Surrogate Fuel Formulation to Improve the Dual-Mode Dual-Fuel Combustion Operation at Different Operating Conditions

Dual-mode dual-fuel combustion is a promising combustion concept to achieve the required emissions and CO₂ reductions imposed by the next standards. Nonetheless, the fuel formulation requirements are stricter than for the single-fuel combustion concepts as the combustion concept relies on the reactivity of two different fuels. This work investigates the effect of the low reactivity fuel sensitivity (S=RON-MON) and the octane number at different operating conditions representative of the different combustion regimes found during the dual-mode dual-fuel operation. For this purpose, experimental tests were performed using a PRF 95 with three different sensitivities (S0, S5 and S10) at operating conditions of 25% load/950 rpm, 50%/1800 rpm and 100%/2200 rpm. Moreover, air sweeps varying ±10% around a reference air mass were performed at 25%/1800 rpm and 50%/1800 rpm. Conventional diesel fuel was used as high reactivity fuel in all the cases. Moreover, commercial 95 RON gasoline was used as reference to compare the different TRFs. The engine settings were managed to adjust the rate of heat release to that found with 95 RON gasoline. To do this, a quality index imposing a maximum deviation of 5% point-to-point between the HRR curves from both fuels was defined. The results suggest that PRF 95 with S0 has the most similar behavior compared to conventional 95 RON gasoline whatever the engine load. As the engine load increases, the sensitivity effect is more noticeable and iso-HRR operation was only possible for S0. At low and medium load, the TRFs present similar engine-out emissions with equal fuel consumption. At full load, the NOx emissions are increased with respect to the reference 95 RON gasoline without fuel consumption benefits. The results from the air variation for the different octane numbers demonstrated that the greatest differences are obtained for low air mass (i.e, higher EGR). In addition, the decrease of the octane number limits the maximum air increase due to the pressure gradients, requiring modifications in the engine settings that increase the soot formation.