## 1 Introduction  
Reactivity controlled compression ignition (RCCI) is a dual fuel low-temperature combustion (LTC) mode that adapts an in-cylinder mixing of at least two fuels of different reactivity to manipulate and improve the combustion phase by stratification [1; 2; 3; 4]. The auto-ignition of the high reactivity fuel (HRF) initiates the combustion while a rise in the temperature and pressure facilitates the burning of the low reactivity fuel (LRF) [5]. The RCCI combustion concept has turned out to be a promising model for the upcoming progeny of internal combustion engines, but is still subject to an in-depth study for its perfection [2; 6]. According to [PERSON] et al. [7] the measures taken to address the nitrogen oxides (NOx) and soot emissions in RCCI resulted in an unacceptable rise in carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions; hence dual direct injection (DDI) was developed to improve the local reactivity index and reduces the amount of fuel entering the crevices as a source of these emissions. According to [PERSON] and [PERSON] [8], use of DDI along with the exhaust gas recirculation (EGR) could extend the RCCI load to a 21 bar indicated mean effective pressure (IMEP), as a consequence of utilizing the piston geometry and important regions by the two injectors. Achieved also was high thermal efficiency of 48.7% with a very low NOx and soot emissions. Utilizing a considerable quantity of the fuel during the first injection timing enables effective control of the combustion by maximizing evaporative cooling in the squish region as similarly reported in [9]. Despite the achievement of the DDI technique, an unacceptable rise in the CO and UHC emission still manifested at low load [10]. Evidences in literature show that, over a decade few researchers such as; [PERSON] and [PERSON] [11], [PERSON] and [PERSON] [8], [PERSON] and [PERSON] [12], [PERSON] et al. [10], [PERSON] and [PERSON] [13], and recently [PERSON] et al. [7] considered the use of dual direct injectors to inject both low and high reactivity fuels directly into the cylinder with the view to reducing the amount of premixed air-fuel mixture in the crevice region using gasoline-diesel and natural gas (NG)-diesel in RCCI combustion. The reduced attractiveness of the DDI technique may attribute to the complexity in the cylinder head modification posing technical challenges to achieving its maximal utilization and practical examination. However, [PERSON] et al. [14] in their review suggested that more of the techniques on the reduction of UHC and CO emissions should be on studies. Likewise, [PERSON] et al. [4] suggested investigations on the various injection approaches for more control in RCCI combustion. Therefore, the quest for further reduction of UHC and CO emission through a simple to achieve, easy to implement and cost-effective modified port injection approach motivated the research on gaseous LRF injection at the valve, which aimed at eliminating air-fuel mixing before entering the cylinder, raising the in-cylinder temperature at low load and reducing the amount of fuel mixture entering the crevices. Comparatively, Figure 1 shows the various LRF delivery methods; conventional premixed port injection [15; 16; 17], dual direct injection [8; 10; 12; 13], and the proposed port injection at the valve for this research. To the best of the authors' knowledge, this article is the first to report this approach. Besides, the benefits of using biodiesel as HRF, in place of conventional diesel in RCCI combustion, are of utmost importance. Biodiesel offers certain advantages to improve fuel stratification, control combustion phase, and extend the load in the RCCI engine. According to [PERSON] et al. [18] direct use of biodiesel, produced from vegetable oils, might result in some technical faults.