Combustion Parametric Investigations of Methanol-Based RCCI Internal Combustion Engine and Comparison with the Conventional Dual Fuel Mode

: Reactivity-controlled compression ignition (RCCI) is an advanced combustion mode. Its uses two fuels with different physical and chemical properties to form a combustible mixture with active stratification. RCCI can flexibly control the combustion process by changing the concentration and activity of the combustible mixture. It can also reduce the emission of NOx and particulate matter in the engine without significantly reducing the thermal efficiency. Among various fuel combinations, methanol as an oxygen-containing fuel, has a high latent heat of vaporization, which is conducive to reducing combustion temperature and achieving low-temperature combustion. This experimental study explores the potential of Methanol-Diesel Reactivity Controlled Compression Ignition (RCCI) in achieving low emissions and high thermal efficiency and compares this with the conventional dual fuel mode. Low-temperature combustions such as Reactivity Controlled Compression Ignition (RCCI) have been shown to be a promising way to reduce pollutants at the exhaust, i.e. NOx and soot emissions. and increase the thermal efficiency of future engines. The methanol to diesel energy share (MDES) could be enhanced to 56% in the RCCI mode with proper setting of the injection parameters from 45% in the dual fuel mode. A higher quantity in the second diesel pulse that occurred close to TDC led to higher thermal efficiency and good combustion stability. Engines working in a dual-fuel mode need special conditions to ignite an air-fuel mixture without a spark plug in a good moment with high combustion efficiency.


Introduction
Rising fuel costs and a focus on the reduction of greenhouse gases has driven the need for increased efficiency from the internal combustion engine.This need for increased efficiency has placed the diesel or CI engine in the spotlight due to its superior fuel efficiency

Suggested Citation
Iqbal, M.Y., Wang, T. Li, G. & Ali, W. (2023).Combustion Parametric Investigations of Methanol-Based RCCI Internal Combustion Engine and Comparison with the Conventional Dual Fuel Mode.European Journal of Theoretical and Applied Sciences, 1(5), 951-961. DOI: 10.59324/ejtas.2023.1(5).82compared to SI engines.However, under conventional diesel operation, regions of the combustion chamber pass through both rich and lean high-temperature regions, forming soot and NOx, respectively.Reactivity-controlled compression ignition (RCCI) is a dual fuel lowtemperature 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 (Wang, et  al., 2016; Salahi, et al., 2017; Reitz, & Duraisamy,  2015; Reitz, & Duraisamy, 2015).The autoignition 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) (Benajes, et al., 2014).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 (Dalha, et al., 2018;Iqbal, et al., 2022).Among the Low-Temperature Combustion (LTC) concepts aimed to reduce NOx and soot emissions, such as Homogeneous Charge Compression Ignition (HCCI) or Premixed Charge Compression Ignition (PCCI), the Reactivity Controlled Compression Ignition (RCCI) combustion has demonstrated great potential.This strategy consists in a combustion of a blend mixture composed by a low-reactivity methanol and a high-reactivity diesel fuel.Simultaneous reduction of particulate matter (PM) and nitrogen oxides (NOx) emissions from diesel exhaust is the key to current research activities.Therefore, new combustion concepts should be developed to meet stringent standards.Nowadays, Low Temperature Combustion (LTC) strategies, including Homogeneous and Premixed Charge Compression Ignition (HCCI/PCCI), are receiving more attention due to their potential for simultaneously reducing soot and NOx emissions.For the HCCI mode, in-cylinder homogeneity may cause rapid combustion by simultaneous ignition throughout the cylinder space and the mixture can be susceptible to great combustion noise.It is also very tough to control the combustion phases.Premixed charge compression ignition (PCCI) ignition has been evolved from the HCCI combustion mode for the sake of better control over the start of combustion (SOC).It is not fully homogeneous like HCCI and achieves the desired ignition delay through extensive use of EGR.Reactivity-controlled compression ignition (RCCI) is the newest approach where multiple fuels of different reactivity are injected at scheduled intervals which governs the reactivity of the charge in the cylinder for the desired combustion duration and magnitude.Mainly, in this approach, relatively low reactive fuel (low cetane number) Such as natural gas or gasoline is injected (port injection) very early in the engine cycle which mixes with the air homogeneously.Later on, a higher reactive fuel like diesel is injected directly into the cylinder; it creates pockets of different air-fuel ratios and reactivity, which govern the onset of combustion at different times and rates.Figure 1 shows the schematic of RCCI combustion strategy.

Figure 1. Schematic of RCCI Combustion Strategy
To create a homogenous air-fuel mixture the conditions in the cylinder are even more demanding.Many concepts of ignition were developed, but the most effective needed the perfect mixing of fuel and air, which is a serious technical challenge.Technical solutions for dualfuel engines cover the complexity of these problems thus leading to the further development of ignition systems in internal combustion engines.Fuel supply systems, their operation strategy, and the shape of the combustion chamber are the most important elements to change and develop for the correct operation of dual-fuel engines.The literature analysis showed a small amount of research carried out to optimize the operation of dual-fuel engines The variety of engines in which a dualfuel system can be used requires much more research about them, and solutions necessary for their correct operation.
Fuel properties play a vital role in kinetically controlled combustion.Bessonette et al. (2007) and Yousefi et al. (2016)reported that highoctane fuels (low reactivity) are more suitable for high loads whereas high-cetane fuels (high reactivity) are good for low loads.Hence, an LTC engine can be made to operate at different loads with fuel combinations that can have controlled reactivities so that the load range can be extended (Saxena, & Maurya, 2017).However, fuel reactivity is also sensitive to operating conditions such as intake charge temperature, pressure and composition.Reactivity controlled compression ignition (RCCI) is a premixed combustion process using two fuels of different reactivities.Less reactive fuels like gasoline, methanol and ethanol have been injected in the intake port to form a premixed charge with air.Another fuel of higher reactivity like diesel which is injected directly into the cylinder during the compression stroke controls the ignition and combustion processes (Ma, 2013;Park, & Yoon, 2016;Saxena, & Maurya, 2020;Kokjohn, et al., 2011).This process also generates a fuel reactivity gradient inside the combustion chamber.The proportions of the two fuels are varied and this leads to better control and extended load range.RCCI is also kinetically controlled and results in almost zero NOx and soot like HCCI.Kokjohn et al., (2011) showed that ignition timing and combustion rate can be controlled by varying the proportions of each fuel and was able to achieve an indicated thermal efficiency of 53% with negligible NOx and soot.It was also reported that flame propagation did not play a significant role due to presence of very lean mixtures during the start of combustion and that the combustion rate is controlled through chemical kinetics like in HCCI (Zhao, et al., 2020; Iqbal, et al., 2020; Li,  et al., 2022; Wu, et al., 2019; Adzuan, & Yusop,  2022; Rangasamy, et al., 2020).Comparatively, Figure 2 shows the various LRF delivery methods; conventional premixed port injection, dual direct injection, 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 the combustion phase, and extend the load in the RCCI engine.

Experimental Set-Up and Measurement
A single-cylinder, four-stroke diesel engine was modified to achieve the biogas delivery at the valve for in-cylinder mixing of the fuels.The technical information on the test equipment is presented in Table 1, while Figure 2 depicted the layout of the engine test rig.

Figure 3. An illustration of the engine test rig
The technical information on the test equipment is presented in Table 1, while Figure 2 depicts the layout of the engine test rig.However, this research investigated the effects of directinjected diesel and port-injected methanol delivered at the valve on the combustion, performance, and emission parameters.The concept aimed at eliminating air-fuel mixing before entering the cylinder, though delivered through the air intake manifold, thereby reducing the amount of mixture entering the crevices.To raise the in-cylinder temperature, thus reducing the pre-mature combustion at low load (4-7 bar IMEP).An injector holder and a delivery hose was developed and coupled to the air intake manifold to enable the achievement of methanol fuel delivery at the valve in the cylinder.The test was conducted to investigate the engine characteristics at high speed and low load (4-7 bar) conditions in a modified approach of port injection at the valve.The manufacturer recommended the operation of the engine at a speed of 2000 rpm and above.A consistent engine speed of 2000 rpm was initially considered based on the manufacturer's recommendation and varied the load from the no-load indicated mean effective pressure (4.5 bar IMEP) to maximum capacity (6.5 bar IMEP) at an interval of 0.5 bar IMEP, which correspond to an increase by 25% engine load.Subsequently, 1600 rpm, 1800 rpm, and 2000 rpm were further investigated in a premixed port injection approach to ascertain the contribution of the high speed selected in elevating some of the emissions.The research reported the engine capacity based on the actual indicated mean effective pressures (IMEP) to reflect the low load range, unlike the usual load percentages.
In Equation 1, mD and mM are the masses consumed by diesel and methanol under certain working conditions are indicated respectively, unit: kg/h; represents the low calorific value of methanol, which is 19.9 MJ/kg; Indicates the low calorific value of diesel, which is 42.6 MJ/kg.
where m D+M is equivalent fuel consumption (kg/h).The effective fuel consumption rate is defined as the mass of fuel that the engine needs to consume per unit of effective work.In a methanol diesel dual-injection engine, the total effective fuel consumption rate is the total fuel consumption per unit of power when two fuels are burned together to produce the following equation: The output power e p of the diesel engine: kW.The effective thermal efficiency of an engine is the ratio of the heat equivalent of the effective power during engine operation to the heat content of the fuel consumed per unit of time.In a methanol-diesel dual-fuel engine, the effective thermal efficiency η is calculated as above.

Results and Discussion
The core of the active controlled compression ignition combustion mode is to form a combustible mixture with concentration stratification by using two fuels with large differences in physical and chemical properties, and control the combustion process in the cylinder by adjusting the concentration stratification of the combustible mixture.In this project, methanol and diesel fuel are two fuels with different physical and chemical properties, and the activities of combustible mixtures formed by different proportions of methanol and diesel fuel are also quite different.In order to essentially explore the influence of fuel activity on the combustion process of RCCI combustion mode (Altun, et al., 2023; Huang, et  al., 2023; Chen, et al., 2021; Huang, et al., 2023;  Jamrozik, et al., 2019), this chapter studies the influence of methanol occupancy ratio on the combustion and emission performance of methanol/diesel RCCI engine by changing the mixing ratio of methanol to diesel in combustible mixture.

Effect of methanol occupancy ratio on engine performance
The simulation is set to 2000r/min, 50% load condition, methanol energy ratio is set to 0%, 30%, 40% 50%, 60%, methanol injection pressure is 15MPa, and methanol injection time is set to 130°CABTDC.According to the experimental data obtained from the engine bench test, the injection parameters of the pilot diesel were set, and the pilot diesel fuel was set to a single injection, the injection time was 11°CABTDC, and the injection pressure was 103.2MPa.Fuel consumption data is obtained through engine bench test, and then the total calorific value of the fuel in the cylinder when the engine burns only diesel under the selected simulation condition is calculated according to the calorific value of the diesel, which is the total calorific value of the fuel in the simulation process, and then distributed to the methanol and diesel fuels in proportion, and the injection amount of methanol and diesel per cycle is calculated according to the calorific values of the two fuels, as shown in Table 2.The trend of combustion pressure in the cylinder with the proportion of methanol is shown in Figure 5.It can be seen from the figure that with the increase of the methanol energy ratio, the peak combustion pressure in the cylinder shows a trend of first increasing and then decreasing, and the crankshaft angle position corresponding to the cylinder pressure peak is gradually advanced, and the peak combustion pressure in the combustion chamber reaches the highest value when the methanol energy ratio is 50%.
The maximum combustion pressure in the cylinder is related to the temperature in the cylinder and the concentration of the combustible mixture.When the proportion of methanol injection is less than 50%, the temperature in the combustion chamber is higher at this time, the influence of methanol evaporation and heat absorption on the temperature in the combustion chamber is weak, and the combustion rate of methanol premix is faster, so when the methanol energy ratio is less than 50%, with the increase of methanol energy ratio, the pressure peak in the cylinder gradually increases, and its corresponding crankshaft angle position is gradually advanced.However, when the methanol energy ratio rises to more than 50%, the methanol content in the combustion chamber is more at this time, and its evaporation heat absorption will cause the temperature in the combustion chamber to drop significantly, inhibit the atomization evaporation of diesel fuel in the combustion chamber, resulting in a decrease in the maximum combustion pressure.

Figure 5. Effect of Methanol Energy Ratio on in-Cylinder Pressure
The changing trend of pressure increase rate with the methanol occupancy ratio is shown in Figure 6.With the increase of methanol energy ratio, the peak pressure increase rate gradually increases.When the methanol mass ratio is less than 50%, diffusion combustion is the main combustion method of fuel in the combustion chamber.With the increase of methanol injection ratio, the number of methanol premixtures formed in the combustion chamber increases, thus accelerating the combustion reaction rate, so the pressure rise rate gradually increases.When the methanol mass ratio is greater than 50%, the temperature in the cylinder drops sharply, resulting in the delay of the ignition initiation point and the increase of the hysteresis period, so the amount of combustible mixture generated during the hysteresis period increases, resulting in a further increase in the pressure rise rate.When the methanol energy ratio is 0, the peak pressure increase rate is 0.7MPa°CA -1 , and when the methanol energy ratio is 60%, the peak pressure increase rate has reached 1.34 MPa°CA -1 , which is about 2.9 times that of the original engine.The change of NOx content and emissions in the cylinder with methanol energy ratio is shown in Figure 7.With the increase of the proportion of methanol, the peak NOx production in the cylinder showed a trend of first increasing and then decreasing, and NOx emissions also showed the same change trend.The main factors influencing NOx production are high temperature, oxygen enrichment, and reaction duration.In the model setting, only the generation of NO is considered, and in the generation of NO, only the generation of hot NO is considered, while hot NO is generated in the front of the flame and in the gases that leave the flame.As an oxygenated fuel, methanol causes an increase in oxygen content in the combustion chamber.When the methanol occupancy ratio is less than 40%, the maximum temperature in the cylinder gradually increases, and the injection of methanol will increase the oxygen content in the combustion chamber, which will lead to an increase in NOx emissions.
When the methanol energy ratio exceeds 40%, with the continuous increase of the methanol energy ratio, although the maximum temperature in the combustion chamber will continue to rise, the inclination degree of the temperature curve in the descending section of the cylinder gradually increases, that is, the high temperature residence time of the fuel in the combustion chamber is gradually shortened, resulting in the decrease of NOx emissions, and with the increase of the methanol energy ratio, the NOx emissions gradually decrease.The distribution of NOx emissions in the cylinder at the time of exhaust valve opening under different methanol occupancy ratios is shown in Figure 8. Corresponding to the temperature field distribution in the cylinder, the maximum temperature in the cylinder is mainly concentrated in the center of the piston, so the NOx in the cylinder is mainly concentrated in the center area of the combustion chamber.It can be seen from the temperature curve in the cylinder that when the methanol energy ratio is 40%, the highest temperature in the cylinder is the highest, so the NOx emissions of the engine at this time are the highest, with the increase of methanol injection, because methanol is an oxygenated fuel, it will increase the oxygen content in the cylinder after entering the combustion chamber, so with the increase of methanol energy ratio, NOx emissions gradually decrease.

Effect of methanol injection time on combustion characteristics Figure 9. Effect of Methanol Injection Timing on In-Cylinder Pressure
From the analysis it can be seen that considering the combustion and emission characteristics of the engine under different methanol energy ratios, it is found that the working condition with a methanol energy ratio of 50% is the optimal working condition of the engine.Therefore, the simulation is set to 2000r/min, 50% load condition, methanol energy ratio is set to 50%, methanol injection pressure is 15MPa, and methanol injection time is set to 130°CABTDC, 100°CABTDC, 70°CABTDC, 40°CABTDC, 10°CABTDC, respectively.The pilot diesel is set to a single injection, the injection time is 11°CA BTDC, and the injection pressure is set to 1 Combustion pressure and pressure rise rate in the cylinder.The trend of pressure change in cylinder at different methanol injection times is shown in Figure 9.It can be seen from the figure that when the alcohol injection time is pushed back from 130°CABTDC to 40°CA BTDC, the generation trend of combustion pressure in the cylinder does not change significantly.The maximum combustion pressure in the cylinder is the highest when the injection time is set to 40°CA BTDC: when the methanol injection time is set to 0°CABTDC, the maximum combustion pressure in the cylinder is the lowest.Different methanol injection times cause methanol to form a mixture in the combustion chamber with a change in concentration, methanol is injected at 130°CA BTDC time, and methanol can form a more homogeneous methanol combustible mixture when there is enough in the combustion chamber: methanol will be in the combustion chamber as the methanol injection time is delayed 03.2MPa.

Conclusion
Based on the existing theory of reactivity controlled compression ignition combustion mode, uses a detailed chemical reaction mechanism to numerically simulate the combustion process of methanol/diesel RCCI engine, and explores the influence of changing parameters such as methanol injection ratio, injection time of methanol and pilot diesel, and fuel injection strategy of pilot diesel on the combustion and emission characteristics of methanol/diesel RCCI engine, and obtains the following conclusions: Compared to conventional diesel engines, methanol/diesel RCCI engines have significantly higher incylinder combustion pressure, pressure increase rate, and average in-cylinder temperature.With the increase of the methanol energy ratio, the combustion phase gradually advances, the peak pressure increases rate and the maximum temperature in the cylinder gradually increase, and the indicated thermal efficiency of the engine reaches the highest value when the methanol energy ratio is 30%.The addition of methanol will lead to a significant increase in HC, SOOT and CO emissions of the engine, but with the increase of methanol energy ratio, HC, SOOT and CO emissions are gradually reduced, NOx emissions first increase and then decrease, and the trad-of relationship between NOx and SOOT emissions is also weakened.

Figure 2 .
Figure 2. Comparative Methods of Low Reactivity Fuel (LRF) Delivery in the Reactivity-Controlled Compression Ignition (RCCI) Engine: (a) Conventional Premixed Port Injection, (b) Dual Direct Injection, and (c) Proposed Injection at the Valve

Figure
Figure 4. Workbench Layout

Figure
Figure 6.Effect of Methanol Energy Ratio on In-Cylinder Pressure Rise Rate

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Figure 7. Trend of In-Cylinder NOx Content and NOx Emission Varied with Methanol Energy Ratio