Comparison of NOx and Smoke Characteristics of Water-in-Oil Emulsion and Marine Diesel Oil in 400-kW Marine Generator Engine

Currently, the exhaust gas of a ship is regulated for nitrogen oxides and sulphur compounds; however, there is no IMO regulation on smoke under discussion. This study investigated the reduction of exhaust gas through ship emulsion fuel, which can simultaneously reduce nitrogen oxides and smoke in ship engines before smoke regulations are established. The combustion and exhaust characteristics were investigated according to the moisture content of emulsion fuel using a 400-kW generator engine. As the water content of the emulsion and the temperature of the combustion chamber increase, micro explosion increases and the combustion period decreases. The nitrogen oxide and smoke from the emulsion fuel used in this study decreased by 7% and 75%, respectively. The nitrogen oxides and soot reductions obtained by the use of emulsion fuel were boosted by micro-explosion of water contained in the fuel during combustion.


Introduction
Large amounts of fuels are consumed in off-road or road vehicles, shipping, agriculture, forestry, and industrial generators. In such cases, diesel engines are preferred over gasoline engines because of their low fuel consumption, high output, rigid structure, and high brake thermal efficiency (BTE) [1]. As such, diesel engines are an important power source for automobiles. However, they are associated with the emission of pollutants, such as carbon monoxide, hydrocarbons, nitrogen oxides, and smoke [2]. Currently, the negative effects of such emissions on the human body such as pneumonia, breast and lung cancer will continue unless technologies for emission reduction are developed [3,4].
Lin et al. [5] investigated diesel engine performance and emission characteristics with oil-in-water emulsion fuel and emulsified fuel containing diglyme (a combination enhancer). They found that the CO level of water/diesel (W/D) emulsified fuel increased because of incomplete combustion caused by steam from water present in the emulsion fuel, but it can be reduced by adding diglyme, which assists combustion. The presence of moisture reduces NOx emissions. Lin et al. [6] investigated diesel engine performance using two-phase and three-phase emulsion fuels. They used moisture contents of 10% and 20% in preparing the emulsified fuel and found that emulsified fuel increased the CO level, but decreased NOx emissions by 56.82% compared to diesel fuel. Subramanian [7] found that reductions in NOx and smoke in water-diesel emulsion fuel were lower compared to those of the injection method. However, the carbon monoxide and hydrocarbon levels were higher with emulsified oil.
Lin et al. [8] investigated the effects of NOx and smoke emission characteristic on the emulsion oil type in water-in-oil and oil-in-water multiple emulsions. The specific fuel consumption, CO emissions As the ambient temperature increases, the duration of micro-explosion increases and the frequency of occurrence increases nearly three times [31]. The lower the atmospheric temperature, the lower the frequency of occurrence of micro explosions. It takes a long time for the droplets to be heated and burned, the evaporation of the droplets occurs in advance, and the size of the droplets is reduced. As the mixing ratio of water and the ambient temperature increase, the frequency of micro-explosions increases. Figure 1 shows a schematic diagram for studying the combustion and exhaust characteristics of three types of emulsion fuel and MDO in an actual ship according to the water content. The 4-stroke turbocharger engine used in this study is a 400-kW generator engine. As shown in Figure 1, the 4-stroke engine consists of a generator engine, a controller panel, a data acquisition system, an exhaust gas analyser, and a pressure sensor. The engine load was adjusted using a load cell in the ship. The pressure sensor was mounted on the No. 6 cylinder through a hole in the combustion chamber, and signals from the data acquisition device, the flow meter and the encoder were simultaneously acquired from the sensor (model 6056 A, Kistler, Winterthur, Switzerland). The resolution of the encoder used in this study is 1CA. The heat generation rate was calculated by applying a zero-dimensional combustion model and averaging combustion data of 100 cycles. 13% and 16% moisture. As the moisture content increased, the calorific value and sulphur content tended to decrease, whereas the density and flash point increased.

Experimental Apparatus
As the ambient temperature increases, the duration of micro-explosion increases and the frequency of occurrence increases nearly three times [31]. The lower the atmospheric temperature, the lower the frequency of occurrence of micro explosions. It takes a long time for the droplets to be heated and burned, the evaporation of the droplets occurs in advance, and the size of the droplets is reduced. As the mixing ratio of water and the ambient temperature increase, the frequency of microexplosions increases.  Figure 1 shows a schematic diagram for studying the combustion and exhaust characteristics of three types of emulsion fuel and MDO in an actual ship according to the water content. The 4-stroke turbocharger engine used in this study is a 400-kW generator engine. As shown in Figure 1, the 4stroke engine consists of a generator engine, a controller panel, a data acquisition system, an exhaust gas analyser, and a pressure sensor. The engine load was adjusted using a load cell in the ship. The pressure sensor was mounted on the No. 6 cylinder through a hole in the combustion chamber, and signals from the data acquisition device, the flow meter and the encoder were simultaneously acquired from the sensor (model 6056 A, Kistler, Winterthur, Switzerland). The resolution of the encoder used in this study is 1CA. The heat generation rate was calculated by applying a zerodimensional combustion model and averaging combustion data of 100 cycles.   The specifications of the engine used in this study, which consists of a six-combustion chamber, a mechanical pump and a four-stroke cycle diesel engine, are presented in Table 2.  Table 3 presents the specifications of the exhaust gas analyser used in this study. The SWG 300 model was used to measure nitrogen oxides exhaust gas, whereas a diesel opacimeter was used to measure smoke.

Experimental Conditions
In this study, the combustion and exhaust characteristics of three types of emulsion oil and MDO were investigated for loads of 100 kW, 200 kW and 300 kW, and moisture contents of emulsion fuel of 10%, 13% and 16%. The experimental conditions of this study are listed in Table 4.  Figure 2 shows the results of comparison of the cylinder pressure and heat release characteristics for a conventional marine diesel engine according to the ratio of emulsified fuels and engine loads. The cylinder pressure and the rate of heat release increased with increasing engine load. The result in Figure 2 shows the characteristics of a conventional marine diesel engine. The figure also shows results of analysis of the combustion and heat release characteristics according to the variation of emulsified MDO with water concentrations of 10%, 13%, and 16%.  Figure 3 shows a comparison of the combustion pressure and heat generation rate with increasing load using emulsion oil with 10% water content. As the water content in the emulsion oil increases, the combustion pressure tends to increase. This is because micro-explosion increases as the ambient temperature increases [32]. Results from this study show that micro-explosion is mostly caused by increase in the combustion pressure and temperature as the load increases.  Figure 3 shows a comparison of the combustion pressure and heat generation rate with increasing load using emulsion oil with 10% water content. As the water content in the emulsion oil increases, the combustion pressure tends to increase. This is because micro-explosion increases as the ambient temperature increases [32]. Results from this study show that micro-explosion is mostly caused by increase in the combustion pressure and temperature as the load increases.  Figure 3 shows a comparison of the combustion pressure and heat generation rate with increasing load using emulsion oil with 10% water content. As the water content in the emulsion oil increases, the combustion pressure tends to increase. This is because micro-explosion increases as the ambient temperature increases [32]. Results from this study show that micro-explosion is mostly caused by increase in the combustion pressure and temperature as the load increases.   Figure 4 shows the combustion pressure and heat generation rate according to the water content of the emulsion under the same load conditions. Overall, the use of emulsion oil with water content of 16% yields the highest combustion pressure. The combustion pressure generally increased due to increase in the moisture content. In particular, emulsion oil with 10% moisture content exhibits very poor heat generation characteristic compared to MDO. The reason for this is unclear; however, it is determined that the engine condition was not satisfied with the current experimental conditions. As shown in Figures 4a,b,c, the cylinder pressure was higher for emulsion fuel containing water  Figure 4 shows the combustion pressure and heat generation rate according to the water content of the emulsion under the same load conditions. Overall, the use of emulsion oil with water content of 16% yields the highest combustion pressure. The combustion pressure generally increased due to increase in the moisture content. In particular, emulsion oil with 10% moisture content exhibits very poor heat generation characteristic compared to MDO. The reason for this is unclear; however, it is determined that the engine condition was not satisfied with the current experimental conditions. As shown in Figure 4a-c, the cylinder pressure was higher for emulsion fuel containing water compared to MDO. This is possibly because the combustion of emulsified fuel containing 16% water is activated by micro-explosion. Furthermore, as the mixing ratio of water and the ambient temperature increase, the frequency of occurrence of micro-explosions increases.

Characteristics of Combustion and Heat Release Rate of Emulsion Fuel According to Moisture Content
Energies 2018, 11, x FOR PEER REVIEW 7 of 15 compared to MDO. This is possibly because the combustion of emulsified fuel containing 16% water is activated by micro-explosion. Furthermore, as the mixing ratio of water and the ambient temperature increase, the frequency of occurrence of micro-explosions increases.

Combustion Duration Characteristics of MDO and EMDO
A zero-dimensional model [33] with air as the working fluid was used to predict the temperature and the net heat release (NHR) inside the cylinder. The total cumulative NHR was obtained by integrating the derivative of the NHR over the relevant crank angle rotation. The start of combustion (SOC) time was defined as the time when 10% of the accumulated heat release occurred. The point of combustion end was defined as 90% of the cumulative heat release. The combustion duration (CD) was defined as the crank angle rotation period between the SOC and the end of combustion.

Combustion Duration Characteristics of MDO and EMDO
A zero-dimensional model [33] with air as the working fluid was used to predict the temperature and the net heat release (NHR) inside the cylinder. The total cumulative NHR was obtained by integrating the derivative of the NHR over the relevant crank angle rotation. The start of combustion (SOC) time was defined as the time when 10% of the accumulated heat release occurred. The point of combustion end was defined as 90% of the cumulative heat release. The combustion duration (CD) was defined as the crank angle rotation period between the SOC and the end of combustion. Figure 5 shows the definitions of SOC and CD. In this study, SOC was used as the combustion timing indicator. Other parameters commonly used to quantify and characterize combustion onset are the maximum pressure (Pmax) timing and maximum pressure rate (dP/d Energies 2018, 11, x FOR PEER REVIEW 9 of 15 Figure 5 shows the definitions of SOC and CD. In this study, SOC was used as the combustion timing indicator. Other parameters commonly used to quantify and characterize combustion onset are the maximum pressure (Pmax) timing and maximum pressure rate (dP/d Ѳ ) max timing. All the pressure related parameters, including SOC, CD, Pmax, and (dP/dѲ) max, were calculated for 100 individual cycles at each point and then averaged. The heat release calculations indicate a strong correlation between calculated and actual pressure trace parameters. Figure 6 shows the results of analysis of the combustion duration according to varying engine loads and moisture content of Emulsified Marine Diesel Oil (EMDO). The combustion period of commercial EMDO fuel was shorter compared to MDO. It is considered that moisture contained in the emulsion oil, as well as minute explosion during combustion boosted combustion. However, as the water content of EMDO increased, the combustion period tended to increase. The combustion period of 10% EMDO was shorter than those of 13% EMDO and 16% EMDO.  Figure 5 shows the definitions of SOC and CD. In this study, SOC was used as the combustion timing indicator. Other parameters commonly used to quantify and characterize combustion onset are the maximum pressure (Pmax) timing and maximum pressure rate (dP/d Ѳ ) max timing. All the pressure related parameters, including SOC, CD, Pmax, and (dP/dѲ) max, were calculated for 100 individual cycles at each point and then averaged. The heat release calculations indicate a strong correlation between calculated and actual pressure trace parameters. Figure 6 shows the results of analysis of the combustion duration according to varying engine loads and moisture content of Emulsified Marine Diesel Oil (EMDO). The combustion period of commercial EMDO fuel was shorter compared to MDO. It is considered that moisture contained in the emulsion oil, as well as minute explosion during combustion boosted combustion. However, as the water content of EMDO increased, the combustion period tended to increase. The combustion period of 10% EMDO was shorter than those of 13% EMDO and 16% EMDO. ) max, were calculated for 100 individual cycles at each point and then averaged. The heat release calculations indicate a strong correlation between calculated and actual pressure trace parameters. Figure 6 shows the results of analysis of the combustion duration according to varying engine loads and moisture content of Emulsified Marine Diesel Oil (EMDO). The combustion period of commercial EMDO fuel was shorter compared to MDO. It is considered that moisture contained in the emulsion oil, as well as minute explosion during combustion boosted combustion. However, as the water content of EMDO increased, the combustion period tended to increase. The combustion period of 10% EMDO was shorter than those of 13% EMDO and 16% EMDO. and the net heat release (NHR) inside the cylinder. The total cumulative NHR was obtained by integrating the derivative of the NHR over the relevant crank angle rotation. The start of combustion (SOC) time was defined as the time when 10% of the accumulated heat release occurred. The point of combustion end was defined as 90% of the cumulative heat release. The combustion duration (CD) was defined as the crank angle rotation period between the SOC and the end of combustion.   Figure 5 shows the definitions of SOC and CD. In this study, SOC was used as the combustion timing indicator. Other parameters commonly used to quantify and characterize combustion onset are the maximum pressure (Pmax) timing and maximum pressure rate (dP/dѲ) max timing. All the pressure related parameters, including SOC, CD, Pmax, and (dP/dѲ) max, were calculated for 100 individual cycles at each point and then averaged. The heat release calculations indicate a strong correlation between calculated and actual pressure trace parameters. Figure 6 shows the results of analysis of the combustion duration according to varying engine loads and moisture content of Emulsified Marine Diesel Oil (EMDO). The combustion period of commercial EMDO fuel was shorter compared to MDO. It is considered that moisture contained in the emulsion oil, as well as minute explosion during combustion boosted combustion. However, as the water content of EMDO increased, the combustion period tended to increase. The combustion period of 10% EMDO was shorter than those of 13% EMDO and 16% EMDO.
The combustion duration of 10% EMDO yielded reduction ratios of 8.3%, 8.5% and 6.3%, that of 13% EMDO yielded 5.5%, 7.1% and 4.4%, and that of 16% EMDO yielded 2.7%, 5.7%, and 3.1% with loads compared to 10%, 13%, and 16% water concentration MDO. Overall, the characteristic of emulsion oil is such that that the combustion period is shortened by 2-8% compared to that of MDO. This is because as the atmospheric temperature increases, micro explosion increases. As a result, the combustion pressure and temperature increase as the load increases. It can be observed that it occurs actively micro explosion phenomena.   Figure 8 shows the fuel consumptions with and without water; Figure 8a shows the fuel consumption with water, whereas Figure 8b shows the fuel consumption without moisture content. The results show that to obtain the same output, the fuel consumption rate of the emulsified fuel was increased, depending on the water content. The low calorific value of the fuel presented in Table 1 indicates that the fuel consumption rate will increase. The combustion duration of 10% EMDO yielded reduction ratios of 8.3%, 8.5% and 6.3%, that of 13% EMDO yielded 5.5%, 7.1% and 4.4%, and that of 16% EMDO yielded 2.7%, 5.7%, and 3.1% with loads compared to 10%, 13%, and 16% water concentration MDO. Overall, the characteristic of emulsion oil is such that that the combustion period is shortened by 2-8% compared to that of MDO. This is because as the atmospheric temperature increases, micro explosion increases. As a result, the combustion pressure and temperature increase as the load increases. It can be observed that it occurs actively micro explosion phenomena. Figure 8 shows the fuel consumptions with and without water; Figure 8a shows the fuel consumption with water, whereas Figure 8b shows the fuel consumption without moisture content. The results show that to obtain the same output, the fuel consumption rate of the emulsified fuel was increased, depending on the water content. The low calorific value of the fuel presented in Table 1 indicates that the fuel consumption rate will increase.  Figure 8 shows the fuel consumptions with and without water; Figure 8a shows the fuel consumption with water, whereas Figure 8b shows the fuel consumption without moisture content. The results show that to obtain the same output, the fuel consumption rate of the emulsified fuel was increased, depending on the water content. The low calorific value of the fuel presented in Table 1 indicates that the fuel consumption rate will increase.  Figure 9 shows the fuel consumption rate characteristics according to the composition of the load and emulsified fuel. The specific fuel consumptions of 10% EMDO decreased by 13.5%, 11.5%, and 5.7%, those of 13% EMDO by 5.1%, 7.3%, and −2.1%, and those of 16% EMDO by 6.5%, 7.4%, and 0% with loads. However, at 75% engine load, the fuel consumption rate of 10% EMDO decreased, whereas the fuel consumption rates of 13% EMDO and 16% EMDO increased.  Figure 10 shows the results of nitrogen oxides reduction of MDO and EMDO, which is emulsion oil with moisture contents of 10%, 13%, and 16%. Nitrogen oxides decreased at 50% and 75% load conditions, but not at 25% load condition. Emulsion oil with 16% moisture content yielded the highest nitrogen oxides reduction. It is considered that this is due to the increase in micro explosion and  Figure 9 shows the fuel consumption rate characteristics according to the composition of the load and emulsified fuel. The specific fuel consumptions of 10% EMDO decreased by 13.5%, 11.5%, and 5.7%, those of 13% EMDO by 5.1%, 7.3%, and −2.1%, and those of 16% EMDO by 6.5%, 7.4%, and 0% with loads. However, at 75% engine load, the fuel consumption rate of 10% EMDO decreased, whereas the fuel consumption rates of 13% EMDO and 16% EMDO increased.  Figure 9 shows the fuel consumption rate characteristics according to the composition of the load and emulsified fuel. The specific fuel consumptions of 10% EMDO decreased by 13.5%, 11.5%, and 5.7%, those of 13% EMDO by 5.1%, 7.3%, and −2.1%, and those of 16% EMDO by 6.5%, 7.4%, and 0% with loads. However, at 75% engine load, the fuel consumption rate of 10% EMDO decreased, whereas the fuel consumption rates of 13% EMDO and 16% EMDO increased.  Figure 10 shows the results of nitrogen oxides reduction of MDO and EMDO, which is emulsion oil with moisture contents of 10%, 13%, and 16%. Nitrogen oxides decreased at 50% and 75% load conditions, but not at 25% load condition. Emulsion oil with 16% moisture content yielded the highest nitrogen oxides reduction. It is considered that this is due to the increase in micro explosion and  Figure 10 shows the results of nitrogen oxides reduction of MDO and EMDO, which is emulsion oil with moisture contents of 10%, 13%, and 16%. Nitrogen oxides decreased at 50% and 75% load conditions, but not at 25% load condition. Emulsion oil with 16% moisture content yielded the highest nitrogen oxides reduction. It is considered that this is due to the increase in micro explosion and decrease in the combustion chamber temperature; as the ambient temperature increases, the latent heat of evaporation in the combustion chamber increases due to evaporation of water. This can be attributed to the suppression of NOx generation owing to the shortened combustion period with decreased combustion temperature caused by evaporation of water, as well as improvement in combustion due to micro-explosion. The conversion of NOx reduction is given as follows:

Combustion and Exhaust Characteristics of MDO and Emulsified Fuel According to Moisture Content
As the water concentration of MDO and load increased, the reduction rate of NO x increased. Overall, the maximum NO x reduction was approximately 9%.
Energies 2018, 11, x FOR PEER REVIEW 12 of 15 decrease in the combustion chamber temperature; as the ambient temperature increases, the latent heat of evaporation in the combustion chamber increases due to evaporation of water. This can be attributed to the suppression of NOx generation owing to the shortened combustion period with decreased combustion temperature caused by evaporation of water, as well as improvement in combustion due to micro-explosion. The conversion of NOx reduction is given as follows: As the water concentration of MDO and load increased, the reduction rate of NOx increased. Overall, the maximum NOx reduction was approximately 9%.  Figure 11 shows the exhaust characteristics for smoke reduction according to the load and water concentration of MDO. In the entire load range, smoke emissions were lower with EMDO than with MDO. The conversion of black carbon reduction is given as follows: We observed that smoke exhibited a decreasing trend with increasing water concentration in MDO and gradually decreased as the loads increased. It is considered that the smoke decreased owing to shortening of the trailing edge due to micro explosion of water contained in emulsified fuel and decrease in combustion duration due to evaporation and increase in heat release.
The emulsified fuel used in this study was produced according to the moisture content. Such W/O type emulsified fuel is produced in the combustion chamber as a result of micro-explosion, thereby finely atomizing the fuel. This phenomenon reduces lead in smoke produced close to complete combustion, and also suppresses the formation of nitrogen oxides by lowering the temperature in the combustion chamber and eliminating the vaporization heat required by water in the combustion chamber. In addition, smoke and NOx emissions decreased as the water content of emulsified fuel increased. The decrease was found to be more significant with high load than with low load. The decrease in smoke and NOx emissions with increasing moisture content suggests  Figure 11 shows the exhaust characteristics for smoke reduction according to the load and water concentration of MDO. In the entire load range, smoke emissions were lower with EMDO than with MDO. The conversion of black carbon reduction is given as follows: We observed that smoke exhibited a decreasing trend with increasing water concentration in MDO and gradually decreased as the loads increased. It is considered that the smoke decreased owing to shortening of the trailing edge due to micro explosion of water contained in emulsified fuel and decrease in combustion duration due to evaporation and increase in heat release.
The emulsified fuel used in this study was produced according to the moisture content. Such W/O type emulsified fuel is produced in the combustion chamber as a result of micro-explosion, thereby finely atomizing the fuel. This phenomenon reduces lead in smoke produced close to complete combustion, and also suppresses the formation of nitrogen oxides by lowering the temperature in the combustion chamber and eliminating the vaporization heat required by water in the combustion chamber. In addition, smoke and NOx emissions decreased as the water content of emulsified fuel increased. The decrease was found to be more significant with high load than with low load. The decrease in smoke and NOx emissions with increasing moisture content suggests improvements in air and fuel mixing, increase in water vapor concentration, increase in water vapor due to decrease in the combustion temperature, and increase in the surface area of droplets due to micro-explosion.
Energies 2018, 11, x FOR PEER REVIEW 13 of 15 improvements in air and fuel mixing, increase in water vapor concentration, increase in water vapor due to decrease in the combustion temperature, and increase in the surface area of droplets due to micro-explosion.

Conclusions
In this study, a 400-kW marine diesel generator engine was investigated using marine MDO. In addition, the combustion and exhaust characteristics were investigated using diesel oil as oil-in-water emulsion. The following conclusions are drawn from this study: (1) EMDO exhibited higher cylinder pressure and shorter combustion duration than MDO. The combustion durations of 10% EMDO decreased by 8.3%, 8.5% and 6.3%, those of 13% EMDO by 5.5%, 7.1% and 4.4%, and those of 16% EMDO by 2.7%, 5.7% and 3.1% with loads compared to MDO with water concentrations of 10%, 13% and 16%. (2) The pure fuel consumptions of 10% EMDO decreased by 13.5%, 11.5%, and 5.7%, those of 13% EMDO by 5.1%, 7.3%, and -2.1%r, and those of 16% EMDO by 6.5%, 7.4%, and 0% with loads compared to MDO. In the case of 75% engine load, the fuel consumption rate of 10% EMDO decreased, whereas those of 13% and 16% EMDO increased. (3) The NOx emissions of EMDO fuel decreased by up to 8% at 75% load compared to MDO. The smoke reduction rate was 75% for 16% EMDO at 75% engine load.

Conclusions
In this study, a 400-kW marine diesel generator engine was investigated using marine MDO. In addition, the combustion and exhaust characteristics were investigated using diesel oil as oil-in-water emulsion. The following conclusions are drawn from this study: (1) EMDO exhibited higher cylinder pressure and shorter combustion duration than MDO.
The smoke reduction rate was 75% for 16% EMDO at 75% engine load. (4) As the water content of emulsified fuel increased, the nitrogen oxide emission and smoke exhaust decreased. The smoke density and emission also decreased with increase in the moisture content of MDO. The reduction of smoke with increasing water content could be attributed to the following factors: (1) reduction in the combustion temperature, (2) the mixture of the surrounding air and the fuel is boosted as the surface area of the droplet increases due to fine explosion of