Computational Analysis on Combustion Control of Diesel Engines Fueled with Ester Alcohol Diesel

Abstract

As the urgency for environmental sustainability escalates globally, the exploration of alternative fuels for diesel engines becomes a crucial endeavor. By combining chemical reaction kinetics and three-dimensional simulation software, the combustion and emission characteristics of a diesel engine fueled with two oxygenated fuels, hydrogenated biodiesel and ethanol, and adopting a multi-stage injection strategy were studied. The combustion mechanism of hydrogenated biodiesel ethanol diesel hybrid fuel was established, and the reaction activity of ester alcohol diesel with different mixing ratios was studied through reaction flow analysis at high and low OH temperatures. The established mechanism was coupled with CFD 2021 three-dimensional simulation software to compare the combustion and emission performance of diesel engines fueled with different ratios of ester alcohol diesel. The results show that as the proportion of ester alcohol mixture increases, at low temperatures, the OH generation rate decreases, the consumption rate increases, and the reaction activity decreases, which is not conducive to the promotion of combustion reaction; at high temperatures, the generation rate of OH increases, the consumption rate decreases, and the reaction activity increases, which is conducive to the promotion of combustion reactions. Compared to diesel, the reaction system activity of mixed fuel is enhanced, and the main peak values of cylinder pressure and instantaneous heat release rate are higher than that of diesel. The diffusion of oil and gas in the cylinder is improved. As the proportion of ester alcohol diesel mixture increases, the oxygen content increases, nitrogen oxides emissions increase compared to diesel, and soot emissions decrease compared to diesel. Soot emissions are mainly distributed in areas with a high equivalence ratio and high temperature, which is consistent with the distribution area of C₂H₂, the precursor of soot generation.

1. Introduction

The increasing global concerns regarding environmental degradation and the impending depletion of conventional fossil fuels have necessitated a focused [1] and collective global effort to explore sustainable [2] and environmentally [3] benign energy alternatives. Diesel engines [4], renowned for their superior efficiency and torque [5], have found extensive applications in transportation and industrial sectors [6]. However, they are associated with substantial greenhouse gas emissions and air pollution [7]. This worrying scenario has instigated an intense search for alternate fuels [8] that can replace traditional diesel with a reduced environmental footprint [9].

Ester alcohol fuels, typically derived from renewable resources [10], have emerged as a promising candidate in this context [11]. They are distinguished by unique physicochemical properties, such as higher oxygen content and lower potential for soot formation [12]. These properties mark them as promising contenders for cleaner and more efficient combustion [13]. Comprehending the combustion process of ester alcohol fuels in diesel engines is crucial to their effective deployment, and this topic is currently a focal point of intensive research [14].

In recent decades, advancements in the realms of thermodynamics, chemical reaction kinetics [15], and computational fluid dynamics (CFD) have been significant [16]. These strides have given rise to sophisticated simulation technologies that incorporate both CFD and chemical kinetics [17], now deemed indispensable for investigating intricate combustion processes within diesel engines [18]. These high-resolution, detailed simulations empower researchers to optimize engine performance [19] and curb emissions significantly [20].

Bio-diesel is one such alternative fuel [21] that has garnered considerable interest due to its physical [22] and chemical properties [23], closely mirroring conventional diesel [9]. It can be employed in modern direct-injection diesel engines [24] with minimal alterations [25], thereby positioning it as a practical [26], immediate solution [27]. Numerous studies validate that bio-diesel can curtail emissions of CO₂ [28], hydrocarbons [29], and soot by up to 50% [30], highlighting its potential in reducing [31] the environmental impact of diesel engines [32]. However, the increase of approximately 10% in NO_x emissions [33] when using bio-diesel remains an issue that warrants resolution [34].

One popular strategy to manipulate combustion characteristics [35] and emissions is the blending of oxygenated fuels [36] with diesel. The research landscape in this domain is extensive and complex [37], with multiple studies delving into various aspects of this approach. For example, Sun [38] conducted a meticulous exploration into the influence of pre-injection and post-injection on the NO_x emissions of bio-diesel engines by leveraging the synergy of experimentation and simulation. Her research elucidated that NO_x emissions diminish with the amplification of pre-main spray intervals, while post-injection augments NO_x emissions. He [39] examined the impact of a 20% volumetric blend of bio-diesel on NO_x emissions from diesel engines under high-speed, full-load conditions, deploying both single-stage and multi-stage injection strategies. His findings unveiled that multi-stage injection tactics could effectively mitigate NO_x emissions, but the engine’s NO_x emission levels after the introduction of a 20% volumetric bio-diesel blend remained elevated compared to pure diesel. Wang [40] and others conducted intricate computational simulations of the in-cylinder soot formation under diesel engine combustion conditions. The results accentuated the significant influence of combustion temperature and mixture equivalence ratio on soot generation. Li [41] and others simulated the effect of two-stage injection on soot emissions. The results evinced that pre-injection can effectively curtail the production of soot precursors, thereby reducing soot emissions.

Despite this progress, research on the concurrent blending of two different oxygenated fuels, like bio-diesel and ethanol, for diesel engine combustion, is relatively scarce. Considering the unique properties of these fuels, their combined use could potentially result in distinctive combustion characteristics and emission profiles. Consequently, gaining a deep understanding of their fundamental mechanisms when used in combination is vital to performance optimization and emission control.

In our study, we intend to bridge this research gap by investigating the impacts of different blending ratios of ester alcohol diesel fuel on the combustion mixture and emissions’ formation. We are utilizing state-of-the-art simulation methods, combining chemical reaction kinetics and three-dimensional CFD, to probe the combustion process from both macroscopic and microscopic perspectives. We hope that our research will pave the way for a deeper understanding of ester alcohol fuels and contribute to the development of more efficient and environmentally friendly diesel combustion strategies.

2. Computational Model Construction and Validation

2.1. Construction of Simplified Chemical Kinetic Model

Prior to amalgamating the mechanistic models of hydrobiodiesel, ethanol, diesel, and other fuel oxidation reactions, we commenced with the distillation and analysis of crucial primitive and main reactions, in addition to key components, in the models of methyl oleate, methyl stearate, ethanol, and n-heptane. Following this, mechanism models were derived through multipolar simplification, utilizing ignition delay and molar concentration of pivotal components’ experimental data as the reference for validating these simplified models. The fire delay period was used as the target parameter, and the error threshold was set at 10% for detailed mechanistic simplification. According to the hierarchical construction method, based on the simplified kinetic mechanism model of the chemical reaction of the characterized components, the simplified mechanism model of hydrogenated biodiesel is constructed. The simplified mechanism model of hydrogenated biodiesel: Methyl stearate and methyl oleate are the two components with the highest proportion in hydrogenated bio-diesel. Based on the composition and important physicochemical properties such as cetane number of hydrogenated bio-diesel, a mixture of 78.7% methyl oleate and 21.3% methyl stearate is used as the surrogate for hydrogenated bio-diesel; this is based on the detailed mechanisms of methyl oleate and methyl stearate constructed by Westbrook et al. [31] under conditions of temperature 800–1500 K, equivalence ratio 0.5–2.0, pressure 0.1–6 MPa, which include 402 components and 16,188 reactions, and 423 components and 17,436 reactions, respectively. By using Directed Relation Graph (DRG), DRG with Error Propagation (DRGEP), and the Sensitivity Analysis Method, a simplified mechanism of methyl oleate and methyl stearate was finally obtained, including 71 components with 273 reactions, and 67 components with 246 reactions. Figure 1 and Figure 2 show the ignition delay periods predicted by the simplified mechanisms of methyl oleate [32] and methyl stearate [34] with experimental values.

The simplified mechanism of hydrogenated bio-diesel was obtained by adding the elementary reactions and components that are unique to methyl oleate, which are different from those in methyl stearate, into the simplified mechanism of methyl stearate, which contains 104 components and 427 reactions. Figure 3 shows the comparison of the ignition delay period predicted by the simplified mechanism of hydrogenated bio-diesel and the experimental values of ignition delay period of methyl oleate measured by Hammill et al. [42], showing a small error between the two.

By using the decoupling method, the simplified mechanisms of ethanol and n-heptane were coupled, and an ethanol-n-heptane mechanism model including 58 components and 178 reactions was obtained. Figure 4 shows the comparison of ignition delay periods predicted by n-heptane-ethanol mixed mechanism with experimental values [43]. The results show that the ignition delay periods predicted by the n-heptane-ethanol mixed mechanism are in good agreement with the experimental values, which proves the correctness of the mechanism construction and can carry out the next work.

Since the surrogate mechanism of hydrogenated bio-diesel is made up of two simplified mechanisms, when coupling the hydrogenated bio-diesel mechanism with ethanol and n-heptane via the decoupling method, it is impossible to conduct mechanism singularity verification and exclude similar components in the mixed mechanism composed of hydrogenated bio-diesel, ethanol, and n-heptane. Therefore, only by using the decoupling method can the two be coupled, finally obtaining a hydrogenated bio-diesel-ethanol-diesel mixed mechanism model containing 162 reactions and 605 steps.

2.2. In-Cylinder Combustion Simulation Model Construction

The test engine is a high-pressure common rail four-cylinder diesel engine, model 4B28V16, and the main technical parameters are shown in

Table 1. Main technical parameters of 4B28V16 diesel engine.

Item	Parameter
Number of cylinders	4
Cylinder diameter	95 mm
Piston stroke	100 mm
Total piston displacement	2.83 L
Calibrated power	75 kW
Calibrated speed	3200 r/min
Maximum torque	250 N·m
Maximum speed	1600~2800 r/min

. The on-site diagram of the engine is shown in Figure 5.

The test prototype adopts a seven-hole injector, which is installed at the center axis of the cylinder, and the seven holes are evenly distributed in the circumferential direction. On the premise of minimizing the computational workload and ensuring the accuracy of the model simulation results, the one-seventh combustion chamber model was selected as the computational domain to carry out the diesel combustion simulation. The one-seventh combustion chamber model was chosen as the computational domain to simulate the combustion process of the diesel engine. Firstly, the three-dimensional combustion chamber model was imported into CONVERGE 2.0 software, and the software was used to simulate the combustion process of the diesel engine. Use the make-surface tool that comes with the software to obtain the ideal orthogonal closed mesh, and then divide the surface mesh into cylinder wall, cylinder surface, and cylinder surface. Then, the surface mesh is divided into cylinder wall, cylinder head, piston, and other boundaries. Figure 6 shows the combustion chamber mesh model, and the boundary conditions for calculation are set as shown in

Table 2. Initial conditions and boundary parameters.

Initial Condition	Boundary Parameter
Cylinder wall temperature	407 K
Cylinder head temperature	450 K
Combustion chamber top temperature	478 K
Initial pressure	1.8 MPa
Initial temperature	403 K

. Engine simulation calculation shows that the crankshaft angle ranges from −60 °CA to 60 °CA, that is, from the moment the inlet valve is closed to the moment the exhaust valve is opened (the top dead center of the compression stroke is defined as 0 °CA, and a negative angle appears before the top dead center, and vice versa, the angle is positive). Table 2 shows the initial condition for simulation calculation. The initial parameter settings of initial fuel temperature, initial pressure, cylinder head, cylinder wall and combustion chamber top temperature are based on the average temperature and experience.

2.3. Fuel Injection Scheme and Model Verification

To investigate the effect of mixing different proportions of hydrogenated biodiesel and ethanol in diesel on the combustion activity and emissions of diesel engines using multi-stage injection scheme, three ratios of mixed fuels were set: HB5E5 (5% by mass each of hydrogenated biodiesel and ethanol), HB10E10 (10% by mass each), and HB15E15 (15% by mass each). The physical and chemical properties of the fuels used are shown in

Table 3. Physical and chemical properties of fuels.

Fuel Properties	Molecular Formula	Oxygen-Containing Functional Groups	Cetane	Lower Calorific Value	Oxygen Content
n-Heptane	nC7H16	/	54	41.8 MJ/kg	0
Hydrogenated biodiesel	C19H36.4O2		71.2	38.42 MJ/kg	9.3
Ethanol	C2H5OH		35	15.78 MJ/kg	53.4

.

Table 4. Structure and cetane number of fatty acid methyl esters.

Lipid Structure Cetane Number	Lipid Structure Cetane Number	Lipid Structure Cetane Number
Methyl Palmitic acid		86
Methyl stearate		104.1
Methyl oleate		62.3
Methyl Linoleic acid		38
Methyl linolenic acid		23

shows the structures and cetane number of various fatty acid methyl ester. These esters have very similar structures and are composed of a long alkyl chain attached to the methyl ester group. The difference lies in the number of double bonds in the chain. The cetane number is affected by the number of branch chains, degree of unsaturation degree, isomerization degree, carbon chain length and other factors, and the degree of unsaturation will directly affect the low-temperature oxidation reaction of methyl ester, which is the main reason for the large difference in the cetane number of these lipids.

Table 5. Compositions of cottonseed methyl ester and hydrogenated cottonseed methyl ester.

Number Retention Time	Min Component	Name	Molecular Formula	Peak Area/%
				CME	PHCME
1	14.61	methyl myristate (C14:0)	C15H30O2	0.13	0.12
2	17.23	methyl palmitate (C16:0)	C17H34O2	23.05	23.07
3	21.19	methyl stearate (C18:0)	C19H38O2	3.17	13.25
4	21.66	Elaidic acid methyl ester (trans-C18:1)	C19H36O2	0	15.47
5	21.91	Cis oleic acid methyl ester (cis-C18:1)	C19H36O2	21.47	30.48
6	23.18	Linoleic acid methyl ester (C18:2)	C19H34O2	49.94	16.06
7	24.65	Methyl linolenic acid (C18:3)	C19H32O2	1.56	0.87
8	25.89	Arachidic acid methyl ester (C20:0)	C21H42O2	0.53	0.55
9	30.79	Behenic acid methyl ester (C22:0)	C23H46O2	0.15	0.13

shows the components and contents of cottonseed biodiesel and hydrogenated cottonseed biodiesel, which are composed of methyl stearate (C18:0), methyl Palmitic acid (C16:0), and unsaturated fatty acid methyl esters such as methyl oleate (cis-C18:1) and methyl Linoleic acid (C18:2).

Table 6. Schemes of fuel injection.

Case	Scheme	SOI-P1	SOI-M	QM	Q1
Case 1	Diesel-9	−28 °CA	−9 °CA	43 mg	1.2 mg
	HB5E5-9	−28 °CA	−9 °CA	43 mg	1.2 mg
	HB10E10-9	−28 °CA	−9 °CA	43 mg	1.2 mg
	HB15E15-9	−28 °CA	−9 °CA	43 mg	1.2 mg
Case 2	Diesel-6	−25 °CA	−6 °CA	43 mg	1.2 mg
	HB5E5-6	−25 °CA	−6 °CA	43 mg	1.2 mg
	HB10E10-6	−25 °CA	−6 °CA	43 mg	1.2 mg
	HB15E15-6	−25 °CA	−6 °CA	43 mg	1.2 mg

shows the fuel injection schemes at different blending ratios of hydrogenated biodiesel–ethanol–diesel and at different injection times, set according to experimental data [44]. The in-cylinder combustion simulation work was carried out at an engine speed of 1800 r/min and an average effective pressure of 0.93 MPa. The main parameters of the fuel injection system of the experimental prototype are the injection timing SOI-P1 (Start of 1st Pilot Injection) and SOI-M (Start of Main Injection) for pre injection and main injection, as well as the corresponding injection quantities for the two stages of injection, which are Q1 and QM, respectively.

Figure 7 shows the comparison of simulated and experimental in-cylinder pressures and instantaneous heat release rates for the pure diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 fuel injection scheme in Table 4. It can be seen that the hydrogenated biodiesel–ethanol–diesel mixture fuel combustion reaction mechanism model is well coupled with CFD, and it can accurately represent the true combustion and emission performance of the mixed fuel.

3. Results and Analysis

3.1. Analysis of Diesel Engine Combustion Process

Figure 8 shows the cylinder pressure curves of diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 and Case 2 fuel injection schemes. As can be seen, due to the two-stage injection scheme, the cylinder pressure curves of all four fuels consist of two pressure peaks. As a pre-injection–main injection scheme is adopted, the pre-injection fuel amount is smaller, leading to difficulties in the formation of the mixture, resulting in lower in-cylinder temperatures and smaller combustion zones after the pre-injected fuel enters the combustion chamber. HB15E15 fuel reaches the first pressure peak first, with the fastest heat release and the highest peak pressure, followed by HB10E10, HB5E5, and diesel fuel reaching the first pressure peak. Compared to pure diesel, the addition of ester-alcohol leads to a higher peak combustion pressure, which grows larger as the proportion of ester-alcohol increases.

Figure 9 shows the peak pressure positions of diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 and Case 2 fuel injection schemes. As can be observed, adding 15% ester-alcohol advances the in-cylinder pressure peak position, while the peak position for pure diesel is delayed. On one hand, this is related to the fuel’s physicochemical properties. On the other hand, from a chemical kinetics perspective, it is known from the analysis of the cross-reaction flux of the hydrogenated biodiesel–ethanol–diesel mixed fuel at low temperatures that a certain amount of active substance OH is needed to trigger the combustion at the initial stage of combustion of the mixed fuel.

Figure 10 shows the instantaneous heat release rate curves of diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 and Case 2 fuel injection schemes. As can be seen, the heat release rate curves of the four fuels all have two heat release peaks. In the initial heat release stage, the positions of the heat release peaks of the ternary fuels are basically consistent with that of diesel. In the main heat release stage, the heat release peak value of the ternary fuel is significantly delayed and higher than that of diesel.

Figure 11 shows the hydroxyl (OH) generation in pure diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 and Case 2 injection schemes. As observed, with the postponement of the overall injection phase, the generation of OH for all four fuels gradually declines, largely due to the consequential retardation of the primary combustion stage and the decrease in combustion rate, thereby reducing the OH formation. This results in a weakened intra-cylinder combustion reactivity and consequent reduction in cylinder temperature.

Figure 12 shows the evolution curves for hydroperoxyl (HO₂), hydrogen peroxide (H₂O₂) and OH under the Case 2 scheme for diesel, HB5E5, HB10E10, and HB15E15 fuels. It is apparent that around −6 °CA, HO₂ and H₂O₂ are generated nearly simultaneously, signaling the onset of low-temperature reactions. Fuel molecules first undergo oxidation to form HO₂, which then partakes in dehydrogenation reactions with other fuel molecules to produce H₂O₂. From the cross-reaction flow analysis of mixed fuels, it can be inferred that H₂O transforms into a more stable H₂O₂ under low-temperature conditions. As the combustion process advances, H₂O₂ rapidly decomposes to produce copious OH, enhancing system reactivity and rapidly increasing the temperature of the combustion reaction system. The accumulated peak values of OH for the four fuels are distinct, with pure diesel fuel displaying the least accumulation of OH and HB15E15 fuel showing the highest.

3.2. Emission Generation Characteristics

The primary source of NO_x emissions in diesel engines arises from the generation of nitric oxide (NO) in high-temperature areas. Figure 13 illustrates the NO_x emission generation curves for pure diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 and Case 2 fuel injection schemes. It is observable that as the injection moment is delayed, the generation of NO_x gradually decreases.

Figure 14 shows the in-cylinder NO_x emission mass changes under the Case 2 scheme for the four fuels: diesel, HB5E5, HB10E10, and HB15E15. It is evident that among the primary products of NO_x emissions, the generation of nitrogen dioxide (NO₂) is relatively more than NO. Upon the addition of hydrogenated biodiesel and ethanol, NO_x emissions rise across the board. The blended fuel’s hydroxyl (OH) generation exceeds that of pure diesel fuel. A high-temperature, oxygen-rich environment stimulates NO_x production, resulting in higher NO_x emissions from blended fuels than pure diesel fuel.

Figure 15 shows the soot generation for diesel, HB5E5, HB10E10, and HB15E15 fuels under the Case 1 and Case 2 injection schemes. As seen, the soot generation is the highest in diesel, which is primarily due to the provision of an oxygen-rich environment by hydrogenated biodiesel and ethanol for the combustion reaction system.

Figure 16 shows the in-cylinder changes in the mass of soot precursors C₂H₂, and soot emissions under the Case 2 scheme for the four fuels: diesel, HB5E5, HB10E10, and HB15E15. In comparison to diesel, the high oxygen content of blended fuels enhances opportunities for oxygen interaction, thus curtailing soot production stemming from oxygen-deficient mixtures. The generation of soot is significantly impacted by hydroxyl (OH). As can be inferred from Figure 9, the generation of OH has diminished under the Case 2 scheme relative to the Case 1 scheme, leading to a decrease in in-cylinder combustion reaction activity. Consequently, soot oxidation is restricted, culminating in an uptick in soot generation.

4. Conclusions

(1)

The addition of ester-alcohol fuel augments in-cylinder combustion reaction activity, promoting the progression of in-cylinder combustion. Furthermore, as the blending ratio of ester-alcohol fuel increases, the advance of the heat release center, CA50, becomes more pronounced. Due to the superior volatility of ethanol, which stimulates the progression of in-cylinder combustion, the peak values of the maximum combustion pressure and instantaneous heat release rate for HB5E5, HB10E10, and HB15E15 fuels surpass that of pure diesel.

(2)

After the inclusion of ester-alcohol, the NO_x emission generation volume of blended fuels escalates in comparison to pure diesel. This increase becomes more significant as the blending ratio of hydrogenated biodiesel and ethanol heightens.

(3)

After the addition of ester-alcohol, the soot generation volume of blended fuels significantly diminishes relative to pure diesel. It is primarily distributed in areas with high equivalence ratios and high temperatures. The distribution of acetylene (C₂H₂) in-cylinder is approximately similar to soot. This phenomenon primarily stems from the high oxygen content of blended fuels, increasing the chance of diesel-oxygen contact, thereby reducing soot production due to oxygen-deficient mixtures.