Abstract
This experimental study was conducted using a single-cylinder compression ignition (CI) engine with a pilot injection strategy to determine the effect of fuel injection pressure and the timing of the second start of injection (SOI2) on combustion and emission characteristics. This experiment used a mixture of 80% commercial gasoline (G80%) and 20% soybean biodiesel (B20%), by volume. The pilot injection strategy was applied with varying SOI2. Meanwhile, the first start of injection (SOI1) was constant at -350° ATDC and 900 bar fuel injection pressure. A range of fuel injection pressures from 400 to 900 bars and varied injection timing from -44 to -36 CA ATDC was applied at SOI2 to analyze the effect of injection timing and injection pressure on combustion characteristics and emissions. The increasing fuel injection pressure of GB20 in early injection timing will cause a longer ignition delay. The autoignition resistance of GB20 and the improvement of spray velocity enhance the wall wetting probability, consequently reducing the autoignition capability as fuel deposits were formed in the cylinder wall vicinity. Closer injection timing to TDC inhibits spray penetration due to higher room pressure and density, causing lower ignition delay. For GB20, 700 bar fuel injection pressure became the turning point in the ignition delay due to a lower fuel penetration velocity as a higher fuel injection pressure was applied. NOx emissions were identified as a sign of high temperature produced during combustion. The lowest CO2 emissions and the longest ignition delay appeared at the 700-bar injection pressure. Because incomplete combustion resulted in fuel deposits in the vicinity of the cylinder and temperature decrease, injection timings earlier than 40°CA BTDC initiated low thermal reaction (LTR) conditions, causing a temperature decrease during combustion.
1. Introduction
In recent years, improved economic stability has increased the capability of individuals to purchase necessities, which has affected the global demand for vehicles. Increased usage of personal vehicles will increase the consumption of fossil fuels. Due to the limited supply of fossil fuels, renewable fuel research is being undertaken in order to avoid a future energy crisis. The production of fuel from animals and vegetables is a promising field of research due to the possibility of relatively rapid reproduction of the raw material. For that reason, the usage of biodiesel as an alternative fuel is very promising, since biodiesel can be produced from vegetable oil and animal fat [1].
The compression ignition (CI) engine has been used widely for research due to its high efficiency compared to the spark ignition (SI) engine. During the injection process, fuel is injected when the air is in a compressed condition. This compressed condition is ideal for combustion, enhancing the performance of the CI engine. The possibility of applying a high compression ratio makes the CI engine popular in vehicles and industrial engines due to its capability to produce more energy than SI engines.
However, CI engines are mainly fueled with diesel fuel, which has a higher contribution to air pollution than gasoline fuel, causing the issuance of strict emissions regulations. For that reason, research into the use of gasoline in CI engines is interesting due to the cleaner fuel characteristics of gasoline than diesel. Putrasari and Lim [2] studied the application of gasoline-biodiesel on the CI engine and found that the duration of combustion and the thermal efficiency were similar to diesel fuels. Studies on the addition of biodiesel to conventional petroleum diesel in CI engines found that the biodiesel content decreases environmental pollution, while slightly increasing the thermal efficiency and the brake power [3]. The low emissions and high thermal efficiency associated with gasoline compression ignition (GCI) engines have made them attractive research subjects [4–10].
To achieve low thermal combustion (LTC) conditions, fuel with a low cetane number and high volatility characteristics are preferable [11–14]. LTC conditions will produce lower temperature during combustion, which results in a decrease in NOx emissions. In addition, LTC will improve air-fuel mixing and ultimately reduce the particulate matter in the emissions [15]. Applying gasoline fuel to GCI engines is known to produce LTC. The octane number on gasoline will become autoignition resistance, and a long ignition delay will be acquired. Consequently, a homogeneous air-fuel mixture and due to volatility characteristics of gasoline decrease the temperature in the chamber and decrease the local equivalence ratio.
In long-term usage, the application of pure gasoline will damage the common rail [16] due to the low lubricity characteristics of gasoline fuel [17]. The addition of biodiesel fuel in the GCI engine is expected to improve the fuel lubricity characteristics and minimize damage to the common rail and injector. Additionally, due to the higher oxygen content of the biodiesel, the combustion process will be improved [18]. Adams et al. [19] investigated the combustion characteristics of GCI engines by mixing conventional petroleum diesel and biodiesel fuel. They demonstrated that the addition of biodiesel to conventional petroleum diesel increased the required intake temperature and increased the combustion stability.
The research on the efficiency of combustion and engine performance improvement has mostly been conducted in recent years. EL-Seesy et al. [20] studied the effect of nanomaterial additives on the diesel-n-heptane mixture in CI engines and found that the addition of biodiesel caused an improvement in engine performance. Şen [21] studied the effect of fuel injection pressure on GCI engines and claimed that the increase of fuel injection pressure caused a decrease in the brake-specific fuel consumption (BSFC) and improved the brake thermal efficiency (BTE) at low speed. The variation of fuel injection pressure produced a varying effect on the combustion behavior. Increasing the injection pressure enhances fuel stratification. Meanwhile, the probability of wall-wetting also increases due to velocity improvement. For that reason, varying injection pressure appears to be a promising strategy for improving combustion characteristics. A higher fuel injection pressure results in smaller fuel droplets, improving the surface area to volume ratio. For that reason, more complete combustion can be achieved due to the increased vaporization capability of the fuel.
Many researchers have conducted studies about GCI engines over the years. Loeper et al. [22] identified and quantified combustion characteristics and emissions based on the variation of a number of engine operation parameters. The parameters studied included inlet temperature, inlet pressure, engine speed, fuel injection pressure, and injection timing/duration. The results showed that low NOx emissions and proper combustion stability could be obtained by manipulating the input parameters. Kodavasal et al. found that the start of injection (SOI) timing also greatly influenced the balance of combustion [23]. By observing the effect of the injection timing on the GCI engine mechanism, it was found that the SOI timing at -30° ATDC produces the most stable combustion characteristics. Meanwhile, a miss-fire was observed when fuel was injected earlier than -42° ATDC. Maintaining the NOx emissions while improving the efficiency is viable by controlling SOI position. However, the improvement of engine noise and emissions, especially hydrocarbon and CO, had been produced due to misfiring and deposit of fuel in the cylinder wall impingement. A study of biodiesel blends with the goal of increasing the lubricity and oxygen content in the fuel also was conducted by Misra and Murthy [18]. Adams et al. [19] studied the combustion and emission characteristics of GCI engine by mixing 5% and 10% biodiesel into the gasoline. Fuel was partially premixed by applying split injection. Due to the great difficulty of supplying a high intake temperature, the study focused on decreasing the intake temperature requirement of combustion at low load condition by adding the biodiesel to the gasoline. However, due to its poor low-temperature properties, the addition of biodiesel has been limited to 20% to maintain the combustion stability [18]. Conventional gasoline fuel containing 25% biodiesel additive was considered the most suitable mixture to be applied to GCI machines without improving the intake temperature or any modifications to the engine [24]. The application of a gasoline-biodiesel blend in a GCI engine by using 20% biodiesel and a single injection strategy has also been studied previously [2]. However, the experiment was conducted without modifying the initial conditions of the engine, which kept the temperature of oil, intake air, and coolant at an ambient temperature of approximately 25° C. The results obtained were unsatisfying, especially on the NOx emissions, which were much greater than for conventional diesel fuel.
The objective of this study is to observe the effect of various injection strategies on the combustion characteristics and emissions of GB20 applied to the GCI engine. The multiple fuel injection model was used, which consists of a pilot and the main injection fueled with the gasoline-biodiesel blend. The timing of the second start of injection (SOI2) was varied in order to obtain the optimal combustion and understand the phenomena while comparing the emission behavior. The model was also combined by varying the high fuel injection pressure at SOI2 and maintaining the input energy by conducting a fuel test of each rail pressure improvement. The combustion characteristics in terms of cylinder pressure, heat release rate, ignition delay, and the emission characteristics are the focus of this study.
2. Methodology
2.1. Fuel Preparation
The main fuel used in this study was a mixture of commercial gasoline and soybean biodiesel. The chemical composition of the soybean biodiesel can be seen in Table 1 [25]. The fuel labeled “GB20” represents gasoline plus 20% biodiesel by volume. The gasoline-biodiesel mixture was processed on a shaking/mixing process for approximately 10 minutes to obtain homogeneity. Due to the stability issues of biodiesel-gasoline blends, the crystalline colloid and phase separation of the blend has been noted in a previous study [26]. The physical properties of fuels tested in this experiment are presented in Table 2, from laboratory tests based on international standards [26]. In order to ensure the quantity of fuel injected into the cylinder, the injection test was carried out on the fuel rate injection device shown in Figure 1. To ensure the accuracy of the measurement, a 1000 fuel injection cycle was injected into the fuel measurement chamber. The 30 bar of pressure was applied to the chamber for the back pressure effect. The weight of the fuel was measured, and the quantity of the fuel per injection was calculated.
2.2. Test Engine Specifications and Experimental Conditions
This study remodeled a four-cylinder engine into a single-cylinder engine production GM 1.9 L direct-injection light-duty compression ignition engine. The production head and valve-train were used in conjunction with a Labeco CLR crankcase. A reentrant bowl design with a lower compression ratio than the current Euro-4 compliant production configuration was used along with a slightly longer connecting rod length required for the crankcase dimensions; all other cylinder geometry was matched to the equivalent four-cylinder engine. Engine specifications are shown in Table 3.
The experimental engine speed can be enhanced up to 4000 RPM by stimulating turbocharger behavior using a compressed air intake and exhaust backpressure valve. The oil, coolant, outlet and inlet temperatures, and pressure can each be varied and can be measured accurately in the laboratory. The fuel rail system uses the BOSCH CP3.3 pump. ETAS INCA calibration software used in conjunction with the production ECU allows for up to five injections per cycle. A production CRIP2-MI injector was used with an experimental 7 × 155° minisac nozzle tip with a flow number of 440 cc/30 s.
A piezoelectric KISTLER 6125B pressure transducer was used to obtain the in-cylinder pressure data. The data resolution was 0.1 CAD, corresponding to 3600 counts per revolution; 200 cycles of data were recorded for all heat release analyses. The combustion noise was recorded using an AVL Combustion Noise Meter (Model A01); meter output was in decibels. Injector current was measured using a Tektronix A622 AC/DC current probe.
The engine was operated at 1500 RPM using a pilot injection strategy. Double fuel system and common-rail system were connected to a single fuel injector in order to provide two different fuel injection pressures in one cycle. A ZB-1100 common-rail PCV driver was used to control the fuel pressure in the common rails. The timing of injection was controlled using a fuel injection driver by applying two injections in one cycle. The timing of SOI1 was injected constantly at -350 ATDC for all conditions and followed by the SOI2, which was varied from -44 to -36 CA ATDC by the fuel split ratio 65/35. In the first injection, the rail pressure was fixed at 900 bar for all injection timings. Meanwhile, at the SOI2, the fuel injection pressures varied from 400 bar to 900 bar. The detail of the operation condition can be seen in Table 4. A higher fuel injection pressure will increase the rate of fuel injection into the chamber. Therefore, in order to obtain a constant energy input of 11 mg, the fuel injection rate test was conducted on an average of 500 cycle injections and repeated three times to ensure the accuracy of the measurement. The intake air temperature was set at 80°C to obtain stable combustion based on the previous study about controlling autoignition combustion of gasoline fuel in a compression ignition engine [22, 27].
3. Results and Discussion
3.1. The Effect of Variation of Fuel Injection Pressure and Timing of SOI2 on Combustion Characteristics
The ignition delay and the start of engine combustion, when operated on GB20 fuel, are shown in Figure 2 with varying fuel injection pressures and timing of SOI2. In this work, the ignition delay is defined as the time interval between fuel injection and the start of combustion. The start of combustion was identified by calculating the pressure rise rate, and the second peak of pressure rise rate has been chosen as the start of combustion in this study, as described by Lee and Song [28]. An increase in fuel injection pressure caused an increase of the main breakup, cavitation. Consequently, the fuel atomization was enhanced, which generally shortened the ignition delay as well as the duration of combustion. The ignition delay determines the quality of the air-fuel mixture inside the chamber, with longer ignition delay improving the quality of the mixture [29]. However, a higher fuel injection pressure will enhance the velocity at which fuel droplets are delivered to the cylinder wall before reaching the ignition point, as a longer ignition delay is applied in the system. In the case of early injection timing, a longer ignition delay will not improve the quality of the combustion due to the better air-fuel mixture, but incomplete combustion will occur as the atomization of fuel will transform into the droplet as the fuel attaches to the cylinder wall, making the fuel harder to ignite. Therefore, understanding fuel spray characteristics is necessary when studying GDI engine systems using early injection. Das et al. [30] have studied the characteristics of fuel spray of GB20 in the constant volume chamber with varied ambient densities and fuel injection pressures of 400-1200 bar. They mentioned that the ambient density affects the penetration characteristics, with an increase in the ambient density posing a strong ambient resistance on the spray development. In real engines, the ambient density will increase when the injection timing is closer to TDC. The improvement of fuel injection pressure causes the spray cone angle near the nozzle to become smaller. This indicates a high amount of fuel mass near the nozzle pushed aside by subsequent droplets. In his study, he found that as rail pressure increases from 400 to 700 bar, the cone angle decreases significantly, but a further injection pressure increase from 700 to 1200 bar leads to a slight decrease, indicating smaller penetration velocity increase as injection pressure increases. Meanwhile, the Reynolds number will increase constantly as the injection pressure increases. A higher Reynolds number implies unstable flow during injection, causing a high possibility of turbulence. The vortex induced by the improvement of the Reynolds number is expected to increase radial dispersion and increase the air flow rate. The decrease in ignition delay when the injection pressure is increased from 700 to 900 bar is due to consistently increasing turbulence as the injection pressure increases while the velocity decreases. Figure 2 shows the effects of varying the fuel injection pressure from 400 to 900 bar; a fuel injection pressure of 700 bar produced the highest ignition delay due to the most significant development of velocity. Meanwhile, for injection pressures from 700 to 900 bar, the turbulent kinetic energy caused the movement path of the fuel to lengthen. Consequently, the development of fuel penetration was lower compared to fuel velocity. The previous study [30] showed the spray velocity of GB20 that was injected at ambient pressure 10 kg/m3 and 400 bar rail pressure. The observation of spray showing the length 85 mm has obtained for 1125 ms. This is in agreement with the results of Adams [31], who characterized the injection pressure effect on a gasoline compression ignition engine using the 1-D transient spray model and KIVA 3D CFD code. In his simulation, he compared the spray penetration of gasoline-based fuel using fuel injection pressures of 465, 785, and 1604 bar at a constant volume and transient rate of injection. He found that at fuel injection pressures from 785 bar to 1604 bar, the rate of fuel velocity increase was the most significant. However, it was much less than 465 bar to 785 bar in the case of penetration length improvement. The development of turbulent kinetic energy and velocity increased exponentially with injection pressure. Meanwhile, the penetration growth rate decreased when fuel was injected above 700 bar. For that reason, it enhanced the autoignition characteristics and decreased the possibility of wall-wetting.
In the pilot injection system, SOI2 was injected near TDC; thus, the SOI2 has limited time to achieve a homogenous air-fuel mixture. Instead, the premixed combustion occurred on SOI1 due to the injection timing taking place at the intake position that the mixture promoted by the intake-turbulence. Moreover, the autoignition resistance characteristic of gasoline prolongs the ignition delay, which increases the quality of the mixture [13, 14, 32]. In a CI engine, fuel will generally be injected between 5 and 20 degrees of crank angle BTDC due to the high viscosity and density of diesel fuel to prevent fuel impingement. However, due to the high volatility, the possibility of an early injection in order to obtain a better air-fuel mixture for gasoline-based fuel is still viable. Figure 3 presents the in-cylinder pressure of GB20 under different fuel injection pressures and injection timing. The maximum in-cylinder pressure trace is also shown to present all the parameters tested in this study. The thermodynamic state of charge information can be taken from the pressure inside the cylinder during the combustion process. By applying the first law of thermodynamics and simplifying some equations, the heat release rate at which the combustion took place can be obtained. The rate of heat release is calculated using the following equation: where is the instantaneous combustion chamber, is the specific heat ratio, and is the in-cylinder pressure. The specific heat ratio of the CI engine ranges from 1.3 to 1.35. Figure 4 shows the heat release rate of GB20 as a function of injection timing and rail pressure using the pilot injection strategy. It has been demonstrated that the variation of the injection timing affects the position of the maximum pressure. Meanwhile, the injection pressure has a great effect on the amount of pressure improvement. In this study, the ignition delay played an essential role in terms of combustion characteristics and emissions. The most extended ignition delay for 700 bar fuel injection pressure resulted in inhibition of pressure development and heat release. Consequently, the lowest in-cylinder pressure and HRR was produced. The main reason was that the fuel impingement increased the restriction of air entry in the initial soot formation process. The decrease of wall temperature due to fuel deposits has also slowed the development of flame during combustion. The chemical reaction rate of the air/fuel mixture was highly affected by the wall temperature due to the temperature gradient zone in the near-surface area. The air/fuel mixture inhibited the thermal energy absorption due to the fuel impingement effect in initial temperature reduction. As a result, fuel evaporation and the fuel/air mixing process were severely hampered as fuel spray developed along the wall, increasing the preparation time for automatic ignition of the fuel mixture [33].
3.2. Effect of Fuel Injection Pressure and SOI2 on Efficiency
By multiplying torque by the engine speed, brake power can be obtained. The indicated power can be calculated from in-cylinder pressure data. The friction loss in the engine system was the reason for brake power being lower than the indicated power. From the indicated power, information about thermal efficiency can be obtained. Figure 5 shows the performance of the engine, including brake power, indicated power, and torque. Meanwhile, the indicated thermal efficiency of the GCI engine from the experiment can be observed in Figure 6. The indicated thermal efficiency was obtained by dividing the indicated power by the energy input per cycle which classically identified the definition following the reference by Pulkrabek [34]. In this work, by multiplying the displacement of volume by the IMEP, the indicated power can be obtained. However, based on the equation proposed by Kalghatgi, the indicated thermal efficiency can be defined as the indicated fuel conversion efficiency [35]. Because all the components of the formula in this work are similar to Ref. [34], the indicated thermal efficiency can be obtained using the following equation: where indicates thermal efficiency (%), indicates work (J/cycle), and indicates input energy (J/cycle).
Figure 6 shows the thermal efficiency of GB20 at different injection timings and fuel injection pressures. From the figure, we could conclude that at injection timing -34° CA ATDC to -40° CA ATDC the thermal efficiency was lower, while injection timing earlier than -40° CA ATDC initiated greater fluctuation of the thermal efficiency. Delaying the injection timing beyond -34° CA ATDC will promote excessive liner spray impingement, while earlier injection timing than -40° CA ATDC will promote the low thermal reaction. Consequently, the quality of combustion was improved. This result is in agreement with Putrasari and Lim [2], who studied combustion characteristics of GCI engines fueled with a gasoline-biodiesel blend. He varied the injection timing from -18° CA ATDC to -65° CA ATDC. In his study, the CA50 graph showed the injection timing of -40° CA ATDC as the lowest point of CA50. The thermal efficiency of combustion can be determined by calculating the energy input and energy output per cycle. By maintaining the heat-release rate profile along with the optimal profile, thermal efficiency can be maximized. A feasible approach to maintain the heat release is to control the CA50, which is one of the most significant indicators to measure the combustion process. The maximum thermal efficiency can be obtained when the CA50 reaches the optimal set-point [36].
Brake-specific fuel consumption (BSFC) identifies the efficiency of an engine that converts fuel or energy input into rotational power at the shaft or crankshaft. In automotive applications, the efficiency of the internal combustion engine can be evaluated by using BSFC. It is calculated by using the ratio of the rate of fuel consumption to the effective power produced from the engine during combustion. Therefore, the brake specific fuel consumption (g/kWh) can be obtained by dividing the fuel mass flowrate (g/h) by the engine output power (W): where (g/h) is the fuel mass flow rate, (W) is effective (brake) engine power, and BSFC (g/kWh) is the brake-specific fuel consumption.
Figure 7 shows the BSFC of the engine during the operation condition. BSFC showed the highest state at a fuel injection pressure of 700 bar. Meanwhile, an increase in injection pressure above 700 bar showed a decrease in fuel consumption, indicating better performance for the same energy input injected. High fuel injection pressure indeed improves the quality of combustion. The decrease in the size of fuel particles at a rail pressure above 700 bar overcame the autoignition resistance of gasoline. The higher Reynolds number disperses the spread of fuel droplets, inhibiting the spray penetration development and minimizing the possibility of wall-wetting. As a result, the air-mixture quality is enhanced, producing an even reaction, and higher-efficiency combustion will be obtained. [37] studied the effect of fuel injection pressure on performance and emissions in compression ignition engines. High fuel injection pressures of 800 to 1400 bar were investigated. The results showed that the in-cylinder pressure increased with increasing fuel injection pressure, producing a higher peak of heat release and shorter combustion duration due to higher in-cylinder temperature and better atomization. The indicated mean effective pressure and the indicated thermal efficiency also increased as higher fuel injection pressure was applied. Increased injection pressure also was associated with decreased HC and soot emissions and increased NOx emissions. In general, the injection pressure of a GDI engine is typically limited to approximately 200 bar in order to prevent excessive wear on the injectors [38]. Therefore, a further study is needed to investigate the effect of injection pressure on the injector and how to minimize it. The volatility characteristics of gasoline fuel make it capable of obtaining good atomization spray on the low common-rail pressure. However, energy efficiency usage is also important in light of the current limitations of fossil fuels as the main energy resource for vehicles.
3.3. Effect of Fuel Injection Pressure and SOI2 on Emissions
Figure 8 shows CO emissions from the GCI engine, which uses GB20 fuel for multiple injections. CO emissions are related to combustion stability and the mass fraction of fuel that reacted with air. The CO emissions increased as the injection pressure increased from 400 bar to 700 bar. As the injection pressure increased from 700 bar to 900 bar, the CO emissions decreased. Furthermore, CO emissions decreased when fuel was injected closer to TDC. When the local equivalence ratio is too lean to permit complete combustion, it will cause the improvement of CO emissions. For that reason, the CO emissions mirror the capability of the engine to perform complete combustion. As discussed in the previous section, early injection timing conditions will promote fuel impingement in the cylinder wall due to longer ignition delay. Fuel droplets that attach to the wall will generate fire in the vertical direction along the cylinder wall, causing the distribution of heat and pressure to focus on the outer wall of the cylinder [33]. Leaving the center of the cylinder at the lower temperature. For that reason, the greater the percentage of fuel attached to the cylinder wall, the more difficult it is for fuel to burn perfectly due to un-even temperature distribution and fuel deposits.
Figure 9 shows NOx emissions from the GCI engine, which uses GB20 fuel for multiple injections. In this work, the initial temperature was raised to above 80°C, which scales with the maximum in-cylinder pressure, gas temperature, and combustion efficiency. Usually, higher oxygen content in the fuel will produce higher NOx emissions in the exhaust product due to the elevated oxygen content increasing the temperature during combustion. However, by using a pilot injection strategy, further reduction of NOx emission can be realized, as reported in a previous study [22]. The temperature of the combustion process will determine the quantity of the NOx emissions. A charge status before ignition is required. Therefore, autoignition resistance fuel characteristics such as gasoline that produce longer ignition delay which expected to obtain thoroughly mixing with air are to be advantageous. Many researchers have investigated the effect of the cetane number on NOx emissions. Some of the results have shown that the improvement of cetane number in the fuel leads to an increase in NOx. Meanwhile, some of the results have shown that by manipulating the combustion strategy, low NOx emissions can be realized by using fuel with a low cetane number [54]. The gasoline fuel has a low cetane number and a much more extensive ignition delay, which is expected to enable operation of the engine with less smoke in higher load conditions compared to diesel fuel, without affecting the CO and NOx emissions or fuel consumption. However, the improvement of the initial temperature reduces the combustion duration and leads to advanced combustion phasing causing bulk in-cylinder temperature. Consequently, the NOx emissions during combustion are enhanced. Moreover, the addition of biodiesel elevates the oxygen content in the fuel mixture. The higher oxygen content in the fuel can improve combustion quality. However, the oxygen is expected to improve the temperature in the combustion chamber, leading to the increased NOx produced by engines fueled with a biodiesel blend [29]. Moreover, an increase in the in-cylinder temperature is expected when a late pilot injection is applied. The radical concentration of the pilot injection mixture interferes with the main injection air-fuel mixing process [39]. For that reason, this condition decreases the ignition delay of the main fuel, which therefore leads to the peak of premixed combustion reduction, the possibility of knocking, and NOx emissions [40]. As shown in Figure 9, injecting fuel closer to the TDC will promote NOx. Deposits of fuel in the vicinity of the cylinder cause incomplete combustion, resulting in lower temperature during combustion, and consequently lower NOx being produced. The improvement of turbulent kinetic energy by increasing fuel injection pressure seems to be the key factor of the temperature decrease during high-pressure injection conditions. However, at fuel injection pressures ranging from 400 bar to 700 bar, high-velocity development promotes wall-wetting, causing lower temperature [33] as incomplete combustion occurs. This result is in agreement with the findings of Jain et al. [41], who studied the effects of injection pressure on a PCCI engine. They found that increasing the fuel injection pressure reduces the production of particulate matter and NOx emissions. The lowest emissions were obtained when 700 bar fuel injection pressure was applied. At injection timing earlier than 40 degrees BTDC, the variation of fuel injection pressure did not show much fluctuation, even though a different quality of combustion was produced as explained in the previous section. The low-temperature reaction (LTR) has a similar effect as using a pilot injection, which controls the combustion rate while improving the stability of combustion by delaying the combustion phasing. Solaka et al. [42] have pointed out that the fraction of LTR can be improved by extending the ignition delay. The LTR phenomenon is closely related to the cool flame phenomenon, because the cool flame is controlled by low-temperature chemistry. When fuel is injected very close to TDC, the LTR phenomenon is barely seen due to high in-cylinder temperature, which is beyond the temperature range suitable for cool flame reactions [43]. For that reason, at late injection timing, the increase of NOx emission will not be significant, as shown in the graph. Figure 10 shows the CO2 emission of GB20 with the variation of injection pressure and injection timing. The more efficient combustion will produce more heat as a result of the higher percentage of the reaction of oxygen and fuel. Contrary to CO emissions, the development of CO2 emissions indicates the perfection of combustion. The more efficient combustion will produce more CO due to a consequence of the higher percentage of the reaction of oxygen and fuel. However, the quantity of the emission will depend on the kind of fuel used in the vehicle.
4. Conclusion
This paper has explored suitable initial pilot injection strategies, including pilot timing and variations in fuel injection pressure under GCI operating engines. The influence of fuel impingement, ignition delay, and the optimal injection strategy was discussed, with the focus on fuel efficiency improvement and reduction of emissions. The main conclusions are as follows: (1)Fuel injection pressure determined the fuel spray penetration and velocity; the key factor of wall-wetting fuel formation consequently affects the ignition delay. In the case of early injection, ignition delay increased with increasing injection pressure. However, for GB20, an injection pressure of 700 bar was the turning for the development of penetration speed much lower than fuel velocity, decreasing the ignition delay(2)Closer SOI2 to TDC decreased the development of spray penetration, which inhibited the development of fuel deposits in the vicinity of the cylinder wall. Consequently, this condition enhanced the autoignition capability, the efficiency of the combustion, and the in-cylinder pressure(3)Earlier SOI2 than 40°CA BTDC enhanced LTR, which improved the stability of combustion as shown by the improvement of CO2 emissions and engine performance, meanwhile keeping the in-cylinder temperature low(4)The fuel injection pressure of 700 bar produced low NOx emissions over a range of injection timings. The deposit of fuel in the vicinity of the cylinder wall due to long ignition delay caused inhibition of combustion temperature development. High fuel injection pressure did not affect the NOx emissions as the early injection promotes the LTC. The development of CO2 emissions could become the indicator of perfect combustion, contrary to CO(5)However, fuel chemical and physical properties also influence the combustion characteristics, which is a limitation of the current study. Differences in the fuel density affect the fuel velocity and spray penetration. Meanwhile, fuel resistance on pressure and temperature determines the position of combustion. Further investigations on different kinds of fuel will improve our understanding of fuel properties that affect the combustion characteristics
Data Availability
The data that supported the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the Industrial Strategic Technology Development Program-Development of low flash point fuel injection system for hazardous emission reduction from small and meddle class ships (Project no.: 20013146) funded By the Ministry of Trade, Industry & Energy (MOTIE), Korea; supported by “Regional Innovation Strategy (RIS)” through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE) (2021RIS-003); and financially supported by the Energy Technology Development Project of the Korea Energy Technology Evaluation and Planning (20182010106370, Demonstration Research Project of Clean Fuel DME Engine for Fine Dust Reduction), Republic of Korea.