## 3 Results and Discussion  
### Cylinder Pressure  
The experimental evaluation of the dual-fuel operation in the SI engine begins with the characterization of the pressure gradients developed within the combustion chamber. This parameter provides a clear perspective of the fuel mode performance while relating the appropriate mixing interaction between the base fuel (gasoline/ABE), hydroxy gas (gaseous fuel), and air. Figure 3 shows the overall behavior of the in-cylinder pressure at different load conditions. Notice that the standalone gasoline operation has been set as the baseline fuel for comparison purposes.  
Figure 3: _Cont_.  
According to the results, the maximum in-cylinder pressure within the engine operating range is achieved by the ABE 5 + HHO with values between 60-135 bar. This result is consistent with the magnification of the laminar flame speed of this blend. Results in Figure 3 assist in the evaluation of the combustion phasing from the different fuel blends. Based on Figure 3a, all the blends present a retarded phasing between 0.2\({}^{\circ}\)-0.6\({}^{\circ}\) compared to the baseline fuel, which implies that a low engine load inhibits the fast combustion for the alternative fuels. The overall pattern encountered in the phasing is consistent with related investigations of ABE at different blend ratios [12]. Next, in Figure 3b, all the tested fuels feature a relatively similar phasing condition. However, at high engine loads (Figure 3c), the enhancement on the laminar flame speed from the oxygenated fuels becomes evident as the combustion phasing is advanced, ranging from 0.8\({}^{\circ}\) to 1.6\({}^{\circ}\) compared to the baseline, which facilitates the completion of combustion and faster peak pressures achievement even before reaching 360\({}^{\circ}\). The latter is a direct indication of a significant improvement in combustion efficiency and thermal performance.  
The pressure curve features an increasing trend as the engine load increases since more fuel is mixed to meet the power demand. Notably, increasing the fuel substitution with ABE (ABE 10) limited the pressure produced during combustion between 30-70 bar, which can be associated with the lower calorific value of this blend. This result agrees with similar investigations related to diesel engine combustion phenomena while operating with biodiesel blends [40]. Another contributor to the reduced pressure of ABE-based blends could be associated with a higher-octane rating that extends the initiation delay and reduces the laminar flame speed. In contrast, hydroxy doping enables a significant improvement in the combustion pressure in both blends (ABE 5/10) up to 75%. This pattern can be explained considering that incorporating hydroxy in the engine facilitates a homogeneous air-fuel mixture. The positive effects on the gasoline octane rating by incorporating HHO in the intake air system can be mentioned as another contributor to the enhanced combustion performance since the compression ratio is maximized. It should be noted that the overall trend of the pressure curves after combustion features a sharp decrement which guarantees that the knocking condition is not reached. On average, the combustion pressure developed in the standalone gasoline operation is higher than that of ABE 5, ABE 10, and ABE 10 + HHO by 12%, 18%, and 24%, respectively. Contrarily, the ABE 5 + HHO shows an enhanced pressure range compared to commercial gasoline between 10-15% for the engine load ranges analyzed.  
Figure 3: Influence of fuel on cylinder pressure for an engine load of (**a**) 50%, (**b**) 75%, and (**c**) 100%.