Spray Behavior, Combustion, and Emission Characteristics of Jet Propellant-5 and Biodiesel Fuels with Multiple Split Injection Strategies

Abstract

This study focuses on an analysis of the spray behavior, combustion, and emission characteristics of jet propellant-5 (JP-5) and biodiesel fuels with single-injection timing and multiple split injection strategies in a common rail direct injection (CRDI) single-cylinder diesel engine system. The analysis includes visualization of the spray and combustion. Multiple split injection strategies (e.g., double, triple, quadruple, and quintuple) were considered by equally distributing the fuel injection amount within the single-injection. Injection of biodiesel has a delayed start (0.2 ms) as well as shorter spray tip penetration compared with JP-5. As the fuel injection timing was approached to the top dead center (TDC), the engine performance and combustion efficiency improved. Retarding the injection timing contributed to an increase in carbon dioxide (CO₂) (JP-5: max. 2.6% up, BD100: max. 1.5% up) and a decrease in carbon monoxide (CO) (JP-5: max. 93% down, BD100: max. 91% down) and nitrogen oxides (NOx) (JP-5: max. 83% down, BD100: max. 82% down). In comparison with JP-5, biodiesel showed disadvantages from the point of its combustion and emission characteristics for all injection timings. The quadruple-injection strategy, in which fuel injection was performed four times, showed excellent combustion, engine performance, and combustion efficiency. The CO₂ emissions were highest with the quadruple-injection strategy (JP-5: 6.6%, BD100: 5.8%). The CO emissions of biodiesel decreased as the pulses of split injection extended, and a significant reduction of 83.8% was observed. NOx increased as the number of split injections increased (JP-5: max. 37% up, BD100: max. 52% up). JP-5 was a longer ignition delay than that of biodiesel from combustion flame visualization results. The final combustion in the multiple-injection strategy showed a typical diffusion combustion pattern.

1. Introduction

The greatest advantage of a diesel engine installed with an electronic fuel injector system and a common rail system is that it is possible to diversify the fuel injection methods through, for example, fuel injection pressure control, timing control, and injection frequency control. Among the various fuel injection methods, controlling the fuel injection timing and injection frequency has remarkable effect on the combustion quality and emission levels under constant engine operating conditions. The fuel injection timing strategy is a very powerful tool that can optimize engine performance and emissions with a constant fuel injection amount. As the fuel injection timing is retarded toward the TDC, higher in-cylinder pressure and in-cylinder temperature are produced. A shorter ignition delay, a reduction in negative work, and a longer combustion period can be achieved under these conditions. Consequently, the engine’s performance and combustion efficiency improve. In terms of emissions, the positive effect of reduced CO and NOx is expected [1,2,3].

Table 1. Studies on the effects of injection timing.

Author(s) [Ref #]	Main Contents
Kulandaivel, D. et al., [4]	Performance, combustion, and emission characteristics of blended biodiesel fuel with retarded injection timing. Delayed injection timing leads to decreased maximum in-cylinder pressure and shortens the overall combustion duration. Significant reductions in NOx emissions observed with retarded injection timing.
Shareef, S.M. et al., [5]	Performance, combustion, and emission characteristics of biodiesel with various injection timings. Retarded injection timing contributes to higher combustion efficiency and lower emissions. Maximum in-cylinder pressure, ROHR, and ROPR decreased as injection timing was retarded.
Rami Reddy, S. et al., [6]	Combustion and emission characteristics of biodiesel with different injection timings. The brake thermal efficiency and maximum in-cylinder pressure are enhanced under advanced injection timing conditions. Significant increases in NOx emissions were confirmed with the advanced injection timing condition.

reviews the results of published research on the influences of fuel injection timing.

The multiple-injection strategy is a concept in which the main injection is shared into double or more injections. This method can further expand the reaction area between the air and combustion flame in the combustion chamber. Due to its affection, the distribution of rich regions, which usually occur in single-injection combustion, is reduced inside the combustion chamber. When the multiple-injection strategy is applied to a diesel engine, the combustion generated during the compression stroke increases the in-cylinder pressure and temperature. Therefore, the ignition delay duration of the main injection is shortened, the ROHR is suppressed, and NOx is deceased. Moreover, the application of multiple injection in diesel engines with a low compression ratio can improve their cold-start capability [7,8,9,10].

Table 2. Studies on the effects of multiple-injection strategies.

Author(s) [Ref #]	Main Contents
How, H.G. et al., [11]	Performance and emission characteristics of blended biodiesel fuels with various injection timing and split injection strategies. Significant reductions in NOx were achieved with delayed injection timing and triple injection.
Park, H. et al., [12]	Analysis of the diesel combustion process from cold-start conditions with multiple-injection strategies. Pilot injection leads to improved diesel vaporization because of the small amount of heat and promotes the evaporation of the subsequent main injection. The triple- and quadruple-injection strategies improve the combustion quality.
Doll, H. et al., [13]	Mixing, ignition, and combustion characteristics in premixed compression ignition with a split injection strategy. Subsequent fuel spray after the earlier injection increases the mixing rate. The split injection strategy contributed to an improvement in the low-temperature oxidation reaction.
de la Garza, O.A. et al., [14]	Spray interaction analysis of various biodiesels under multiple-injection strategy conditions. Lower spray cone angle and area and longer spray tip penetration from biodiesels were found compared with ULSD. The main injection had higher spray parameters compared with the pilot injection.

reviews the results of published research on the application of multiple-injection strategies.

Jet propellant (JP-5) is used for naval aircraft, while JP-8 is used for ground-based military aircraft. Although the physical properties of JP-5 and JP-8 are similar, JP-5 has a higher flash point and is mainly stored in naval vessels. Kerosene-based military jet fuel has a lower density, viscosity, and cetane number compared with conventional diesel fuel. These physicochemical properties increase the ROHR in the premixed combustion stage by lengthening the ignition delay. The higher vaporization of JP-5 results in rapid mixture formation, whereas its low cold flow plugging point (CFPP) ensures fuel supply to the engine in low-temperature environments [15,16].

Table 3. Studies on the effects of JP-5 and military aviation fuel.

Author(s) [Ref #]	Main Contents
Lee, J.W. et al., [17,18]	Spray, combustion, and emission characteristics of JP-8 fuel. JP-8 has asymmetric behavior, shorter spray length, and a larger spray angle than diesel fuel in a VOC injector. JP-8 has longer ignition delay and higher heat release than diesel fuel. NOx emissions of JP-8 are greater than those of diesel fuel.
Uyumaz, A. et al., [19]	Performance and emission characteristics of JP-8 and biodiesel fuels. Ignition delay became shorter as the ratio of added biodiesel increased. Lower cetane number of JP-8 causes an increase in ignition delay duration. Specific fuel consumption reduces as the engine load increases in test fuels. More NOx is generated as the amount of biodiesel increases.
Lee, J.W. et al., [20]	Combustion and emission analysis of JP-8 with multiple-injection strategies. Combustion phase of JP-8 is delayed compared with diesel fuel with the same injection strategy. NOx emissions increased significantly with JP-8 in single-injection mode; however, the formation rate was suppressed under multiple pilot injection.
Labeckas, G. et al., [21]	Combustion, engine performance, and exhaust characteristics of JP-8 and biodiesel blends. Ignition delay and ROHR decreased as the proportion of added biodiesel portion increased in JP-8. The NOx formation rate increased as the biodiesel content of JP-8 increased. Addition of biodiesel increased CO emissions.
Hissa, M. et al., [22]	Research into the combustion of various alternative fuels, including kerosene. Combustion parameters showed very similar results for all test fuels except RME. Except for renewable naphtha, all test fuels, including kerosene, can be used in nonroad diesel engines.

reviews the results of published research on the applications of diesel engines with JP-5 and military jet fuel.

Biodiesel is an eco-friendly alternative fuel that has a higher density, viscosity, cetane number, and a lower heating value compared with diesel fuel. A disadvantage of biodiesel is the atomization of fuel droplets, which weakens the formation of the mixture and burns in a locally fuel-rich region, resulting in incomplete combustion. These fuel droplets cause the reduced spray tip penetration and engine power of biodiesel. The oxygen component of biodiesel plays a significant role in shortening the ignition delay; however, more NOx is formed as the addition ratio of biodiesel increases. Various studies have shown that CO decreases and NOx increases when pure biodiesel or biodiesel blend oil is used in a diesel engine [23,24,25]. One study reported that the physical properties of biodiesel at a low injection pressure caused poor spray and atomization, which resulted in incomplete combustion. As a result, CO increased and NOx decreased [26,27,28].

Table 4. Studies on the effects of biodiesel.

Author(s) [Ref #]	Main Contents
Kattimani, S.S. et al., [29]	Thermal efficiency characteristics of biodiesel blended fuels with different injection strategies and various compression ratios. Biodiesel blended fuel has slightly reduced thermal efficiency compared with pure diesel.
Nguyen, T. et al., [30]	Spray, combustion, performance, and emission characteristics of fish-oil biodiesel blended with diesel fuel. Biodiesel had longer spray length and lower engine power than pure diesel. An increase in the ratio of fish-oil biodiesel reduced CO emissions and increased NOx formation.
Sahoo, P.K. et al., [31]	Comparative evaluation of the performance and emission characteristics of various biodiesel fuels. Lower CO emissions were observed with one of the test fuels among various biodiesels. In general, the NOx emissions generated from biodiesel were greater than those of diesel fuel.
Xue, J. et al., [32]	Higher or lower CO and NOx emissions from biodiesel compared with diesel fuel were found under various engine operation conditions after reviewing a significant amount of literature.

reviews the results of published research on biodiesel applications.

Comparative analyses of the combustion and air pollutant characteristics of using JP-5 and biodiesel with various multiple injection strategies in common rail direct injection (CRDI) diesel engines have yielded limited results. These two fuels are believed to be applicable to diesel engines for propulsion and power generation. JP-5 is a fuel used in naval aircraft with limited access to the general public. In particular, previous studies must be considered when JP-5 and biodiesel fuels with large differences in properties are applied to marine diesel engines, including naval vessels equipped with an electronic injector.

The present study was conducted on CRDI single-cylinder diesel engine installed with an electronic injector to evaluate the combustion and emission characteristics of JP-5 and biodiesel fuels. Single-injection timing and multiple split injection strategies were introduced to diversify the fuel injection methods in this experimental study. The multiple split injection strategy was divided into four modes: double, triple, quadruple, and quintuple. The combustion characteristics of JP-5 and biodiesel were analyzed using combustion parameters such as in-cylinder pressure, ROHR, indicated mean effect pressure (IMEP), engine torque, and combustion efficiency. The emission characteristics were analyzed by measuring substances such as oxygen (O₂), carbon dioxide (CO₂), CO, and NOx emissions. In addition, the spray development and combustion process were visualized in an optically accessible experimental system.

2. Experimental Setup and Strategy

2.1. Experimental System

A schematic diagram of the experimental systems used to evaluate the spray behavior, combustion, and emission characteristics of JP-5 and biodiesel fuels is shown in Figure 1. The functions of the main devices that constitute the experimental system are shown in

Table 5. Function of the main devices constituting the experimental system.

Instruments	Main Function	Model
Test engine	Single-cylinder CRDI diesel engine allowing visualization of the combustion	MD-SDE-100, Mobiltek
22 kW electric motor	Precise control of the speed of the test engine	HV2 induction motor, Hyosung
Injection controller	Injection control	ZB-5100 peak and hold driver, Zenobalti
Sensor-based exhaust gas analyzer	Measuring emissions such as O2, CO2, CO, and NOx	Testo-350K, Testo
Piezoelectric pressure sensor	Measuring the in-cylinder pressure	Type 6056A, Kistler
Amplifier	Measuring the in-cylinder pressure	Type 5018, Kistler
Combustion analyzer	Analyzing the combustion characteristic	MT-7000S, Mobiltek
High-speed camera	Recording the spray and combustion behavior	Fastcam SA3, Photron
Torque meter	Measuring the torque generated by the engine	T8 ECO, Interface
Engine control system	Labview-based programmed control of engine characteristics such as fuel injection pulse duration, engine speed, and fuel injection pressure
Visualization chamber	Analyzing the spray behavior process

.

A high-performance light source (Xeon 1000, Optical System) was used to obtain a clear image of the spray’s behavior. A high-pressure fuel tube coupled with a sac-type injector mounted in the CVC (constant volume chamber) for spray visualization was directly connected to the common rail of the test engine. In addition, the fuel injection in the spray chamber is synchronized with the test engine. An extended piston, quartz, and 45° mirror were installed to the test engine for visualization of the combustion flame with JP-5 and BD100. The main specifications of the experimental research engine are summarized in

Table 6. Main specifications of the experimental research engine.

Engine type		4-stroke, single-cylinder, CRDI
Bore × stroke		83 mm × 92 mm
Compression ratio		17.7:1
Displacement		498 cc
Number of valves		two intakes, two exhausts
Valve timing	IVO	BTDC 7 CA
	IVC	BTDC 43 CA
	EVO	BBDC 52 CA
	EVC	ATDC 6 CA

.

2.2. Test Fuels

The JP-5 used for naval aircraft and biodiesel were the test fuels used in this study. JP-5 was supplied by naval vessels carrying aircraft. JP-5 has a lower distillation temperature, density, cetane number, and kinematic viscosity compared with biodiesel, but does not have a lower heating value. In particular, its CFPP shows very low properties below –35 °C.

Table 7. Main physiochemical properties of the test fuels.

Test Fuel Property		JP-5	Biodiesel
Carbon (wt%)		85.80	77.71
Hydrogen (wt%)		14.07	13.05
Oxygen (wt%)		0	9.24
Distillation temperature (°C)	10% recovery	190.5	330.5
	50% recovery	205.5	334.3
	90% recovery	233.9	343.0
Density (kg/m3)		801.0	882.7
Cetane number		48.2	56.3
Kinematic viscosity (mm2/s)		1.356	4.341
Surface tension (N/m)		0.025	0.032
Lower heating value (kJ/kg)		42,950	36,910
Flash point (°C)		62.0	154.0
Pour point (°C)		−51.0	0.0
Cold flow plugging point (°C)		−35.0 or less	0.0

lists the main physicochemical properties of the JP-5 and biodiesel fuels.

2.3. Injection Strategy and Experimental Conditions

The injection strategies applied in this study were single and multiple split injection. In the case of the single-injection, the fuel injection timing was changed from before top dead center (BTDC) with a crank angle (CA) of 30 to BTDC 5 CA. The amount injected in the single-injection strategy was equally divided according to the number of injections in the multiple split injection strategy. The injected amount of fuel was determined based on the total heating value (m_fQ_LEV = 429.5 J). In the multiple split injection strategy, the first injection timing was selected as BTDC 20 CA, where NOx was formed in large amounts with the single-injection strategy. Figure 2 shows the multiple split injection strategies introduced in this research.

The injected fuel was supplied by the common rail of the test engine to visualize the spray behavior of the test fuel. The start of energizing (SOE) of the fuel injector and the start of shooting by a high-speed camera were synchronized for accurate analysis of the spray behavior process. The energy generated by the engine was dissipated by a braking resistor connected to an electric motor. The in-cylinder pressure was averaged over 300 cycles to minimize cycle-to-cycle variations. Detailed test conditions, including visualization of the spray behavior process and combustion, are listed in

Table 8. Experimental conditions.

Experiments	Parameters		Conditions
Spray behavior	Pinj		40 MPa
	Pamb		0.1 MPa
	Ambient temperature		Room temperature
	mf	JP-5	10 mg
		Biodiesel	11.64 mg
	Image record		15,000 frames/s with a 256 × 256 resolution
	Light source		High-performance optical lamp
Combustion and emission	Engine speed		1100 rpm
	Pinj		40 MPa
	Coolant temperature		353 K
	Total heat release (mfQLEV)		429.5 J
	Injection strategy		Presented in Figure 2
Combustion visualization	Pinj		40 MPa
	Coolant temperature		353 K
	Image record		16,000 frames/s with a 256 × 256 resolution

.

3. Results and Discussions

3.1. Macroscopic Spray Behavior Characteristic

Figure 3b shows images of the spray behavior of the JP-5 and biodiesel. It has been reported that JP-8 has an asymmetric spray shape owing to the strong turbulence of the internal nozzle from a valve orifice covered-type injector [33]. However, it is generally accepted that the sac-type injector has a symmetrical spray [34]. In this study, the spray patterns of the two fuels showed symmetrical results. After the SOE, the start of injection (SOI) of biodiesel was 0.2 ms later than that of JP-5 (JP-5: 0.467 ms; biodiesel: 0.667 ms). Among the physical properties of fuel, it is believed that the physical characteristics of biodiesel, such as higher kinematic viscosity and surface tension, are the main causes of SOI delay. As the spray developed, JP-5 had a longer spray tip penetration. These differences can be attributed to the influence of fuel properties. JP-5 has a lower density, distillation temperature, and kinematic viscosity compared with biodiesel. These properties weaken the surface tension of the fuel more, which causes disequilibrium into the injector hole [17,35].

Figure 3c shows the results of the spray tip penetration based on the images acquired by the high-speed camera. The spray tip penetration by the definition (Figure 3a) was calculated by analyzing the photos obtained by high speed camera from injection tip to the end of visualization window. Under a constant fuel injection pressure, JP-5 showed longer spray tip penetration than biodiesel. According to the spray model based on momentum theory, the spray tip penetration is strongly dependent on the fuel density and the average injection velocity [36,37]. Biodiesel, with its relatively higher density, can be expected to have longer spray tip penetration owing to this model. However, it is thought that the delayed SOI of biodiesel fuel, which has a greater viscosity and surface tension, leads to shorter spray length.

3.2. Combustion and Emission Characteristics with Single Injection Strategy

Figure 4 shows the influences of fuel injection timing on the maximum in-cylinder pressure, maximum ROHR, engine torque, and fuel conversion efficiency.

As expected, as the fuel injection timing approached toward the TDC and the maximum in-cylinder pressure and ROHR tended to decrease, the engine torque and fuel conversion efficiency increased. The maximum in-cylinder pressure and ROHR decreased because the premixed combustion intensity decreased as the fuel injection timing was delayed. Under advanced injection conditions, the combustion gas acts as the negative work in the compression stroke process, which reduces the engine torque and fuel conversion efficiency. Therefore, it can be concluded that the IMEP and combustion efficiency improved in the retarded injection region. The results for biodiesel shown in Figure 5 were lower than those of JP-5 regardless of the fuel injection timing, even when the total heating value (m_fQ_LEV = 429.5 J) was constant. The lower cetane number of JP-5 lengthened the ignition delay and strengthened the premixed combustion intensity. This has a significant affection on the magnitude of in-cylinder pressure and ROHR. The ignition delay of JP-5 lengthened the combustion period, thus increasing the positive work during the expansion stroke.

Figure 5 shows the O₂, CO₂, CO, and NOx characteristics of JP-5 and biodiesel with different fuel injection timings.

Biodiesel contains oxygen, but its spray angle was smaller than that of JP-5; hence, the formation of a mixture with the surrounding air was weakened. This meant that the proportion of air participating in combustion was lower than that of JP-5. Therefore, the O₂ level in the exhaust gas of biodiesel was higher than in that of JP-5, regardless of the fuel injection timing. The CO₂ emissions increased because the combustion efficiency improved as the fuel injection time was retarded. CO emissions, which are mainly caused by insufficient oxidation reaction and incomplete combustion, decreased as the fuel injection timing moved toward the TDC. Higher in-cylinder pressure and temperature shortened the ignition delay and reduced cylinder wall wetting. This increased the oxidation rate and reduced incomplete combustion, which was a major cause of the CO reductions. Compared with JP-5, the biodiesel produced higher CO emissions. The atomization of fuel droplets and mixture formation were disadvantageous compared with JP-5 because of the high viscosity of biodiesel [37].

For this reason, a locally fuel-rich area is formed in the combustion chamber, which is a major cause of incomplete combustion. NOx is a hazardous substance that is dependent on the combustion gas temperature. As the fuel injection timing was delayed, the maximum in-cylinder pressure and ROHR decreased, which led to a reduction in the combustion gas temperature. The greater vaporization rate and longer ignition delay of JP-5 increased the mixture formation rate, which caused a large amount of heat to be released in the premixed combustion phase. In addition, the faster mixture formation of the high evaporation rate reduced the locally fuel-rich condition, thereby increasing the amount of heat released and the combustion gas temperature.

3.3. Combustion and Emission Characteristics with Multiple Split Injection Strategy

Figure 6 shows the results of the in-cylinder pressure and ROHR curves for JP-5 and biodiesel for the multiple split injection strategies.

It was confirmed that the in-cylinder pressure and ROHR were superior for JP-5, which has an excellent evaporation rate and fast mixture formation. Although the divided injection quantity was reduced, the in-cylinder pressure tended to increase as the number of injection pulses increased. The in-cylinder pressure was higher than that of the single-injection strategy during the expansion stroke owing to the post-combustion effect of the final injection at the TDC. As the number of injections extended, the maximum in-cylinder pressure showed a tendency to increase; however, the maximum ROHR decreased, as expected. In particular, the maximum in-cylinder pressure increased to a level similar to that of single combustion when quintuple split combustion was applied. The increase in the pulse of consecutive fuel injections suppressed the maximum ROHR. However, three or four consecutive injections before the final injection formed a richer mixture that burned rapidly, increasing the in-cylinder pressure.

To verify the effects of the multiple split injection strategy, the maximum in-cylinder pressure, the rate of pressure rise (ROPR), the ROHR, and the mean gas temperature were compared with the single-injection strategy, as shown in Figure 7. The maximum in-cylinder pressure increased as the number of equally divided injections increased. Compared with the single-injection strategy, JP-5 and biodiesel were reduced by at least 2.5% and 0.6%, respectively. The ROPR, which is directly related to the combustion noise of the diesel engine, decreased as the number of multiple split injections increased. As a result of using multiple injections, the fuel injection quantity was divided into small portions and the ROHR was suppressed. The maximum mean gas temperature was calculated using the following equation [38,39].

$$T\left(\theta \right)=\frac{P\left(\theta \right)V\left(\theta \right)}{m_{intake}R},m_{intake}=v_l{\rho }_{air}{V}_{IVC},v_l=\frac{1}{2}\left\{sin\left[360\left(\frac{\theta }{{\theta }_{vod, intake}}-\frac{1}{{\theta }_{vod,\frac{intake}{exhaust}}{\theta }_{\frac{IVO}{EVO}}}-\frac{1}{4}\right)\right]+1\right\}$$

(1)

where P(θ), V(θ), and T(θ) are the in-cylinder pressure, cylinder volume, and in-cylinder temperature, respectively, $m_{intake}$ is in-cylinder air mass at the IVO point, ${\rho }_{air}$ is the air density, $V_{IVC}$ is the cylinder volume at IVC, $v_l$ is the function of valve lift, ${\theta }_{vod, intake}$ is period of IVO, ${\theta }_{vod, exhaust}$ is the duration of EVO, and ${\theta }_{IVO}$ and ${\theta }_{EVO}$ are the IVO angle and EVO angle, respectively.

Figure 8 shows the results of the IMEP, engine torque, fuel conversion efficiency, and combustion efficiency for the multiple split injection strategies. Fuel conversion efficiency was simulated as the ratio of fuel chemical energy(m_fQ_LEV) to engine work. In addition, combustion efficiency was defined as the proportion of fuel chemical energy to the heat release generated by combustion. Compared with the single-injection strategy, the quadruple-injection mode showed superior engine performance. Considering that the first fuel injection timing was BTDC 20 CA, the higher heat release during the compression stroke before the TDC changed through heat loss and negative work. This indicates a penalty that cannot be converted into effective work. The accumulated heat release was reduced because the overall combustion duration was shorter than that of the multiple split injection strategy. This reduced the combustion performance, engine performance, and combustion efficiency. As seen in Figure 8, the triple-injection strategy had the lowest results among the multiple-injection strategies for the following reasons. First, the last injection quantity was smaller than that of the double-injection mode, which reduced the combustion period. Each injected mass in the triple-injection strategy was 33.3% of that of the single-injection strategy. Second, each injection quantity of the quadruple and quintuple-injection strategies was smaller than that of the triple-injection strategy; however, the dwell time between fuel injection timings was more reduced than that of the triple-injection strategy. Owing to the shorter dwell time and the increased number of injections, the mixture burned in a rich condition. The results of the quintuple-injection strategy in Figure 8 are considered inferior to those of the quadruple-injection strategy because the fuel injection quantity is less than that of the quadruple strategy.

Figure 9 shows the O₂ and CO₂ emission levels of the multiple-injection strategies. The O₂ concentration was measured to be higher in biodiesel than in JP-5, similar to the results of the fuel injection timing research. The low O₂ concentration indicated that the oxidation reaction proceeded actively. Compared with the single-injection strategy, the O₂ concentration in the multiple split injection strategy was low, with the lowest found in the quadruple strategy. This result proves that the oxidation reaction rate was the highest, with the quadruple strategy. The increase in the number of injections formed a rich mixture and promoted the oxidation reaction with O₂, which improved the combustion efficiency. CO₂ is proportional to the combustion rate of the injected fuel. The quadruple strategy, which had the lowest O₂ level in the exhaust gas, had the highest CO₂ emission levels.

Figure 10 shows the CO and NOx emission levels of the multiple-injection strategies. With the single, double, and triple-injection strategies, the CO emissions of biodiesel were greater than those of JP-5. Interestingly, less CO reduction was observed in the quadruple and quintuple strategies compared with JP-5. As presented in Figure 10, the O₂ concentration of biodiesel was higher than that of JP-5 when the multiple split injection strategy was applied. Although biodiesel has a lower carbon and higher oxygen component than JP-5, the increase in CO emissions can be explained by the oxidation reaction rate being lower than that of JP-5. With the quadruple strategy, the mixture became richer, and combustion under richer conditions increased CO emissions. Biodiesel is believed to be more effective at reducing CO than JP-5 through activating oxidation with the oxygen inclusion of the fuel. Thermal NOx from an internal combustion engine is governed on the combustion gas temperature, and the in-cylinder pressure is the dominant parameter in the combustion gas temperature. The results in Figure 7 confirm that the maximum in-cylinder pressure and mean gas temperature increased as the number of split injections increased. With the multiple split injection strategy, a higher maximum ROHR was not identified after the first maximum heat release. The multiple combustion gradually increased the in-cylinder pressure, resulting in the highest in-cylinder pressure after the TDC. This played a major role in increasing the combustion gas temperature. The NOx emissions for the double-injection strategy decreased by up to 50.8% and 55.5% for JP-5 and biodiesel, respectively, compared with the single-injection strategy. NOx emissions were observed to increase gradually with a large amount of fuel injection pulse.

3.4. Combustion Visualization

Figure 11 shows continuous images of the combustion flame of JP-5 and biodiesel fuels with the single-injection and double split injection strategies.

It can be confirmed that the ignition delay of biodiesel was significantly shorter for the single-injection strategy. This is because the SOI of biodiesel is slower than that of JP-5; however, the oxygen component of the fuel promotes rapid ignition. In addition, it was confirmed that the flame of JP-5 spread faster after ignition, with an excellent evaporation rate and rapid mixture formation. In the double split injection mode, the fuel injected from BTDC 20 CA ignited first in the biodiesel in the form of scattered flames. This can be seen as the natural luminosity in the images from after ATDC 4.5 CA. The fuel injected from the TDC has a diffusion combustion form; as a result, the ROHR was suppressed. JP-5 had a longer ignition delay than biodiesel when the natural luminosity was used to define the start of combustion in the images of the combustion process. However, the flame distribution throughout the cylinder after ignition was wider than that of biodiesel because the flame propagation speed was very fast.

4. Conclusions

In this experimental study, the spray behavior, combustion, and emission characteristics of JP-5 and biodiesel and their combustion process were analyzed through visualization in an experiment applying single-injection and multiple split injection strategies in an optically accessible single-cylinder CRDI diesel engine. The following important conclusions were drawn from this study:

I. The sac-type injector spray exhibited a symmetrical pattern, regardless of the fuel properties. The SOI of the fuel was delayed because biodiesel had a higher density, kinematic viscosity, and surface tension than JP-5. JP-5 had better spray tip penetration than biodiesel.

II. Combustion characteristics such as the maximum in-cylinder pressure and ROHR decreased as the injection timing approached the TDC; however, the engine performance improved owing to the increase in the positive work. Combustion under retardation conditions is a direct cause of increased CO₂ emissions with a large amount of O₂ participation. CO decreased because the oxidation reaction was activated as the injection timing was retarded. NOx reduction was observed as the combustion gas temperature decreased. Biodiesel had disadvantages in terms of combustion and emission characteristics compared with JP-5, regardless of the fuel injection timing.

III. As the number of split injections extended, the maximum in-cylinder pressure and gas mean temperature increased, while the opposite was observed for the maximum ROPR and ROHR. The quadruple-injection strategy showed the best combustion, performance, and efficiency, including the highest O₂ consumption and CO₂ emissions. The CO emissions of JP-5 decreased for the double and triple-injection strategies; however, an increase in CO with the quadruple strategy was observed again. A significant reduction in CO of up to 83.8% for biodiesel was achieved as split injection pulses increased. An increase in the number of split injections raised the combustion gas temperature, which led to an increase in NOx production from JP-5 and biodiesel.

IV. JP-5, with its excellent evaporation rate, has a longer ignition delay compared with biodiesel, which promotes faster mixture formation inside the cylinder. The rapid flame propagation of JP-5 after ignition enables a combustion process similar to that of biodiesel. Combustion flame by fuel injection was observed to be typical diffusion combustion from the multiple-injection strategy.

Experimental studies on BD100 are working on a CRDI single-cylinder diesel engine to improve the combustion and emission characteristics using multi-stage pre-injection strategies under cold-start conditions. The multi-stage pre-injection strategy can improve the combustion quality and emissions. Combustion quality under the multi-stage injection strategy can be analyzed using RI (ringing intensity) theory. In addition, in the near future, we plan to research the simultaneous reductions in PM and NOx using different injection strategies with a PM analyzer and a urea injection system.