###### Abstract  
Compression ignition (CI) engines are widely used in modern society, but they are also recognized as a significative source of harmful and human hazard emissions such as particulate matter (PM) and nitrogen oxides (NOx). Moreover, the combustion of fossil fuels is related to the growing amount of greenhouse gas (GHG) emissions, such as carbon dioxide (CO\({}_{2}\)). Stringent emission regulatory programs, the transition to cleaner and more advanced powertrains and the use of lower carbon fuels are driving forces for the improvement of diesel engines in terms of overall efficiency and engine-out emissions. Ethanol, a light alcohol and lower carbon fuel, is a promising alternative fuel applicable in the dual-fuel (DF) combustion mode to mitigate CO\({}_{2}\) and also engine-out PM emissions. In this context, this work aims to assess the maximum fuel substitution ratio (FSR) and the impact on CO\({}_{2}\) and PM emissions of different nozzle holes number injectors, 7 and 9, in the DF operating mode. The analysis was conducted within engine working constraints and considered the influence on maximum FSR of calibration parameters, such as combustion phasing, rail pressure, injection pattern and exhaust gas recirculation (EGR). The experimental tests were carried out on a single-cylinder light-duty CI engine with ethanol introduced via port fuel injection (PFI) and direct injection of diesel in two operating points, 1500 and 2000 rpm and at 5 and 8 bar of brake mean effective pressure (BMEP), respectively. Noise and the coefficient of variation in indicated mean effective pressure (COV\({}_{\text{IMEP}}\)) limits have been chosen as practical constraints. In particular, the experimental analysis assesses for each parameter or their combination the highest ethanol fraction that can be injected. To discriminate the effect on ethanol fraction and the combustion process of each parameter, a one-at-a-time-factor approach was used. The results show that, in both operating points, the EGR reduces the maximum ethanol fraction injectable; nevertheless, the ethanol addition leads to outstanding improvement in terms of engine-out PM. The adoption of a 9 hole diesel injector, for lower load, allows reaching a higher fraction of ethanol in all test conditions with an improvement in combustion noise, on average 3 dBA, while near-zero PM emissions and a reduction can be noticed, on the average of 1 g/kWh, and CO\({}_{2}\) compared with the fewer nozzle holes case. Increasing the load insensitivity to different holes number was observed. Keywords:dual-fuel; compression ignition engine; ethanol; nozzle-hole number; soot emissions +
Footnote †: journal: Applied Softthe transition to a more advanced form of propulsion (such as hybrid cars, electric vehicles, fuel cells, etc.) developed for fuel economy improvement and reduction in exhaust emissions is very far to be complete. As a result, stringent emission legislation and higher oil prices push world Original Equipment Manufacturers (OEMs) to investigate more efficient engines. Compression ignition (CI) engines, due its efficiency, robustness, flexibility and low operation cost, are widely used in modern society in many operative fields such as passenger vehicles, industrial or agricultural equipment, or as stationary sources of energy. The main concern using diesel engines is related to the drawback of higher emissions of particulates matter (PM) and nitrogen oxides (NOx). The typical diesel PM-NOx trade-off makes it difficult to control these pollutant emissions simultaneously [2]. In this context, to meet the more stringent emissions directions [3] (setting new CO\({}_{2}\) emission standards for cars and vans, Euro 6 emissions standard regulates particulate and gaseous emissions), modern diesel engines are equipped with complex and costly after-treatment systems, to reduce PM emissions and selective catalytic reduction that injects ammonia or urea into exhaust gases stream to reduce NOx [4].