Optimization of Marine Two-Stroke Diesel Engine Based on Air Intake Composition and Temperature Control

: The inﬂuence of gas intake temperature, composition and the volume concentration of each gas component on diesel engine combustion, emission and the output power was studied by building a calculation model of the B&W 6S35ME-B9 marine two-stroke low-speed diesel engine, followed by a comprehensive optimization exploration. The results showed that under 295 K and 18.5% O 2 of intake gas, the engine’s NOx emission is only 4.5 g/kWh and reduced to 58% from the normal air gas intake condition. Moreover, their power output is very similar. In addition, the effect of CO 2 or H 2 O added into the intake of the diesel engine on the performance of the diesel engine can be compensated by reducing the intake temperature. At the intake temperature of 295 K, the engine’s NOx emission with 20.58% O 2 , 77.42% N 2 and 2% H 2 O is 8.62 g/kWh, and 9.06 g/kWh under 20.79% O 2 , 78.21% N 2 and 2% CO 2 . It is lower than 11.77 g/kWh, which is under normal intake conditions (315 K, 21%O 2 and 79%N 2 ). The power output is also similar to the normal intake condition. Therefore, the comprehensive optimization of gas intake temperature, composition and concentration can effectively optimize the diesel engine’s performance in terms of combustion, emission and power output. The research results have an important reference value for the optimization of diesel engine performance. combustion pressure, and the power output of diesel engines have the same variation trend. Therefore, the emission parameters and work performance of diesel engines can be effectively changed by adjusting the oxygen concentration in the cylinder. Or vice versa, the development of air separation technology, the study of oxygen volume fraction in internal combustion engine intake is effectively promoted. combustion pressure, and


Introduction
The two-stroke marine low-speed diesel engine is widely used in ships because of its low fuel consumption, high efficiency, good economy and power output performance [1,2]. With the deterioration of the global environment and the rise of people's awareness of environmental protection, the emission of the diesel engine has attracted extensive attention, that of the marine diesel engine in particular [3,4]. The International Maritime Organization (IMO) and other maritime organizations have put forward more rigorous legal requirements regarding marine diesel engine emissions [5][6][7]. Therefore, the marine diesel engines' emission and power output performance have become the focus of ship owners' attention, and new technologies are urgently needed to comprehensively optimize diesel engines' performance.
In recent years, the development of new combustion technologies to reduce engine emissions has been very active. The combustion optimization technology of the internal combustion engine has become a hot topic in the industry at home and abroad [8,9]. The examples of such technology are as follows: low temperature intake [10], oxygen-enriched combustion [11], free-nitrogen combustion [12], nitrogen-enriched combustion [13], exhaust gas recirculation (EGR) [14], intake with humidification [15], etc. Studies have shown that the diesel engine's combustion efficiency improves by using low intake temperature to reduce the intake air gas density and increase charge coefficient [16,17]. Therefore, modern marine diesel engines mostly use inter-coolers to lower the temperature of engines' intake [18]. The increase of intake temperature can effectively increase the temperature of the end of compression, shorten the period of ignition delay and improve the combustion process, although these cause the reduction of the amount of fresh air in the cylinder [19]. It has been proven that nitrogen reduction to enrich the oxygen concentration of the engines' air intake can significantly enhance its combustion efficiency and power output [20,21]. However, its NOx emissions increase simultaneously. Studies revealed that exhaust gas recirculation, nitrogen-enriched, or humidification can effectively reduce NOx emissions at the expense of the diesel engine's economy [22,23]. As a result, there appears to be a trade-off relationship between diesel engines' economy and NOx emissions [24,25].
It has been revealed that the optimal no-smoke emission can be lowered by oxygenenriched and EGR intake at the following full-load conditions: 1600 rpm, 30-40% EGR rate and 21.5-22.5% oxygen concentration; 2200 rpm, 20-45% EGR rate or 22-24% oxygen concentration [26]. Minimum total emissions can be achieved at 22% O 2 , 2% CO 2 and 76% N 2 intake conditions [27]. Compared with normal air intake conditions, NOx-soot emissions can be lowered by intake air humidification, EGR and shortened fuel injection time, and the indicated specific fuel consumption can be minimized as well [28]. A study has shown NOx-soot emission can be effectively reduced by using EGR and optimizing intake air temperature [29]. At present, there are few studies on the optimization of the combination of intake air temperature, composition and concentration for marine diesel engines, and further systematic research needs to be conducted.
In this paper, based on the working process calculation model of the B&W 6S35ME-B9 marine two-stroke diesel engine, the power output and emission characteristics of diesel engine are studied by adjusting the intake temperature, intake composition (O 2 \N 2 \CO 2 \H 2 O), concentration, to find the way of reducing NOx emission with little effect on its power output, and without affecting the external work capacity. It provides an important reference value for the optimization of air pollution emission and combustion performance for marine diesel engines.

Calculation Model
The model has taken the pressurized inter-cooled and two-stroke marine diesel engine of the B&W 6S35ME-B9 as the research object. The diesel engine's NOx emission meets the requirement of IMO Tier II NOx emission which is 14.4 g/kWh. The engine's technical parameters are shown in Table 1, which has the input parameter values for the calculation model. The working process calculation model is established in GT-Power as shown in Figure 1. According to relevant studies, the DIJET combustion model suitable for direct injection diesel engines was selected to study the combustion and emission performance of the diesel engine [8].   The calculation model was checked and verified by taking the parameters of diesel engine speed, power, cylinder pressure, intake and exhaust temperature, intake and exhaust pressure and emissions at 100%, 75%, 50% and 25% load conditions. Figure 2 shows the comparison between the actual and simulated cylinder pressure curves of diesel engines under various working conditions, indicating that the calculated values are consistent with the experimental values. Key thermodynamic and emissions parameters under various working conditions are shown in Table 2. The diesel engine's emission values were collected by AVL i60 detection equipment. The cylinder pressure data were collected by Kistler pressure sensor, and then the average cylinder pressure of 100 cycles was used as the experimental value for this study. Other experimental data were collected by a special acquisition system. The deviations of pressure, temperature, NOx and CO2 emissions The calculation model was checked and verified by taking the parameters of diesel engine speed, power, cylinder pressure, intake and exhaust temperature, intake and exhaust pressure and emissions at 100%, 75%, 50% and 25% load conditions. Figure 2 shows the comparison between the actual and simulated cylinder pressure curves of diesel engines under various working conditions, indicating that the calculated values are consistent with the experimental values. Key thermodynamic and emissions parameters under various working conditions are shown in Table 2. The diesel engine's emission values were collected by AVL i60 detection equipment. The cylinder pressure data were collected by Kistler pressure sensor, and then the average cylinder pressure of 100 cycles was used as the experimental value for this study. Other experimental data were collected by a special acquisition system. The deviations of pressure, temperature, NOx and CO 2 emissions are all less than 5%. Therefore, this calculation model can be used to study the combustion and emission performance of this type of marine diesel engine.

Calculation, Results and Analysis
To facilitate the study, the connection between the hollow cooler in the original model and the scavenging air main pipe was disconnected. Furthermore, the intake temperature, intake gas type and the volume percentage of each gas component were directly modified and set as the boundary conditions of the intake of scavenging air main pipe. Based on the initial working conditions of 75% diesel engine load, the results are as follows: 129 rpm with 10.285 g fuel injection for one cylinder each time, scavenge pressure is 0.29 MPa, scavenge temperature is 315 K, and air intake (21% O 2 and 79% N 2 ). The following calculation studies were carried out. First, the influences of single factors on diesel engine power output, NOx, CO and soot were studied by changing the intake temperature or the volume percentage of N 2 in the intake. Second, a combined optimization of intake temperature and N 2 volume percentage was conducted to find the optimization method of diesel engine performance. Third, based on the aforementioned two aspects of research regarding the replacement of N 2 in the intake gas with H 2 O or CO 2 , studies of the influence of H 2 O or CO 2 on diesel engine performance were performed.

Influence of Intake Temperature
Taking the air of 21% O 2 percent with 295 K, 315 K and 335 K as the initial intake conditions, the influence of scavenging temperature on the diesel engine's power output and emissions performance was analyzed by comparing the calculated results. As shown in Figure 3, the increase of intake temperature effectively increases the temperature of cylinder gas in the diesel engine compression stage. However, the rate of the temperature rise in the cylinder after top dead center (TDC) remains almost unchanged. Moreover, the beginning time of fuel combustion and heat release does not change with the change of intake temperature, either. That is because the modern two-stroke marine diesel engine adopts a high compression ratio and fuel injection at high pressure. Furthermore, the gas temperature in the cylinder at the injection time after TDC is high enough to ensure fuel particle refinement and ignition combustion, so the difference in the intake gas temperature is not big enough to significantly affect the start time of the ignition combustion. The rise of the gas temperature in the cylinder is conducive to the propagation of the combustion flame and accelerates and improves the combustion process, thus leading to the increase of the maximum combustion temperature in the cylinder. The temperature of the gas in the cylinder is also significantly higher in the later period of combustion, which is consistent with the conclusion in reference [16]. However, Figure 4 shows that the compression pressure and the maximum combustion pressure decrease with the increase of intake temperature. This is because the rise of temperature leads to the reduction of gas in the cylinder. The gas compression pressure is lower under the same compression workload. The combustion efficiency lowered by the reduction of oxygen in the cylinder is more than the effect of high-temperature optimized combustion, so its maximum combustion pressure is lower. Its power output and NOx emission decrease with the increase of intake temperature, while its soot and CO emissions increase. Therefore, the marine diesel engine can optimize the scavenge temperature by adjusting the cooling effect of the air cooler according to its sailing area, to reduce the emission of NOx while not affecting its power output. The minimum intake temperature is limited by seawater temperature, and the maximum sweep temperature is limited by the performance of the diesel engine and supercharger. In addition, Figure 5 shows that, under the normal air conditions, the engine's NOx emission is 11.77 g/kWh with 315 K intake, and the NOx emission is 11.9 g/kWh at 295 K and 11.6 g/kWh at 335 K, indicating that the effect of reducing NOx emission simply by changing intake gas temperature is not good.
osphere 2022, 13, x FOR PEER REVIEW g/kWh at 295 K and 11.6 g/kWh at 335 K, indicating that the effect of r sion simply by changing intake gas temperature is not good.    At the intake temperature of 315 K, the volume fractions of oxyg the intake gas were adjusted. We analyzed the diesel engine's cylinde sure and emissions by the O2 percentage of the intake gas in the calcu 19%, 21% and 23%. The results are shown in Figures 6 and 7. Soot is 6.89 g/kWh, NOx is 5.67 g/kWh, the highest combustion pressure is output power is 2476 kW under the conditions of 19% O2 and 81% N2.

Influence of Oxygen Volume Fraction
At the intake temperature of 315 K, the volume fractions of oxygen and nitroge the intake gas were adjusted. We analyzed the diesel engine's cylinder combustion p sure and emissions by the O2 percentage of the intake gas in the calculation model s 19%, 21% and 23%. The results are shown in Figures 6 and 7. Soot is 0.63 g/kWh, C 6.89 g/kWh, NOx is 5.67 g/kWh, the highest combustion pressure is 15.2 MPa, and output power is 2476 kW under the conditions of 19% O2 and 81% N2. Soot is 0.14 g/k

Influence of Oxygen Volume Fraction
At the intake temperature of 315 K, the volume fractions of oxygen and nitrogen in the intake gas were adjusted. We analyzed the diesel engine's cylinder combustion pressure and emissions by the O 2 percentage of the intake gas in the calculation model set at 19%, 21% and 23%. The results are shown in Figures 6 and 7. Soot is 0.63 g/kWh, CO is 6.89 g/kWh, NOx is 5.67 g/kWh, the highest combustion pressure is 15.2 MPa, and the output power is 2476 kW under the conditions of 19% O 2 and 81% N 2 . Soot is 0.14 g/kWh, CO is 5.13 g/kWh, NOx is 11.77 g/kWh, the highest combustion pressure is 15.8 MPa, and the output power is 2510 kW under air (21% O 2 and 79% N 2 ) intake conditions. Under the conditions of 23% O 2 and 77% N 2 intake, the engine's soot is 0.08 g/kWh, CO is 3.84 g/kWh, NOx is 17.86 g/kWh, the highest combustion pressure is 16.3 MPa, and the output power is 2527 kW. Therefore, the in-cylinder combustion is improved with the increase of oxygen concentration, the highest combustion pressure, and power increase, whereas the engine's soot and CO emissions decrease. However, the improvement of the combustion process causes the increase of the combustion temperature in the cylinder, thus accelerating the NOx generation in the cylinder. It is consistent with the research conclusions in reference [11,20]. At 295 K and 335 K, the emission parameters, maximum combustion pressure, and the power output of diesel engines have the same variation trend. Therefore, the emission parameters and work performance of diesel engines can be effectively changed by adjusting the oxygen concentration in the cylinder. Or vice versa, the development of air separation technology, the study of oxygen volume fraction in internal combustion engine intake is effectively promoted.
concentration, the highest combustion pressure, and power increase, where soot and CO emissions decrease. However, the improvement of the comb causes the increase of the combustion temperature in the cylinder, thus ac NOx generation in the cylinder. It is consistent with the research conclusion [11,20]. At 295 K and 335 K, the emission parameters, maximum combustion the power output of diesel engines have the same variation trend. Therefore parameters and work performance of diesel engines can be effectively chan ing the oxygen concentration in the cylinder. Or vice versa, the developme ration technology, the study of oxygen volume fraction in internal combus take is effectively promoted.  Based on the research discussed above, the decrease of intake tempera crease of intake oxygen volume concentration can effectively improve t combustion process and the power output of diesel engines, though causin in NOx emissions. With the increase of intake temperature or the decrease gen concentration, the NOx emission of the diesel engine can be reduced, output is lower as well. The intake temperature and the intake oxygen co diesel engine power output and NOx emissions are not the same. We can formance of diesel engines through either the combination of high intake tem oxygen enrichment or low intake temperature and oxygen deficiency. soot and CO emissions decrease. However, the improvement of the comb causes the increase of the combustion temperature in the cylinder, thus a NOx generation in the cylinder. It is consistent with the research conclusio [11,20]. At 295 K and 335 K, the emission parameters, maximum combustion the power output of diesel engines have the same variation trend. Therefor parameters and work performance of diesel engines can be effectively chan ing the oxygen concentration in the cylinder. Or vice versa, the developme ration technology, the study of oxygen volume fraction in internal combus take is effectively promoted.  Based on the research discussed above, the decrease of intake tempera crease of intake oxygen volume concentration can effectively improve t combustion process and the power output of diesel engines, though causin in NOx emissions. With the increase of intake temperature or the decrease gen concentration, the NOx emission of the diesel engine can be reduced output is lower as well. The intake temperature and the intake oxygen co diesel engine power output and NOx emissions are not the same. We can formance of diesel engines through either the combination of high intake te oxygen enrichment or low intake temperature and oxygen deficiency. Based on the research discussed above, the decrease of intake temperature or the increase of intake oxygen volume concentration can effectively improve the in-cylinder combustion process and the power output of diesel engines, though causing the increase in NOx emissions. With the increase of intake temperature or the decrease of intake oxygen concentration, the NOx emission of the diesel engine can be reduced, but its power output is lower as well. The intake temperature and the intake oxygen concentration of diesel engine power output and NOx emissions are not the same. We can study the performance of diesel engines through either the combination of high intake temperature and oxygen enrichment or low intake temperature and oxygen deficiency. to conditions 2, 3 and 4 in Figures 8 and 9, respectively. The results were compared with the normal intake condition 1 (315 K, 21% O 2 ). As shown in Figure 8, the compression pressure in condition 1 is higher than the one in condition 2 and there is a slight difference in compression terminal pressure in conditions 2, 3 and 4, which is consistent with the results from Section 3.1.2. In working condition 2, oxygen-enriched air intake is used to promote combustion and compensate for the weakening of the combustion process caused by the intake temperature increase, so its maximum combustion pressure is similar to that in condition 1. Compared with conditions 1-3, O 2 concentration increases from 23% to 29%, the combustion in the cylinder is further improved, and the maximum combustion pressure increases significantly.

High Intake Temperature and Enriched Oxygen
Under the intake temperature of 335 K and changed volume percentag with 23% O2, 29% O2 and 40% O2 were used as boundary conditions, whic to conditions 2, 3 and 4 in Figures 8 and 9, respectively. The results were c the normal intake condition 1 (315 K, 21% O2). As shown in Figure 8, the pressure in condition 1 is higher than the one in condition 2 and there is a sli in compression terminal pressure in conditions 2, 3 and 4, which is consi results from Section 3.1.2. In working condition 2, oxygen-enriched air int promote combustion and compensate for the weakening of the combu caused by the intake temperature increase, so its maximum combustion pres to that in condition 1. Compared with conditions 1-3, O2 concentration incre to 29%, the combustion in the cylinder is further improved, and the maximu pressure increases significantly.  As shown in Section 3.1.1, the increase in intake temperature will red engine's power output and NOx emissions. Figure 9 shows that the engine's increased by oxygen percentage changes from 23% to 29% at 335 K. How lower than the normal intake condition 1. Although its combustion press condition is higher than the normal intake conditions, its power output is s that of 23% O2. The increase of oxygen was not enough to make up for the in rise of intake temperature on the diesel engine's power output, and it is diffi the power output under normal intake conditions. The engine's NOx emis exponentially with oxygen percentage from 23% to 29%, and its influence i

High Intake Temperature and Enriched Oxygen
Under the intake temperature of 335 K and changed volume percentag with 23% O2, 29% O2 and 40% O2 were used as boundary conditions, whic to conditions 2, 3 and 4 in Figures 8 and 9, respectively. The results were c the normal intake condition 1 (315 K, 21% O2). As shown in Figure 8, th pressure in condition 1 is higher than the one in condition 2 and there is a sl in compression terminal pressure in conditions 2, 3 and 4, which is cons results from Section 3.1.2. In working condition 2, oxygen-enriched air in promote combustion and compensate for the weakening of the combu caused by the intake temperature increase, so its maximum combustion pre to that in condition 1. Compared with conditions 1-3, O2 concentration incre to 29%, the combustion in the cylinder is further improved, and the maximu pressure increases significantly.  As shown in Section 3.1.1, the increase in intake temperature will red engine's power output and NOx emissions. Figure 9 shows that the engine's increased by oxygen percentage changes from 23% to 29% at 335 K. How lower than the normal intake condition 1. Although its combustion press condition is higher than the normal intake conditions, its power output is s that of 23% O2. The increase of oxygen was not enough to make up for the i rise of intake temperature on the diesel engine's power output, and it is diff the power output under normal intake conditions. The engine's NOx emis exponentially with oxygen percentage from 23% to 29%, and its influence As shown in Section 3.1.1, the increase in intake temperature will reduce the diesel engine's power output and NOx emissions. Figure 9 shows that the engine's power output increased by oxygen percentage changes from 23% to 29% at 335 K. However, it is still lower than the normal intake condition 1. Although its combustion pressure at 40% O 2 condition is higher than the normal intake conditions, its power output is still lower than that of 23% O 2 . The increase of oxygen was not enough to make up for the influence of the rise of intake temperature on the diesel engine's power output, and it is difficult to achieve the power output under normal intake conditions. The engine's NOx emission increases exponentially with oxygen percentage from 23% to 29%, and its influence is significantly greater than the one of intake temperature. Therefore, it is difficult to optimize the performance of diesel engines' power output and emissions through the combination of high temperature and oxygen-enriched intake.

High Intake Temperature and Low Oxygen
Under the intake temperature of 295 K, changed volume percentage of O 2 and N 2 with 20% O 2 , 19% O 2 and 18% O 2 were used as boundary conditions, which corresponds to conditions 2, 3 and 4 in Figures 10 and 11, respectively. The results were compared with the normal intake condition 1 (315 K, 21% O 2 ). As shown in Figure 10, the compression pressure of conditions 2, 3 and 4 are higher than condition 1, which is consistent with Section 3.1.2. Moreover, low oxygen intake can effectively slow down the combustion process and reduce the maximum combustion pressure. The maximum combustion pressure is even lower than the one in condition 1.

High Intake Temperature and Low Oxygen
Under the intake temperature of 295 K, changed volume percentage with 20% O2, 19% O2 and 18% O2 were used as boundary conditions, whic to conditions 2, 3 and 4 in Figures 10 and 11, respectively. The results were c the normal intake condition 1 (315 K, 21% O2). As shown in Figure 10, the pressure of conditions 2, 3 and 4 are higher than condition 1, which is c Section 3.1.2. Moreover, low oxygen intake can effectively slow down the co cess and reduce the maximum combustion pressure. The maximum combu is even lower than the one in condition 1.   Figure 11 shows that the power output and NOx emission both decr oxygen concentration changes from 20% O2 to 18% O2 at 295 K intake tempe is also consistent with the conclusion in Section 3.1.2. The engine's power dition 2 is higher than the normal intake in condition 1, and its NOx emissi of that in condition 1. With the oxygen concentration further reduced to emission is only 3.41 g/kWh less than 3.44 g/kWh which is the requiremen emission Tier Ш. However, its power output has 12.5 kW lower than the condition. Therefore, when the intake temperature is 295 K and the O2 volu Atmosphere 2022, 13, x FOR PEER REVIEW

High Intake Temperature and Low Oxygen
Under the intake temperature of 295 K, changed volume percentage with 20% O2, 19% O2 and 18% O2 were used as boundary conditions, whic to conditions 2, 3 and 4 in Figures 10 and 11, respectively. The results were c the normal intake condition 1 (315 K, 21% O2). As shown in Figure 10, th pressure of conditions 2, 3 and 4 are higher than condition 1, which is c Section 3.1.2. Moreover, low oxygen intake can effectively slow down the co cess and reduce the maximum combustion pressure. The maximum combu is even lower than the one in condition 1.   Figure 11 shows that the power output and NOx emission both decr oxygen concentration changes from 20% O2 to 18% O2 at 295 K intake temp is also consistent with the conclusion in Section 3.1.2. The engine's power dition 2 is higher than the normal intake in condition 1, and its NOx emissi of that in condition 1. With the oxygen concentration further reduced to emission is only 3.41 g/kWh less than 3.44 g/kWh which is the requiremen emission Tier Ш. However, its power output has 12.5 kW lower than the condition. Therefore, when the intake temperature is 295 K and the O2 volu   Figure 11 shows that the power output and NOx emission both decrease when the oxygen concentration changes from 20% O 2 to 18% O 2 at 295 K intake temperature, which is also consistent with the conclusion in Section 3.1.2. The engine's power output in condition 2 is higher than the normal intake in condition 1, and its NOx emission is only 50% of that in condition 1. With the oxygen concentration further reduced to 18%, its NOx emission is only 3.41 g/kWh less than 3.44 g/kWh which is the requirement of IMO NOx emission Tier III. However, its power output has 12.5 kW lower than the normal intake condition. Therefore, when the intake temperature is 295 K and the O 2 volume fraction is adjusted to about 18.5%, the engine's NOx emission can be effectively reduced by 58% to 4.5 g/kWh, which is slightly higher than the emission of Tier III. Moreover, its power output remains unchanged with normal intake condition 1, as shown in position 'a' of the line in Figure 11. Therefore, the performance of the diesel engines can be effectively optimized by low temperature and low oxygen intake combination.

Effects of Different Intake Components
Based on the previous research results, the effects of different gas components on the diesel engine power output and emissions performance were explored via low temperature and low oxygen. According to the characteristics of the marine diesel engine, it can recycle exhaust gas or directly add water steam from the boiler to dilute the oxygen concentration in the intake air. Different proportions of CO 2 or H 2 O gas into the intake were set in the calculation model. The calculated power output and NOx emissions are shown in Figure 12, its conditions from 1 to 6 are as follows:

Effects of Different Intake Components
Based on the previous research results, the effects of different gas comp diesel engine power output and emissions performance were explored via ture and low oxygen. According to the characteristics of the marine diesel recycle exhaust gas or directly add water steam from the boiler to dilute th centration in the intake air. Different proportions of CO2 or H2O gas into th set in the calculation model. The calculated power output and NOx emissio in Figure 12, its conditions from 1 to 6 are as follows: 1 (295 K, 20% O2 and (295 K, 20% O2 and 79.4% N2), 3 (295 K, 20% O2 and 80% N2), 4 (295 K, 2 77.42% N2), 5 (295 K, 20.79% O2 and 78.21% N2), 6 (315 K, 21% O2 and 79% N As shown in Figure 12, when CO2 or H2O gas is added in the same pro conditions remaining unchanged), the engine's power output and NOx em conditions are both lower than those in H2O gas conditions, which is caused ence in the thermal physical properties of CO2 or H2O gas. The heat abs combustion process in the cylinder is different as well. The comparison b tions 2 and 3 shows that nitrogen is replaced by 0.06% CO2 or H2O gas whe centration is 20%. The engine's power output and NOx emission are lower and N2 in condition 3 (2541 kW and 8.56 g/kWh), condition with H2O gas is 8.13 g/kWh, and the lowest in the condition with CO2. In condition 2, repla in the intake with 0.06% CO2, the engine's power output is 2511 kW, wh higher than the power output of 2510 kW in the normal intake condition 6 NOx emission is only 7.39 g/kWh, which is significantly lower than 11.77 normal intake condition. Therefore, NOx emission can be reduced by reduc temperature and the low oxygen intake combination optimization, while the power output fluctuates only slightly.
According to the comparative analysis of conditions 4, 5 and 6 in Figur ure 11, there are some intake conditions (adding CO2, H2O gas or N2 to redu tration at 295 K) which make the diesel engine output unchanged and N As shown in Figure 12, when CO 2 or H 2 O gas is added in the same proportion (other conditions remaining unchanged), the engine's power output and NOx emission in CO 2 conditions are both lower than those in H 2 O gas conditions, which is caused by the difference in the thermal physical properties of CO 2 or H 2 O gas. The heat absorption of the combustion process in the cylinder is different as well. The comparison between conditions 2 and 3 shows that nitrogen is replaced by 0.06% CO 2 or H 2 O gas when oxygen concentration is 20%. The engine's power output and NOx emission are lower than pure O 2 and N 2 in condition 3 (2541 kW and 8.56 g/kWh), condition with H 2 O gas is 2529 kW and 8.13 g/kWh, and the lowest in the condition with CO 2 . In condition 2, replacing nitrogen in the intake with 0.06% CO 2 , the engine's power output is 2511 kW, which is slightly higher than the power output of 2510 kW in the normal intake condition 6. However, its NOx emission is only 7.39 g/kWh, which is significantly lower than 11.77 g/kWh in the normal intake condition. Therefore, NOx emission can be reduced by reducing the intake temperature and the low oxygen intake combination optimization, while ensuring that the power output fluctuates only slightly.
According to the comparative analysis of conditions 4, 5 and 6 in Figure 12 with Figure 11, there are some intake conditions (adding CO 2 , H 2 O gas or N 2 to reduce O 2 concentration at 295 K) which make the diesel engine output unchanged and NOx emissions reduced. Based on the studies carried out, it is confirmed that the 4.5 g/kWh NOx emission is the smallest, which is achieved by adding nitrogen to reduce O 2 concentration to 18.5%.