Parallel to the rapidly increasing industrial developments worldwide, several challenges have emerged. The quest for alternative fuels to enhance the power capacity and fuel efficiency of internal combustion engines (ICE) in more environmentally friendly conditions has gained significant momentum. ICEs, a common engine type used in vehicles for many years, primarily rely on fossil fuels. However, the combustion of these fuels releases polluting gases into the environment through the exhaust, with known adverse effects on global warming, air pollution, and both the environment and human health. Compounding these issues is the fact that fossil fuel reserves are limited.
Physical and Chemical Properties of Methanol
In this part, general information is given about the physical and chemical properties of methanol, which is used as a renewable alternative fuel in ICEs. Methanol is a type of alcohol that is colorless, has a barely perceptible odor, and is significantly toxic. It can especially affect the nervous system and cause blindness. Even though it is unlikely, taking it orally can have fatal consequences. Its vapor can penetrate a person through the lungs, and methanol liquid can penetrate the skin. Gasoline and diesel are not equally dangerous. The reason is that their taste and smell make them noticeable much more quickly than methanol. Gasoline and diesel fuels should also be used with caution, but they are not as dangerous as methanol [
7].
Methanol is an alcohol fuel, such as ethanol, butanol, and propanol. The most important advantage of alcohol fuels is their low viscosity compared with diesel fuel. Therefore, it can be easily injected, atomized, and mixed with air. In addition, the high laminar combustion rate, which can ensure that the combustion process ends earlier, increases the thermal efficiency of the engine. Less emissions occur due to high oxygen content and low sulfur content [
8]. Some chemical and physical properties of methanol and diesel fuels are shown in
Table 1 [
9,
10,
11].
Many recent studies focus on low-temperature combustion (LTC) strategies that can reduce emissions and improve fuel economy. Three common strategies for achieving LTC are known as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and reactivity-controlled compression ignition (RCCI).
Recent experimental and simulation results show that RCCI is a more promising technology than the other two strategies, HCCI and PCCI. This method is important in terms of providing more effective ignition control and having a low maximum ROPR. Additionally, RCCI has been observed to maintain low emissions and high fuel efficiency simultaneously [
12]. RCCI, defined as a combustion technology in dual-fuel engines, involves the utilization of at least two fuels with differing reactivity levels for in-cylinder fuel blending. This method employs a multiple injection strategy and an optimal EGR (exhaust gas recirculation) rate to regulate in-cylinder reactivity. The ultimate goal is to enhance combustion phasing, duration, and magnitude, consequently resulting in improved brake thermal efficiency (BTE) while minimizing both NO
x and smoke emissions [
13,
14]. The RCCI combustion system demonstrates its versatility by operating effectively over a wide range of engine loads from 4.6–14.6 bar gross IMEP. This operational range achieves nearly negligible levels of NO
x and smoke emissions, ensuring compliance with regulatory standards. Additionally, it maintains acceptable ROPR and minimal ringing intensity while delivering remarkably high indicated efficiency [
15,
16].
There are many publications in the literature on the use of methanol and alcohol fuels in ICEs. However, the methods and results of some publications on the use of methanol and alcohol fuel in RCCI combustion strategies are summarized below:
In their experimental study, Panda and Ramesh conducted research to obtain low emissions and high BTE of the RCCI engine using methanol/diesel fuel. Tests were carried out on a single-cylinder, common-rail water-cooled diesel at a constant speed of 1500 rpm and an average indicated effective pressure of 5 bar. Energy sharing from methanol to diesel can be increased from 45% in dual-fuel mode to nearly 56% in RCCI mode by appropriately adjusting the injection parameters. In this case, while the BTE increased from 36% to 38%, NO
x emission was found to be 95% lower in RCCI mode. Smoke emissions have been reduced by 78%. In order to increase the BTE to 42%, more than 45% ES from methanol to diesel is required after heating the intake air to approximately 85 °C. In this case, it was observed that while NO
x emissions decreased, carbon monoxide (CO) and HC emissions increased [
17].
Agarwal et al. used fuels with different methanol premixed ratios (M30, M50, M80) in a two-cylinder, common-rail direct-injection and turbocharged RCCI engine and compared them with diesel fuel. Bsec decreased between M30 and M80. HC and CO increased in more methanol fractions. CO
2 and smoke emissions decreased compared with conventional diesel fuel. While NO
x emissions decreased in M30 and M50 fuels, they increased in M80 fuels [
18].
Duraisamy et al. examined the effects of methanol/diesel and methanol/PODE dual-fuel RCCI combustion in a three-cylinder, common-rail direct injection, turbocharged diesel engine. The tests were carried out at 3.4 bar brake mean effective pressure (BMEP) and 1500 rpm engine speed. When the results were examined, the researchers observed that the ID lengthened as the mass of methanol increased in both methanol/diesel and methanol/PODE processes. With the increase of methanol mass fraction, NO
x and smoke emissions for RCCI combustion decreased significantly. However, it was determined that HC and CO emissions increased slightly [
19].
Hassan et al. conducted a study using methanol and diesel fuels in a single-cylinder, 2000 rpm constant engine speed, air-cooled engine. In the experimental procedure, pure diesel and methanol content by mass of 7%, 14%, and 21% (MD7, MD14, and MD21) were used as fuel. In the experimental results, it was observed that bsec and bsfc decreased in MD7 and MD14 but increased in MD21 fuel compared with pure diesel fuel [
20].
In their study, Huang et al. conducted research on a four-cylinder, turbocharged, direct-injection RCCI engine at a constant speed of 1800 rpm. They examined the effects of EGR and different methanol substitution rates on combustion, performance, and emissions of methanol/diesel fuels. HC and CO emissions increased with the increase of bsfc at 90% load. As the methanol substitution rate increased from 0% to 30%, bsfc decreased by 3.29%. Increased HC and CO emissions were observed with the use of EGR. Significant improvements were observed in NO
x emissions by 73.6%, with a 30% increase in the methanol substitution rate [
21].
In their study, Liu et al. conducted an experimental study on a turbocharged, intercooled four-cylinder RCCI engine using diesel/methanol dual fuel at a low load. In the study conducted at 1800 rpm and 30% load, HC emissions decreased when the methanol content increased from 40% to 60% at low loads. It was observed that NO
x and smoke emissions increased [
22].
When the literature studies are examined, it is noted that the effects of ID, CD, engine performance parameters such as bsfc, bsec, and exhaust emissions have not been investigated in the methanol/diesel dual-fuel RCCI engine, at a constant engine speed (1750 rpm), in four different ELs (40, 60, 80, and 100 Nm), and with different methanol energy fractions (M12, M19, and M26).