Reactivity controlled compression ignition (RCCI), homogenous control compression ignition (HCCI) and pre-mixed controlled compression ignition (PCCI) are considered as three methods of low temperature combustion (LTC) as methods to have lower emissions in the direction of lower temperature and lean combustion [
1]. In fact, these three methods provide a more diluted mixture of fuels in higher homogeneity to have more lean and complete combustion [
2]. Among these three methods, RCCI engines, which use two different fuels with two different reactivities, in addition to having a more diluted mixture, also have the possibility to have more complete combustion due to low reactivity fuel (LRF) [
3,
4,
5]. In this type of engines, usually low reactivity fuel imports to the cylinder from the intake port and high reactivity fuel injects to the cylinder directly. As such limitations lead lean combustion on engine operational condition to have more stability and efficiency, there are different strategies in the direction of providing better combustion in these types of engines [
6]. Cetane number and mixture of fuel reactivity are two important, effective factors in the combustion quality in RCCI engines. There are various research on using different fuels and mixtures, collated in C. Bae et al.’s 2017 review; research on various fuels, mutant compression ratio, exhaust gas recirculation usage and glow plug as well as injecting pilot fuel has been investigated [
7]. Geometry changes have also been investigated by researchers to provide more homogenous mixture beside higher combustion quality. In 2010, Higashino et al., used a thermal isolated pre-chamber in an HCCI natural gas (NG) CI engine [
8]. Roethlisberger et al., 2009, investigated the roles of pre-chamber on a spark ignition (SI) engine where more smooth working and lower in-cylinder temperature were observed in their research [
9]. Investigating on throat size effects on combustion of an SI engine was numerically performed by Stanislaw Szwaja et al. in 2013 [
10]. Zheng et al., 2005, used pre-chamber to investigate various properties such as mixture formation, fluid flow and nitrogen oxides emissions in a CI engine [
11]. A comparison study on using pre-chamber was done by Salahi et al., in 2015 in a case of numerical works in a natural gas/diesel RCCI engine, where they investigated effects of pre-chamber on combustion quality and exhaust emissions [
12]. In 2018, Kakoee et al., investigated the effects of additives (Hydrogen) on engine performance and emissions in a NG/Diesel RCCI engine [
4]. Further, in their recent works in 2019 they investigated an RCCI engine equipped with a pre-chamber in case of hydrogen addition effects on two various fuel mixtures, NG/diesel and NG/DME, and their results show different behavior of these two mixture fuels against hydrogen addition [
2,
3]. In the case of geometry analysis, Balasubramanian et al., performed an investigation on combustion and emission characteristics by applying 11 various bowl geometry in a numerical study research [
13].
In the way of using alternative fuel mixtures in RCCI engines, Moradi et al., in 2020 investigated hydrogen addition influences on a NG/diesel RCCI engine, and they observed that hydrogen addition reduces unburned hydrocarbons (UHC) and carbon dioxides (CO
2) while increasing nitrogen oxides (NO
x) [
14]. These results were also obtained in Kakoee et al.’s investigations on hydrogen additions [
2,
3]. Because of the use of two different fuels with various reactivities, changing fuels ratio in RCCI engines was attractive for researchers, where Olsen et al., in 2007 studied effects of NG percentages on combustion characteristics and exhaust emission on a NG/Diesel RCCI engine [
15]. They found that there are more UHC and CO
2 besides lower NO
x in higher percentages of natural gas. Bio-diesel, as a high reactivity fuel, was also used by Ghareghani et al., to investigate fuel properties such as Cetane number influences on combustion in a NG/Bio-diesel RCCI engine [
16]. Injection timing is also an alternative research focus to study engine power output and emissions, and there are various investigations in this field. An RCCI engine fueled with gasoline/n-heptane was investigated in 2015 by Zhu et al., to obtain effects of injection timing and fuels ratio [
17]. Han et al., in 2018, studied various start of injection strategies and energies input properties on a NG/diesel RCCI multi-cylinder, heavy-duty engine [
18]. Another experimental research was done by Soloiu et al., in 2013 to investigate injection timing of HRF to determine its effect on emissions and engine output of an N-butanol/Biodiesel RCCI engine [
19]. The combustion properties of a2-butanol/diesel RCCI engine were studied by Pan et al., in 2017 to investigate the effect of 2-butanol percentage on engine emissions and efficiency. Jesus Benajes et al., made an experimental investigation on injection timing of four different low reactivity fuels in high and low engine load, and they found that the effects of SOI angle have different influences on emissions from high to low loads; this was observed also in other properties such as IMEP [
20]. Influences of fuel fractions and injection timing were studied in an RCCI gasoline/biodiesel numerical model by J. Li et al., where they showed that advanced SOI caused an advance in start of combustion and also increased NO
x emissions; this change decreased HC and CO emissions [
21]. Fuel injection parameters were also modeled in 2016 by M. Nazemi et al., and spray angle, SOI timing and fuel injection pressure was investigated in this research. More advanced SOI produce lower NO
x and higher amounts of HC and CO [
22]. A light duty diesel/gas RCCI engine was studied by Kamran Poorghasemi et al., in 2017.It was observed in their research that gross indicated efficiency increases with advancing SOI, where CA50 also advanced, and emission was influenced by changing main SOI where in NO
x decreased with SOI advancing, in contrast to the increase of HC and CO. Advancing in main SOI led to more delay in start of combustion [
23]. Double injection timing strategy was investigated by Jing Li, et al., in 2018 while a main topic in their research was Dwell contact CAD effects on engine performance and emissions [
24]. Various port injected fuel with n-heptane as high reactivity fuel were investigated during injection timing strategy in 2015 by Lifeng Zhu et.al. Five various injection angles were investigated and this research shows advancing in SOC due to advancing in start of injection. It was also observed that more advancing SOI has more ignition delay in CAD. Exhaust NO emission show lower amounts in more advanced SOI where the amount of HC and CO increases during advancing start of injection. Higher temperature and pressure was also observed in more advanced SOI [
25]. An optimization work on injection parameter combustion chamber geometry was studied by Jie Liu. et al., in 2018. More than 200 case data were calculated during the optimization process, which has various data in start of combustion related to three various combustion chambers. In their results, more NO was observed in more advanced SOI besides higher HC and CO [
26]. Iso-Butanol/Diesel RCCI heavy-duty engine was studied numerically by Mobasheri et al., in 2018 and they showed that, in three various injected pressure, there are different behavior of NO
x emission during advancing SOI, until 50 CAD aTDC where the amount of NO
x decreased, but in SOI between 50 and60 the amount of NO
x increased. CO and HC emission also have the same condition against these SOI changes. Advanced SOI increased temperature until50 CAD aTDC, in which maximum pressure and temperature occurred in this point also; after this crank angle temperature and pressure decreased [
27]. Peak pressure rise rate and combustion stability were investigated against injection timing in an RCCI engine by Saxena M.R et al., in 2018. In this study, two different blends of high low reactivity fuels were investigated against SOI variation. This experimental research showed that advancing in SOI led to lower pressure and in-cylinder temperature. There was also a minimum peak pressure rise rate at 30 CAD before top dead center(bTDC).IMEP was increased during SOI advancing in two fuel blending [
28].
Dimethyl ether as a HRF is produced from various methods such as natural gas, waste products, crude oils and biomasses. Its characteristic is approximately the same as liquefied petroleum gases, although its high reactivity made it attractive for researchers to use it in CI engines. In 2007, Kong et al., investigated a NG/DME RCCI engine numerically with detailed chemical kinetic solution [
29]. Arcoumaniset et al., in 2008 studied different various works to show DME usage in CI engines, spark ignition and rotary applications [
30]. As it was mentioned before, Liu et al., has done research on a NG/DME RCCI with hydrogen as additive [
13]. In addition to the comparative study of NG/Diesel and NG/DME by Kakoee et al. [
4], usage of DME in various percentages to obtain optimum HRF fuel percentage and research on its behavior against hydrogen addition was done in 2019 [
2,
3].
In the current study, effects of injection of HRF (dimethyl-ether) were investigated numerically on the performance of NG/DME RCCI engine. Combustion characteristics and emissions output besides engine efficiency have been studied in various start of injection (SOI) in single and split injection strategy. In all studied cases, total input energy kept constant at 485 J. A computational fluid dynamics (CFD) model of engine was used in CONVERGE (Convergent Science, Madison, WI, USA) with coupling in Chemkin Solver to investigate the effects of SOI on emissions, efficiency and combustion characteristics. In this research modeling, chemical kinetic reactions of fuels were modeled with a set of 893 reactions and 130 species mechanism where DME decomposition was also considered in this mechanism. Equivalence ratio is about 0.3 and ratio of HRF and LRF is 1.4 (kg-kg). Numerical results were validated with an experimental setup of a Ricardo E6/MS one-cylinder research engine equipped with a Commet MK.V pre-chamber. In three various strategies of injection, one stage, 50%-50% split strategy and 80%-20% split strategy, an optimum SOI was obtained based on numerically optimum output emission. Investigation on injection strategies based on emission performances for each applied strategy and optimum injection angle were obtained in this research.