Effects of Fuel Octane Rating, Ignition Timing, and Injection Timing on Idle Exhaust Emissions of Electronic Fuel-Injected Motorcycles Fueled with Gasoline–Ethanol Blends (E10 and E40)

Recent technological developments in motorcycle engines have shifted from carburetor-based systems toward electronically controlled fuel injection (FI) systems. Compared with conventional carburetors, FI technology enables more precise regulation of the air-fuel ratio (AFR), resulting in improved combustion efficiency, lower fuel consumption, and reduced exhaust emissions [1, 2]. In FI engines, various sensors transmit operational data to the electronic control unit (ECU) for fuel and ignition control. The emergence of programmable ECU has further expanded calibration flexibility, allowing ignition timing and fuel injection timing maps to be modified according to specific operating requirements. Previous studies have demonstrated that such recalibration significantly influences carbon monoxide (CO) and hydrocarbon (HC) emissions in fuel-injected motorcycles [3]. For gasoline–ethanol (gasohol) blends, appropriate adjustment of ignition timing, injection timing, compression ratio, and fuel octane rating is required to maintain combustion stability and improve emission performance [4, 5].

Previous studies have shown that injection parameters and combustion phasing strongly influence engine performance, combustion characteristics, and exhaust emissions [6]. Evidence from alternative-fuel engines further indicates that injection timing, compression ratio, air–fuel ratio, and fuel composition jointly affect ignition delay, flame propagation, combustion temperature, and emission formation [7]. Programmable engine management strategies have also been reported to improve combustion stability under varying operating conditions [8]. Gasohol blends have attracted considerable attention because ethanol possesses a high-octane number and inherent oxygen content, both of which contribute to improved combustion efficiency and lower CO and HC emissions under suitable operating conditions [9-11]. In Indonesia, commercially available gasoline grades include RON 90, RON 92, and RON 98, providing practical alternatives for gasoline–ethanol blending and engine calibration studies [12, 13]. Higher octane ratings generally improve knock resistance, potentially enabling more favorable ignition phasing and more complete combustion.

Ethanol, a renewable biofuel derived from biomass fermentation, possesses an octane number of approximately 111 and contains around 35% oxygen by mass [14-16]. These physicochemical properties influence laminar flame speed, ignition delay, and oxidation kinetics in spark-ignition engines [17]. Besides fuel modification, emission reduction strategies also include optimized engine calibration approaches and combustion control techniques. Previous studies have shown that these strategies improve combustion stability while reducing CO and HC emissions and maintaining overall engine performance under gasoline–ethanol operating conditions [18, 19]. From a combustion chemistry perspective, complete combustion ideally converts fuel into carbon dioxide (CO₂) and water vapor (H₂O), whereas incomplete combustion produces CO and unburned HC [20]. Engine calibration and combustion phasing influence mixture preparation and combustion completeness, particularly for gasoline–ethanol blends [21]. Elevated HC emissions are generally associated with flame quenching, wall wetting, and incomplete oxidation, whereas increased CO concentrations indicate locally rich mixtures where oxygen availability is insufficient for complete carbon oxidation to CO₂ [22, 23]. Consequently, exhaust emission formation is governed by the interaction among fuel chemistry, mixture preparation, combustion temperature, and ignition phasing. Despite extensive investigations of gasohol blends under part-load and performance-oriented operating conditions [9, 10], comparatively limited attention has been devoted to steady idle operation. Under idle conditions, low turbulence intensity, reduced combustion temperature, and mixture stratification increase the likelihood of incomplete combustion. Furthermore, previous studies have generally evaluated either fuel composition or ECU calibration independently rather than systematically investigating their combined interaction under idle emission compliance conditions.

Therefore, this study evaluates the combined effects of fuel octane rating (RON 90, RON 92, and RON 98), gasohol blending ratios (E10 and E40), ignition timing, and fuel injection timing on idle combustion stoichiometry and exhaust emissions. A full-factorial experimental design was employed to identify fuel–calibration combinations associated with compliance with the adopted idle CO and HC emission limits under the investigated idle operating conditions.

3.1 Combustion stoichiometry

For clarity, the stoichiometric calculations presented in this section employ representative surrogate hydrocarbon compounds to illustrate the influence of ethanol blending on theoretical combustion behavior. Commercial gasoline is a complex mixture of numerous hydrocarbons; therefore, the molecular formulas assigned to RON 90, RON 92, and RON 98 are intended solely as representative surrogate compounds for illustrative stoichiometric calculations and should not be interpreted as the actual chemical compositions of commercial gasoline fuels.

Based on these representative surrogate formulations, the stoichiometric analysis presented in Table 8 illustrates how increasing ethanol content influences the elemental composition of the fuel blends. The surrogate gasoline compounds contain only carbon and hydrogen atoms, whereas ethanol (C₂H₅OH) contributes chemically bound oxygen atoms. Consequently, increasing the ethanol blending ratio progressively increases the oxygen content of the representative fuel composition.

Using these surrogate formulations, the theoretical stoichiometric oxygen demand was calculated, as summarized in Table 9. The calculations indicate that surrogate hydrocarbon with higher carbon numbers require greater theoretical oxygen demand for complete combustion, whereas ethanol requires comparatively less external oxygen because part of the required oxygen is already contained within its molecular structure. Accordingly, the representative E10 and E40 surrogate blends exhibit corresponding changes in theoretical oxygen demand and combustion products.

The combustion equations, oxygen requirements, and theoretical AFR presented in Tables 8-10 are intended solely as representative stoichiometric calculations for comparative analysis and do not represent the exact combustion chemistry or molecular composition of commercial RON 90, RON 92, and RON 98 gasolines, whose compositions vary with refinery processes and formulations. Nevertheless, these calculations provide a framework for interpreting experimental emission results by illustrating how fuel composition influences theoretical oxygen demand, combustion products, and AFR.

Finally, Table 10 summarizes the theoretical combustion stoichiometry and corresponding AFR values derived from the representative surrogate formulations. The representative gasoline surrogates require approximately 15.0-15.3 AFR for complete combustion, whereas ethanol requires approximately 9.0 AFR because of its chemically bound oxygen. Consequently, the representative E10 blends exhibit theoretical AFR values ranging from 14.8 to 15.2, while the representative E40 blends range from 13.8 to 14.3. These theoretical AFR values are consistent with the reduced external oxygen demand as ethanol content increases.

Table 8. Representative surrogate fuel composition used for stoichiometric illustration

Table 9. Theoretical stoichiometric oxygen requirements and combustion products calculated using a representative surrogate hydrocarbon

Although actual engine operation is influenced by factors such as fuel atomization, in-cylinder mixture formation, ignition timing, fuel injection timing, and ECU control strategies, the stoichiometric calculations provide a useful theoretical reference for interpreting the experimental trends observed in AFR and exhaust emissions under the investigated idle operating conditions. The progressive reduction in theoretical AFR with increasing ethanol content therefore reflects the lower theoretical air requirement for complete combustion and provides a scientific basis for interpreting the combustion characteristics and emission behavior discussed in the following sections.

Table 10. Representative combustion stoichiometry and theoretical air-fuel ratio (AFR)

3.2 Hydrocarbon emissions

As illustrated in Figure 2, HC emissions ranged from 200.7 to 1,337.7 ppm across the 27 idle operating conditions. Based on the adopted idle HC emission limit of 1,000 ppm, 22 operating conditions (81.5%) complied with the criterion, whereas the remaining five conditions (18.5%) exceeded the limit. The exceedances occurred in Orders 5, 6, 7, 16, and 17, indicating that excessive HC emissions were confined to a limited number of operating conditions. Among the 24 factorial treatment conditions, the lowest HC emission was obtained with Gasohol E40 (RON 98) at an ignition timing of 3° BTDC and a fuel injection timing of 350° ATDC (Order 26), producing 318.7 ppm. In contrast, the highest HC emission was recorded for Gasohol E10 (RON 90) at 7° BTDC and 355° ATDC (Order 5), reaching 1,337.7 ppm.

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Figure 2. Hydrocarbon (HC) emissions under the 27 investigated idle operating conditions

For the E10 blends, RON 92 and RON 98 satisfied the adopted idle HC emission limit under all four timing combinations. HC emissions ranged from 315.3 to 522.7 ppm for RON 92 and from 244.7 to 339.7 ppm for RON 98. In contrast, RON 90 exhibited substantially higher HC emissions, with three of the four operating conditions exceeding the adopted limit (Orders 5–7). For the E40 blends, RON 90 remained fully compliant under all operating conditions, with HC emissions ranging from 375.0 to 711.3 ppm. RON 92 exhibited the least favorable HC performance within the E40 group, producing HC emissions between 544.3 and 1054.0 ppm, with two operating conditions (Orders 16 and 17) exceeding the adopted limit. RON 98 maintained satisfactory HC performance, with emissions ranging from 304.0 to 579.3 ppm across all four timing combinations.

The experimental results further indicate that the combination of 7° BTDC ignition timing and 355° ATDC fuel injection timing generally resulted in higher HC emissions, particularly for lower-octane fuels. Conversely, advancing the fuel injection timing to 350° ATDC generally reduced HC emissions under comparable operating conditions, suggesting improved combustion completeness. Overall, the results demonstrate that compliance with the adopted idle HC emission limit depends on an appropriate combination of fuel octane rating, ethanol blending ratio, and coordinated ignition and fuel injection timing under idle operating conditions.

3.3 Carbon monoxide emissions

As illustrated in Figure 3, 11 of the 27 idle operating conditions (40.7%) satisfied the adopted idle CO emission limit of 3.0 vol.%, whereas the remaining 16 conditions (59.3%) exceeded the limit. Compared with HC emissions, CO control represented the more restrictive requirement for achieving overall idle emission compliance. Among the 24 factorial treatment conditions, the lowest CO emission was obtained with Gasohol E40 (RON 98) at an ignition timing of 3° BTDC and a fuel injection timing of 350° ATDC (Order 26), producing 0.57 vol.%. In contrast, the highest CO emission was recorded for Gasohol E40 (RON 92) at 7° BTDC and 355° ATDC (Order 17), reaching 7.42 vol.%.

For the E10 blends, RON 92 consistently complied with the adopted idle CO emission limit under all four timing combinations, with CO emissions ranging from 1.45 to 2.24 vol.%. In contrast, RON 90 exhibited substantially higher CO emissions, ranging from 4.50 to 7.27 vol.%, and none of the four operating conditions satisfied the adopted limit. RON 98 showed intermediate performance, with CO emissions ranging from 2.23 to 4.43 vol.%, where two of the four operating conditions achieved compliance.

For the E40 blends, RON 98 produced the lowest overall CO emissions, ranging from 0.57 to 5.34 vol.%, with two operating conditions (Orders 26 and 27) satisfying the adopted idle CO emission limit. RON 90 exhibited CO emissions between 1.87 and 5.44 vol.%, with one compliant operating condition (Order 15). In comparison, RON 92 produced CO emissions ranging from 3.93 to 7.42 vol.%, and none of the operating conditions complied with the adopted limit. Across all fuel grades, advancing the fuel injection timing to 350° ATDC generally reduced CO emissions relative to the corresponding 355° ATDC setting, indicating improved combustion efficiency under idle operation. Overall, the results demonstrate that compliance with the adopted idle CO emission limit depends on the combined effects of fuel octane rating, ethanol blending ratio, and coordinated ignition and fuel injection timing.

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Figure 3. Carbon monoxide (CO) emissions under the 27 investigated idle operating conditions

3.4 Carbon dioxide emissions

As illustrated in Figure 4, CO₂ concentrations at idle ranged from 4.7 to 12.6 vol.%, indicating considerable variation in combustion characteristics among the investigated operating conditions. Among the 24 factorial treatment conditions, the highest CO₂ concentration was obtained with Gasohol E40 (RON 98) at an ignition timing of 3° BTDC and a fuel injection timing of 350° ATDC (Order 26), producing 12.4 vol.%. In contrast, the lowest CO₂ concentration was observed for the same fuel at 3° BTDC and 355° ATDC (Order 24), with only 4.7 vol.%. These results demonstrate that combustion characteristics were influenced by the combined effects of fuel properties and ignition–injection timing.

Higher CO₂ concentrations were generally associated with lower CO and HC emissions, suggesting more complete combustion under idle operating conditions. Most operating conditions with CO₂ concentrations above approximately 11.6 vol.% satisfied the adopted idle CO and HC emission limits, whereas lower CO₂ concentrations were frequently accompanied by increased CO and HC emissions. For the E10 blends, RON 92 maintained relatively stable CO₂ concentrations between 10.8 and 11.7 vol.% across all timing combinations, corresponding to consistently low HC emissions and full compliance with the adopted idle HC emission limit. In contrast, RON 90 exhibited lower CO₂ concentrations, reaching a minimum of 7.8 vol.%, which coincided with the highest HC emission recorded in the present study.

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Figure 4. Carbon dioxide (CO₂) concentrations under the 27 investigated idle operating conditions

The E40 blends exhibited greater variation in CO₂ concentration than the E10 blends. CO₂ concentrations ranged from 7.3 to 11.6 vol.% for RON 90, from 6.7 to 11.7 vol.% for RON 92, and from 4.7 to 12.4 vol.% for RON 98. Across all fuel grades, advancing the fuel injection timing to 350° ATDC generally increased CO₂ concentrations relative to the corresponding 355° ATDC setting, indicating improved combustion completeness. Overall, the results suggest that CO₂ concentration is a useful indicator of combustion quality and is generally associated with improved idle emission performance under the investigated operating conditions.

3.5 Residual oxygen emissions

As illustrated in Figure 5, residual O₂ concentrations at idle ranged from 0.46 to 9.31 vol.%, indicating substantial variation in oxygen utilization among the investigated operating conditions. Among the 24 factorial treatment conditions, the lowest residual O₂ concentration was obtained with Gasohol E10 (RON 92) at an ignition timing of 3° BTDC and a fuel injection timing of 350° ATDC (Order 14), producing 0.46 vol.%. In contrast, the highest residual O₂ concentration was observed for Gasohol E40 (RON 98) at 3° BTDC and 355° ATDC (Order 24), reaching 9.31 vol.%.

Most operating conditions that satisfied the adopted idle CO and HC emission limits exhibited residual O₂ concentrations between approximately 0.7 and 1.8 vol.%, although one compliant lean-burn condition showed a higher O₂ concentration. For the E10 blends, RON 92 maintained relatively stable residual O₂ concentrations ranging from 0.46 to 1.65 vol.% across all timing combinations, corresponding to consistently low CO and HC emissions. In contrast, RON 90 exhibited a wider O₂ range (0.71–4.27 vol.%), with the highest value occurring at 7° BTDC and 350° ATDC, where elevated residual oxygen was accompanied by increased HC emissions.

The E40 blends exhibited greater variation in residual O₂ concentration than the E10 blends. Residual O₂ ranged from 1.09 to 4.27 vol.% for RON 90, from 1.73 to 5.89 vol.% for RON 92, and from 0.66 to 9.31 vol.% for RON 98. Across all fuel grades, advancing the fuel injection timing to 350° ATDC generally reduced residual O₂ relative to the corresponding 355° ATDC setting, suggesting more effective oxygen utilization during combustion. Overall, the results indicate that residual O₂ is a useful indicator of combustion quality and, when interpreted together with AFR, CO₂, CO, and HC, provides additional insight into combustion completeness under idle operating conditions.

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Figure 5. Residual oxygen (O₂) concentrations under the 27 investigated idle operating conditions

3.6 Air-fuel ratio

As illustrated in Figure 6, AFR values ranged from 11.7 to 21.2 across the 27 idle operating conditions, indicating substantial variation in mixture preparation under different fuel blends and ECU timing settings. For reference, the stoichiometric AFR of conventional gasoline is approximately 14.7:1. Gasohol E10 (RON 92) generally maintained AFR values between 14.0 and 14.7, corresponding to comparatively lower CO and HC emissions. In contrast, Gasohol E10 (RON 90) exhibited a wider AFR range (11.7–14.3), accompanied by higher CO and HC emissions under several operating conditions.

The highest AFR (21.2) was observed for Gasohol E40 (RON 98) at an ignition timing of 3° BTDC and a fuel injection timing of 355° ATDC (Order 24). This operating condition was accompanied by 9.31 vol.% O₂, 4.7 vol.% CO₂, and 4.10 vol.% CO, suggesting that combustion deviated substantially from the conditions associated with lower emissions. However, the underlying cause could not be established because no additional combustion diagnostic measurements were performed.

Most operating conditions that satisfied the adopted idle CO and HC emission limits were clustered around AFR values between approximately 14.0 and 15.5. In comparison, the E40 blends exhibited a wider AFR distribution than the E10 blends. AFR values ranged from 13.0 to 14.7 for Gasohol E40 (RON 90), from 11.8 to 16.7 for Gasohol E40 (RON 92), and from 12.5 to 21.2 for Gasohol E40 (RON 98). Advancing the fuel injection timing to 350° ATDC generally produced a narrower AFR distribution than the corresponding 355° ATDC setting under the investigated operating conditions.

Overall, operating conditions with AFR values closer to the stoichiometric range generally exhibited lower exhaust emissions under idle operation. Nevertheless, AFR alone was insufficient to explain emission behavior, indicating that fuel properties, ethanol blending ratio, and coordinated ignition and fuel injection timing also contributed to the observed combustion characteristics.

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Figure 6. Air-fuel ratio (AFR) under the 27 investigated idle operating conditions

3.7 Emission compliance status by experimental order

To facilitate comparison of the emission results presented in the preceding sections, Table 11 provides a comprehensive summary of the 27 experimental operating conditions. The table compiles the measured AFR, CO, HC, CO₂, and residual O₂ concentrations together with the corresponding compliance status based on the adopted idle CO (<3.0 vol.%) and HC (<1,000 ppm) emission limits.

As shown in Table 11, 11 operating conditions satisfied both the adopted idle CO and HC emission limits, 11 conditions satisfied only one of the two criteria, and the remaining five operating conditions exceeded both limits. This consolidated presentation provides a comprehensive overview of the experimental dataset and facilitates direct comparison among all investigated operating conditions.

Within the present experimental dataset, the operating conditions that satisfied both emission limits generally exhibited AFR values between approximately 14.0 and 15.5, accompanied by CO₂ concentrations above approximately 11.6 vol.% and residual O₂ concentrations below approximately 1.8 vol.%. These observations indicate that compliant operating conditions tended to cluster within these ranges under the investigated idle operating conditions. Nevertheless, these values represent empirical trends observed in the present study and should not be interpreted as universal combustion criteria.

The operating conditions classified as partial compliance generally satisfied either the CO or HC emission limit but not both simultaneously. In most cases, HC emissions remained below the adopted limit, whereas CO concentrations exceeded the allowable threshold, indicating that CO control represented the more restrictive requirement for achieving overall idle emission compliance. This behavior suggests that the evaluated fuel blends and ECU calibration settings influenced combustion characteristics differently under idle operation.

Five operating conditions were classified as non-compliant, exceeding both the adopted idle CO and HC emission limits. These operating conditions were generally associated with either relatively rich combustion or markedly lean operating conditions. For example, Operating Condition 24 exhibited an AFR of 21.2, residual O₂ of 9.31 vol.%, CO₂ of 4.7 vol.%, and CO of 4.10 vol.%. This emission pattern differed substantially from that observed for the compliant operating conditions. However, because no additional combustion diagnostic measurements were performed, the underlying cause could not be established, and no definitive conclusion regarding the combustion mechanism can be drawn from the present results.

Overall, Table 11 provides a unified presentation of the complete emission dataset and supports the consistency of the interpretations presented in the preceding sections. Within the scope of the present idle experiments, the results suggest that compliance with the adopted idle CO and HC emission limits was associated with appropriate combinations of fuel properties, ethanol blending ratio, ignition timing, fuel injection timing, and combustion conditions approaching the stoichiometric AFR. Further investigations under broader engine operating conditions are required before these observations can be generalized beyond the present experimental conditions.

3.8 E10 compliance

The compliance performance of the E10 fuel blends varied according to fuel grade and ECU calibration, as summarized in Table 12. Of the twelve E10 operating conditions evaluated, five satisfied both the adopted idle CO and HC emission limits. Among the three fuel grades, Gasohol E10 (RON 92) demonstrated the highest level of compliance, with all four timing combinations meeting both emission criteria. CO emissions ranged from 1.45 to 2.24 vol.%, while HC emissions ranged from 394.0 to 522.7 ppm, indicating consistently stable emission performance under the investigated idle conditions.

In contrast, none of the four Gasohol E10 (RON 90) operating conditions satisfied both emission limits simultaneously. Elevated CO emissions were observed under all timing combinations, while HC concentrations exceeded the adopted limit under three operating conditions (Orders 5–7). The highest emissions occurred at 7° BTDC ignition timing and 355° ATDC injection timing, where CO reached 7.27 vol.% and HC reached 1,337.7 ppm.

Table 11. Summary of idle emission characteristics and compliance status

Table 12. Fully compliant E10 operating conditions

Gasohol E10 (RON 98) achieved compliance in two of the four operating conditions. The compliant conditions were obtained using 7° BTDC ignition timing, whereas both operating conditions with 3° BTDC ignition timing exceeded the adopted CO emission limit despite maintaining HC concentrations below 1,000 ppm. These results indicate that the emission performance of the investigated E10 blends depended on the combined effects of fuel grade and ignition–injection timing under idle operation.

3.9 E40 compliance

The emission performance of the E40 fuel blends also depended on fuel grade and ECU calibration, as summarized in Table 13. Among the twelve E40 operating conditions, three satisfied both the adopted idle CO and HC emission limits.

Gasohol E40 (RON 98) exhibited the highest compliance among the E40 blends, with two of the four operating conditions satisfying both emission criteria. The lowest emissions were obtained at 3° BTDC ignition timing and 350° ATDC injection timing, producing 0.57 vol.% CO and 318.7 ppm HC. In contrast, the operating condition using 3° BTDC ignition timing and 355° ATDC injection timing exhibited an AFR of 21.2, residual O₂ of 9.31 vol.%, CO₂ of 4.7 vol.%, and CO of 4.10 vol.%, differing substantially from the emission characteristics observed under the compliant operating conditions.

None of the four Gasohol E40 (RON 92) operating conditions satisfied both adopted idle emission limits. Although several operating conditions maintained HC concentrations close to the adopted limit, CO emissions remained above 3.0 vol.% under all investigated timing combinations. The least favorable emission performance occurred at 7° BTDC ignition timing and 355° ATDC injection timing, producing 7.42 vol.% CO and 1054.0 ppm HC.

Gasohol E40 (RON 90) achieved compliance in one of the four operating conditions. The compliant condition was obtained at 3° BTDC ignition timing and 355° ATDC injection timing, producing 2.43 vol.% CO and 375.0 ppm HC, whereas the remaining operating conditions exceeded either the CO limit or both adopted emission limits. Overall, the results indicate that the emission performance of the investigated E40 blends was strongly influenced by the interaction between fuel grade and ECU timing under idle operating conditions.

Table 13. Fully compliant E40 operating conditions

3.10 Least favorable operating condition

Among the 27 operating conditions investigated, the least favorable emission performance was observed for Gasohol E10 (RON 90) with 7° BTDC ignition timing and 355° ATDC injection timing, as summarized in Table 14. This operating condition exceeded both the adopted idle CO and HC emission limits, producing 7.27 vol.% CO and 1,337.7 ppm HC.

Compared with the corresponding RON 90 baseline condition, CO increased from 0.60 to 7.27 vol.%, while HC increased from 200.7 to 1,337.7 ppm. In addition, CO₂ decreased to 7.8 vol.% and residual O₂ was measured at 1.60 vol.%, accompanied by an AFR of 11.7. Collectively, these emission characteristics differed substantially from those observed under the compliant operating conditions and were consistent with relatively incomplete combustion under the investigated idle condition.

The observed emission pattern suggests that the combination of low-octane fuel, E10 blending, and the selected ignition and injection timing was less favorable than the other operating conditions evaluated in this study. However, because no in-cylinder combustion diagnostics or knock measurements were performed, the underlying combustion mechanism cannot be determined from the present dataset. These findings indicate that the investigated calibration combination was not suitable for satisfying the adopted idle CO and HC emission limits under the experimental conditions considered.

Table 14. Emission characteristics of the least favorable operating condition

3.11 Most favorable operating condition

The most favorable emission performance among the investigated operating conditions was obtained using Gasohol E40 (RON 98) with 3° BTDC ignition timing and 350° ATDC injection timing, as summarized in Table 15. This operating condition produced the lowest measured CO concentration (0.57 vol.%) together with an HC concentration of 318.7 ppm, satisfying both adopted idle CO and HC emission limits while providing the largest compliance margin observed in the present study.

Table 15. Emission characteristics of the most favorable operating condition

Relative to the corresponding RON 98 baseline condition, CO decreased from 1.31 to 0.57 vol.%, whereas HC increased slightly from 257.3 to 318.7 ppm while remaining well below the adopted emission limit. In addition, CO₂ reached 12.4 vol.%, residual O₂ was 1.55 vol.%, and the measured AFR was 15.3, indicating combustion characteristics consistent with those generally associated with compliant operating conditions under idle engine operation. This combination of emission parameters suggests that the selected operating condition promoted relatively efficient combustion while maintaining all measured emissions within the adopted idle limits.

Within the scope of the present investigation, this operating condition exhibited the most favorable combination of AFR and exhaust emission characteristics among the 27 operating conditions evaluated. Overall, the experimental results demonstrate that idle exhaust emissions were strongly influenced by the combined effects of fuel grade, ethanol blending ratio, ignition timing, and fuel injection timing. Of the 27 operating conditions investigated, only 11 satisfied both adopted idle emission limits, highlighting the importance of appropriate fuel selection and ECU calibration for achieving improved emission performance under the investigated idle conditions. These findings further indicate that favorable emission performance depended on the combined optimization of fuel characteristics and engine calibration rather than on any single operating parameter. The implications of these findings and their relationship to combustion behavior are discussed in the following section.

This study evaluated 27 idle operating conditions to examine the effects of fuel octane rating, gasoline–ethanol blend composition, ignition timing, and fuel injection timing on exhaust emission characteristics. The results indicate that compliance with the adopted idle CO and HC emission limits depended on appropriate combinations of fuel properties and ECU calibration under the investigated idle conditions.

Six principal findings were obtained. First, fuel grade was identified as a key factor affecting idle emission performance. Gasohol E10 (RON 92) satisfied the adopted idle emission limits under all evaluated ignition and injection timing combinations, whereas Gasohol E10 (RON 90) achieved compliance in only one of the four tested timing configurations and produced the highest HC emission when operated with 7° BTDC ignition timing and 355° ATDC injection timing. Second, two promising calibration strategies were identified. Gasohol E10 (RON 92) exhibited consistently compliant performance across the investigated timing settings, while Gasohol E40 (RON 98) combined with 3° BTDC ignition timing and 350° ATDC injection timing produced the lowest measured emissions, with CO and HC concentrations of 0.57 vol.% and 318.7 ppm, respectively. The 350° ATDC injection timing was frequently associated with improved emission performance, indicating that injection timing influenced idle combustion characteristics. Third, none of the Gasohol E40 (RON 92) operating conditions satisfied both adopted idle emission limits, suggesting that this fuel–timing combination was less favorable than the other evaluated configurations. Fourth, operating conditions satisfying both emission limits generally exhibited AFR values between approximately 14.0 and 15.5, together with CO₂ concentrations above approximately 11.6 vol.% and residual O₂ concentrations below approximately 1.6 vol.%. Within the present dataset, compliant cases tended to cluster within these ranges, indicating an association between near-stoichiometric combustion and improved idle emission performance. Fifth, HC emissions showed greater sensitivity than CO emissions to variations in fuel properties and ECU calibration. Finally, the combination of Gasohol E10 (RON 90), 7° BTDC ignition timing, and 355° ATDC injection timing produced the least favorable emission performance among all evaluated operating conditions.

The present findings provide experimental evidence that fuel-specific ECU calibration can improve idle exhaust emission performance. In particular, the combination of Gasohol E40 (RON 98), 3° BTDC ignition timing, and 350° ATDC injection timing demonstrated the most favorable emission characteristics, whereas the consistently compliant performance of Gasohol E10 (RON 92) suggests greater calibration flexibility under idle operation. Conversely, the poorer performance observed for several low-octane fuel configurations indicates that additional calibration optimization may be required before practical implementation.

The conclusions of this study are limited to steady idle operation. Further investigations under cold-start, transient, and loaded engine conditions, together with evaluations of catalyst performance, fuel compatibility, engine durability, and real-world driving cycles, are necessary before broader engineering recommendations or regulatory implications can be established.