Hybrid-electric vehicle (HEV) fuel economy test procedures require that the net energy change (NEC) of the battery not interfere with measuring accurate fuel consumption results. SAE J1711-2010 required the NEC to 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 an imbalance of one unit of battery energy can translate into a likely fuel consumption error of roughly three units.
A standard S-Factor, a dimensionless ratio of marginal fuel change to marginal NEC change, was introduced in J1711-2023 to improve SOC correction procedures. The method improves upon the previous J1711 (2010) 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 potentially 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 estimate 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 reviews the data and simulation work used to identify practical S-Factor values, evaluates the resulting correction accuracy across vehicle types and test cycles, and demonstrates that a single standardized S-Factor provides consistent results within an expanded NECFR window.