Work

Determinants of Test-to-Reality CO2 Gaps in European PHEVs: The Limited Role of Battery Capacity

Year: 2026

Type: article

Abstract: Plug-in hybrid electric vehicles (PHEVs) are expected to reduce fleet CO₂ emissions, yet their real-world performance often deviates substantially from type-approval expectations. This study examines whether traction battery capacity provides an independent explanatory signal for the test-to-reality CO₂ gap (gap%), or whether it primarily acts as a proxy for market segmentation and usage patterns. Using European on-board fuel and energy consumption monitoring (OBFCM) records for 457,555 PHEV observations (2021–2023) from 14 manufacturers, we estimate nested fixed-effects models and introduce engineered usage proxies describing charge-depleting operation (EUR), hybrid utilization intensity (HI), energy-into-battery intensity (EDE), and a real-world to type-approval fuel-consumption ratio proxy (ELP). Battery capacity alone explains limited variation in gap% (R² = 0.075), while adding segment/year/manufacturer fixed effects increases R² to 0.203 and adding usage proxies increases it to 0.826, with the battery coefficient attenuating from 19.6 to 8.9 percentage points per kWh. Allowing non-linear battery terms via cubic B-splines yields only a modest additional improvement (R² ≈ 0.829), although the conditional shape is non-monotonic. Importantly, the battery–gap association is strongly segment-dependent, ranging from −22.1 pp/kWh in medium vans to +10.5 pp/kWh in large cars. Robustness checks using model-identifier fixed effects (MS_Cn) with standard errors clustered by MS_Cn further attenuate the battery effect (p ≈ 0.085), whereas ELP remains strongly associated with gap%. Overall, battery capacity is informative for compliance analytics mainly as a proxy variable capturing segmentation and real-world usage, rather than a universal lever of PHEV CO₂ performance.

Source: Vehicles

DOI: 10.3390/vehicles8030060

Authors: Maksymilian Mądziel, Paulina Kulasa, Tiziana Campisi

Institutions: Rzeszów University of Technology, Università degli Studi di Enna Kore

Language: English

Topic: Vehicle emissions and performance

Subfield: Automotive Engineering

Field: Engineering

Domain: Physical Sciences

SDGs: Climate action, Affordable and clean energy, Industry, innovation and infrastructure

Open Access status: gold

Estimated APC paid: $1,930