with minimal error. The method also allows the NECFR window to be doubled (e.g., from ±1% to ±2%) while improving the fidelity of final fuel consumption results compared with the legacy procedure. This paper will review the data and modeling used to identify practical S-factor values, examine the insensitivity of HEV fuel consumption results to the exact choice of value, and discuss how expanding the NECFR window can facilitate HEV designs that employ more effective use of electric propulsion. Alternative approaches, including cycle-specific and worst-case S-factors, will also be considered.