Influence of fuel formulation on exhaust emissions from gasoline direct injection vehicle

Influence of fuel formulation on exhaust emissions from gasoline direct injection vehicle Tawfiq Al Wasif-Ruiz a,* , Paloma ´ Alvarez-Mateos b , Jos´ e Alberto S´ anchez-Martín a , María Guirado a , Carmen Cecilia Barrios-S´ anchez a a Research Centre for Energy, Environment and Technology (CIEMAT), Avda. Complutense, 40, 28040 Madrid, Spain b Universidad de Sevilla, Departamento de Ingeniería Química, C/Profesor García Gonz´ alez, s/n, 41012 Sevilla, Spain ARTICLE INFO Keywords: Exhaust emissions Nanoparticles Real driving emissions Fuel formulation Air quality ABSTRACT Fuel formulations are adjusted seasonally to optimize engine performance and emissions control. This study examines the impact of these changes on emissions from a Euro 6 vehicle during a real driving emissions cycle in Madrid, Spain. Keeping temperature constant, we compared winter and summer gasoline. Results show that using winter gasoline in summer increases nitrogen oxides emissions by 12.6 %, while in winter, it raises particulate emissions by 17.2 % compared to summer gasoline. Additionally, in a typical scenario—using summer gasoline in summer and winter gasoline in winter—particle emissions were 17.7 % higher with winter gasoline. These findings highlight the need to refine fuel formulations, as internal combustion engines will continue to coexist with electric and hybrid vehicles, especially in freight transport, agriculture, and rural mobility. Understanding how seasonal gasoline variations affect emissions is crucial for developing strategies to reduce environmental impact and improve air quality year-round. 1. Introduction Transportation remains one of the most significant contributors to air pollution globally, with emissions from vehicles causing severe environmental and public health impacts [1,2,3,4]. Despite government efforts to regulate exhaust emissions through policies like the Euro standards, which have notably reduced NO x and PM2.5 emissions, the global warming crisis persists [5,6,7]. In the UK, for example, transportation accounts for approximately 30 % of particle emissions and 40.5 % of NO x emissions in Europe, with road transport being the primary contributor [8,9,10]. The global automotive fleet is undergoing a transition toward cleaner technologies. However, Internal Combustion Engines (ICE) continue to dominate the majority of vehicles on the road. For instance, the U.S. Energy Information Administration (EIA) reports that in 2020, electric vehicles accounted for only 0.7 % of the global light-duty vehicle fleet, a figure projected to increase to 31 % by 2050, indicating that a significant proportion of vehicles will still rely on ICEs in the coming decades. Moreover, the International Energy Agency (IEA) notes that although the adoption of electric vehicles is on the rise, estimates suggest that by 2030, they will represent approximately 40 % of global light-duty vehicle sales, highlighting the continued coexistence of ICE vehicles in the near future. Internal combustion engines remain the primary power source for both light-duty and heavy-duty vehicles. In recent years, advancements in control strategies and combustion technologies have significantly enhanced their thermal efficiency. Researchers have been exploring innovative approaches to optimize engine performance. For instance, Dahham et al. [11] demonstrated that implementing advanced control systems can fine-tune combustion and reduce pollutant emissions. Similarly, Koehler et al. [12] examined the role of hybridization and homogeneous charge compression ignition, highlighting their potential to further improve engine efficiency. These developments underscore the need to continue refining internal combustion technologies—not only to meet environmental regulations but also to contribute to better air quality. Further complicating this issue is the dominance of ultrafine particles (≤2.5 μ m) in vehicular exhaust emissions, which pose significant health risks, leading to around 4 million deaths annually due to cardiopulmonary diseases [13,14]. To address these concerns, Euro regulations have continuously tightened exhaust emission limits, driving advancements in engine technology, such as direct injection systems that, while * Corresponding author. E-mail address: [email protected] (T. Al Wasif-Ruiz). Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc https://doi.org/10.1016/j.fuproc.2025.108215 Received 17 February 2025; Received in revised form 25 March 2025; Accepted 3 April 2025 Fuel Processing Technology 272 (2025) 108215 Available online 6 April 2025 0378-3820/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). improving fuel efficiency, have also led to increased NO x emissions and finer particulate emissions [15,16]. However, while these regulations have successfully reduced exhaust emissions under laboratory conditions, real-world scenarios, as highlighted by the Dieselgate scandal, reveal discrepancies that necessitate more reliable measurement methods [17,18]. The introduction of the Real Driving Emissions (RDE) procedure aimed to address this by simulating typical driving conditions. Yet, factors like ambient temperature and seasonal fuel formulations remain underexplored in their impact on emissions. Additionally, the seasonal variations in fuel composition, driven by the need to optimize engine performance under different ambient temperatures, introduce another layer of complexity in managing vehicle emissions. Winter fuels, formulated with higher Reid Vapor Pressure (RVP) to facilitate cold starts, can lead to different emission profiles compared to summer fuels, which are adjusted to prevent excessive vaporization [19]. The impact of these seasonal adjustments on particulate exhaust emissions, particularly in the context of Gasoline Direct Injection (GDI) engines, is a critical area that warrants further investigation ([20]; [21]). Moreover, the chemical composition of gasoline, particularly the aromatic content, significantly influences particulate exhaust emissions. Studies have shown that high levels of aromatics can exacerbate fine particulate emissions [22], while the addition of oxygenated compounds can reduce the number of particles due to its low boiling point and the presence of bound oxygen [23]. These findings underscore the importance of understanding the interplay between fuel composition and exhaust emission characteristics to develop strategies that can reduce the environmental impact of internal combustion engines. Given this context, the present study focuses on evaluating the impact of seasonal gasoline variations on particulate and NO x emissions from a modern gasoline engine under real-world driving conditions. 2. Material and methods The experimental measurements were carried out using a Skoda Yeti 1.2 gasoline vehicle, specially modified into a mobile laboratory. This vehicle is equipped with a turbocharged engine featuring direct stratified injection and an inline four-cylinder configuration. Key specifications influencing emissions include a power output of 110 HP at 5600 rpm, torque of 175 Nm at 1550–4100 rpm, and an engine displacement of 1197 cm 3 . It meets Euro 6 emissions standards and uses a Three-Way Catalytic (TWC) aftertreatment system, though it lacks a particulate filter. To evaluate the impact of seasonal fuel variations on nanoparticle and NO x emissions, the study followed the Real Driving Emissions (RDE) cycle. Each test covered a distance of 76 km, starting from a cold engine, and was repeated on different days with consistent weather conditions. For fuel type testing, the vehicle’s tank was emptied, and the vehicle was driven briefly with the new gasoline to allow the Engine Control Unit (ECU) to adjust. The driving route, referred to as the Torrelodones-route (Fig. 1), was designed to represent typical traffic density and road conditions in Spain, encompassing a highly populated metropolitan area including Madrid and Torrelodones. This route includes urban, rural, and motorway segments, all compliant with RDE regulation requirements. In this study, emissions were measured using advanced instruments integrated into the mobile laboratory. For particle concentration and size distribution, the Engine Exhaust Particle Sizer (EEPS-3090, TSI) was used. This device detects particles ranging from 5.6 to 560 nm in diameter, utilizing a unipolar diffusion charger to electrically charge particles based on their size. The charged particles are directed toward sensing electrodes, where their charge is transferred upon contact, generating a current. This current is then amplified by electrometers, digitized, and read by a microcontroller, providing detailed particle size distribution data. For gas emissions, the On-Board Emissions Measurement System (OBS-2200, HORIBA) was employed. This system includes vibrationresistant analyzers capable of measuring concentrations of various gases, including CO, CO 2 , THC, and NO x . Specifically, CO and CO 2 concentrations are measured using a Non-Dispersive Infrared (NDIR) analyzer, which detects gas concentrations through the absorption of infrared light at specific wavelengths. THC levels are determined by a Flame Ionization Detector (FID), which ionizes hydrocarbons in a flame and measures the resulting ions as an electrical signal. NO x emissions are measured using a Chemiluminescent Detector (CLD), which detects light emitted from the reaction of NO x with ozone, providing highly sensitive and accurate measurements. These systems are integrated with GPS data and additional external signals, enabling real-time data logging and analysis. This setup ensures precise and reliable measurements, allowing synchronization of emissions data with vehicle location and operating conditions. The mobile laboratory is also equipped with diagnostic tools that continuously monitor vehicle performance, ensuring that all data collected are both accurate and reflective of real-world driving conditions. The on-board system diagram, illustrated in Fig. 2, offers a comprehensive overview of the entire emissions measurement setup, detailing the interplay between the various instruments and their role in capturing real-time emissions data. This diagram visually represents the sophisticated technology employed in this study, highlighting the intricate processes involved in accurately measuring vehicle emissions. To investigate the influence of gasoline type on Gasoline Direct Injection engine emissions, statistical analyses were performed using nonparametric methods due to the lack of normality and homoscedasticity in the data. The Kruskal-Wallis test, with a significance level of 5 %, was used to determine the significance of differences in emissions. Correlation analyses were conducted using Spearman’s rho to assess the Fig. 1. Map of Torrelodones-route. T. Al Wasif-Ruiz et al. Fuel Processing Technology 272 (2025) 108215 2 association between variables. The analyses were conducted using IBM SPSS® software (V. 11.5), with emissions considered significant if the pvalue was less than 0.05. The gasoline used in the experiments was analyzed using various instrumental analytical techniques. Proton Nuclear Magnetic Resonance (^ 1 H NMR) spectroscopy was employed to determine the composition, using a Bruker Avance NEO 500 MHz spectrometer. Gas chromatography-mass spectrometry (GC–MS) was performed with a TSQ8000 mass spectrometer to identify individual gasoline components. Other important parameters analyzed include the octane number, distillation curve, vapor pressure, and elemental composition. The octane number was determined through ^ 1 H NMR spectroscopy, while the distillation curve was obtained following the ASTM 85 standard using a Herzog HDA628 device. Vapor pressure was measured using the GRABNER INSTRUMENTS MINIVAP VPXpert device according to ASTM 5188. Elemental analysis was conducted to determine the percentages of carbon, hydrogen, oxygen, sulfur, and nitrogen in the gasoline, using a CHNS-932 elemental analyzer. The chemical composition and properties of the two gasoline formulations are summarized in Table 1. These include key parameters such as aromatic and oxygenate content, vapor pressure, and boiling point, which influence combustion behavior and emission characteristics. 3. Results and discussion 3.1. Gas emissions This section focuses on the impact of fuel type on gas emissions, comparing tests performed with summer gasoline (SG) and winter gasoline (WG) under summer conditions (SSG and SWG, respectively). This approach isolates the effect of fuel composition, maintaining ambient temperature as a constant variable. In terms of CO emissions, SSG generates higher concentrations of carbon monoxide compared to SWG, despite maintaining similar air-fuel ratios (15.82 for SSG and 15.54 for SWG) (Fig. 3). This difference is primarily due to the higher aromatic content in SSG, which has a lower vapor pressure compared to the olefins and paraffins present in WG. The lower vapor pressure of aromatics hinders the combustion process, promoting CO formation and reducing CO 2 generation (Fig. 4). Additionally, the higher viscosity of aromatics leads to poorer atomization of the air-fuel mixture, exacerbating incomplete combustion. On the other hand, SWG produces higher CO 2 emissions due to its greater paraffin content, which enhances combustion efficiency. The presence of ethanol in WG also contributes to more complete combustion [23]. However, a crucial factor influencing CO 2 emissions is the lower aromatic content of WG, which reduces the fuel’s energy content Fig. 2. On-board system diagram. Table 1 Gasoline specifications. Composition WG SG Paraffins (%) 54.22 50.69 Aromatics (%) 24.48 25.38 Olefins (%) 12.53 10.61 MTBE (%) 0.5 0.56 ETOH (%) 5.56 4.17 Oxygenates (%) 3.00 9.35 Elemental analysis WG SG C (wt%) 52.40 ±0.05 47.96 ±1.88 H (wt%) 8.63 ±0.08 8.20 ±0.01 O (wt%) 38.97 ±0.27 43.84 ±2.23 S (wt%) <0.01 <0.01 N (wt%) <0.02 <0.02 Properties WG SG Vapor pressure (kPa) 74.21 ±0.28 58.01 ±0.16 Initial boiling point (◦C) 20.33 ±0.35 27.49 ±0.50 T10 (◦C) 55.81 ±0.88 56.87 ±0.93 T50 (◦C) 98.23 ±1.20 102.72 ±0.70 T90 (◦C) 132.4 ±1.40 134.35 ±1.30 T. Al Wasif-Ruiz et al. Fuel Processing Technology 272 (2025) 108215 3 and carbon-to‑hydrogen (C/H) ratio. This lower energy content leads to increased fuel consumption to achieve equivalent power output, directly increasing CO 2 emissions [24]. This interplay between aromatic content, energy density, and combustion efficiency explains the divergent impacts of fuel type on CO and CO 2 emissions. Furthermore, WG consistently produces higher THC (Fig. 5) and NOx (Fig. 6) emissions compared to SSG. The higher paraffin content in WG, specifically designed to facilitate ignition in cold climates, contributes not only to less effective atomization and incomplete combustion but also to additional loss mechanisms. In particular, a portion of the unburned mixture escapes into the exhaust port during the gas exchange process, and another fraction is absorbed by the lubricating oil, being subsequently released during the expansion stroke, especially under cold start conditions. These combined factors significantly contribute to the elevated THC emissions observed in WG. In addition to the influence of fuel composition, the formation of NOx is strongly affected by the oxygen concentration in the combustion chamber, the combustion temperature, and the residence time of the reactants at high temperature. A higher oxygen concentration promotes the oxidation of nitrogen, while elevated combustion temperatures and longer residence times enhance the conversion of nitrogen to NOx. In the case of WG, the lower aromatic content and higher paraffin content can result in a leaner mixture and increased combustion temperatures, as well as longer durations at these temperatures, thereby contributing to the higher NOx emissions observed compared to SSG. Conversely, the higher aromatic content in SSG tends to lower the adiabatic flame temperature, reducing NOx formation [25]. These interrelated factors underscore the complexity of NOx formation and highlight the importance of optimizing both engine calibration and fuel formulation to minimize NOx emissions. Additionally, a segmented analysis of emissions was conducted based on the driving area (urban, rural, and highway) during the RDE cycle. The results reveal significant differences depending on the type of gasoline used: •Urban Area: o Winter Gasoline (WG): Average emissions recorded were 0.0184 for CO, 11.98 for CO₂, 5.14 for THC, and 9.20 for NOₓ. o Summer Gasoline (SG): Average values were 0.0305 for CO, 10.50 for CO₂, 2.71 for THC, and 8.81 for NOₓ. These results indicate that in urban environments, WG leads to lower CO emissions but higher CO₂, THC, and NOₓ emissions compared to SG. •Rural Area: o WG: Recorded averages were 0.0453 for CO, 14.51 for CO₂, 27.66 for THC, and 9.80 for NOₓ. o SG: Values obtained were 0.0976 for CO, 13.09 for CO₂, 49.58 for THC, and 8.00 for NOₓ. Here, SG produces nearly twice as much CO and THC as WG, while WG exhibits slightly higher CO₂ and NOₓ emissions. •Highway: Fig. 3. Influence of gasoline specifications on CO emission. Fig. 4. Influence of gasoline specifications on CO 2 emission. Fig. 5. Influence of gasoline specifications on THC emission. Fig. 6. Influence of gasoline specifications on NO x emission. T. Al Wasif-Ruiz et al. Fuel Processing Technology 272 (2025) 108215 4 o WG: Average emissions were 0.0333 for CO, 12.43 for CO₂, 13.88 for THC, and 9.53 for NOₓ. o SG: Values recorded were 0.0642 for CO, 11.65 for CO₂, 11.90 for THC, and 9.57 for NOₓ. In this setting, while differences are more moderate, WG tends to generate less CO but higher CO₂ levels, with NOₓ emissions remaining nearly constant. These findings highlight the influence of driving conditions on emission behavior, suggesting that engine response to fuel formulation varies according to operating conditions and driving environment. 3.2. Particle emissions This section analyzes how fuel type affects particle emissions. Its effect was examined by conducting tests with winter gasoline and summer gasoline under winter conditions (WSG and WWG, respectively), again considering the entire RDE cycle, including cold starts, to observe the cumulative influence on particle emissions. The results show that WWG produces 17.2 % more particles than WSG (Fig. 7). This can be attributed to several factors, including differences in fuel composition, combustion efficiency, and the operational temperature of the catalyst, which are all influenced by the aromatic and ethanol content of the fuels. First, particle emissions are strongly influenced by the atomization, evaporation, and combustion processes within the engine. WG, which contains fewer aromatic compounds than SG, releases less heat in the combustion chamber under the same fuel injection conditions, causing the catalyst to operate at lower temperatures and thus become less efficient. Additionally, the higher ethanol content in WWG lowers the combustion chamber temperature, leading to poorer fuel atomization and a less homogeneous air-fuel mixture. This incomplete combustion favors particle formation[20]. In contrast, WSG, with its higher content of oxygenated compounds, facilitates better fuel evaporation—thanks to the low boiling points of these compounds—and more complete combustion due to the oxygen present within the fuel molecules. This reduces the concentration of key intermediate species that are known to promote soot formation [22,23]. Finally, in the most common scenario, using summer gasoline in summer and winter gasoline in winter, when comparing tests conducted with WWG and SSG, we find that fuel type exerts a significant influence on particle emissions. Specifically, WWG results in 17.7 % more particle emissions than SSG (Fig. 8). Furthermore, as observed in Fig. 8, while WWG contributes to a greater total particle count, SSG emits more particles in the smaller size range (6–8 nm). This highlights the importance of evaluating not only the total number of particles but also their size distribution, as smaller particles have been shown in studies, such as Hakkarainen et al. [22], to possess higher oxidative potential and greater adverse health effects, including increased risks of respiratory and cardiovascular diseases. Potential mitigation strategies could involve optimizing fuel formulations to reduce ultrafine particle emissions or implementing advanced after-treatment technologies to capture these particles before release. To summarize the relationship between fuel composition and its impact on combustion and emissions, Table 2 is presented. This table synthesizes how different fuel components influence combustion efficiency and pollutant formation, providing a clear overview of the correlations observed in this study. 4. Conclusion The experimental results demonstrate a significant influence of fuel type on the emissions of gaseous pollutants, based on the complete RDE cycle (including cold starts) to capture the overall impact on emissions. When keeping the temperature constant (in this case, summer tests were selected) and analyzing the effect of different fuels, the results show that summer gasoline produces higher levels of CO compared to winter gasoline under the same ambient temperature conditions, and winter gasoline produces higher levels of THC and NO x compared to summer gasoline under the same ambient temperaure. Specifically, SG emits 14.2 % more CO, and WG emits 9.9 % more THC, and 12.6 % more NO x than WG. However, in terms of particle emissions, winter gasoline leads to Fig. 7. Influence of gasoline specifications on particle size distribution. Fig. 8. Comparison of size distribution: WWG vs. SSG. Table 2 Influence of fuel components on emissions. Fuel Component Effect on Combustion Impact on Emissions Paraffins (WG >SG) Better atomization and vaporization, promoting more complete combustion. Lower CO emissions but higher NOx due to increased combustion temperatures. Aromatics (SG >WG) Less efficient combustion, lower flame temperature. Higher CO and ultrafine particle emissions, but lower NOx. Olefins (WG > SG) Higher reactivity, facilitating ignition. Can increase soot and particle formation. Ethanol (WG >SG) Improve air-fuel mixing and reduce combustion temperature. Lower particle emissions, but possible increase in CO and THC under certain conditions. Vapor pressure (WG >SG) Influences fuel vaporization and ignition ease. Can affect CO and NOx formation depending on ambient temperature. T. Al Wasif-Ruiz et al. Fuel Processing Technology 272 (2025) 108215 5 higher emissions, likely due to poorer atomization and vaporization of the fuel, which worsens the combustion process compared to SG. The lower aromatic content in WG, while reducing energy density and increasing CO 2 emissions, also contributes to higher particle emissions by influencing the combustion process. This occurs due to several factors, including reduced heat release, lower catalyst temperatures, and less efficient fuel atomization, all of which negatively affect the engine’s operational efficiency. These findings underscore the intricate relationship between aromatic content and emissions: while higher aromatics can help lower NO x emissions, they may simultaneously increase CO and ultrafine particle emissions. Therefore, optimizing fuel formulations becomes essential to strike the right balance. This could involve tailoring aromatic levels to meet specific regional emission priorities, such as reducing ultrafine particles in urban areas or minimizing NO x in regions with stricter air quality regulations. The segmented analysis by driving area reveals that fuel formulation choice (WG vs. SG) has a distinct impact on gaseous emissions. In urban and highway conditions, WG leads to lower CO emissions compared to SG but tends to produce higher CO₂ levels and, in some cases, increased THC and NOₓ emissions. On the other hand, in rural areas, SG generates significantly more CO and THC emissions, despite showing lower CO₂ and NOₓ levels. These findings highlight the importance of using modal and segmented analyses in emission studies, as they provide a more detailed understanding of how emissions vary across different driving environments. Incorporating this data could help develop optimized fuel formulations that minimize the environmental impact of vehicles based on real-world operating conditions. In real-world vehicle use, the most common scenario is to use summer gasoline in summer and winter gasoline in winter. Under these conditions, the experiments revealed that WWG results in 17.7 higher particle emissions compared to SSG, reinforcing that fuel type has a substantial impact on particle emissions. These differences highlight the inherent trade-offs in seasonal fuel designs, prioritizing ignition in winter or reduced vaporization in summer, which affect overall emission balances. To address these challenges, future efforts could focus on refining fuel compositions to optimize combustion year-round and engaging with vehicle manufacturers to enhance engine technologies less susceptible to seasonal fuel variations. This approach would contribute to reducing emissions and mitigating their environmental impact. Finally, it is essential to consider the role that vehicle manufacturers’ calibration of load control systems (such as the air-fuel ratio) plays in influencing emissions from internal combustion engines. Enriching the fresh mixture during winter aims to boost NO x post-treatment by increasing exhaust gas temperatures and, consequently, the temperature of the three-way catalyst. However, this process also leads to higher emissions of other pollutants, such as CO and THC. This study is pioneering in evaluating the impact of seasonal gasoline variations on emissions from a Euro 6 vehicle during a real driving emissions cycle in Madrid, Spain. Unlike previous research conducted under laboratory conditions, our work provides crucial real-world data on how winter and summer gasoline formulations influence NOx and particulate emissions. These findings are essential for developing fuel optimization strategies that enhance air quality and reduce environmental impact throughout the year. Moreover, they have significant implications for future emission regulations, particularly as countries adopt stricter standards to mitigate air pollution. The results underscore the necessity of refining fuel formulations tailored to regional climate conditions, optimizing combustion efficiency, and minimizing emissions. Furthermore, as the automotive industry transitions toward hybrid and electric vehicles, our insights can guide the development of new fuel blends that improve efficiency and reduce pollutants in internal combustion engines, particularly in sectors where these engines remain essential, such as freight transport and rural mobility. Availability of data and materials The datasets used or analyzed during the current study are available from the corresponding author on reasonable requests. CRediT authorship contribution statement Tawfiq Al Wasif-Ruiz: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Paloma ´ Alvarez-Mateos: Methodology, Investigation, Formal analysis. Jos´ e Alberto S´ anchez-Martín: Methodology. María Guirado: Investigation, Formal analysis. Carmen Cecilia Barrios-S´ anchez: Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. References [1] A. Eisenkopf, C. 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