Investigation of the Impact of Combustion Chamber Geometry on Engine Combustion and Emission Performance Under Various Fuel Injection Timings With Biodiesel Blending

Abstract

ABSTRACT

This study uses AVL FIRE 2020 R1 software for simulation and experimental verification to deeply analyze the impact of combustion chamber geometry and biodiesel on diesel engine performance at different injection timings. The study found that: With the advancement of injection timing, the indicated fuel consumption rate, cylinder pressure and NOx emissions of the two combustion systems increased, while the indicated thermal efficiency, temperature and Soot emissions decreased accordingly; The blending of low calorific value biodiesel will increase the indicated fuel consumption rate of the two combustion systems, but at the same time it can effectively reduce NOx and Soot emissions; The T: Turbocharger, C: Charger air cooling, D: Diesel particle filter (TCD) combustion system improves the utilization rate of cylinder air due to its unique combustion chamber geometry, thereby improving combustion performance. Compared with the Omega combustion system, the indicated thermal efficiency of the TCD combustion system increased by 6.16% to 8.38% and the indicated fuel consumption rate decreased by 5.80% to 7.73% when burning four types of fuel. In addition, the in‐cylinder pressure and temperature increased, and it performed better in reducing Soot emissions. The research results show that the TCD combustion system can effectively improve the combustion and emission performance of diesel engines, provide data support for the development of diesel engine combustion systems and the combustion of oxygen‐containing fuels in plateau environments, and provide an important reference for energy conservation and emission reduction.

Introduction

Nowadays, new energy technology is developing rapidly, but the diesel engine still remains the main power unit for current agricultural machinery, engineering machinery, and military equipment due to its high thermal efficiency [1, 2]. With the increasingly severe energy crisis and environmental pollution problems, the development of safe, clean, and efficient internal combustion engines is of great significance [1, 3–5]. The combustion of oxygenated fuels [6–8] and the optimization of the combustion chamber's shape have become important technological means to improve the thermal efficiency of diesel engines and reduce the emission of pollutants [9–11]. Biodiesel [12] is considered a high-quality alternative to diesel because it has similar physical and chemical properties to diesel, can be blended with diesel in any ratio, is renewable, does not contain sulfur, and does not require structural changes to the diesel engine [13, 14]. However, its higher oxygen content can cause the adverse effect of elevated NOx emissions [15]. Changes in the geometry of the combustion chamber affect the mixing of the cylinder mixture, and the quality of this mixture significantly impacts the in-cylinder combustion process, thereby affecting power, economy, and emission performance [16–19]. There are numerous domestic and international studies on the use of oxygenated fuels [20–22] and on the optimization of combustion chamber geometry [23–26]. Pullagura et al. [27] investigated the effects of graphene nanoplatelets (GNPs) in water-diesel emulsified fuel (WDEF) on the performance, combustion characteristics, and emissions of diesel engines, particularly under variable injection timing. The results indicated that the addition of 60-ppm GNPs to WDEF, with an injection timing advanced to 25° before top dead center (BTDC), significantly improved engine performance, combustion characteristics, and emission indicators. Amin et al. [28] investigated the effect of diesel injection timing on the low-speed, high-load combustion performance of a natural gas/diesel dual-fuel engine using numerical simulation. They pointed out that advancing the diesel injection timing can significantly improve thermal efficiency and reduce the emission of pollutants, such as CH4. Çelik and Önder Özgören [29] studied blends of soybean biodiesel, hazelnut biodiesel, and diesel and found that the exothermic rate and peak in-cylinder combustion pressure of these blends were reduced. This reduction was related to the low calorific value of biodiesel. Singh et al. [30, 31] investigated the effects of adding catalysts (fly ash and zeolite) during the pyrolysis of plastics on the yield and quality of biofuels. They also tested the engine performance of diesel and plastic pyrolysis oil mixed fuels. The results indicated that although the addition of catalysts reduced the liquid yield, it improved the quality of the oil. The combustion performance and emission indicators of the mixed fuel were comparable to diesel, and even showed some improvement. This suggests that using fuel derived from waste plastics in internal combustion engines is feasible and can reduce environmental pollution. Furthermore, they used Momordica charantia (L.) seeds [32] as an alternative feedstock for biodiesel production and employed calcium oxide (CaO) derived from duck eggshells as a novel heterogeneous catalyst for the esterification reaction. This achieved a 96.8% conversion rate of methyl esters. They also conducted a comprehensive analysis of the performance and emission characteristics of the resulting biodiesel. Compared to diesel, NOx emissions were significantly reduced, while other emission indicators were similar. Lastly, they utilized response surface methodology and full factorial design for multiobjective optimization, successfully enhancing the performance of diesel engines using microalgae Spirulina (L.) [33] as fuel and reducing emissions. Benjumea et al. [34] investigated the performance and combustion characteristics of a turbocharged diesel engine fueled with pure palm oil biodiesel (B100) and diesel No. 2 (B0) at altitudes of 500 and 2400 m. The study found that palm oil biodiesel can improve engine performance at high altitude. Zhang et al. [35] modeled a diesel engine using AVL-Fire software and investigated the combustion and emissions of diesel/biodiesel at different loads. As the biodiesel blend ratio increased, the maximum cylinder pressure decreased, the instantaneous exothermic rate gradually increased, and NOx emissions increased, while hydrocarbon (HC), carbon monoxide (CO), and particulate matter (PM) emissions decreased. Bao et al. [36] designed four different combustion chamber shapes to study and numerically analyze the effects on the engine's combustion process, performance, and pollutant emissions. The results show that different combustion chamber shapes have a slight effect on the start of combustion, and the differences in in-cylinder pressure become more pronounced as combustion progresses. Higher turbulent kinetic energy improves airflow motion and promotes the formation of in-cylinder mixtures. However, increasing the depth of the combustion chamber recesses reduces NOx formation at the cost of fuel economy. According to Karthickeyan [37], diesel engine retrofit technology is one of the current development directions in the field of diesel engine research. This technology aims to promote the realization of complete combustion in diesel engines. Two new types of combustion chambers were designed: annular and trapezoidal combustion chambers. It was found that annular combustion chambers can better mix air and fuel, leading to more complete fuel combustion. Şener and Gül [38] studied the shape optimization of a compression ignition engine under the guidance of computational fluid dynamics (CFD). The aim was to optimize the combustion efficiency of diesel fuel while maintaining engine power and torque. Zhang et al. [39] determined that the geometry of the corrugated combustion chamber can enhance the performance of diesel engines using ethanol and biodiesel blends. Khan, Panua, and Bose [40] investigated the effects of spray angle and combustion chamber geometry on the mixing, combustion, and emission characteristics of a direct injection diesel engine. They used three different combustion chamber geometries and four injection angles: 150°, 155°, 160°, and 165°. They found that a suitable combustion chamber geometry can provide better diesel engine performance. Therefore, modifying the diesel engine by changing the combustion chamber geometry is a good option.

To investigate the effects of combustion chamber geometry and biodiesel on engine performance, this study examines a specific type of diesel engine. AVL FIRE 2020 R1 software is employed to model a prototype ω combustion chamber and an optimized TCD (T: Turbocharger, C: Charge Air Cooler, D: Diesel Particulate Filter) combustion chamber. The performance of these engines is evaluated when they operate on various fuel blends: D100 (pure diesel), B10 (10% biodiesel by volume and 90% diesel), B20 (20% biodiesel by volume and 80% diesel), and B50 (50% biodiesel by volume and 50% diesel), respectively. The injection advance angles for the above two combustion chambers with 7°, 9°, 11°, 13°, and 15° BTDC were calculated and verified by the plateau altitude test at 2000 m above sea level, respectively. Through three-dimensional simulation technology, this study explored the cylinder temperature, cylinder pressure, indicated power, indicated thermal efficiency (ITE), indicated fuel consumption rate, and NOx and Soot emission characteristics of two combustion systems when biodiesel was mixed in the 2000-m plateau environment. This study can not only provide data support for the optimization design of diesel engine combustion chamber systems in plateau areas, but also provide an important reference for the study of the combustion characteristics of oxygen-containing fuels in plateau environments. These research results will help guide the development of diesel engine combustion chamber systems in plateau areas, optimize the combustion process, reduce NOx and Soot emissions, and provide a scientific basis for achieving energy conservation and emission reduction goals.

Establishment of Simulation Model

Simulation Model Parameter Setting

The engine utilized in this study is an inline four-cylinder, four-stroke diesel engine. The main parameters of the engine are presented in Table 1, which features a deep pit ω-type combustion chamber. The testing was conducted in Kunming, Yunnan Province, at an altitude of 2000 m, under the condition of 1800 revolutions per minute (rpm) at full load. The simulation model parameters were set in accordance with the experimental control parameters. Initial conditions and simulation boundary conditions are detailed in Table 2, while Table 3 outlines the selection of each submodel (Figure 1).

Table 1 Main parameters of the test engine.

Menu	Numerical
Combustion chamber type	Direct injection ω-type combustion chamber
Bore × Stroke (mm × mm)	80 × 92
Compression ratio	16.5
Calibration power/KW	58(2500 r/min)
Maximum torque/(N·m)	265(1800 r/min)
Number of spray holes	6
Hole Diameter/mm	0.122
Injector hole angle/°	156°

Table 2 Simulation initial and boundary conditions.

Parameter name	Numerical
Initial pressure	2.418 bar
Initial temperature	340 K
Injection start–end moment	353°–378°CA
Single-hole circulating oil injection volume	9.55 mg
Clamping angle of oil bundle	156°

Table 3 Sub-model selection.

Submodel type	Select model name
Turbulence models	k-zeta-f model
Spray model	Wave model
Combustion model	ECFM-3Z model
Soot generates models	Kinetic model
NOx generation modeling	Extend Zeldovich model

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Test Oil

Biodiesel and diesel were blended in specific volume ratios to create B10 (10% v/v biodiesel and 90% v/v diesel), B20 (20% v/v biodiesel and 80% v/v diesel), B50 (50% v/v biodiesel and 50% v/v diesel), and pure diesel D100 (100% v/v diesel) blends. The physical and chemical properties test of diesel and biodiesel was completed by the staff of the Oil Product Analysis Institute of Sinopec Group. The experimental test equipment mainly includes: DSY-006B pour point, solidification point, cloud point, cold filter point tester, BSH-2 closed cup flash point tester, kinematic viscosity test constant temperature bath, DSY-020 petroleum product copper sheet turbidity tester, DSS-1 petroleum product moisture tester, DRD-100 automatic distillation tester, and so on. The main physical and chemical properties of these different fuels are presented in Table 4.

Table 4 Physical and chemical properties of different fuels.

Fuel	Quality indicators
D100	B100	B10	B20	B50
Oxygen mass fraction/%	0	10	1	2	5
Low calorific value/MJ·kg−1	42.85	39.50	42.51	42.19	42.18
Kinematic viscosity (40°C)/mm²/s	3.67	6.01	4.51	4.71	5.31
Density(20°C)/kg/m³	821.8	946.6	827.4	832.7	849.6
Cetane number	53.6	60.4	54.3	55.3	57.6
Sulfur mass fraction/%	0.0234	0.0013	0.0187	0.0143	0.0105
Acidity/(mgKOH·(100Ml)−1)	4.49	8.37	3.03	4.19	5.49
Copper sheet corrosion (50°C, 3 h)/level not more than	1	1	1	1	1
Flash point/°	63	130	73	74	80
Freezing point/°	−7	−1	−7	−5	−4
Boiling point/°	185	220	142	145	161

Simulation Model and Method

The established combustion chamber model was simulated and analyzed using CFD software AVL-FIRE 2020 R1. This software is widely used in the field of engine simulation for its high accuracy and reliability. The model establishment parameters are the same as the engine parameters, ensuring the consistency of the simulation results with the actual engine performance. To save the calculation cost, the simulation analysis only considers the flow and combustion in the cylinder from the closing of the intake valve to the opening of the exhaust valve. The intake valve opening time is 232° CA ATDC, and the exhaust valve opening time is 507° CA ATDC. The selection of these two moments is based on the working cycle characteristics of the engine and the design requirements of the combustion chamber. Considering that the six nozzles of the injector are evenly arranged and symmetrical in the circumference, a 1/6 combustion chamber model is established as the calculation object. This simplified model can reduce the amount of calculation while maintaining sufficient accuracy to capture the main flow and combustion characteristics in the combustion chamber. The model uses hexahedral grids, which are widely used because of their regular structure and high calculation efficiency. The average size of the main grid is determined to be about 0.5 mm, which can both ensure the accuracy of the calculation and control the calculation cost. The total number of meshes for the two types of combustion chambers is 20740 for the prototype ω combustion chamber and 19890 for the TCD combustion chamber. The selection of these mesh numbers is based on the balance between simulation accuracy and computing resources. Figures 2 and 3 are structural schematics and model mesh schematics of the two types of combustion chambers. These diagrams provide an intuitive display of the combustion chamber and a detailed view of the mesh division, which helps to understand the geometric structure of the combustion chamber and the construction of the simulation model.

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Validation of the Model

The initial boundary conditions are introduced into the selected computational model, and the operating conditions for the combustion of four fuels at 1800 r/min and 100% load are also specified for numerical simulation of the prototype diesel engine. The simulated cylinder pressure and heat release rate are compared with experimental results, as shown in Figure 3. The graphs of cylinder pressure and heat release rate from the simulation results are essentially in agreement with those from the experimental data. Consequently, the mathematical model employed in this study exhibits a good fit to the diffusion combustion process of the diesel engine. The discrepancy between the simulated and experimental data is confined to within 5%, ensuring the accuracy of the model, which is thus validated for subsequent calculations. In this paper, the positive and negative values of the crankshaft angle indicate the position relative to the top dead center (TDC) of the compression stroke; a negative value signifies a position before TDC, while a positive value indicates a position after TDC.

Results and Discussion

In this study, the research objectives are the combustion and emission characteristics of two distinct combustion systems in diesel engines operating in a plateau environment. The fuels selected for investigation are D100, B10, B20, and B50, representing various blends of biodiesel with diesel fuel. The study aims to explore the combustion and emission characteristics of these biodiesel blends within the two combustion systems under the specific conditions of a 2000-m altitude plateau environment. A three-dimensional simulation approach is employed to analyze these characteristics.

Combustion Characteristics

Indicated Power

Indicated power refers to the indicated work consumed in the actual cycle per unit time, which is one of the important indicators for evaluating engine performance. Figure 4 shows the indicated power of the two combustion systems blended with biodiesel at different injection timings. As shown in the figure, with the advancement of injection timing, the indicated power of the two combustion systems when burning four fuels, D100, B10, B20, and B50, is reduced. At the same time, with the increase of the proportion of biodiesel, the indicated power of the two combustion systems also decreases. This is because the kinematic viscosity of biodiesel itself is higher than that of diesel. As the proportion of biodiesel in the mixed fuel increases, the viscosity of the mixed fuel also increases, resulting in a decrease in the atomization degree of the fuel after injection, and the diffusion of the oil–gas mixture to the entire combustion chamber becomes worse, which in turn affects the combustion effect. Compared with the Omega combustion system, when burning D100, B10, B20, and B50 fuels, the indicated power of the TCD combustion system increased by 8.17%, 7.74%, 7.18%, and 6.25%, respectively. It can also be seen from the figure that no matter what fuel is used and under what injection timing conditions, the indicated power of the TCD combustion system is higher than that of the Omega combustion system. This is because the diversion effect of the TCD combustion system can accelerate the oil-gas mixing in the cylinder. The higher the speed, the more obvious the improvement effect of the oil-gas mixing in the cylinder, which is conducive to the rapid and full combustion of the fuel, thereby increasing the indicated power.

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Indicative Specific Fuel Consumption (ISFC)

The ISFC is defined as the amount of fuel consumed per unit of indicated work and serves as a critical metric for evaluating an engine's fuel efficiency and economy. Figure 5 illustrates the ISFC for biodiesel blends in two different combustion systems at various injection timings. As depicted in the figure, the ISFC for both combustion systems increases with more advanced injection timing. Specifically, for the TCD combustion system, the ISFC is reduced by 7.55%–7.73%, 7.10%–7.20%, 6.70%–6.92%, and 5.80%–5.88% when burning D100, B10, B20, and B50 fuels, respectively, compared to the Omega combustion system. As the injection timing is advanced, the fuel injected into the cylinder has more time to atomize and mix with the air to form a uniform combustible mixture. The surface area of the atomized fuel droplets increases, making it easier for the fuel to evaporate. The evaporation of the droplets absorbs heat, thereby reducing the temperature in the cylinder. The earlier the injection timing, the longer the evaporation time, resulting in a higher indicated fuel consumption rate. This is because the early injection allows the fuel to have more time to mix with the air, forming a more uniform mixture, thereby improving combustion efficiency. At the same time, the droplet evaporation absorbs heat to help reduce the temperature in the cylinder, reduce heat loss, and further improve the indicated fuel consumption rate. Regardless of the diesel fuel type, the TCD combustion system consistently exhibits a lower ISFC than the Omega system. This is due to the TCD system's enhanced combustion performance, which is achieved by increasing in-cylinder air utilization, a result of its unique geometry compared to the Omega system. Furthermore, as the biodiesel content in the fuel blend increases, the ISFC for both combustion systems also increases. For the Omega combustion system, the ISFC when fueled with D100 is reduced by 3.36%, 3.24%, 3.01%, 2.94%, and 2.96%, respectively, compared to the TCD system when fueled with B50 at all injection timings. This increase in ISFC is attributed to the lower calorific value of biodiesel, which further decreases as the biodiesel percentage in the blend rises.

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Indicative Thermal Efficiency

ITE is the ratio of the actual thermal energy converted within the engine to the theoretical thermal energy of complete fuel combustion, serving as a pivotal parameter for assessing the performance of an internal combustion engine. Figure 6 illustrates the ITE of biodiesel blends in two combustion systems at various injection timings. As depicted in the figure, the ITE of the two combustion systems decreases with more advanced injection timing. Specifically, the ITE of the TCD combustion system is enhanced by 8.17%–8.38%, 7.64%–7.76%, 7.18%–7.43%, and 6.16%–6.25%, respectively, compared to the Omega combustion system when burning D100, B10, B20, and B50 fuels. This enhancement is attributed to the increased time for fuel-air mixing and atomization as injection timing advances, leading to a more homogeneous combustible mixture. The increased surface area of atomized fuel droplets facilitates faster evaporation, which in turn absorbs heat and lowers the cylinder temperature. The earlier the injection timing, the longer the evaporation time, resulting in a lower ITE. The TCD combustion system consistently exhibits a higher ITE than the Omega system, irrespective of the diesel fuel type. This is due to the TCD system's unique geometry, which improves in-cylinder air utilization and combustion performance compared to the Omega system. Furthermore, as the biodiesel content in the fuel blend increases, the ITE of both combustion systems decreases. For the Omega combustion system, the ITE for D100 decreases by 0.56%, 0.68%, 0.91%, 0.99%, and 0.97%, respectively, compared to the TCD combustion system for B50 at various injection timings. Biodiesel has a lower calorific value than conventional diesel. As the proportion of biodiesel in a fuel blend increases, the calorific value of the entire fuel blend decreases. Since calorific value is an important indicator of the energy content of a fuel, a decrease in the calorific value of a fuel blend directly affects the energy released during combustion. Therefore, as the proportion of biodiesel increases, the calorific value of the fuel blend decreases, which in turn leads to a decrease in the ITE of the two combustion systems. This is because thermal efficiency is a measure of the efficiency of converting fuel energy into useful work, and a lower calorific value means that the same mass of fuel releases less energy, thereby reducing the overall thermal efficiency of the combustion system.

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In-Cylinder Pressure

In-cylinder pressure is the force exerted on the cylinder wall by the high-temperature and high-pressure gases produced during fuel combustion. This pressure is influenced by the duration of the stagnant combustion phase and the characteristics of the combustion mixture. The peak in-cylinder pressure is determined by the combustion rate during the premixed combustion stage, which in turn is affected by the engine's operating conditions and the composition of the fuel intake [41]. In-cylinder pressure is a critical factor in the engine's power output and significantly impacts the engine's performance and efficiency. Figure 7 illustrates the in-cylinder pressures for the Omega and TCD type combustion systems with biodiesel blends at various injection timings. As depicted from Figure 7a–d, regardless of the type of diesel fuel combusted, the in-cylinder pressure of the TCD combustion system consistently exceeds that of the Omega system. Furthermore, the in-cylinder pressures for both combustion systems escalate with the advancement of injection timing. This phenomenon can be attributed to the distinctive structure of the TCD combustion system, which enhances the quality of the oil–gas mixture within the cylinder and the combustion characteristics, thereby resulting in higher in-cylinder pressures compared to the Omega system. With the advancement of injection timing, the in-cylinder pressure and temperature of the fuel decrease upon injection, leading to an extended stagnation period. This allows the injected fuel ample time to mix with air, forming a more homogeneous fuel-air mixture that enhances combustion and consequently increases in-cylinder pressure. A similar trend was observed by Li, Zhang, and Li [42].

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As shown in Figure 7e, the in-cylinder peak pressure of both combustion systems increases with the advance of injection timing, because the advance of injection timing allows the fuel injected into the cylinder more time to mix with the air and form a more uniform oil–gas mixture. This good mixing quality improves the combustion process in the cylinder, resulting in an increase in the in-cylinder peak pressure. The advance of injection timing may also affect the ignition timing and combustion rate, because the mixing of fuel and air starts earlier, which may lead to a more rapid combustion process, thereby generating higher pressure in the combustion chamber. On the other hand, as the proportion of biodiesel in the mixed fuel increases, the in-cylinder peak pressure decreases. This is because biodiesel itself has a higher kinematic viscosity and surface tension, which leads to slower evaporation, atomization, and fragmentation. As the proportion of biodiesel increases, the overall kinematic viscosity and surface tension of the mixed fuel increase, which affects the formation and combustion efficiency of the oil–gas mixture, making the in-cylinder peak pressure of the biodiesel mixed fuel lower than that of pure diesel.

Average Temperature in the Cylinder

The in-cylinder temperature refers to the temperature of the gas mixture within the cylinder (comprising air and fuel) or the products of combustion during the engine's combustion process. It typically reaches its peak within the engine's combustion chamber, particularly during the combustion phase. The in-cylinder temperature is a critical parameter that significantly influences engine performance and efficiency. Figure 8 illustrates the in-cylinder temperatures for the Omega and TCD combustion systems when blended with biodiesel at various injection timings. As depicted from Figure 8a–d, regardless of the type of diesel fuel combusted, the in-cylinder temperature of the TCD combustion system is consistently higher than that of the Omega system. Moreover, the in-cylinder temperature for both systems decreases with the advancement of injection timing. This reduction is attributed to the inflow effect of the TCD combustion system, which enhances the mixing of oil and gas within the cylinder, thereby increasing the air utilization rate and promoting complete fuel combustion, leading to higher in-cylinder temperatures.

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With the advancement of injection timing, the average in-cylinder temperature curves for both combustion systems exhibit an earlier and steeper increase in the −20° to 0° CA ATDC range, peaking in the 15°–20° CA ATDC range, and then decreasing as injection timing advances further. The increased stagnation period due to advanced injection timing allows for a more homogeneous fuel mixture and extended atomization and volatilization times for the fuel, which in turn absorbs some of the heat, resulting in a reduction of in-cylinder temperature.

Figure 8e shows that the in-cylinder peak temperature of both combustion systems decreases with the advance of injection timing. This is because the advance in injection timing allows the fuel injected into the cylinder more time to mix with the air and form a more uniform oil–gas mixture. This good mixing quality improves the combustion process in the cylinder, reduces the formation of local high-temperature areas, and leads to a decrease in the in-cylinder peak temperature. On the other hand, as the proportion of biodiesel in the mixed fuel increases, the in-cylinder peak temperature also decreases. This is because biodiesel itself has a lower calorific value and cetane number. As the proportion of biodiesel increases, the calorific value and cetane number of the mixed fuel decrease. A lower calorific value means that the same mass of fuel releases less energy, while a lower cetane number may cause combustion delays. The two work together to reduce the in-cylinder temperature of the fuel–fuel mixed fuel. A similar trend was also observed by Li et al. [43].

Equivalent Ratio Distribution

The fuel equivalence ratio distribution was observed by slicing, and the oil–gas mixing process of the Omega combustion system and the TCD combustion system at different injection timings and when burning different biodiesels were analyzed. Figure 9 illustrates the equivalence ratio distribution in combustion systems using biodiesel blends at different injection timings for both the Omega and TCD types. As observed in the figure, at a crankshaft rotation angle of 7° CA ATDC, in the Omega combustion system, the fuel spray spreads from the side wall of the combustion chamber to the top gap and bottom area after impacting the wall. The advance of injection timing leads to an increased stagnant combustion period, allowing more time for the fuel to mix with air and increasing the fuel diffusion area, particularly in the top gap of the combustion chamber. This effect is more pronounced in the TCD combustion system.

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In the TCD combustion system, the fuel spray, after impacting the wall, is guided by the ring-shaped bumps to flow into the inner chamber along the bottom arc section of the combustion chamber towards the center, and into the outer chamber along the bottom of the shallow disk, colliding with the side wall to form a jet that develops towards the bottom of the cylinder head and diffuses in the area around it. In the TCD combustion system, the oil beam has a secondary impact on the wall after hitting the wall, which expands the fuel diffusion area in the cylinder and improves the oil-gas mixing quality. However, in the Omega combustion system, after hitting the wall, the fuel mainly diffuses along the side wall of the combustion chamber to the top gap and the bottom area of the combustion chamber. The fuel diffusion area is smaller than that of the TCD combustion system, and the fuel distribution is concentrated near the wall. Therefore, the oil-gas mixing quality is poor, the combustion performance is reduced, and the Soot emission increases.

Biodiesel, due to its inherently high kinematic viscosity, density, and surface tension, causes an increase in these properties of the fuel blend as the proportion of biodiesel increases. This results in a deterioration of the in-cylinder equivalence ratio distribution for the fuel blends in both combustion systems compared to that of pure diesel. However, the TCD combustion system, with its unique geometry, results in a wider in-cylinder fuel diffusion area and more homogeneous oil/gas mixing, leading to a more favorable equivalence ratio distribution compared to the Omega system. Comparing the distribution of fuel equivalence ratio under different working conditions, it can be seen that the oil-gas mixing quality of the TCD combustion system is better than that of the Omega combustion system under different fuels and injection timings.

Temperature Distribution Map

Figure 10 illustrates the temperature distribution in combustion systems using biodiesel blends at different injection timings for both the Omega and TCD types. It is observed that at a crankshaft rotation angle of 10° CA ATDC, regardless of the type of diesel fuel, the in-cylinder temperature distribution field expands with the advancement of injection timing, indicating an increase in temperature. This expansion is attributed to the earlier injection timing, which increases the proportion of premixed combustion and the stagnation period, leading to a greater amount of fuel-air mixture at the initial stage of combustion. A higher fuel–air mixture ratio provides more ignition points and energy, accelerating the combustion rate and the diffusion combustion process. Consequently, more fuel–air mixture burns and releases heat at the top dead center, causing the temperature peak to rise and reach the maximum value. The peak in-cylinder temperatures of the TCD combustion system when burning D100, B10, B20, and B50 are 2181.49, 2167.23, 2152.03, and 2113.70 K, respectively, while the peak in-cylinder temperatures of the Omega combustion system when burning D100, B10, B20, and B50 are 1988.35, 1978.04, 1967.81, and 1939.91 K, respectively.

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The calorific value of the blended fuel decreases as the proportion of biodiesel in the fuel mixture increases due to the lower calorific value of biodiesel itself. This results in a poorer in-cylinder temperature distribution for the two combustion systems burning the blended fuel compared to that of pure diesel. However, the TCD combustion system, with its unique geometrical structure, has a wider fuel diffusion area in the cylinder, leading to more homogeneous oil/gas mixing and more complete fuel combustion. This results in a higher in-cylinder temperature compared to the Omega combustion system.

Velocity Distribution in the Cylinder

To better analyze the in-cylinder oil–gas mixing process of the two combustion systems burning mixed diesel fuel at different injection timings, the in-cylinder velocity distribution following fuel wall impact is examined. As depicted in Figure 11, at a crankshaft rotation angle of 6° CA ATDC, wall impingement has occurred in both combustion systems, resulting in the formation of a low-velocity region at the impingement location. The low-speed zone in the TCD combustion system appears at the rounded corner of the annular protrusion, with a speed of about 40 m/s. There are high-speed flow areas above 100 m/s near the walls on both sides of the protrusion, while a large area of low-speed area below 40 m/s is formed in the fuel wall-impacting area of the Omega combustion system, indicating that the annular protrusion structure of the TCD combustion system can play a good flow guidance effect, and the energy loss of fuel hitting the wall is smaller than that of the Omega combustion system, thereby alleviating the problems of fuel wall accumulation and wall-attached combustion. In the Omega combustion system, after the fuel impacts the wall, the oil jet spreads along the side wall of the combustion chamber to the top gap and the bottom area. As the injection process continues and the piston moves downward, more fuel flows to the bottom than to the top gap. Subsequently, the amount of fuel flowing to both areas becomes similar, and a cyclone is formed at the bottom area of the combustion chamber, near the center.

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In the TCD combustion system, after the fuel impacts the wall, the oil jet develops in two chambers: the inner and outer. The oil jet, after hitting the wall, spreads to the internal and external chambers. With the progression of the injection process and the downward movement of the piston, the fuel flowing into the inner chamber diffuses from the bottom arc section of the combustion chamber towards the center. The fuel flowing into the outer chamber develops along the bottom surface of the shallow disk, collides with the side wall, and forms a jet that moves towards the bottom of the cylinder head, diffusing into the surrounding area and forming cyclone flows on the right and left sides. This facilitates the complete diffusion of the fuel and the formation of a more homogeneous oil–gas mixture.

As the proportion of biodiesel in the fuel mixture increases, the in-cylinder flat velocity decreases for both combustion systems. This reduction is attributed to the higher kinematic viscosity of biodiesel. As the proportion of biodiesel in the fuel blend increases, the viscosity of the blend also increases, leading to poorer diffusion of the fuel blend in the combustion chamber compared to pure diesel fuel. Consequently, this results in lower in-cylinder fuel velocities for both combustion systems when firing the blended fuel compared to those firing pure diesel fuel.

Emission Characterization

During the operation of a diesel engine, the exhaust gases emitted contain a variety of hazardous substances. These include CO, carbon dioxide (CO₂), and HC, which are primarily produced due to insufficient combustion. Additionally, PM, soot, and nitrogen oxides (NOx) are generated by the incomplete combustion of fuel [44]. These harmful gases can pollute the environment and adversely affect human health [45, 46]. Therefore, the objective of this paragraph is to investigate the emission patterns of pollutants from biodiesel blended with different combustion systems through simulation.

Nox Emissions

NOx in diesel exhaust is mainly composed of NO and contains a small amount of NO₂. NO molecules contain free radicals, so it is an extremely unstable weakly toxic gas. NO can be quickly oxidized into NO₂ when it comes into contact with oxygen, and NO₂ is a highly irritating and toxic gas that causes great harm to the human body. In diesel exhaust emissions, NO₂ accounts for only a small part, and the vast majority is NO. This is mainly because NO₂ decomposes into NO at high temperatures. At a temperature of about 650°C, NO₂ will completely decompose into O₂ and NO. Due to the high temperature in the diesel cylinder, when discussing nitrogen oxides in exhaust emissions, the main focus is on NO. NOx in diesel exhaust is mainly composed of NO, and there is a small amount of NO₂. The generation of NOx in diesel engines is mainly controlled by the thermodynamic NO mechanism, especially the Zeldovich mechanism [47–50]. Its reaction mechanism is as follows: 1$<math altimg="urn:x-wiley:20500505:media:ese32000:ese32000-math-0001" display="block" wiley:location="equation/ese32000-math-0001.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi>N</mi><mn>2</mn></msub><mo>\unicode{x0002B}</mo><mi>O</mi><mo>\unicode{x021D4}</mo><mi mathvariant="italic">NO</mi><mo>\unicode{x0002B}</mo><mi>N</mi><mo>,</mo></mrow></mrow></math>$ 2$<math altimg="urn:x-wiley:20500505:media:ese32000:ese32000-math-0002" display="block" wiley:location="equation/ese32000-math-0002.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mi>N</mi><mo>\unicode{x0002B}</mo><msub><mi>O</mi><mn>2</mn></msub><mo>\unicode{x021D4}</mo><mi mathvariant="italic">NO</mi><mo>\unicode{x0002B}</mo><mi>O</mi><mo>,</mo></mrow></mrow></math>$ 3$<math altimg="urn:x-wiley:20500505:media:ese32000:ese32000-math-0003" display="block" wiley:location="equation/ese32000-math-0003.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mi>N</mi><mo>\unicode{x0002B}</mo><mi mathvariant="italic">OH</mi><mo>\unicode{x021D4}</mo><mi mathvariant="italic">NO</mi><mo>\unicode{x0002B}</mo><mi>H</mi><mo>.</mo></mrow></mrow></math>$

The formation mechanism is that the trivalent bond of nitrogen is relatively stable and requires high-temperature excitation and high activation energy to promote the positive reaction. According to this formula, the production of Zeldovich NO is not closely related to its fuel, but is determined by five chemical components (O, H, OH, N, and O₂). The formation mechanism is that the trivalent bond of nitrogen is relatively stable, and high-temperature excitation and high activation energy are required to promote the positive reaction. According to this formula, the generation of Zeldovich NO has little to do with its fuel, but is determined by five chemical components (O, H, OH, N, and O₂). This mechanism depends on the local high temperature in the cylinder, the low air-fuel ratio and the long high-temperature residence time [51]. Therefore, the peak temperature in the cylinder has a significant effect on NOx generation [52].

Figure 12 illustrates the NOx generation for two combustion systems when biodiesel is blended at injection timings of −7°, −9°, −11°, −13°, and −15° CA ATDC. As depicted in the figure, NOx generation for both systems increases with more advanced injection timing and decreases as the proportion of biodiesel in the blend rises. Notably, the TCD combustion system exhibits significantly higher NOx generation compared to the Omega system.

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Conditions that are favorable for NOx formation include high temperature, rich oxygen, and sufficient reaction time. Advancing the injection timing will lead to increased NOx emissions, which is attributed to the extension of the premixed combustion phase in the cylinder. The extension of the premixed combustion phase means that the mixing of fuel and air begins earlier, resulting in an increase in the local high-temperature area during the combustion process, thereby increasing the temperature in the cylinder. In addition, earlier injection timing causes the combustion center to be closer to the top dead center, which not only increases the temperature in the cylinder, but also prolongs the residence time of the combustion products in the combustion chamber. This extended residence time provides more favorable conditions for the formation of NOx, because the formation rate of NOx is closely related to temperature and oxygen concentration, and high-temperature and oxygen-rich environment and longer reaction time are all conducive to the formation of NOx. Biodiesel, due to its high oxygen content, low sulfur content, and lower calorific value, helps reduce combustion temperatures as its proportion in the fuel mixture increases. The high oxygen content improves combustion efficiency and reduces the high-temperature areas produced by incomplete combustion. The low sulfur content reduces the formation of sulfur oxides, which form at high temperatures and help reduce combustion temperatures. At the same time, the lower calorific value means that less energy is released for the same mass of fuel, which also helps reduce combustion temperatures. This reduction in combustion temperature is not conducive to the formation of NOx, because the formation of NOx is associated with high temperatures and oxygen-rich environments. The increase in oxygen content in the mixture, combined with the reduction in sulfur and calorific value, mitigates the conditions that favor the formation of NOx, thereby reducing NOx formation.

Furthermore, the unique geometry of the TCD combustion system enhances in-cylinder air utilization, leading to higher in-cylinder temperatures and, consequently, increased NOx generation.

Nox Mass Fraction

Figure 13 illustrates the NOx mass distribution for biodiesel blending in the two combustion systems at injection timings of −7°, −9°, −11°, −13°, and −15° CA ATDC, with a crankshaft turn angle of 20° CA ATDC. It is observed that in the ω combustion system, NOx is predominantly generated in the pit region of the combustion chamber. When the nozzle injects fuel, the fuel oil beam strikes the wall at the marquee and is deflected towards the bottom region of the combustion chamber and the top gap region.

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In contrast, in the TCD combustion system, NOx is primarily generated in the inner and outer chamber regions of the combustion chamber at 20° CA. Upon fuel injection, the fuel oil beam impacts the ring bump, diverting towards the inner and outer combustion chambers. The beam that is deflected towards the outer chamber continues to impact the side wall, forming a jet directed towards the bottom surface of the cylinder head. This jet disperses into the area surrounding the bottom of the cylinder head.

Due to the higher viscosity of biodiesel, which results in slower flow, at 20° CA, the fuel in the ω-combustion system is predominantly deposited in the bottom region of the combustion chamber and the top-gap region. Conversely, in the TCD combustion system, the fuel is mainly deposited in the inner chamber region, the outer chamber region, and the top-gap region. As the piston continues to move downward and the combustion process advances, the fuel in the TCD combustion system is more uniformly distributed and diffused compared to that in the Omega combustion system. Furthermore, as the proportion of biodiesel in the fuel blend increases, NOx emissions gradually increase.

Soot Emissions

In the combustion process of diesel engines, incomplete combustion of HCs results in the formation of soot. Soot production is exacerbated when the fuel is not uniformly mixed with air or when the combustion temperature is excessively high. Soot is a primary emission from diesel engine combustion and is highly detrimental, existing as fine particles and being a major source of carbon deposits. As depicted in Figure 14, the Soot emission results for two combustion systems using four types of fuels at five different injection timings are plotted.

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The soot emissions of the two combustion systems decrease with the advancement of the injection timing and with the increase of the proportion of biodiesel in the mixed fuel, and the soot emissions of the TCD combustion system are much lower than those of the Omega combustion system. This is because as the injection timing is advanced, the mixing time of the fuel in the combustion chamber increases, thereby improving the uniformity of the mixture, facilitating more complete combustion, and reducing the generation of soot. At the same time, the evaporation of the fuel in the combustion chamber absorbs part of the heat, resulting in a decrease in the temperature in the combustion chamber, which in turn inhibits the generation of soot. In addition, the increase in the proportion of biodiesel in the mixed fuel, due to its high oxygen content, helps to improve combustion efficiency, reduce incomplete combustion, and thus reduce the generation of soot. The TCD combustion system may be able to more effectively reduce the generation of soot due to its more optimized design, so its soot emissions are much lower than those of the Omega combustion system.

Biodiesel itself contains a high oxygen content, so the oxygen content of the mixed fuel increases as the proportion of biodiesel increases. This increased oxygen content helps to improve combustion efficiency, promote more complete combustion, reduce incomplete combustion, and thus inhibit the generation of soot. Due to its special geometric structure, the TCD combustion system increases the air utilization rate in the cylinder, which promotes more complete combustion relative to the Omega combustion system. This more complete combustion helps to achieve an effect that is not conducive to the generation of soot, because the generation of soot is usually related to incomplete combustion. Therefore, the TCD combustion system has an advantage in reducing soot emissions.

Soot Mass Fraction

Figure 15 illustrates the Soot mass distribution for two combustion systems when blending biodiesel at injection timings of −7°, −9°, −11°, −13°, and −15° CA ATDC, with a crankshaft rotation angle of 20° CA ATDC. The plots reveal that, as the piston descends and the fuel injection occurs, the Soot concentration field for both combustion systems burning the four fuels exhibits a similar change process. Concurrently with the initiation and progression of combustion, the quantity of Soot increases, with its formation conditions being high-temperature and oxygen-deficient environments, predominantly in the high-temperature, oil-rich regions.

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In the Omega combustion system, Soot is primarily generated in the piston crevice area, the bottom of the combustion chamber pit, and along the two sides of the center of the combustion chamber. The confined space in the piston crevice is detrimental to airflow, leading to localized oxygen deficiency. Additionally, the higher temperature in this area results in a higher concentration of Soot. As the proportion of biodiesel in the fuel blend increases, the oxygen content of the fuel blend also increases, which to some extent mitigates Soot emissions.

The TCD combustion system, with its distinctive raised molding structure, diverts the fuel oil beam into the inner and outer combustion chambers. The fuel entering the outer chamber impinges on the side wall, forming a jet that moves toward the bottom of the cylinder head and spreads to the area surrounding the cylinder head's base. This action expands the fuel diffusion area, enhances the quality of oil and gas mixing, and increases the air utilization rate, thereby inhibiting the growth of soot.

Conclusion

In this paper, the effects of combustion chamber geometry and blended biodiesel on engine performance at different injection timings are investigated. The main conclusions are as follows.

• 1.

  As the proportion of biodiesel in the mixed fuel increases, the indicated fuel consumption of the two combustion systems increases and the thermal efficiency decreases. Specifically, the indicated fuel consumption of the Omega combustion system when burning D100 is 3.36%–2.96% lower than that of the TCD combustion system when burning B50, and the ITE is also reduced by 0.56%–0.97%, respectively. In addition, the peak pressure and temperature in the cylinder change with the advance of injection timing and decrease with the increase of the proportion of biodiesel. The amount of NOx generated increases with the advance of injection timing, but decreases with the increase of the proportion of biodiesel. The NOx generation of the TCD combustion system is much higher than that of the Omega combustion system. Soot emissions decrease with the advance of injection timing and decrease with the increase of the proportion of biodiesel. The Soot emissions of the TCD combustion system are much lower than those of the Omega combustion system.

• 2.

  As the injection timing is advanced, the indicated fuel consumption of the two combustion systems increases and the thermal efficiency decreases. Specifically, compared with the Omega combustion system, the indicated fuel consumption of the TCD combustion system is reduced by 7.55% ~ 7.73%, 7.10% ~ 7.20%, 6.70% ~ 6.92% and 5.80% ~ 5.88% when burning D100, B10, B20, and B50 fuels, respectively. The ITE is increased by 8.17% ~ 8.38%, 7.64% ~ 7.76%, 7.18% ~ 7.43%, and 6.16% ~ 6.25%, respectively. In addition, the cylinder pressure of the two combustion systems increases and the temperature decreases. The NOx generation increases with the advance of the injection timing but decreases with the increase of the biodiesel ratio. The NOx generation of the TCD system is higher than that of the Omega system. The Soot emission decreases with the advance of the injection timing and decreases with the increase of the biodiesel ratio. The Soot emission of the TCD system is lower than that of the Omega system.

• 3.

  This study found that the TCD combustion system can produce relatively strong turbulence due to its unique combustion chamber geometry, improve the air utilization rate in the cylinder, and enable it to have better oil and gas mixing. Compared with the prototype Omega combustion system, the combustion performance of the TCD combustion system is improved, and the Soot emission reduction effect is significant.

Acknowledgments

This work was supported by the Scientific Research Fund project of Education Department of Yunnan Province (2024Y605); the Agricultural Joint Special Project of Yunnan Provincial Department of Science and Technology (202301BD070001-077); the Agricultural Joint Special Project of Yunnan Provincial Department of Science and Technology (202301BD070001-257); the Yunnan Provincial High-level Talent Cultivation and Supporting Program + “Young Top Talents” Project (YNWR-QNBJ-2018-066); Yunnan Province Thousand Talents Program + “Industrial Talents” Project (YNQR-CYRC-2019-001).

Conflicts of Interest

The authors declare no conflicts of interest.

References

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