Effect of Isobutanol/Gasoline Blends on the Performance and Emissions of an Otto Engine-Powered Vehicle Operating on a Driving Cycle

Internal combustion engines (ICEs) have been used in transportation, power generation, agriculture, aerospace, and other industries for over a century. These engines are distinguished by their wide power ranges and compact structures. Fossil fuels have been used in the transportation sector, accounting for 80% of the energy used. However, these fuels are the primary source of environmentally harmful polluting greenhouse gas emissions, mainly involving carbon dioxide (CO₂), carbon monoxide (CO), unburned hydrocarbons (HCs), and nitrogen oxides (NO_X) [1].

Global efforts have been undertaken to mitigate the impact of fossil fuels. The United Nations Conference on Climate Change (COP28) addressed this issue by establishing goals to control these emissions. In addition, researchers have explored innovative technologies that contribute to reducing the discharge of pollutants into the atmosphere [2]. In this regard, some additives, including antioxidants, detergents, antiknock agents, or bio alcohols such as ethanol, butanol, and methanol, can be easily used as fuels in conventional ICEs [3,4]. This is because the physicochemical properties of the fuels obtained from renewable sources, such as lower heating value (LHV), octane number, and latent heat of vaporization, are like those of fuels such as gasoline [5,6]. Therefore, these fuels have been utilized in ICEs or mixed with gasoline for commercial purposes [7], with ethanol notably standing out as a primary renewable replacement [8]. Despite the extensive use of ethanol, new studies have identified alternative bioalcohols that have superior thermal properties, produce less harmful pollutants, and have comparable production costs, such as n-heptanol, isobutanol, methanol, among others [9].

Specifically, isobutanol has a higher-octane number than ethanol [10]. Similarly, it has an air/fuel ratio close to 11.1, which allows for the entry of more fuel per unit volume displaced, thus minimizing the impact of power loss caused by its lower LHV [11]. Elfasakhany [12] found that using 10 vol% of isobutanol mixed with gasoline reduced CO and HC emissions by about 30% and 21%, respectively, compared to using pure gasoline in an ICE running at 2600 rpm. However, the CO and HC emissions were comparable to those of gasoline at moderate speeds (2900 rpm). According to another study by the same researcher [13], ternary blends of isobutanol, ethanol, and gasoline reduced the emissions of HC, CO, and CO₂ by 15%, 20%, and 34%, respectively, compared with pure gasoline. Sharma and Agarwal [14] showed that by adjusting variables such as ignition advance, ignition duration, and injection time, performances comparable to those obtained with gasoline can be achieved when using these alcohols, while simultaneously reducing polluting gas emissions.

Generally, using alcohols in spark ignition engines results in a reduction in the emission of pollutants, such as HC and CO [15]. However, there are instances when emission rates are higher than those reached with fossil fuels, which can be typically associated with issues involving incomplete combustion [16]. For instance, Dernotte et al. [17] studied the effects of 20%, 40%, 60%, and 80% isobutanol–gasoline blends on emissions. They found an increase in CO emissions, although the HC emissions were reduced by approximately 5% compared to gasoline, when blends of 20% and 40% butanol were used.

Most of the referenced studies have focused on testing ICEs under steady-state conditions. Hence, information under transient modes is limited, even though the transportation industry mainly operates under dynamic conditions, where emissions and performance vary as functions of load and vehicle speed [18]. In addition, there are no reports on the use of isobutanol in engines that simulate operation under dynamic conditions or driving cycles. This is noteworthy despite the potential advantages of using isobutanol. Hence, this study evaluates for the first time the performance and emissions of a vehicle equipped with an indirect injection engine throughout the worldwide harmonized light test cycle (WLTC), which was conducted using a chassis dynamometer. The study was done while contemplating the usage of isobutanol at different concentrations in blends with commercial gasoline. The isobutanol–gasoline blends were 10%, 20%, and 30% (v/v).

The tests were carried out in dynamic conditions in accordance with the WLTC cycle. The details of the experimental setup are provided below.

Blends of isobutanol and commercial Colombian gasoline containing 10 vol% of ethanol (E10) was used. Table 1 summarizes the key characteristics of isobutanol, ethanol, and gasoline. In particular, the motor octane number (MON) and research octane number (RON) are used to characterize the ignition quality of gasoline.

Three isobutanol/gasoline blends, designated B10, B20, and B30, were considered in this study. Table 2 lists the properties of said fuels. These were determined using homogeneous volumetric blends assumptions.

Testing was conducted using a 1.6L vehicle, which uses a spark ignition engine with port fuel injection (PFI). Table 3 lists the main technical characteristics of the vehicle.

An MD-Gas-5C gas analyzer was used to measure polluting gas emissions. This is mostly composed of non-dispersive infrared sensors (NDIR) and electrochemical cells. Table 4 presents the key technological features. A combustion analysis was performed to obtain instantaneous fuel consumption and determine the emission indices.

The experimental setup used in this study is shown in Fig. 1. The setup includes a Mustang Chassis Dynamometer of the MD150 series. The technical specifications for this dynamometer are listed in Table 5. Testing was performed to obtain the initial information necessary to assess the mechanical performance of the vehicle, including torque, engine speed, wheel speed, and power.

The monitoring of a WLTC driving cycle has been programed using the dynamometer. This cycle is divided into four sub-cycles that are designated based on the top speeds accomplished [20]. In this case, the low sub-cycle has a maximum speed of 60 km/h; the medium sub-cycle has the highest speed of 75 km/h; the high sub-cycle has a top speed of 75 km/h; and finally, the extra-high sub-cycle has the top speeds of 130 km/h.

The oil temperature was checked using a K-type thermocouple to confirm that the engine always started from the same conditions prior to each test, with 80 °C chosen as the temperature indicator for starting the test. Similarly, a Siemens SITRANS FC MASS2100 flowmeter with an FC300 transmitter connected to a National Instruments NI USB-6009 data acquisition system was used to assess the fuel consumption. Each test mode included three replicates in order to obtain reliable results.

Figure 2 illustrates the fuel economy with respect to various isobutanol and gasoline combinations for each of the WLTC sub-cycles. The figure shows that the performance increases as the amount of isobutanol in the blend increases until a maximum is reached with the use of B20, which improved the performance by up to 6% in comparison with operation with E10. This can be attributed to the improvement in combustion quality caused by the higher oxygen content and faster combustion rate of isobutanol [21], as alcohols contribute OH groups that cause the fuel to burn quickly, even under poor combustion conditions [22], as the commercial gasoline blend with isobutanol can significantly influence the formation of free radicals during combustion, primarily due to the chemical composition of isobutanol and its interaction with both the hydrocarbons present in the gasoline and itself [23]. This behavior occurred in the low-speed sub-cycle, whereas up to 4% higher yields relative to E10 were attained for the higher-speed sub-cycles.

Furthermore, the oxygenated functional groups in isobutanol disrupt the typical reaction pathways of hydrocarbon combustion in gasoline, leading to an increased generation of radicals such as OH, HO₂, and others intermediates. These functional groups are highly reactive and susceptible to decomposition under high-temperature combustion conditions [24]. Additionally, the decomposition of the hydroxyl group facilitates the cleavage of C–O and C–H bonds within the isobutanol molecule, resulting in the generation of OH radicals [25]. Consequently, the presence of these radicals can enhance the oxidation of other fuel components, leading to more complete and efficient combustion [26].

Contrary to what has been reported in certain studies analyzing steady-state performance [27], a 20 vol% drop in the LHV, induced by the presence of alcohols, cannot result in an increase in fuel consumption. The decrease in LHV under these conditions cannot exceed 5%; however, its mixing and autoignition qualities are increased [28], where the electronic control unit (ECU) of the engine takes advantage particularly under dynamic operating conditions.

The fuel economy decreased by up to 7% compared to that of E10 when the blend reached 30% substitution (B30). In this case, the LHV was approximately 6.7% lower than that of gasoline, as shown in Table 2. Therefore, a significant amount of isobutanol/gasoline blend was added to the cylinder to achieve high speeds [29]. Furthermore, the presence of O–H bonds in alcohols can also affect heat transfer and flame dynamics at high volumetric concentrations, thus modifying flame temperature and overall reaction rate of the combustion process [30]. This can be attributed to the cooling effect of alcohols in the combustion chamber due to their high enthalpy of vaporization compared to gasoline, prolonging the physical and chemical delay times of the pilot fuel, thus yielding negative results in fuel economy [31]. This may decrease combustion efficiency and consequently reduce energy output [32]. Another factor contributing to the behavior of B30 is the deviation of the ECU management strategy from its typical operational zone, leading to an inability to bring the air/fuel ratio closer to the stoichiometric range, irrespective of the injected fuel quantity.

Under typical operating conditions, the ECU of a vehicle changes its engine operational settings, directing it toward areas of higher efficiency [33]. This can be confirmed by examining the oxygen emissions during each sub-cycle (Fig. 3); the oxygen emissions when isobutanol/gasoline B20 blends are employed are equivalent to those achieved with E10, although the existence of isobutanol suggests that its molecules contain more than 5.6 wt% of oxygen. Nevertheless, the oxygen emissions with B30 are greater in each of the sub-cycles, indicating that the control action of the ECU involves increasing the amount of fuel injected to close the gap between the injected fuel and stoichiometric mixture.

Figure 4 shows the specific HC emissions of each WLTC sub-cycle, with the bars indicating the relative mass emissions in relation to the distance traveled. The results demonstrated that the specific HC emissions decreased as the speed increased. This is because the temperature increases at higher speeds, which enhances the combustion performance and facilitating a higher degree of fuel molecule oxidation in the rich regions of the combustion chamber [34].

Furthermore, increasing the amount of isobutanol in the fuel blends reduced the HC emissions. However, comparisons of the sub-cycles revealed that the variations between the fuels were not statistically significant, except for the low sub-cycle, where the HC emissions decreased by 12% with B30 compared to E10.

Comprehensive data on HC evolution are provided in Fig. 5, which displays the instantaneous and accumulative HC emissions with respect to time, corresponding to the complete cycle duration. Figure 5 shows a statistically significant difference in the accumulative emissions, where the mass emissions of HC at the end of the cycle for each isobutanol/gasoline blend decreased with increasing alcohol concentration. This is because, in Fig. 4, the emissions in each sub-cycle are evaluated, whereas the accumulated data provide a more accurate representation of the overall emissions of the vehicle operating cycle. Therefore, Fig. 5 demonstrates the trend of instantaneous emissions increasing as vehicle speed increases because cycle accelerations cause emission peaks that increase when the cycle demands higher acceleration shifts. Despite the emission peaks shown in Fig. 5, the overall specific emissions of each sub-cycle were lower. Consequently, the effect of acceleration peaks has a more significant influence on accumulative emissions because when strong accelerations are displayed, the curve exhibits more pronounced slope changes.

Thus, the use of isobutanol resulted in 5%, 7.4%, and 10.5% decrease in the overall HC emissions for the B10, B20, and B30 blends, respectively, compared with the use of E10, indicating the favorable effect of this alcohol for complete combustion. These results were due to the oxygen supply from isobutanol, especially in the fuel-rich zones of the combustion chamber [35]. In addition, the WLTC cycle includes acceleration and deceleration intervals, which introduce variability in engine operating conditions and highlight the importance of isobutanol during combustion. This outcome is consistent with that reported in the literature [36].

Moreover, Fig. 6 shows that the CO emissions follow the same trend as the HC emissions because both emissions are related to incomplete combustion. Therefore, they decrease as the vehicle speed increases and as the amount of isobutanol in the blend increases. The most significant CO reductions occurred in the extra-high sub-cycle, where CO reductions of 14.8%, 13%, and 22.6% were achieved for B10, B20, and B30, respectively. This result is mostly due to the oxygenation of the fuel caused by the generation of OH groups during isobutanol combustion [37], and to the high alcohol octane value [19].

Figure 7 shows instantaneous and cumulative CO emissions. The initial part of the cycle (low sub-cycle) exhibited minimal CO production. Thus, HC emissions were more intense than CO emissions at the beginning of the cycle when the engine started. Then, CO emissions intensified with the variation in speed that the subsequent sub-cycles entail, and the slope of the accumulated emissions curve increased significantly, as evident at the points of largest change in acceleration. This abrupt change in regime prevents the injection system from performing instantaneous adjustments to bring combustion to high-efficiency areas, resulting in a greater amount of carbon monoxide.

However, the use of isobutanol decreases the total mass of CO at the end of the cycle by 3.1%, 8.1%, and 10.2% for B10, B20, and B30, respectively. This is consistent with the trend observed for HC emissions, indicating that isobutanol has a beneficial effect on fuel oxidation and consequently a positive environmental impact. These results are mostly attributable to the oxygen delivered to the combustion chamber by isobutanol, particularly in the rich zones, where lower C/H and stoichiometric A/F ratios are incorporated, as indicated by certain studies [36]

Finally, both the performance and emission results validate the viability of isobutanol as a suitable fuel for spark ignition engines, especially in vehicles such as the model used in this study [14]. This model is commercially acceptable but has limited fuel injection technology and minimal enhancements for gas treatment.

In this experimental study, the performance and emissions of a typical Colombian vehicle using a spark ignition engine fueled with gasoline isobutanol blends were analyzed. Consequently, the following results were reached:

• – Incorporating up to 30 vol% of isobutanol in blends yields high fuel economy. The results indicate comparable or even superior efficiency compared to gasoline, with gains of up to 6% are observed when running with B20. However, the ECU method restricts the performance outcomes because it responds to excessive O₂ emissions by making adjustments that lead to increased fuel consumption.

• – Even with a drop in the LHV of an isobutanol/gasoline blend below 5%, improvements in the fuel ignition process can still be achieved, resulting in improved specific fuel consumption. This underscores the potential use of isobutanol as fuel for spark ignition engines.

• – The use of a B30 mixture reduces the total HC emissions by up to 10.5% owing to improvements in fuel oxygenation and H/C and A/F stoichiometric ratios. However, a substantial accumulation of fuel is observed at the end of the cycle, primarily due to the periods of maximum acceleration, because a less precise quantity of HC is emitted in the higher-speed sub-cycles.

• – The CO results follow the same trend as the HC results, and the use of isobutanol results in reductions of up to 10.2% in the total CO emissions with the B30 blend. This reduction was attributed not only to the oxygen contribution from the alcohol to combustion but also to the promotion of OH radical generation by isobutanol.

The authors wish to express their gratitude to Institución Universitaria Pascual Bravo for financing the research project “Evaluación teórico experimental del efecto de mezclas isobutanol/gasolina en un motor ciclo Otto típico del parque automotor colombiano” identified with the code IN202111.

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.