## 1 Introduction
However, the opposite effect was observed during the WLTC cycle, where the serial-hybrid vehicle demonstrated a 1.7% reduction in fuel consumption and improved emissions performance. Further insights into hybrid vehicle testing are provided by [PERSON] et al. [23], who found that HEVs are more economical than conventional vehicles while producing nearly the same power. The use of thermoelectric elements to recover heat from the exhaust and convert it into electricity also demonstrates potential for improving vehicle efficiency [24]. Compared to conventional gasoline vehicles, HEVs can reduce cold-start extra emissions by 30% to 85% [25], although a significant amount of pollutants is still emitted during the cold-start phase. In a test involving a hybrid hydrogen gasoline-engine-powered passenger car under the NEDC, [PERSON] et al. found that using hydrogen to start a spark-ignition (SI) engine effectively reduces HC and CO emissions [26]. Compared to WLTC, significantly higher fuel consumption and CO\({}_{2}\) emissions during RDE testing have been consistently reported for various vehicle types [27; 28]. Furthermore, some studies suggest that hybrid vehicles may emit more particulates, without particle filters, than their conventional counterparts [29]. The shift towards renewable energy sources has become a key component in addressing climate change and reducing reliance on fossil fuels. Among these alternatives, biofuels have garnered significant attention for their potential to lower carbon emissions. Derivedfrom biological materials, biofuels like bioethanol and biodiesel can be used in internal combustion engines either as standalone fuels or blended with traditional gasoline or diesel [30]. Many common liquid fuels (gasoline, diesel, etc.) are made from hydrocarbons that have accumulated in the earth over many years. In contrast, ethanol is produced from renewable bio-based feedstocks. Like other fuels, ethanol produces CO\({}_{2}\) when burned in ICE, but in the plant materials used to produce ethanol, this CO\({}_{2}\) is converted into organic matter through photosynthesis. Ethanol production recovers about 40% of the organic matter accumulated in vegetation. The production waste is returned to the soil to fertilise it, increasing crop yields and reducing soil erosion. By returning 1% of organic matter back to the soil, each hectare of land recycles 40 t CO\({}_{2}\) without releasing it into the atmosphere [31]. Once in the ground, ethanol is rapidly degraded by micro-organisms due to its biological origin, unlike hydrocarbons of mineral origin [32]. The WLTC offers a thorough evaluation of biofuel performance across various driving conditions for biofuel testing. This includes cold-start emissions, which are particularly important for biofuels due to differences in combustion temperatures and the potential for increased emissions during engine warm-up. A review [33] identified biodiesel, hydrotreated vegetable oil (HVO), bioethanol, and biomethane as the most relevant biofuels in Europe. In 2019, the most consumed biofuels in EU28 transport were biodiesel and HVO, accounting for 80.5% of the energy share, followed by bioethanol (18%) and biomethane (1.5%) [34]. Studies [35] have shown that biofuels generally have a lower climate impact compared to diesel and gasoline, with average greenhouse gas emission reductions varying by biofuel type: 70% for biohydrogen, 63% for biomethane, 41% for pure biodiesel, and between 54% and 7% for bioethanol, depending on the blend percentage (100% to 10%). Few studies have specifically examined how different concentrations of ethanol in fuel blends affect exhaust emissions in modern SI engines [36; 37] or how ethanol-containing blends influence emissions from modern flex-fuel vehicles (FFVs) [37; 38; 39]. FFVs are designed to operate with standard gasoline and any ethanol-containing blend (both hydrous and anhydrous). In Europe, FFVs can run on standard gasoline (E5, which contains 5% ethanol) or blends with up to 85% ethanol (E85) during summer or up to 75% (E75) in winter.