Effects of Plateau Environment on Combustion and Emission Characteristics of a Plateau High-Pressure Common-Rail Diesel Engine with Different Blending Ratios of Biodiesel

Abstract

Taking a plateau high-pressure common-rail diesel engine as the research model, a model was established and simulated by AVL FIRE according to the structural parameters of a diesel engine. The combustion and emission characteristics of D, B20, and B50 diesel engines were simulated in the plateau atmospheric environment at 0 m, 1000 m, and 2000 m. The calculation results show that as the altitude increased, the peak in-cylinder pressure and the cumulative heat release of diesel decreased with different blending ratios. When the altitude increased by 1000 m, the cumulative heat release was reduced by about 5%. Furthermore, the emission trend of NO, soot, and CO was to first increase and then decrease. As the altitude increased, the mass fraction of NO emission decreased. As the altitude increased, the mass fractions of soot and CO increased. Additionally, when the altitude was 0 m and 1000 m, the maximum temperature, the mass fraction of OH, and the fuel–air ratio of B20 were higher and more uniform. When the altitude was 2000 m, the maximum temperature, the mass fraction of OH, and the fuel–air ratio of B50 were higher and more uniform. Lastly, as the altitude increased, the maximum combustion temperature of D and B20 decreased, and combustion became more uneven. As the altitude increased, the maximum combustion temperature of B50 increased, and the combustion became more uniform. As the altitude increased, the fuel–air ratio and the mass fractions of OH and NO decreased. When the altitude increased, the soot concentration increased, and the distribution area was larger.

1. Introduction

With the intensification of air pollution and the energy crisis, the diesel engine represents the main source of energy consumption and air pollution, and the requirements for combustion optimization and emission reduction are gradually increasing [1,2]. Biodiesel is a renewable fuel that can replace petroleum by transesterification of oil crops such as soybean, rapeseed, cotton, and palm. Currently, the main preparation methods include the chemical synthesis of biodiesel and the biological enzymatic synthesis of biodiesel. The use of vegetable oil as a substitute fuel oil according to local conditions is a major adjustment of the raw material route. It is the major trend in the development of new energy in the world.

Biodiesel has become a research hotspot in the field of internal combustion engines due to its abundant raw material sources, renewability, and low-sulfur advantages [3]. Biodiesel refers to the renewable diesel fuel that is made from vegetable oil, animal fat, and recycled catering industry waste fats and oils as raw materials under the action of a catalyst through a cool exchange process that can replace petrochemical diesel [4,5]. The carbon chain length was reduced by cool exchange reaction, micro-emulsification, high-temperature pyrolysis and other methods, the viscosity of the oil was also reduced, and then its fluidity and vaporization performance were improved to meet the requirements as a fuel [6]. Yuqiang Li et al. [7,8] studied the new skeletal mechanism for diesel–n-butanol blend combustion in an engine. Jiaqiang E et al. [9] conducted an effects analysis on the diesel soot continuous regeneration performance of a rotary microwave-assisted regeneration diesel particulate filter. Qingsong Zuo et al. [10] predicted the performance and emissions of a spark ignition engine fueled with butanol–gasoline blends using support vector regression. Zhang Z. et al. [11] studied the effects of an Fe₂O₃-based DOC and SCR catalyst on the combustion and emission characteristics of a diesel engine fueled with biodiesel. Zhiqing Zhang et al. [12,13,14] studied the effects of boiling heat transfer, low-level water addition, and proportion on combustion and emission characteristics. Biodiesel is environmentally friendly, nontoxic and degradable, and it can be directly added and used without modifying the diesel engine [15,16].

As the altitude increases, the atmospheric pressure and oxygen content decline rapidly, resulting in the deterioration of diesel fuel economy and power performance, as well as an increase in pollutant emissions. Therefore, it is of great significance to study the effect of altitude changes on its combustion and emission characteristics [17,18,19,20,21]. The choice of fuel is also particularly important [22,23,24,25,26,27]. For example, a mixture of biodiesel and diesel fuel has a better combustion effect in the plateau environment [28]. There are also many studies on the application of biodiesel abroad, such as factors affecting NO_X emissions and emission reduction measures [29], as well as the effect of ethanol–diesel–biodiesel mixed fuel on combustion and emissions in premixed low-temperature combustion [30]. The structure and fuel injection parameters of the diesel engine also affect its combustion and emission characteristics [31,32,33,34,35,36,37]. Wang et al. [38] studied the modeling and optimization of a light-duty diesel engine at high altitude with a support vector machine and a genetic algorithm. Zhang et al. [39] studied the effects of oil pressure on a high-altitude diesel engine. Liu et al. [40] studied the research status and development trend of two-stage turbocharging control for a high-altitude diesel engine. The plateau environment reduces the pressure and oxygen content, and the application of biodiesel can help it burn fully and reduce exhaust emissions.

The aim was to study the effects of plateau environment on the combustion and emission characteristics of a diesel engine with different blending ratios of biodiesel. Taking a plateau high-pressure common-rail diesel engine as the research model, according to the structural parameters of the engine, its model was established and simulated by AVL FIRE software. In the plateau atmosphere, the combustion process and emission characteristics of three fuel engines of pure diesel (D), 20% biodiesel (B20), and 50% biodiesel (B50) were simulated and compared at altitudes of 0 m, 1000 m, and 2000 m. The research in this article can provide a reference for the application of biodiesel to diesel engines working in plateau environments.

2. Mathematical Model

2.1. Basic Governing Equation

The fluid flow in an engine cylinder is very complicated. All fluid flows satisfy three basic conservation laws. The basic equations of the working process of the cylinder are as follows:

(1)

Mass conservation equation

$$\frac{\partial \rho }{\partial t}+\frac{\partial (\rho u)}{\partial x}+\frac{\partial (\rho v)}{\partial y}+\frac{\partial (\rho w)}{\partial z}=0,$$

(1)

$$\frac{\partial \rho }{\partial t}+div(\rho u)=0,$$

(2)

$$\frac{\partial \rho }{\partial t}+\nabla ·(\rho u)=0.$$

(3)

(2)

Momentum conservation equation

$$\{\begin{array}{l}\frac{\partial (\rho u)}{\partial t}+div(\rho u\overline{u})=div(\mu gradu)-\frac{\partial p}{\partial x}+S_u\\ \frac{\partial (\rho v)}{\partial t}+div(\rho v\overline{u})=div(\mu gradv)-\frac{\partial p}{\partial y}+S_v\\ \frac{\partial (\rho w)}{\partial t}+div(\rho w\overline{u})=div(\mu gradw)-\frac{\partial p}{\partial z}+S_w\end{array},$$

(4)

where $\mu$ is the dynamic viscosity of the fluid, p is the pressure on the fluid microelement body, and S_u, S_v, and S_w represent the generalized source terms in the equation.

(3)

Energy conservation equation

$$\frac{\partial (\rho T)}{\partial t}+div(\rho \overline{u}T)=div(\frac{k}{c_p}gradT)+S_T,$$

(5)

where T is the temperature, c_p is the specific heat capacity, and k is the heat transfer coefficient of the fluid, S_T is the internal heat source of the fluid and the part of the fluid mechanically converted into heat energy due to the viscous effect, which also becomes the viscous dissipation term.

Synthesizing the basic equations, there are six unknowns (u, v, w, p, T, and ρ). It is also necessary to add a state equation linking p and ρ, so that the system of equations can be closed.

$$p=p(\rho ,T).$$

(6)

2.2. Turbulence Model

The formation and combustion process of the mixed gas in the cylinder of an internal combustion engine was carried out in a turbulent state. The turbulence determines the distribution and transportation state of each component in the cylinder. In order to more truly reflect the complex and transient airflow movement in the cylinder, it is necessary to choose a suitable turbulence model. The k–ε–f four-equation model selected for the simulation calculation in this study has good accuracy and stability and is suitable for the diesel engine used in the experiment. The model calculation formula is

$$\{\begin{array}{l}\rho \frac{Dk}{Dt}=\rho (P_k-\epsilon )+\frac{\partial }{\partial x_j}[(\mu +\frac{{\mu }_t}{{\sigma }_k})\frac{\partial k}{\partial x_j}]\\ \rho \frac{D\epsilon }{Dt}=\rho \frac{C_{\epsilon 1}^{\ast }{P}_k-C_{\epsilon 2}\epsilon }{T}+\frac{\partial }{\partial x_j}[(\mu +\frac{{\mu }_t}{{\sigma }_k}\frac{\partial \epsilon }{\partial x_j})]\\ \rho \frac{D\xi }{Dt}=\rho f+\rho \frac{\xi }{k}{P}_k[(\mu +\frac{{\mu }_t}{{\sigma }_{\xi }})\frac{\partial \xi }{\partial x_j}]\end{array},$$

(7)

where $\xi$ = v²/k, v_t = $\xi$v²/ε, T is the turbulence time scale, L is the turbulence length scale, and C^*_ε1 is the correction coefficient of the governing equation.

2.3. Spraying Model

The spraying model adopts the DDM (discrete droplet method), which can be calculated by the following formula:

$$N_{d_j}=\frac{\delta m_i{\phi }_j}{{\displaystyle \sum {\phi }_j\pi {\rho }_{Fu}{d}_j^3/6}},$$

(8)

where $\delta m_i$ is the fuel injection volume of each segment, ${\rho }_{Fu}$ is the fuel density, and ${\phi }_j$ is the probability of the number of oil droplets with a nominal diameter d_j in all oil droplets.

2.4. Combustion Model

The ECFM-3Z model was used as the correlation flame model. The chemical reaction kinetic mechanism is

$$C_n{H}_m{O}_k+(n+\frac{m}{4}-\frac{k}{2})O_2\to nC{O}_2+\frac{m}{2}{H}_{2}O,$$

(9)

$$C_n{H}_m{O}_k+(\frac{n}{2}-\frac{k}{2})O_2\to nCO+\frac{m}{2}{H}_2,$$

(10)

where n, m, k, and D are the flame surface formed by turbulent stretching, the product of average flow stretching, the effect of surface thermal expansion and curvature, and the failure constant, respectively.

2.5. Emission Model

2.5.1. NO_X Generation Model

The NO_X generation model adopted was the extended Zeldovich NO_X model, whose reaction mechanism can be expressed as

$$O+N_2\underset{k_{b1}}{\overset{k_{f1}}{\leftrightarrow }}NO+N,$$

(11)

$$N+O_2\underset{k_{b2}}{\overset{k_{f2}}{\leftrightarrow }}NO+O,$$

(12)

$$N+OH\underset{k_{b3}}{\overset{k_{f3}}{\leftrightarrow }}NO+H.$$

(13)

The generation rate of NO_X can be calculated by the following formula:

$$\begin{array}{cc}\hfill \frac{d(NO)}{dt}& =k_{f1}(O)(N)-k_{b1}(NO)(N)+K_{f2}(N)(O_2)-k_{b2}(NO)(N)\hfill \\ & +k_{f3}(N)(OH)-k_{b3}(NO)(H)\hfill \end{array}$$

(14)

where k_f1, k_f2, and k_f3 are the forward reaction rates of the three reactions, respectively, and k_b1, k_b2, and k_b3 are the reverse reaction rates of the three reactions, respectively.

2.5.2. Soot Generation Model

The soot generation model adopted was the kinetic soot model, which can be calculated by the following formula:

$$\begin{array}{cc}\hfill \frac{d{m}_s}{dt}& =\frac{d{m}_{sf}}{dt}-\frac{d{m}_{so}}{dt}\hfill \\ & =A_{sf}{p}^{0.5}\exp (-\frac{E_{sf}}{R_{u}T})M_{form}-A_{so}\frac{6}{{\rho }_s{D}_s}{R}_{total}M{W}_c{M}_s\hfill \end{array}$$

(15)

where m_s represents the unit mass of soot, m_sf represents the mass of formed soot, m_so represents the mass of oxidized soot, A_sf represents the Arrhenius index factor for soot formation, E_sf represents the activation energy, R_u represents the gas constant, M_form represents the soot generation Mass, A_so represents the soot oxidation Arrhenius index factor, D_s represents the nominal smoke particle diameter, MW_c represents the molar mass of carbon, and M_s represents the total mass of soot particles.

2.6. Simulation Condition Setting

The Diesel Engine HD module was used in AVL FIRE for simulation calculation. In this study, the top dead center was defined as 720° CA, using the Simple/Piso algorithm. The fuel injection advance angle was 6° CA. The circulating fuel injection mass was 8 mg. Without changing the number of injections, the injection advance angle, and the circulating fuel volume, the pressure (80 kPa, 90 kPa, and 100 kPa) was used to simulate different altitudes (2000 m, 1000 m, and 0 m) and different blending ratios of biodiesel (pure diesel D, 20% biodiesel B20, and 50% biodiesel B50). Its effects on the combustion and emission characteristics of internal combustion engines were analyzed.

3. Methodology

3.1. Model Building

According to

Table 1. Structural parameters of combustion chamber.

Project	Parameter
Cylinder arrangement	Inline
Bore × stroke (mm)	80 × 92
Connecting rod (mm)	150
Number of cylinders	4
Fuel injection type	Direct injection
Number of injection holes	4
Injection advance angle (°CA)	6
Compression ratio	16
Torque (N·m)	235
Rated speed(r/min)	2000

showing the combustion chamber parameters of a certain type of diesel engine, the model was established by FIRE ESE Diesel, and a numerical simulation was carried out. The fuel nozzle of the engine had four identical nozzle holes. In order to reduce the difficulty and time of calculation, one-fourth of the entire combustion chamber was taken as the calculation domain due to its symmetry. It was converted from a two-dimensional grid to a three-dimensional grid (top dead center and bottom dead center), as shown in Figure 1. The average grid size was 0.8 mm.

3.2. Grid Independence

Since the grid size is very sensitive to the effect of numerical results and calculation time, it is necessary to study the effect of grid size on the calculation results. Therefore, this work studied the grid independence of the geometric model of a combustion chamber of a diesel engine. The geometric model with three different mesh sizes (0.6 mm, 0.8 mm, and 1 mm) was generated by FIRE ESE Diesel, as shown in Figure 2. It can be clearly seen from Figure 2 that the number of grid cells of the model gradually decreased, and the size of the grid in the axial and radial directions gradually increased. Figure 3 shows the in-cylinder pressure curves of different grid sizes. It can be seen from Figure 3 that the in-cylinder pressure of different mesh sizes reached a peak at the same crank angle. There was a small difference in the in-cylinder pressure at the later stage of combustion. The results show that it was reasonable and reliable to use a 0.8 mm grid to solve the model under the balance of calculation accuracy and time.

3.3. Model Verification

Figure 4 shows the comparison between the test value and the calculated value of the cylinder pressure. Compared with the experimental results under the same operating conditions in [2], the calculated results and the experimental values had a high degree of coincidence, and the average relative error of the numerical model was less than 10%, indicating that the three-dimensional engine model, initial conditions, and boundary conditions constructed in this study were good. The settings were relatively accurate and could meet the simulation calculation requirements. The numerical model was reliable and reasonable.

4. Simulation Results and Analysis

4.1. Effect of Different Plateau Environments on Diesel Combustion with Different Blending Ratios

The effect of different plateau environments on pressure in the cylinder with different blending ratios is shown in Figure 5. It can be seen that the cylinder pressure fluctuated with the proportion of biodiesel and altitude. As the altitude increased, the peak in-cylinder pressure of D decreased. For example, the cylinder pressure at 1000 m increased about 7.6% compared with 2000 m. This was due to the altitude increasing and the atmospheric pressure decreasing, resulting in a decrease in cylinder pressure and temperature. In particular, the oxygen content and exhaust back pressure decreased, and the excess air coefficient decreased. When the altitude was the same, the in-cylinder pressure of B20 was the highest, followed by B50, and D. For example, the cylinder pressure of B20 increased about 3.4% compared with B50. This was due to the lower calorific value of biodiesel. Therefore, the cylinder pressure decreased with the increase in biodiesel content in the blending fuel. More specifically, greater biodiesel content in the blended fuel resulted in a lower fuel heat value and a lower in-cylinder pressure. However, the self-oxygen content (10%) of biodiesel supplied the required oxygen, making combustion more complete. Thus, the in-cylinder pressure of B20 was the highest.

The effect of different plateau environments on the cumulative heat release of diesel with different blending ratios is shown in Figure 6. As the altitude increased, the cumulative heat release of diesel with different blending ratios decreased. For example, the cumulative heat release of 1000 m increased 5.5% compared with 2000 m. When increasing by 1000 m, the cumulative heat release decreased by about 5%. When the altitude was the same, as the proportion of biodiesel increased, the cumulative heat release gradually decreased. This was due to the calorific value of biodiesel being lower than that of pure diesel. As the altitude increased, the pressure and the oxygen content of the air decreased, resulting in a decrease in intake air volume and intake oxygen content and an increase in its fuel–air ratio. The appearance of incomplete combustion eventually led to a decrease in the cumulative heat release. When the altitude was 2000 m, the cumulative heat release was abnormal, mainly because the self-oxygen content (10%) of biodiesel supplemented the required oxygen, making combustion more complete. The cumulative heat release increased. It can be seen from Figure 6 that the heat release trend was basically the same. Thus, different altitudes and blending ratios had little effect on the combustion duration and ignition delay. The altitude had a greater effect on the cumulative heat release than the blending ratio.

4.2. Effect of Different Plateau Environment on Diesel Emission with Different Blending Ratios

With the increasingly stringent emission regulations, the requirements for reducing the emissions of pollutants have become more strict. Diesel engines, especially in the plateau environment, are more likely to produce large amounts of pollutants, such as NO_X and soot. This work mainly studied the effect of different plateau environments on diesel emission with different blending ratios, thus providing guidance for the emission reduction of diesel engines in plateau environments.

The effect of different plateau environments on the NO emissions of diesel with different blending ratios is shown in Figure 7. The conditions for NO generation were high temperature, oxygen enrichment, and combustion duration. The trend of NO emission first increased and then decreased. This was mainly due to the existence of Reactions (9)–(13). These three reactions are all reversible reactions. The temperature in the initial stage of combustion and the oxygen content was high. The mass fraction of NO increased sharply. The oxygen concentration decreased ay the later stage of combustion. The reaction proceeded in the reverse direction, and the mass fraction of NO decreased. As the altitude increased, the mass fraction of NO emissions decreased. This was mainly due to the increase in altitude and the decrease in air oxygen content and atmospheric pressure, resulting in insufficient combustion; hence, the mass fraction of NO emissions was lower. When the altitude was the same, the mass fraction of NO emissions of B20 was the highest, followed by B50 and D.

The effect of different plateau environments on the soot emissions of diesel with different blending ratios is shown in Figure 8. High temperature and lack of oxygen are the two main factors in soot formation. The trend of soot emission first increased and then decreased. As the altitude increased, the mass fraction of soot emission increased. This was mainly due to the increase in altitude and the decrease in oxygen content, leading to internal hypoxia (lack of oxygen) and an increase in soot emission. When the altitude was the same, the mass fraction of soot emission of B50 was the highest, followed by D and B20. This was mainly due to biodiesel itself containing a small number of oxygen atoms, which could reduce the fuel–air ratio, make combustion more complete, and inhibit the formation of soot. However, the viscosity of the mixed fuel increased, the atomization quality decreased, the flame internal combustion air equivalent ratio increased, and incomplete combustion of soot increased. Thus, the soot mass fraction of B50 was the highest.

The effect of different plateau environments on the CO emissions of diesel with different blending ratios is shown in Figure 9. The overall trend of CO emissions firstly increased sharply and then decreased slowly. This was due to the low temperature in the cylinder at the beginning of combustion and the increase in the fuel–air ratio, resulting in a sharp increase in CO emission. When the combustion began, the temperature in the cylinder increased sharply. CO was rapidly oxidized to CO₂; hence, the mass fraction of CO decreased rapidly. As the altitude increased, the mass fraction of CO emission increased. This was mainly due to the increase in altitude and the decrease in oxygen content, which led to insufficient combustion in the cylinder and an increase in CO emissions. When the altitude was the same, as the blending ratio increased, the mass fraction of CO emissions decreased. This was mainly due to biodiesel itself containing a small number of oxygen atoms, which was conducive to the combustion and oxidation of CO. Hence, the mass fraction of CO emissions decreased.

4.3. Effect of Different Plateau Environments on the Distribution of Diesel Emissions with Different Blending Ratios

Changes in altitude closely affect the cylinder pressure and fuel–air ratio, which greatly affect the combustion effect (temperature, generation of intermediate products, emissions, and distribution in the cylinder). The emissions of D, B20, and B50 fuels at three altitudes (0 m, 1000 m, and 2000 m) were analyzed, and the slice corresponding to the highest combustion pressure in the cylinder (crank angle = 730° CA) was selected as the research object.

The effect of different plateau environments on temperature field of the cylinder with different blending ratios is shown in

Table 2. Effect of different plateau environments on temperature field of cylinder with three blending ratios.

Altitude (m)	Temperature (K)	Three Blending Ratios of Biodiesel
		D	B20	B50
0
1000
2000

. As the altitude increased, the maximum combustion temperature of D and B20 decreased, and combustion became more uneven. As the altitude increased, the maximum combustion temperature of B50 increased, and the combustion became more uniform. When the altitude was 0 m and 1000 m, the maximum temperature of B20 was higher and more uniform. This was mainly due to the higher calorific value of diesel. When the altitude was 2000 m, the maximum temperature of B50 was higher and more uniform. This was mainly due to biodiesel itself containing a small number of oxygen atoms. A higher altitude was correlated with lower environmental pressure and lower oxygen content. Therefore, the B50 combustion effect was better.

The effect of different plateau environments on the OH distribution field of cylinders with different blending ratios is shown in

Table 3. Effect of different plateau environments on OH distribution field of cylinder with three blending ratios.

Altitude (m)	The Mass Fraction of OH	Three Blending Ratios of Biodiesel
		D	B20	B50
0
1000
2000

. The OH group was mainly formed rapidly at the high-temperature reaction stage. Its concentration affected the oxidation reaction of soot. It can be seen from the comparison of Table 2 and Table 3 that OH groups were mainly distributed in high-temperature areas. As the altitude increased, the OH group concentration decreased and the distribution became more uneven. When the altitude increased, the mass fraction of OH decreased. When the altitude was 0 m and 1000 m, the mass fraction of the OH group of B20 was higher and more uniform. This was mainly due to the higher calorific value of diesel fuel, more sufficient reaction, and larger high-temperature area. When the altitude was 2000 m, the mass fraction of the OH group of B50 was higher and more uniform. This was mainly due to biodiesel itself containing a small number of oxygen atoms. The pressure and the oxygen content at high altitudes decreased, resulting in hypoxia (lack of oxygen); thus, the combustion effect of B50 was better.

The effect of the plateau environment on the distribution field of the fuel–air ratio with different blending ratios is shown in

Table 4. Effect of plateau environment on distribution field of fuel–air ratio with three blending ratios.

Altitude (m)	Fuel–Air Ratio	Three Blending Ratios of Biodiesel
		D	B20	B50
0
1000
2000

. As the altitude increased, the fuel–air ratio and the distribution area increased. The fuel–air ratio of D at 1000 m was 5.2, and the distribution area was the largest. When the altitude was 0 m and 1000 m, the fuel–air ratio of B20 was higher, and the distribution area was larger. This was mainly due to the poor spray atomization effect of biodiesel. When the altitude was 2000 m, the fuel–air ratio of B50 was higher and the distribution area was larger. This was mainly due to the biodiesel itself containing a small number of oxygen atoms. This compensated for the increase in fuel–air ratio in the cylinder caused by high fuel viscosity and poor atomization; hence, B50 had a better combustion effect.

The effect of different plateau environments on the NO distribution field of the cylinder with different blending ratios is shown in

Table 5. Effect of different plateau environments on NO distribution field of cylinder with three blending ratios.

Altitude (m)	The Mass Fraction of NO	Three Blending Ratios of Biodiesel
		D	B20	B50
0
1000
2000

. The NO generation area was mainly in high-temperature and oxygen-deficient areas. As the altitude increased, the mass fraction of NO and the distribution area decreased. When the altitude was the same, the mass fraction of NO of D was higher, and the distribution area was larger. This was mainly due to the higher calorific value of diesel, more sufficient reaction, and the larger high-temperature area. Moreover, biodiesel itself contains a small number of oxygen atoms. The decrease in oxygen content led to a lack of oxygen; therefore, the combustion effect of B20 was better.

The effect of different plateau environments on the soot distribution field of the cylinder with different blending ratios is shown in

Table 6. Effect of different plateau environments on soot distribution field of cylinder with three blending ratios.

Altitude (m)	The Mass Fraction of Soot	Three Blending Ratios of Biodiesel
		D	B20	B50
0
1000
2000

. Soot was mainly distributed in the position where the wall in the pit of the combustion chamber and the position of the squeeze area along the top of the piston came into contact. This was because fuel deposits were formed after the sprayed fuel hit the wall, and local high temperature and lack of oxygen caused a large amount of soot. In the squeeze area, the piston moved down. The fuel in the cylinder moved toward the squeeze area where the airflow gradually increased, and part of the fuel was sucked. Entering the squeeze zone increased the fuel–air ratio in this area, and local oxygen deficiency led to an increase in the amount of soot. While the combustion pit area generated a large number of OH radicals, the soot was quickly oxidized, resulting in a smaller amount of soot. As the altitude increased, the mass fraction of soot increased, and the distribution area was larger. When the altitude was the same, the mass fraction of soot of D was higher, and the distribution area was larger. This was mainly due to biodiesel itself containing a small number of oxygen atoms. The pressure at higher altitudes decreased, and the oxygen content decreased, leading to hypoxia (lack of oxygen). Therefore, the combustion effect of B50 was better, and the mass fraction of soot emission was reduced.

5. Conclusions

To overcome the energy crisis and and environmental problems, it is necessary to investigate effects of plateau environment on combustion and emission characteristics of a plateau high-pressure common-rail diesel engine with three blending ratios of biodiesel. In this work, a CFD model was developed and verified in AVL FIRE. In addition, the effects of different blending ratios on the combustion and emission characteristics of the diesel engine were studied in terms of cylinder pressure, cumulative heat release, NO, OH, fuel–air ratio, CO, and Soot. According to the above analysis, B20 was the optimal diesel–biodiesel blend fuel. The main conclusions are as follows:

(1)

As the altitude increased, the peak in-cylinder pressure and the cumulative heat release of diesel with different blending ratios decreased. When the altitude was the same, the in-cylinder pressure of B20 was the highest, followed by B50 and D. When the altitude was the same, as the proportion of biodiesel increased, the cumulative heat release decreased. The change in altitude had a greater effect on the cumulative heat release than the change in the blending ratio. When the altitude increased by 1000 m, the cumulative heat release was reduced by about 5%.

(2)

The emission trend of NO, soot, and CO was to first increase and then decrease. As the altitude increased, the mass fraction of NO emission decreased. When the altitude was the same, the mass fraction of NO was consistent with the change trend of the cylinder pressure, with B20 being the highest, followed by B50 and D. As the altitude increased, the mass fractions of soot and CO increased. When the altitude was the same, the mass fractions of soot of B50 were the highest, followed by D and B20. When the altitude was the same, as the blending ratio increased, the mass fraction of CO decreased.

(3)

When the altitude was 0 m and 1000 m, the maximum temperature, the mass fraction of OH, and the fuel–air ratio of B20 were higher and more uniform. When the altitude was 2000 m, the maximum temperature, the mass fraction of OH, and the fuel–air ratio of B50 were higher and more uniform. When the altitude was the same, the mass fraction of NO of D was higher, and the distribution area was larger. When the altitude was the same, the soot concentration of D was higher, and the distribution area was larger.

(4)

As the altitude increased, the maximum combustion temperature of D and B20 decreased, and combustion became more uneven. As the altitude increased, the maximum combustion temperature of B50 increased, and the combustion became more uniform. As the altitude increased, the fuel–air ratio, the mass fractions of OH and NO decreased. As the altitude increased, the soot concentration increased, and the distribution area was larger.