Nitrogen Oxides and Particulate Matter from Marine Diesel Oil (MDO), Emulsiﬁed MDO, and Dimethyl Ether Fuels in Auxiliary Marine Engines

: Exhaust gases from ships and automobiles have a signiﬁcant impact on people and the environment. As a result, diesel engines used in land and marine vehicles are gradually being restricted, and low-carbon engines are under development. This study considers marine diesel oil (MDO) that is used in ships to meet the emission regulations required by the International Maritime Organization. This investigation explores the method and application technology for the reduction of nitrogen oxides (NO x ) and particulate matter using emulsiﬁed fuel and mass-produced dimethyl ether (DME) fuel, which are analyzed. When comparing emulsiﬁed fuel and DME fuel to MDO, which is a ship oil, NO x are reduced by 20–45% and the particulate matter is reduced by 60–97%. When emulsiﬁed fuel containing moisture is used, the combustion chamber temperature is lowered due to the optimal expansion by moisture contained in the fuel. The particulate matter is also reduced. When DME fuel is used, it reduces the particulate matter by more than 97% in comparison with the existing MDO fuel and the emulsiﬁed fuel. The conditions are believed to be suitable for combustion and they can be satisﬁed by supplying oxygen during post-combustion.


Introduction
Exhaust gases from ships and automobiles have a significant impact on people and the environment. As a result, diesel engines used in land and marine vehicles are gradually being restricted and low-carbon engines are currently being developed. In accordance with the regulations of the International Maritime Organization (IMO) [1], the emission control area is determined in accordance with international conventions for the prevention of pollution from ships. Emissions must be managed in emission control areas. Due to the limitations of exhaust gases, especially nitrogen oxides (NO x ) emissions, internal combustion engines are being applied to reduce NO x and particulate matter (PM) in exhaust gases using the 'optimal emulsifying fuel composition'. Therefore, it should be noted that estimation and forecasting of diesel engine emissions is very important in this context [2]. Moreover, predicting emissions from diesel engines in real time is not an easy task. With the development of powerful, accurate, and fast predictive algorithms, diesel engine exhaust can be controlled in real time.
Meanwhile, studies on NO x reduction using emulsified fuels have been actively conducted. Lim et al. [3,4] showed a deterioration in the fuel efficiency at low loads when the test diesel engine was operated by dispersing water in light oil with a mixer using ultrasonic and shear force without using additives, but at high loads. Favourable operating conditions, stabilization of fuel economy, and a high NO x reduction rate were confirmed. However, by mixing water, light oil, and additives, and applying an emulsified fuel in oil-in-water, the fuel efficiency is improved by reducing NO x , which dramatically reduces the particulate matter. For this reason, by mixing water particles in light oil, the combustion chamber temperature decreases due to micro-explosions caused by the phase change of water. In addition, the post combustion combusts rapidly due to atomization, thereby significantly reducing the particulate matter [5][6][7][8][9][10][11]. Specifically, heavy fuel oil (HFO), which is used as a ship fuel, has high density characteristics relative to on-shore fuels; hence, it is easy to emulsify, and has the feature of separating oil and water; thus, it is highly practical. Kawasaki Heavy Industries applied emulsion fuel to bulk carriers of 58,000 tons (MAN B&W 6S50MC-C, 8630 kW/116 rpm) and succeeded in continuous operations [12] for 2740 h. This suggests the possibility of using emulsion fuel for vessels. This emulsion fuel was proven to be effective in reducing the particulate matter, as well as NO x . Murayama et al. [6] confirmed the reduction effect of 48% using emulsion fuel. However, since the method of using the emulsion fuel has a reduction rate of NO x that does not meet the IMO regulation value, the disadvantage is that it is difficult to achieve the target value with this method alone.
For a long time, the search has been on to find a fuel for compression engines that is environmentally friendly and can be easily stored and transported. Dimethyl ether (DME, CH 3 -O-CH 3 ) [11] is an alternative fuel for diesel engines that has a low auto ignition temperature. It also has good evaporative performance when sprayed into the combustion chamber, and it has a higher cetane number (> 55) than diesel fuel. In particular, DME is an oxygen-containing fuel that consists of 34.8 wt.% oxygen. As there is no direct coupling between carbon and carbon in the fuel characteristics, almost no particulate matter is emitted in the diesel engine [12][13][14][15][16][17][18][19]. Due to these advantages, it is possible to apply a large amount of exhaust gas recirculation (EGR) without much difficulty to the engine. This can greatly reduce NO x and has many excellent characteristics as an alternative fuel for diesel engines [19][20][21][22][23][24].
Industrially mass-produced DME is a very promising alternative fuel for diesel engines [25][26][27]. DME has a simple molecular structure, C-O-C without a C-C bond. DME also has a very high cetane number (> 55), high oxygen content (34.8 mass%), and better atomization, combustion, and fuel economy than diesel [28,29]. DME is also known as a potentially very clean fuel that does not have much smoke, and can be burned without particulate matter emissions. In addition, a high EGR allows NO x emissions to be controlled to meet increasingly stringent regulatory standards [30] for application to diesel engines. These properties include the bulk modulus, low calorific values, and viscosity. The bulk modulus and viscosity values of DMEare much lower than diesel's bulk modulus and viscosity values. This leads to higher pressures, intense pressure fluctuations, and steam leaks in fuel delivery systems (e.g., high-pressure pumps, common rails, injectors). In order to solve this problem, it is not possible to use a fuel supply device for general diesel; thus, a fuel supply system dedicated to DME must be applied. [31,32]. A realistic alternative is biodiesel [33][34][35][36], which is an alternative oxygenated fuel for diesel engines. Biodiesel is gaining momentum due to its sustainability, good exhaust quality, and biodegradability [37][38][39][40]. In DME engines, a good lubricity and a high calorific biodiesel complement the properties of DME and eliminate the use of lubricating additives by mixing biodiesel.
Developing new combustion techniques to improve performance and reduce the emissions is a difficult and expensive option. Some notable works on homogeneous charge compression ignition as a potential combustion technology for DME combustion have shown promising results with a simultaneous reduction of NO x and particulate matter emissions [29,[41][42][43]. Other possible methods for achieving complete combustion and less emissions include a controlled fuel injection strategy [44][45][46][47][48][49][50][51], fuel injector configuration, injection pressures, and employing combustion after treatment processes such as EGR, oxidation catalysts, and particle filters [52,53]. In this review paper, the effect of injection strategies, fuel additives, and exhaust gas after treatment techniques on emissions are thoroughly examined in order to suggest the optimum methods for reducing emissions. In the end, a summary is drawn to show the direction in which research on DME as an alternative fuel should be focused to achieve the goals of reduced emission and better combustion.
Finally, comparative values of NO x emissions from DME compression ignition engines and those from diesel fuel vary depending on the engine conditions and the fuel supply system. Some studies found that NO x emissions are lower, while other studies have reported the opposite [54,55]. To lower NO x emissions, some researchers reported that DME combustion results in lower NO x emissions [56][57][58] than diesel combustion. The reasons for this include a lower heating value, the higher heat of vaporization, a shorter ignition delay, a reduced amount of fuel that is injected during the ignition delay period, and a decreased amount of fuel burned during the premixed burning phase. As a result, this can lead to a lower peak combustion temperature. In regard to higher NO x emissions, it is possible that a higher amount of NO x can be produced from DME than from diesel fuel for an early injection start. This is because the duration of the peak temperature would be longer in the initial combustion period due to the shorter ignition delay of DME. When injection retardation is optimized for each fuel, NO x from DME is lower than that of diesel fuel [57].
In this study, the experimental data and numerical analysis results were compared and analyzed using the existing marine diesel engine's experimental data and AVL BOOST (www.avl.com/boost). The numerical analysis was conducted on the characteristics of NO x and particulate matter reduction in marine engines using emulsified fuel and DME fuel according to the content of fuel and water used in the marine engine. In general, a study was conducted on the exhaust characteristics of oil-in-water emulsion fuel through marine fuel, moisture, and additives. In addition, combustion and exhaust characteristics based on the excess air ratio according to the compression ratio change of the turbocharger were analyzed using emulsified fuel with a moisture content of 16%. In addition, the characteristics of NO x and particulate matter emission from DME fuel and emulsion fuel similar to diesel fuel were compared and analyzed. This study analyzed the combustion and exhaust characteristics based on the hole diameter and the injection timing of the nozzle of a general marine diesel engine. This research was focused on combustion and exhaust reduction based on the optimal hole diameter and the injection timing.

Experimental Method
Regarding the engine used in this study, a 600-kW-class generator engine was constructed, as shown in Figure 1. The experimental apparatus includes Encoder, which can measure the number of revolutions of the engine on the crankshaft, and a pressure sensor (model 6056 A, Kistler, Winterthur, Switzerland) on cylinder 1 to measure the pressure in the combustion chamber, and measures the combustion pressure. In addition, to measure the flow rate of the incoming fuel, a flow meter, a load regulator, and a system capable of measuring NO x and particulate matter were installed at the outlet of the exhaust outlet. Tables 1-3 show the engine specifications, exhaust gas measurement devices, and experimental conditions used in this study, respectively. Table 4 shows the experimental conditions of the study. The key properties of emulsified marine diesel oil (MDO), DME, and marine diesel fuel are shown in Table 4 [8,57,59]. In addition to the advantages above, it has a low carbon-to-hydrogen ratio (C:H), with the chemical formula CH 3 -O-CH 3 .

Numerical Analysis Method
The software adopted for the simulation was AVL BOOST©, version 2019.1(AVL List GmbH, Graz, Austria) which provides a graphical user interface (GUI) with icons representing the components of the internal combustion engine (ICE). For the engine in Figure 1, a simulation model was constructed using icons as shown in Figure 2a, and once all necessary data was collected, the model was built in AVL BOOST™ software [60] and turbocharger (TC1) model was controlled on the air flow rate of the turbocharger, as shown in Figure 2b. Figure 2c shows the combustion chamber pressure and heat generation rate characteristics for the experimental and numerical results ( Figure 2a). First, the experimental results and numerical values using MDO show similar results. Through the test results and numerical analysis results, the combustion and heat generation rates according to water content were compared after verification. The numerical results in Figure 2c show that the maximum combustion pressure increases with increasing water content. Furthermore, based on the heat generation rate characteristics, the numerical results show that the combustion pressure decreases as the water content increases, and combustion is actively performed [61].    In the simulation model, initial and boundary conditions were established by modelling cylinders, turbochargers, valves or heat exchangers, and engine components. The modelled engine configuration considered reference cylinder 1 (C1), the main engine characteristics for the spatial distribution of the cylinders, namely, the explosion sequence C1-C2-C4-C6-C5-C3 and the firing angle of each cylinder. The model's C1 (AVL BOOST™) is associated with Element 1 Engine 1 (E1) and defines the engine type, operating speed, moment of inertia, and brake average effective pressure (BMEP) used. The combustion method adopts an experimental mixed control combustion (MCC) AVL combustion model that predicts the amount of heat released (ROHR) and NO x emissions based on the amount of fuel in the cylinder and turbulent kinetic energy from injection.
AVL combustion model that predicts the amount of heat released (ROHR) and NOx emissions based on the amount of fuel in the cylinder and turbulent kinetic energy from injection.   (Test: Experimental data, Base: Numerical data, P: combustion pressure, R: heat release, WE10: water emulsion including 10% of moisture concentration) Figure 3 displays the results of the combustion pressure characteristics of the marine engine according to the injection timing change using MDO, emulsified MDO (EMDO), and DME fuels. Compared with the combustion pressure of MDO, the pressure ratio of EMDO and the turbocharger containing 16% water increases, and the combustion pressure increases when DME fuel is used. As the moisture content of the water increases, it is believed that the moisture contained in the EMDO fuel increases the combustion pressure due to volume expansion because of the water's phase change. For the case of DME fuel, the injection timing shows similar combustion characteristics for 2.0 CA BTDC; however, the combustion pressure tends to increase significantly as the injection timing advances.

Results and Investigation
3.1. Combustion Characteristics in Accordance with MDO, EMDO, and DME Fuels Figure 3 displays the results of the combustion pressure characteristics of the marine engine according to the injection timing change using MDO, emulsified MDO (EMDO), and DME fuels. Compared with the combustion pressure of MDO, the pressure ratio of EMDO and the turbocharger containing 16% water increases, and the combustion pressure increases when DME fuel is used. As the moisture content of the water increases, it is believed that the moisture contained in the EMDO fuel increases the combustion pressure due to volume expansion because of the water's phase change. For the case of DME fuel, the injection timing shows similar combustion characteristics for 2.0 CA BTDC; however, the combustion pressure tends to increase significantly as the injection timing advances. Figure 4 illustrates the ship engine's rate of heat release according to the injection timing change using MDO, EMDO, and DME fuels. After considering the heat generation rate results while using MDO fuel, there is a rapid heat generation rate and a longer post-combustion property compared with EMDO and DME fuels, which causes particulate matter generation. In addition, NO x generation and post-combustion increase as the combustion chamber temperature increases due to rapid combustion. When EMDO contains moisture, it somewhat slows the combustion characteristics and shortens the post-combustion characteristics compared with the basic MDO. For this reason, NO x and particulate matter are expected to decrease. In addition, DME fuel contains 30% or more oxygen. This shows the heat generation rate at the same injection time in comparison with the existing MDO. In addition, it shows a very gentle heat generation rate and the post combustion is rapidly shortened. Therefore, it is believed that NO x and particulate matter will be reduced very rapidly. Based on the characteristics of combustion pressure and the heat generation rate using various fuels, it is possible to analyze the emissions of NO x and particulate matter.  Figure 3. Combustion pressure history of the cylinder in accordance with MDO, EMDO, and DME fuels for 100% engine loads. Figure 4 illustrates the ship engine's rate of heat release according to the injection timing change using MDO, EMDO, and DME fuels. After considering the heat generation rate results while using MDO fuel, there is a rapid heat generation rate and a longer post-combustion property compared with EMDO and DME fuels, which causes particulate matter generation. In addition, NOx generation   Figure 5 depicts the combustion characteristics according to the location where the highest combustion pressure occurs for the various fuels, injection timing, and turbocharger pressure ratio. Using the results obtained from Figures 3 and 4, the characteristics of combustion were analyzed through the generation characteristics of the highest combustion pressure. First, when comparing the characteristic curve of the highest combustion pressure obtained using MDO fuel, most of the EMDO and DME fuels show the location of the highest combustion pressure that is perceived. This causes the tendency of the highest combustion pressure to advance through the rapid combustion of MDO fuel. As mentioned earlier, it is expected that the NOx will increase due to the increase in the combustion chamber temperature due to rapid combustion. In the case of DME fuel, the position of the highest combustion pressure is perceived in the low load region and the high load region in comparison to the MDO fuel. When the injection timing is 12.5CA BTDC, the location of the highest combustion pressure is perceived. This phenomenon is considered to occur due to the small volume elastic modulus and a longer fuel injector period because the density of fuel is less than 30% compared to the MDO through the composition of the fuel mentioned in Table 3. When the pressure ratio of the EMDO and the turbocharger is changed, the highest combustion pressure is perceived as the moisture content in the fuel increases and the pressure ratio of the turbocharger increases. These phenomena also show that the combustion chamber temperature is lowered due to atomization of the fuel and micro-explosions during the phase change due to moisture. In addition, post-combustion is rapidly exhibited due to atomization. It is believed that NOx and particulate matter can be reduced at the same time through the control of the combustion state. As mentioned earlier, the fuel's DME,  Figure 5 depicts the combustion characteristics according to the location where the highest combustion pressure occurs for the various fuels, injection timing, and turbocharger pressure ratio. Using the results obtained from Figures 3 and 4, the characteristics of combustion were analyzed through the generation characteristics of the highest combustion pressure. First, when comparing the characteristic curve of the highest combustion pressure obtained using MDO fuel, most of the EMDO and DME fuels show the location of the highest combustion pressure that is perceived. This causes the tendency of the highest combustion pressure to advance through the rapid combustion of MDO fuel. As mentioned earlier, it is expected that the NO x will increase due to the increase in the combustion chamber temperature due to rapid combustion. In the case of DME fuel, the position of the highest combustion pressure is perceived in the low load region and the high load region in comparison to the MDO fuel. When the injection timing is 12.5CA BTDC, the location of the highest combustion pressure is perceived. This phenomenon is considered to occur due to the small volume elastic modulus and a longer fuel injector period because the density of fuel is less than 30% compared to the MDO through the composition of the fuel mentioned in Table 3. When the pressure ratio of the EMDO and the turbocharger is changed, the highest combustion pressure is perceived as the moisture content in the fuel increases and the pressure ratio of the turbocharger increases. These phenomena also show that the combustion chamber temperature is lowered due to atomization of the fuel and micro-explosions during the phase change due to moisture. In addition, post-combustion is rapidly exhibited due to atomization. It is believed that NO x and particulate matter can be reduced at the same time through the control of the combustion state. As mentioned earlier, the fuel's DME, which is an oxygen fuel, is a phenomenon in which the density and volumetric modulus of the fuel are small, since the position of the highest combustion pressure is perceived even though the injection timing is advanced. which is an oxygen fuel, is a phenomenon in which the density and volumetric modulus of the fuel are small, since the position of the highest combustion pressure is perceived even though the injection timing is advanced. Figure 5. Location of the peak combustion pressure in a cylinder chamber in accordance with MDO, EMDO, and DME fuels. Figure 6 shows the results of nitrogen tetrachloride for emulsified fuel according to the injection timing change, turbocharger compression ratio, and the moisture content for MDO, EMDO, and DME fuels. First, when considering the emulsifying fuel characteristics based on the moisture content, the overall NOx emissions are reduced in comparison to MDO fuel. The reason for this is that the moisture contained in the emulsified fuel is lowered in the combustion chamber due to the volume expansion and the latent heat effect of evaporation due to the phase change according to the temperature. It is hypothesized that the combustion was active due to the promotion of atomization due to the microexplosion of water and fuel. In the case of emulsified fuel with a moisture content of 16%, the pressure of the turbocharger was adjusted to increase the amount of intake air. Along with the microexplosion, sufficient air was injected to promote the combustion of emulsified fuel. This is considered to be a factor that can be controlled for NOx emission by controlling the compression ratio of the emulsified fuel and the turbocharger. In the case of DME, which contains oxygen, it emits more NOx than conventional MDO, especially at a low load. This can generate a large amount of NOx due to an increase in the pressure of the high combustion chamber. In addition, a plethora of NOx can be produced by increasing the temperature of the combustion chamber due to the injection timing. In Figure 5. Location of the peak combustion pressure in a cylinder chamber in accordance with MDO, EMDO, and DME fuels. Figure 6 shows the results of nitrogen tetrachloride for emulsified fuel according to the injection timing change, turbocharger compression ratio, and the moisture content for MDO, EMDO, and DME fuels. First, when considering the emulsifying fuel characteristics based on the moisture content, the overall NO x emissions are reduced in comparison to MDO fuel. The reason for this is that the moisture contained in the emulsified fuel is lowered in the combustion chamber due to the volume expansion and the latent heat effect of evaporation due to the phase change according to the temperature. It is hypothesized that the combustion was active due to the promotion of atomization due to the micro-explosion of water and fuel. In the case of emulsified fuel with a moisture content of 16%, the pressure of the turbocharger was adjusted to increase the amount of intake air. Along with the micro-explosion, sufficient air was injected to promote the combustion of emulsified fuel. This is considered to be a factor that can be controlled for NO x emission by controlling the compression ratio of the emulsified fuel and the turbocharger. In the case of DME, which contains oxygen, it emits more NO x than conventional MDO, especially at a low load. This can generate a large amount of NO x due to an increase in the pressure of the high combustion chamber. In addition, a plethora of NO x can be produced by increasing the temperature of the combustion chamber due to the injection timing.

NO x Reduction with EMDO, MDO, and DME Fuels
In order to eliminate this cause, under the same conditions as the injection timing of the MDO, NO x were reduced to a very low level compared to the combustion conditions of other emulsified fuels. It is believed that the cause of this is that fuel containing 30% oxygen and the density of the fuel are low; hence, rapid combustion is avoided. To reduce the amount of NO x , it is considered to be a very suitable fuel for reducing NO x and the particulate matter by adjusting the compression ratio of the emulsion fuel and the turbocharger. This can also be achieved by controlling the injection period by increasing the injection timing of the DME fuel and the effective cross-sectional area of the nozzle hole. order to eliminate this cause, under the same conditions as the injection timing of the MDO, NOx were reduced to a very low level compared to the combustion conditions of other emulsified fuels. It is believed that the cause of this is that fuel containing 30% oxygen and the density of the fuel are low; hence, rapid combustion is avoided. To reduce the amount of NOx, it is considered to be a very suitable fuel for reducing NOx and the particulate matter by adjusting the compression ratio of the emulsion fuel and the turbocharger. This can also be achieved by controlling the injection period by increasing the injection timing of the DME fuel and the effective cross-sectional area of the nozzle hole. Figure 6. NOx emissions according to MDO, EMDO, and DME fuels in terms of injection timing, turbocharger compression ratio, and moisture concentration. Figure 7 shows the results of reducing nitrogen tetrafluoride for emulsified fuel according to the injection timing change, the turbocharger compression ratio, and the moisture content for MDO, EMDO, and DME fuels. From these results, the NOx reduction characteristics of emulsified fuel and the emulsified fuel with a moisture content of 16% according to the moisture content show that the NOx are reduced by up to 20% in comparison to the MDO fuel. NOx are reduced due to the increase in the moisture content and the increase in the compression ratio of the turbocharger. This is due to the atomization of the fuel that is caused by micro-explosions in the fuel, which is attributed to the volume expansion of the moisture. In addition, the volume expansion is caused by the phase change of the moisture. However, in the case of DME fuel, the reduction of NOx can change rapidly with the injection timing. When the injection timing relies on the injection timing of the existing MDO fuel, Figure 6. NO x emissions according to MDO, EMDO, and DME fuels in terms of injection timing, turbocharger compression ratio, and moisture concentration. Figure 7 shows the results of reducing nitrogen tetrafluoride for emulsified fuel according to the injection timing change, the turbocharger compression ratio, and the moisture content for MDO, EMDO, and DME fuels. From these results, the NO x reduction characteristics of emulsified fuel and the emulsified fuel with a moisture content of 16% according to the moisture content show that the NO x are reduced by up to 20% in comparison to the MDO fuel. NO x are reduced due to the increase in the moisture content and the increase in the compression ratio of the turbocharger. This is due to the atomization of the fuel that is caused by micro-explosions in the fuel, which is attributed to the volume expansion of the moisture. In addition, the volume expansion is caused by the phase change of the moisture. However, in the case of DME fuel, the reduction of NO x can change rapidly with the injection timing. When the injection timing relies on the injection timing of the existing MDO fuel, there is a reduction of up to 40%. From these results, the result of adjusting the injection timing and increasing the area of the pore diameter of the fuel nozzle is displayed in Figure 8. As a result, if the nozzle hole diameter is 0.396 mm, the nozzle diameter is suitable for NO x reduction. It is considered reasonable to determine these properties by comparing their correlation with the particulate matter. there is a reduction of up to 40%. From these results, the result of adjusting the injection timing and increasing the area of the pore diameter of the fuel nozzle is displayed in Figure 8. As a result, if the nozzle hole diameter is 0.396 mm, the nozzle diameter is suitable for NOx reduction. It is considered reasonable to determine these properties by comparing their correlation with the particulate matter.  . NO x reduction rate according to the MDO, EMDO, and DME fuels for the conditions of injection timing, turbocharger compression ratio, and moisture concentration. Figure 9 presents the results of the emission characteristics of the emulsified fuels and the DME fuels dependent on the DME and the moisture content. Overall, the results show a decrease when using the existing MDO fuel, emulsified fuel, and DME fuel. This is because the production of particulate matter was suppressed due to the rapid combustion of post-combustion, which can be predicted from the heat generation rate curve. Figure 10 demonstrates the results of Figure 9 in terms of showing the reduction of particulate matter based on MDO fuel. When 16% of the moisture is contained and the compression ratio of the turbocharger is increased, the analysis reveals that the reduction rate of the particulate matter is reduced to 70%. As a result, the increase in the excess air ratio due to the increase in the compression ratio of the turbocharger affects the reduction of NO x ; however, the result is somewhat slower for the reduction of particulate matter. When DME fuel is used, it reduces the particulate matter by more than 97% by comparing the results of using the existing MDO fuel and the emulsified fuel. The reason for this is believed to be that the conditions, which are suitable for combustion, are satisfied by supplying the oxygen required for post-combustion.  Figure 9 presents the results of the emission characteristics of the emulsified fuels and the DME fuels dependent on the DME and the moisture content. Overall, the results show a decrease when using the existing MDO fuel, emulsified fuel, and DME fuel. This is because the production of particulate matter was suppressed due to the rapid combustion of post-combustion, which can be predicted from the heat generation rate curve. Figure 10 demonstrates the results of Figure 9 in terms of showing the reduction of particulate matter based on MDO fuel. When 16% of the moisture is contained and the compression ratio of the turbocharger is increased, the analysis reveals that the reduction rate of the particulate matter is reduced to 70%. As a result, the increase in the excess air ratio due to the increase in the compression ratio of the turbocharger affects the reduction of NOx; however, the result is somewhat slower for the reduction of particulate matter. When DME fuel is used, it reduces the particulate matter by more than 97% by comparing the results of using the existing MDO fuel and the emulsified fuel. The reason for this is believed to be that the conditions, which are suitable for combustion, are satisfied by supplying the oxygen required for postcombustion. Figure 11 illustrates the results of the reduction ratio of the particulate matter according to the ratio of the pore size of the nozzle and the injection timing using DME fuel. In comparison with the results that were obtained in Figure 8, as the pore diameter increases, the tendency for the particulate matter to increase is demonstrated. In addition, the trade-off relationship of the particulate matter reduction with respect to the injection timing is also perceived. When the pore diameter is 0.27 mm, it has no significant effect on the characteristics of the particulate matter discharge, depending on the injection timing. However, when the pore diameter is 0.396 mm, the emission of the particulate matter increases by more than 0.27 mm. As there is a particulate matter discharge based on the pore diameter and the injection timing, there is a trade-off relationship. Figure 8. NO x reduction rate according to injection timing and the ratio of the nozzle hole diameter with DME fuels. Figure 11 illustrates the results of the reduction ratio of the particulate matter according to the ratio of the pore size of the nozzle and the injection timing using DME fuel. In comparison with the results that were obtained in Figure 8, as the pore diameter increases, the tendency for the particulate matter to increase is demonstrated. In addition, the trade-off relationship of the particulate matter reduction with respect to the injection timing is also perceived. When the pore diameter is 0.27 mm, it has no significant effect on the characteristics of the particulate matter discharge, depending on the injection timing. However, when the pore diameter is 0.396 mm, the emission of the particulate matter increases by more than 0.27 mm. As there is a particulate matter discharge based on the pore diameter and the injection timing, there is a trade-off relationship. Figure 9. Particulate matter emissions according to MDO, EMDO, and DME fuels due to injection timing, turbocharger compression ratio, and moisture concentration.

Conclusion
In this study, NOx and particulate matter were simulated with MDO, EMDO, and DME within a 16% moisture concentration in accordance with the change in the turbocharger compression ratio and the DME fuels by the AVL BOOST simulation program. We first obtained the experimental data by using the MDO fuel with a 600-kW marine auxiliary engine and then analyzed three types of fuel specifications based on the combustion and emission characteristics of marine diesel fuels. This study revealed the following findings: 1) Based on the results of the heat generation rate in accordance with the change in the injection timing using MDO, EMDO, and DME fuels, the MDO fuel shows a rapid heat generation rate and a longer post-combustion property in comparison with EMDO and DME fuels. 2) For the case of emulsified fuel with a moisture content of 16%, the pressure of the turbocharger is adjusted to increase the amount of intake air. In addition to the micro-explosions, sufficient air is injected to promote the combustion of emulsified fuel. This is considered a factor that can be controlled for NOx emission by controlling the compression ratio of the emulsified fuel and the turbocharger. 3) By increasing the water content of the emulsified fuel, NOx are reduced. The NOx are reduced due to the lower combustion temperature caused by the latent heat of evaporation from the phase change of water in the fuel. 4) By increasing the water content of the emulsified fuel, the particulate matter has a reduction efficiency of up to 60% or more. When DME fuel is used, it reduces the particulate matter by Figure 11. Particulate matter reduction rate according to injection timing and the ratio of the nozzle hole diameter with DME fuels.

Conclusions
In this study, NO x and particulate matter were simulated with MDO, EMDO, and DME within a 16% moisture concentration in accordance with the change in the turbocharger compression ratio and the DME fuels by the AVL BOOST simulation program. We first obtained the experimental data by using the MDO fuel with a 600-kW marine auxiliary engine and then analyzed three types of fuel specifications based on the combustion and emission characteristics of marine diesel fuels. This study revealed the following findings: (1) Based on the results of the heat generation rate in accordance with the change in the injection timing using MDO, EMDO, and DME fuels, the MDO fuel shows a rapid heat generation rate and a longer post-combustion property in comparison with EMDO and DME fuels. (2) For the case of emulsified fuel with a moisture content of 16%, the pressure of the turbocharger is adjusted to increase the amount of intake air. In addition to the micro-explosions, sufficient air is injected to promote the combustion of emulsified fuel. This is considered a factor that can be controlled for NO x emission by controlling the compression ratio of the emulsified fuel and the turbocharger. (3) By increasing the water content of the emulsified fuel, NO x are reduced. The NO x are reduced due to the lower combustion temperature caused by the latent heat of evaporation from the phase change of water in the fuel.
(4) By increasing the water content of the emulsified fuel, the particulate matter has a reduction efficiency of up to 60% or more. When DME fuel is used, it reduces the particulate matter by more than 97% when comparing the results of using the existing MDO fuel and the emulsified fuel. The conditions that are suitable for combustion are believed to be satisfied because of the supplied oxygen that is required for post-combustion.