*3.4. Engine Performance*

The study points out the brake specific fuel consumption (BSFC) overall trend in Figure 6 that is driven to examine the fuel mass consumption per power unit while providing a global perspective of this parameter.

**Figure 6.** Diagram of Brake Specific Fuel Consumption (BSFC).

According to the results, the implementation of oxygenated blends (ABE 5 and ABE 10) promotes higher fuel consumption when compared to the standalone gasoline operation. The latter is a direct consequence of the lower energy density and lower calorific value relative to gasoline. Therefore, a higher fuel amount is required to obtain the same unit power, which escalates the BSFC. For comparison, increasing the ABE ratio fosters the BSFC up to 15%. This pattern is in line with other investigations [12,43]. Note that the ABE-based blends might influence the octane rating rise, supported by longer ignition delays (Figure 4c) that critically limits the power output, thus magnifying BSFC. The fuel conversion efficiency might be a determinant factor altering the fuel metrics since it was corroborated in the in-cylinder pressure curves that at low engine loads, the combustion center retarded up to 0.6◦. Therefore, the altered combustion phasing further deteriorates fuel conversion efficiency, thus increasing fuel consumption. The investigation of Nithyanandan [12] leads to similar findings when implementing blends of ABE 20–40. In this sense, the integration of control strategies that facilitates spark timing could be a feasible solution to avoid the magnification of BSFC in dual-fuel operation [44,45].

The highest fuel consumption was achieved by ABE 10, followed by the ABE 5, Gasoline, ABE 10 + HHO, and ABE 5 + HHO. This result demonstrates that HHO enrichment surpasses the increment in the fuel metrics derived from ABE replacement implementation. The latter implies that gaseous fuel promotes chemical conversion efficiency while acting as a heat intensifier, as verified in Figure 5.

## *3.5. Emission Characteristics*

This section aims to examine the influence of dual-fuel operation on the overall emissions of the SI engine. Reducing greenhouse emissions facilitates an eco-friendly operation in the ICEs. The study mainly examines the pollutant levels of CO, HC, NOx, and smoke opacity while variating fuel operation mode and the engine load.

## 3.5.1. CO Emissions

First, the overall carbon monoxide emissions (CO) for all the tested fuels are displayed in Figure 7 at different load conditions.

**Figure 7.** CO emissions for fuels tested.

The load condition features an inverse relation with the CO emission levels. The engine load plays a central role in the emissions behavior since, at high engine loads, it can be reached a non-oxygenated condition that promotes CO formation. However, the high CO formation remains high at low engine loads due to a highly lean mixture that hinders fuel burning. Thus, the flame cannot be maintained due to the limited propagation speed. Specifically, the ABE implementation reduces CO emissions between 11% to 33% compared to gasoline, depending on the engine load. The HHO implementation enables a further reduction of up to 22%. The enhanced behavior from the fuel blends results from promoting complete combustion, meaning that more CO is converted into CO2. Additionally, the ABE incorporation facilitates oxidation due to the increased laminar flame speed and the leaning effect of its oxygenated nature. The higher volatility derived from the acetone in the ABE blends further promotes CO minimization. Li et al. [46] encountered that the ABE features a post-flame oxidation trend that reduces CO levels.

The implementation of HHO doping in the intake air minimizes CO levels, which can also be explained considering the direct oxygen enrichment in the air/fuel mixture. CO emission is directly linked to the air/fuel ratio within the engine and the fuel consumption. Hence, since hydroxy enrichment reduces the BSFC (Figure 6), fewer CO levels are evidenced in the exhaust gases.

It is worth underlying the trade-off of the proposed fuel methodology since the CO minimization resulted in intensified CO2 formation. Considering that international regulations claimed for integral solutions to meet CO2 standards, it triggers a collateral cost impact in the proposed dual-fuel technology due to the imminent necessity to engage additional methods to promote a sustainable operation in future scenarios. Di Blasio et al. [13] have pointed out the predominant role of advanced fuel injection systems in this goal. Moreover, the latter technology is accompanied by additional improvements in fuel economy and combustion noise that reinforce its implementation. Undoubtedly, implementing such sophisticated fuel injection systems increases the investment cost that can be only supported by substantial fuel savings while maintaining high-efficiency operation. Moreover, considering the effect of combustion phasing, the necessity to control spark timing to promote higher combustion efficiencies requires in-depth exploration of engine design characteristics and fuel injection systems that foster a techno-economic operation.

## 3.5.2. HC Emissions

Figure 8 shows the overall emissions of hydrocarbons (HC) as a function of the engine load.

**Figure 8.** HC emissions for fuels tested.

In terms of unburned hydrocarbons, pure gasoline features the highest emissions levels with nearly 0.042 g • kWh−1. Dual-fuel operation facilitates HC minimization since ABE reduces emissions levels while hydroxy doping upgrades this share. Noticeably, HC emissions are significantly lower than CO levels but still represents an undesired pollutant that affects human health and air quality. The overall trend of HC emissions can be explained based on the same fundamentals of CO emissions. It is important to note that

alcohols additives (ABE) feature improved oxygenated characteristics that further improve combustion efficiency and promote a homogeneous air/fuel mixture, which reduces HC formation. These concluding remarks are consistent with that of Masum et al. [10], which unravels the effect of engine speed on ABE blend overall performance. The enhanced laminar flame speed of ABE and HHO is another contributor supporting HC minimization.

## 3.5.3. NOx Emissions

Figure 9 shows NOx formation for all the tested fuels as a function of engine load. The increment in combustion temperature sets the appropriate conditions to promote nitrogenates oxidation, thus producing NOx. Therefore, it can be stated that NOx formation depends significantly on the in-cylinder temperature, the concentration of oxygen in the fuel, and the residence time of the reaction.

**Figure 9.** NOx emissions for fuels tested.

Based on the results, the maximum NOx emissions are reached at a full engine load. Specifically, ABE implementation facilitates reducing the overall emissions of NOx, but HHO upgrades this share. The latter implies that the magnification of the in-cylinder temperature provided by the hydroxy doping suppresses the positive effect of the ABE. The enhanced behavior of ABE standalone blends can be attributed to the higher vaporization heat that limits the air-fuel mixture temperature in the intake stage, which reflects on reduced combustion temperature, thus reducing the potential of NOx formation. This pattern is consistent with the experimental outcome of a similar investigation that directly measures the temperature at the intake valve closing [12,46].

The highest NOx emission levels were achieved by the ABE 5 + HHO, followed by pure gasoline, ABE 5, ABE 10 + HHO, and ABE 10. Incorporating the oxygenated compounds in the air/fuel mixture reduces NOx emissions between 0.36 to 1.87 g • kWh−<sup>1</sup> compared to gasoline. In contrast, the integration of hydroxy doping in the intake air for the ABE 5 case maximizes the emissions levels between 0.16 to 0.27 g • kWh−<sup>1</sup> but ABE 10 + HHO remains behind the baseline fuel during all the engine loads. The higher oxygen content in the air/fuel mixture can directly contribute to the rise in NOx emissions by incorporating HHO in the fuel blends.

It is essential to mention the trade-off between the effect of HHO doping based on fuel economy and emission levels. Based on the fuel metrics (Figure 6), the integration of hydroxy gas minimizes fuel consumption, which ratifies prospective benefits in terms of fossil-fuel depletion and economic viewpoints. Moreover, since ABE is replacing up to 10% of fossil fuel, the net fuel saving escalates based on a global perspective. In the counterpart, hydroxy doping stimulates NOx formation, as corroborated in this section. A potential solution to mitigate the impact of NOx emissions magnification could be integrating waste

heat recovery (WHR) technologies that further enable a higher fuel utilization ratio. The integration of exhaust gas recirculation systems can be another feasible opportunity to promote the sustainable operation of ICEs.

## 3.5.4. Smoke Emissions

The emissions section concludes with the smoke opacity emission depicted in Figure 10 for the different tested fuels.

**Figure 10.** Smoke emission for fuels tested.

According to the results, the emissions levels of smoke opacity are higher at low engine loads. In this sense, as the engine load increases from 50% to 100%, the emissions are reduced up to 54%, demonstrating that smoke emissions depend primarily on the engine operating conditions. Interestingly, the standalone gasoline operation features the highest smoke formation in the exhaust stream. On the contrary, implementing alcohol compounds and HHO doping promotes smoke opacity minimization. This result implies that the extension of the oxygenated conditions limits the emission levels. In other words, it is observed that by only using ABE additive in the blends, a decent reduction of smoke is achieved when compared to the baseline fuel, and the HHO replacement furthers enlarge the smoke drop. Specifically, gasoline presents up to 9% of smoke emissions, followed by ABE 5 between 4.2 to 8.3% and ABE 10 with less than 7.6%. The implementation of hydroxy enrichment in ABE 5 and ABE 10 provides a further reduction of up to 0.3% and 0.4%, respectively.

Exhaust after-treatment technologies have reported salient results towards emissions reduction. However, the main advantage of the proposed dual-fuel methodology relies on the simplified implementation that does not intervene in engine structure and requires negligible modification on its functionality. The latter elucidate clear advantages from a techno-economic viewpoint and the importance of promoting dual-fuel operation in future platforms of ICEs.

#### 3.5.5. Fuel Energy Distribution

This section concludes by examining the fuel energy distribution within the engine for the tested fuels. Accordingly, Figure 11 summarizes the fuel energy allocation based on the power output and exhaust gases directly measured on the experimental assessment. Notice that the energy loss comprises lubrication oil, refrigerant, among other energy losses such as blow-by gas that limit energy conversion [34].

**Figure 11.** Energy distribution in the SI engine, (**a**) load 50%, (**b**) load 75% and (**c**) load 100%.

According to the results, increasing the engine load intensifies the energy contribution of both the power output and exhaust gases. This result is consistent with WHR applications where higher loads facilitate the fuel utilization ratio. The exhaust streams feature higher temperatures which corroborate the imminent escalation of NOx emission as depicted in Figure 9 [3,5]. However, the negative pattern of increasing the engine load is the significant rise in fuel consumption, as ratified in Figure 6. The results demonstrated that almost half of the fuel energy results in energy losses and other sources. Partial fuel substitution with ABE reduces the heat recovery potential from exhaust gases compared to conventional gasoline, which can be associated with the lower calorific value of this fuel blend.

Overall, ABE 10 + HHO features the best performance on the power output distribution from the tested fuels ranges in all the engine load conditions. An average increment of nearly 2.4% can be achieved compared to the baseline fuel. Contrarily, the maximum heat potential in the exhaust gases is higher in the gasoline fuel, with an improvement between 0.6–0.9% compared to the rest tested fuels. Subsequently, Figure 12 describes the behavior of thermal efficiency for the different fuel blends.

**Figure 12.** Thermal efficiency for different types of fuels.

According to the results, the thermodynamic efficiency directly relates to the engine load while representing a sensitivity between 4–6%. The latter can be attributed to non-optimized combustion derived from lower *HRR* and pressure ranges. This pattern is consistent with similar applications in diesel engines operating in dual-fuel mode [15,16]. Moreover, the comparative assessment between fuel blends demonstrates that the addition of ABE in gasoline endorses engine efficiency, while hydroxy doping favors efficiency escalation. For the tested conditions, the maximum thermal efficiency of 31.81%, 32.49%, 32.52%, 32.63%, and 33.01% was obtained when operating with gasoline, ABE5, ABE5 + HHO, ABE10, and ABE10 + HHO, respectively. The above behavior is mainly attributed to a complete combustion process due to the presence of ABE and hydroxy, which is reflected in the reduction of CO emissions (see Figure 7).

Figure 13 shows the exergy distribution of the engine for the different operating conditions and fuel blends.

**Figure 13.** Exergy distribution in the SI engine, (**a**) load 50%, (**b**) load 75% and (**c**) load 100%.

Results show that exergy destruction represents the greatest share from the sources analyzed. The latter can be attributed to several factors such as residual fuel mixing, turbulence flow instabilities during combustion, among other irreversibilities that limit the chemical energy conversion [47]. Besides, as the engine load increases, both the useful exergy (mechanical power) and exhaust gas exergy rise, which implies improved combustion efficiency as both internal and external irreversibilities decrease [47,48]. The engine reaches a maximum of 29.9%, 30.6%, 30.9%, 31.0%, and 31.4% for the useful exergy when implementing gasoline fuels ABE5, ABE5 + HHO, ABE10, and ABE10 + HHO, respectively.
