J. Cent. South Univ. (2020) 27: 2338-2352

DOI: https://doi.org/10.1007/s11771-020-4453-4

Effect of equivalence ratio on diesel direct injection spark ignition combustion

CHEN Zheng(陈征)^{1, 2}, QIN Tao(覃涛)^{1, 2}, HE Ting-pu(何廷谱)^{1, 2}, ZHU Li-jing(朱立靖)^{1, 2}

1. State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China;

2. Research Center for Advanced Powertrain Technology, Department of Energy and Power Engineering, Hunan University, Changsha 410082, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Aviation heavy-fuel spark ignition (SI) piston engines have been paid more and more attention in the area of small aviation. Aviation heavy-fuel refers to aviation kerosene or light diesel fuel, which is safer to use and store compared to gasoline fuel. And diesel fuel is more suitable for small aviation application on land. In this study, numerical simulation was performed to evaluate the possibility of switching from gasoline direct injection spark ignition (DISI) to diesel DISI combustion. Diesel was injected into the cylinder by original DI system and ignited by spark. In the simulation, computational models were calibrated by test data from a DI engine. Based on the calibrated models, furthermore, the behavior of diesel DISI combustion was investigated. The results indicate that diesel DISI combustion is slower compared to gasoline, and the knock tendency of diesel in SI combustion is higher. For a diesel/air mixture with an equivalence ratio of 0.6 to 1.4, higher combustion pressure and faster burning rate occur when the equivalence ratios are 1.2 and 1.0, but the latter has a higher possibility of knock. In summary, the SI combustion of diesel fuel with a rich mixture can achieve better combustion performance in the engine.

Key words: diesel; direct injection; spark ignition; equivalence ratio; combustion; knock

Cite this article as: CHEN Zheng, QIN Tao, HE Ting-pu, ZHU Li-jing. Effect of equivalence ratio on diesel direct injection spark ignition combustion [J]. Journal of Central South University, 2020, 27(8): 2338-2352. DOI: https://doi.org/10.1007/s11771-020-4453-4.

1 Introduction

Aviation piston engines have some unique advantages such as smaller volume, lower weight, simpler structure, and more convenient maintenance, which are very obvious in low power range compared to turbojets, turboshafts, turboprops, turbofans, jets and other types of engine, so they have been widely used in the area of small aero-engines with power less than 100 kW. The aviation piston engines usually use aviation gasoline as fuel by means of spark ignition (SI) combustion, but gasoline vapor, which is easy to ignite or even cause explosion, has poor safety. In recent years, heavy-fuel as the fuel of aviation piston engines is being paid more attention and studied [1]. In the area of aviation, heavy-fuel mainly refers to aviation kerosene or light diesel fuel, which has higher flash point and lower volatility than aviation gasoline, and is easier to be stored and treated. Compared with the traditional aviation gasoline piston engines, aviation heavy- fuel piston engines have obvious advantages including higher security, better fuel availability, and meeting the requirements of general aviation and military fuel integrated logistics. For this purpose, North Atlantic Treaty Organization (NATO) and the United States (US) military have implemented the “Single Fuel Forward” policy, which requires a common fuel for both aircraft and ground equipment [2-4].

Due to the compression properties of aviation heavy-fuel, compression ignition (CI) mode is usually used to make it ignite and burn [4-8]. And the compression ignition heavy-fuel engines may be the alternative power for unmanned aerial vehicles (UAVs) and general aviation [5]. Some small aircraft heavy-fuel CI piston engines have been successfully applied to UAVs in US, such as MQ-5B Hunter, Warrior, and ScanEagle [1]. It is reported that JP-8 (a representative heavy-fuel in US) can be used directly in unmodified diesel engines without any significant problems [6, 7]. In addition, ARKOUDEAS et al [4] tested the potential of the JP-8 addition with biodiesel in improving the particulate matter emissions of CI engines. Furthermore, CHEN et al [8] also carried out a research to compare the combustion and emission of RP-3, kerosene-pentanol blends and diesel in a compression ignition engine. Different with JP-8, aviation kerosene RP-3, consisting of saturated hydrocarbons (92.1% by volume) and aromatic hydrocarbons (7.9% by volume), has been widely used for military and civil aviation in China [9, 10]. However, CI engines need high compression ratio, which leads to high structural strength requirement, large volume and weight, and low power-to-weight ratio. For the area of small aviation, therefore, the practicality and flexibility are reduced.

Unlike traditional compression combustion [11], the spark ignition (SI) of aviation heavy-fuel is another important technical route in the development of aviation piston engines. However, little research has been shown. SUHY et al [12] investigated the use of kerosene fuel on a spark ignited two-stroke engine and claimed that the vortex pneumatic atomizer helped to improve engine performance when using kerosene fuel. FALKOWSKI et al [13] also tested JP-5 aviation fuel in a SI two-stroke engine. And their results showed that the performance of the engine burning JP-5 was not much different from that of a gasoline SI engine at low speeds and loads, but it was degraded at higher speeds and loads due to the appearance of knocking combustion. GROENEWEGEN et al [14] thought by the experimental study of a small SI engine fueled by JP-8 and biofuel that heavy-fuel could meet power requirement and lower fuel consumption if cold start, atomization, and knock problems could be solved. Moreover, an air-assist direct-injection (AADI) system was developed for direct-injection (DI) gasoline engines with charge stratification [15]. And then this system was used in a spark-ignited heavy-fuel engine and can keep the engine at 85% of the gasoline engine power [16]. HU et al [17, 18] and BEI et al [19] also adopted a similar low pressure AADI system to investigate the effects of injection parameters on direct-injection heavy- fuel SI engines, respectively. In addition, LIU et al [20] evaluated the control strategies of an AADI engine burning diesel fuel for cold start because heavy-fuel had higher viscosity, lower volatility and poorer evaporation compared to gasoline. SINGH et al [21] also compared the cold start performance of a two-stroke outboard engine operating on multi-fuel including JP-5/JP-8, gasoline, diesel, kerosene, ethanol, and biodiesel. However, most of the aviation heavy-fuel research focused on kerosene, rarely involving diesel fuel, which is more suitable for small aviation application in the army due to fuel uniformity requirement.

Although the above works demonstrate the feasibility of using heavy-fuel in spark ignition engines, it should be noted that the maximum load (i.e. full-load) operation is very difficult because of knocking combustion which conventionally results from the end-gas auto-ignition [22]. Compared with gasoline, the octane rating and the auto-ignition temperature of heavy-fuel are relatively low and the maximum laminar flame propagation velocity of heavy-fuel/air mixture is lower than gasoline/air mixture at room temperature and atmospheric pressure [23, 24], which lead to a greater tendency to knock for heavy-fuel SI engines. HU et al [18] experimentally studied the influence of injection start time, spark advance angle, and excess air ratio on a direct injection piston aviation kerosene engine and confirmed that the flame propagation velocity of aviation kerosene is slower and anti-knock property is worse compared to gasoline. LI et al [25] investigated the effects of ignition timing, exhaust gas recirculation (EGR), and mixture concentration on the knocking combustion of a kerosene spark-ignition engine by a three- dimensional simulation. Moreover, WANG et al [26] and NING et al [27, 28] studied knock suppression of a spark-ignition engine fueled with kerosene via water injection and the effects of compression ratio and injection timing on combustion and emissions of a two-stroke direct- injection spark ignition (DISI) engine fueled with aviation kerosene by experiments, respectively. However, there are few studies on diesel spark ignition combustion and knocking combustion in the world.

As a reference for subsequent experiments of heavy-fuel SI engines, the possibility and problems of switching from gasoline direct-injection (GDI) combustion to diesel DISI combustion is evaluated. Light diesel fuel, as a kind of heavy-fuel, is injected directly into the cylinder by original GDI system and ignited by spark plug. And the effects of equivalence ratio on diesel DISI combustion and knock are primarily studied in the paper.

2 Modelling methodology

2.1 Key model description

2.1.1 Turbulence model

The working process in the engine cylinder is a complicated turbulent process in which fuel and air mix and burn quickly. Compared to Reynolds Averaged Navier-Stokes (RANS) model where all turbulence scales are treated statistically by taking the time average for three-dimensional unsteady Navier-Stokes (NS) equations, Large-eddy simulation (LES) model can calculate the large- scale three-dimensional time-dependent turbulence structure in a single realization of the flow while only the small-scale turbulence need be treated statistically [29]. Therefore, LES model enables to capture the large scale unsteady turbulent coherent structure and instantaneous variations of complicate flow field in engines, which can be expected to be more accurate and reliable than RANS models for flows [30]. As shown in Table 1, LES shows the local variation of flow field and equivalent ratio in the combustion chamber of the investigated GDI engine well. Therefore, LES model is adopted in the simulation.

2.1.2 Combustion model

Combustion process in internal combustion (IC) engines belongs to complex turbulent combustion, where the burning rate of the mixture depends on turbulent reaction rate. Besides being affected by the reaction mechanism of reactant itself, the reaction rate has a remarkably close relation with the external influence factors such as temperature, pressure and concentration. Therefore, the key problem to be solved in combustion model is how to consider the interaction between turbulence and chemical reaction. The coherent flame model (CFM) is adapted to premixed and non-premixed conditions based on laminar flamelet concept [31]. Based on the CFM, the extended coherent flame model (ECFM) is developed to describe combustion in direct-injection spark ignition (DISI) engines [31, 32]. It not only contains all the features of the standard CFM but also is coupled to the spray model to satisfy the simulation of stratified combustion. In consequence, the ECFM allows a more accurate estimation of unburned thermodynamic condition because it improves the determination of laminar flame speed used in normal combustion on the flame front but also supports the implementation of both empirical and reduced kinetics auto-ignition models for knock detection [33]. In the study, therefore, the ECFM is adopted as the combustion model.

Table 1 Comparisons of simulated results of RANS and LES

2.1.3 Knock model

Knock phenomenon limits the improvement of the compression ratio of DISI engines, and even affects the power and economy of engines. Numerical simulation is an effective method to study the knock problem. An appropriate knock model can play a key role in both model validation and the accuracy of the calculation results. In the current research, the shell knock model is adopted to simulate auto-ignition phenomena in the spark ignition engine. With the model, the auto-ignition chemistry is reduced to the eight-step chain branching reaction scheme incorporated into four reaction processes [34, 35], which are shown as follows:

Initiation:

CH+O₂→2R^*                            (1)

Propagation:

R^*→R^*+PR　　                         (2)

R^*→R^*+BA                             (3)

R^*→R^*+IA　                            (4)

R^*+IA→R^*+BA    　                    (5)

Branching:

BA→2R^*      　                      (6)

Termination:

R^*→out  　　　                         (7)

2R^*→out                                (8)

where CH represents the hydrocarbon fuel; R is the radical; BA is the branching agent; IA is the intermediate agent; and PR is the product.

Furthermore, a dimensionless parameter, namely the knock factor K, which is derived from an output of the shell knock model, is used as an indicator of the possibility of knocking. And it can reflect the probability of knock occurrence, depending on a criterion determined by the mixture fraction, progress variable, temperature, and intermediate product mass fraction [31]. The larger the K value, the higher the possibility of knock.

2.2 Model validation

In order to verify the spray model of diesel DISI combustion simulation, in this study, a spray test of a direct-injection gasoline injector that had six holes with the bore diameter of 0.2 mm was performed with a RON 93 gasoline fuel in a constant volume bomb. With high-speed camera and Schlieren method, the spray behavior was recorded at an injection pressure of 10 MPa and a back pressure of 0.22 MPa. Based on the test, a three-dimensional numerical model which adopted the WAVE break-up model was established by CFD software FIRE to simulate the spray development and penetration of gasoline. The experimental and simulated spray patterns are shown in Table 2, as well as the spray penetration in Figure 1. As we can see, the simulated results are in good agreement with the test, which indicates the effectiveness of the spray model related to the nozzle and condition studied in this paper.

To understand the combustion behavior of diesel in a DISI engine, furthermore, a real engine combustion system, which contained the intake port and combustion chamber, was modeled by using the FIRE software. The engine specifications and CFD models are shown in Table 3 and Figure 2, respectively. For analysis purposes, the positions of cross and longitudinal profiles are indicated in Figure 3. In the CFD model, high quality meshes consisting of orthogonal hexahedron were generated in most regions by FIRE front processor, besides a small number of unstructured grids in the boundary and narrow surface area. To maintain a balance of resolution and computational cost, the mesh size in the intake process was appropriately enlarged, and the one in the compression and combustion process was reduced, in which the fuel injection and ignition areas are further refined. As a result, the mesh sizes are 0.3 mm around the injector, spark plug, valve and end-gas region, and the other grids are about 0.5-1.0 mm in the compression and combustion processes, but it increases to the maximum of 2 mm in the intake process. To simplify the simulation, especially, the meshes of intake port section were removed after intake valve close (IVC). As shown in Figure 3, accordingly, the grid quantities at both intake bottom dead center (BDC) and compression top dead center (TDC) are around 300000.

Table 2 Experimental and simulated spray development of gasoline fuel

Figure 1 Experimental and simulated spray penetration of gasoline fuel

Table 3 Engine specifications

Figure 2 Computational model:

Figure 3 Profile position diagram:

The simulation was started from IVC to EVO. Computational models used in the simulation, referred to Ref. [31], are listed in Table 4. Furthermore, an experiment related to the gasoline DISI combustion was carried out at an engine speed of 2000 r/min and a BMEP of 1.0 MPa, in which gasoline was direct injected into the cylinder by a six-hole injector with the bore diameter of 0.2 mm at an injection pressure of 10 MPa. In the meanwhile, the cylinder pressure was acquired to validate the relevant computational models.Figure 4 indicates the experimental and simulated combustion pressures. It is observed that the experimental and simulated pressure curves are in good agreement, which ensures the validity of the proposed models.

Table 4 Models in simulation

Figure 4 Computational and simulated cylinder pressures of gasoline fuel

3 Results and discussion

3.1 Fundamental understanding of diesel DISI combustion

Based on model validation, the spray and combustion behaviors of diesel direct injection spark ignition are simulated and evaluated with the same initial and boundary conditions as the gasoline fuel. Figure 5 shows atomized Sauter mean diameter (SMD) and evaporation process of diesel and gasoline. As can be seen from the comparison in the figure, the SMD of diesel is significantly larger than that of gasoline, so the evaporation rate of diesel is slower than that of gasoline, which has a negative effect on the formation of diesel/air combustible mixture. Furthermore, the comparison of the local equivalence ratio distribution of diesel and gasoline fuels is depicted in Table 5. It is observed that at the time of ignition, the local equivalence ratio distribution of gasoline/air mixture is clearly more uniform than that of diesel/air mixture. As shown in Figure 4, the slower evaporation rate of diesel fuel causes its more uneven equivalence ratio distribution.

The changes of combustion pressure and heat release rate with crank angle are compared for the DISI combustion of gasoline and diesel in Figure 6(a), as well as temperature in Figure 6(b).

Figure 5 Comparison of (a) SMD and (b) evaporation process of diesel and gasoline

Table 5 Comparison of local equivalence ratio of diesel and gasoline at ignition of 709°CA

Figure 6 Cylinder pressure and heat release rate vs. crank angle (a) and mean gas temperature vs. crank angle (b)

As can be seen from the figures that the ignition time of gasoline DISI is slightly earlier and its peak of heat release rate is higher compared to diesel DISI, so the peak combustion pressure and temperature of gasoline are also higher than those of diesel.

Moreover, the combustion temperature distributions in the cylinder for the DISI combustion of diesel and gasoline fuels are depicted in Table 6. It is observed visually that the combustion temperature of gasoline DISI in the middle of the cylinder is higher than that of diesel DISI at the same moment. And the high temperature of gasoline DISI is more widely distributed with the flame propagation compared to diesel. Those phenomena could be mainly attributed to two reasons. On the one hand, diesel fuel ignites more difficultly compared to gasoline fuel because of the higher ignition energy of diesel. On the other hand, diesel fuel has higher density and larger carbon molecule number compared to gasoline fuel, resulting in slower chain reaction rate.

Table 6 Temperature distributions for DISI combustion of diesel and gasoline fuels (K)

The position and quantity of the knock factor K is depicted in Table 7 by color scale. Referred to Ref. [36], the K value exceeds 5 as the threshold for knock. It is observed that at the same crank angle, the red regions of the K over 5 of diesel fuel, which mainly appears at the edge of the combustion chamber near the exhaust valves, are obviously larger than those of gasoline fuel.

Furthermore, the knock area ratio, which is calculated by the total area of all the meshes with the K over 5 divided by the total mesh area of the model, is used to show the possible range of knocking within the whole combustion chamber. Figure 7 displays the comparisons of the maximum K value and knock area ratio for DISI combustion of diesel and gasoline fuels. It can be seen that the maximum K value and the knock area ratio of diesel DISI combustion are higher than those of gasoline DISI combustion, which indicates the higher possibility of knock of diesel in DISI combustion compared to gasoline. The cause of that result may be attributed to the lower octane number and auto-ignition temperature of diesel fuel.

3.2 Effect of equivalence ratio on diesel DISI combustion

The impact of equivalence ratio on DISI combustion of diesel fuel is studied by numerical simulation in the section. And all the initial and boundary conditions are consistent with previous studies except the equivalence ratio. The variations of combustion pressure, heat release rate and temperature with equivalence ratio are depicted in Figure 8. It is observed that the maximum peak pressure appears at an equivalence ratio of 1.2, and the second is the stoichiometric combustion with the equivalence ratio 1.0. Furthermore, the peaks of heat release rate and temperature are also the highest at the equivalence ratio of 1.2. The result is like that of aviation kerosene investigated by HU et al [18], who have reported the flame propagation velocity of aviation kerosene SI combustion is the fastest when the equivalence ratio is around 1.2. In addition, kerosene fuel burns faster in a rich mixture than gasoline.

Table 7 Knock factor K distributions for DISI combustion of diesel and gasoline fuels

Figure 7 Comparison of knocking possibility of diesel and gasoline fuels

Further, it can also be seen from Figure 8 that the peak values of combustion pressure, heat release rate and temperature in the cylinder will decrease no matter whether the equivalent ratio is increased or decreased from 1.2. And the lowest pressure, heat release rate and temperature occur when the equivalence ratio is 0.6. The above phenomena may be attributed to the combination of the following three reasons. Firstly, the rich mixtures with equivalence ratio over 1 have more radicals formed which are necessary to maintain combustion. Furthermore, the rate of radical formation is greater than the rate of chain breaking, which causes the faster heat release as well as the higher combustion pressure and temperature. Nevertheless, an over rich mixture may cause the increased possibility of chain breaking due to the increased probability of radical collision and the decreased oxygen content, so the equivalence ratio of 1.4 causes the slower combustion and the lower combustion pressure and temperature. Secondly, for fuels with high viscosity such as diesel, as mentioned above, the evaporation rate is slower than that of gasoline, so diesel direct injection in the cylinder is likely to lead to the uneven distribution of fuel. Local equivalence ratio distributions at ignition for different overall equivalence ratios are shown in Figure 9. It can be seen that under the same overall equivalence ratio, local equivalence ratio distribution is inhomogeneous in the whole combustion chamber. And the higher equivalence ratio occurs mainly in the middle of the combustion chamber, near the spark plug, due to the interaction between the spray and combustion chamber of the GDI engine. Moreover, the larger the overall equivalent ratio, the richer the diesel/air mixture in the middle of the combustion chamber, which will affect the ignition and combustion process of diesel spark ignition. Lastly, the lean mixtures with equivalence ratio less than 1 have less fuel, which leads to less heat release. Moreover, an over lean mixture causes a significant lack of exothermic heat, and hereby the combustion pressure and temperature are obviously decreased at an equivalence ratio of 0.6. In addition, it is also observed in Figure 8(c) that the rich mixtures cause the obvious decrease of gas temperature at the end of compression, which may be attributed to the heat absorption of fuel during evaporation.

Figure 8 Effects of equivalence ratio on:

The variations of combustion temperature distribution in the cylinder with the combustion process and equivalence ratio are depicted in Table 8. By comparing the combustion temperatures of the mixtures with different equivalence ratios, it can be seen that the combustion temperature is almost possible knock region occurs in the condition of equivalence ratio 1.2. On the contrary, the smallest possible knock region appears at an equivalence ratio of 0.6 due to the lowest combustion temperature, as shown in Table 8.

Table 9 shows the effects of equivalence ratio on the distribution of possible knock region represented by knock factor K. It can be seen that the possible knock region of the K over 5 is the largest at an equivalence ratio of 1.0 compared to other equivalence ratios. And the second largest the highest when the equivalence ratio is 1.0 and 1.2, and the distribution of the high temperature zone is the widest. Moreover, the combustion temperature at an equivalent ratio of 0.6 is the lowest due to the very lean fuel/air mixture.

The effects of equivalence ratio on the maximum K value and knock area ratio are depicted in Figure 10. It is observed that the maximum K and knock area ratio first rise and then reduce when the equivalence ratio varies from lean to rich. And the maximum K and knock area ratio occur at an equivalence ratio of 1.0, which indicates the highest possibility of knock. Moreover, the diesel DISI combustion with an equivalence ratio of 1.2 can obviously decrease the maximum K and knock area ratio. The reason is that the diesel mixture with an equivalence ratio of 1.2 has faster combustion rate, which can reduce the time that diesel flame spreads and then decrease the auto-ignition possibility of end mixture gas.

In summary, the diesel mixture with an equivalence ratio of 1.2 has fastest flame propagation velocity. What’s more, the knock tendency of an equivalence ratio of 1.2 is lower than an equivalence ratio of 1.0. From the above two points, therefore, the SI combustion of diesel fuel with a rich mixture will be more appropriate if emissions are not considered. For some specific purposes, such as the military, emissions are not a major concern.

Figure 9 Effect of overall equivalence ratio on local equivalence ratio distribution at ignition of 709°CA

Table 8 Effect of equivalence ratio on temperature distribution in the cylinder (K)

4 Conclusions

In the study, CFD simulation of diesel direct-injection spark ignition (DISI) combustion was carried out to provide a deep understanding of diesel SI combustion behavior and the effect of equivalence ratio. In the simulation, the computational models were calibrated by test data from a GDI engine, and the simulation result verified the accuracy and effectiveness of the proposed models. Based on the calibrated models, furthermore, the combustion characteristics of diesel DISI were investigated.

Compared to the gasoline DISI combustion, the flame propagation speed of diesel DISI combustion is slower, and its combustion pressure and temperature are lower. Moreover, the knocking possibility of diesel fuel in SI combustion is much larger.

For a diesel/air mixture with an equivalence ratio of 0.6 to 1.4, higher combustion pressure and faster burning rate occur when the equivalence ratios are 1.2 and 1.0 compared to the other mixtures. However, higher possibility of knock happens when the equivalence ratio is 1.0. Therefore, the spark ignition combustion of diesel fuel in the engine should use a rich mixture to obtain better combustion performance.

Table 9 Effect of equivalence ratio on possible knock K in cylinder

Figure 10 Effect of equivalence ratio on knocking possibility

Nomenclature

AADI

Air-assist direct injection

BDC

Bottom dead center

CFD

Computational fluid dynamics

CFM

Coherent flame model

CI

Compression ignition

DI

Direct injection

DISI

Direct injection spark ignition

DNS

Direct numerical simulation

ECFM

Extended coherent flame model

ECU

Electronic control unit

EGR

Exhaust gas recirculation

EVO

Exhaust valve open

HC

Hydrocarbon

IC

Internal combustion

IVC

Intake valve closure

IVO

Intake valve open

K

Knock factor

LES

Large eddy simulation

NS

Navier-Stokes

RANS

Reynolds averaged Navier-Stokes

RON

Research octane number

SI

Spark ignition

SMD

Sauter mean diameter

TDC

Top dead center

UAVs

Unmanned aerial vehicles

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(Edited by ZHENG Yu-tong)

中文导读

当量比对柴油直喷点燃燃烧的影响

摘要：在小型航空领域航空重油点燃式活塞发动机受到越来越多的关注。航空重油指的是航空煤油或者轻质柴油，相比汽油燃料在使用和储存上都更安全。而柴油燃料更适合应用在陆地上的小型航空发动机中。本文采用数值模拟研究去评估汽油直喷点燃转换到柴油直喷点燃的可能性。其中柴油采用原有的汽油直喷系统直接喷入气缸并通过火花点燃。在模拟中，计算模型通过一台汽油直喷发动机的实验数据进行标定，并验证了模型的有效性。进而，在验证模型的基础上，研究了柴油直喷点燃燃烧特性。研究结果表明，柴油直喷点燃燃烧过程比汽油慢，并且柴油在点燃燃烧中的爆震倾向更高。对当量比从0.6到1.4的柴油/空气混合气，当量比为1.2和1.0时的燃烧压力更高，燃烧速率更快，但当量比为1.0的爆震可能性更高。综合来看，柴油燃料在发动机中的点燃燃烧采用浓混合气能获得更好的燃烧性能。

关键词：柴油；直喷；点燃；当量比；燃烧；爆震

Foundation item: Project(2018JJ2041) supported by the Science and Technology Project in Hunan Province, China; Project(szjj2019-008) supported by the Open Research Subject of Key Laboratory of Fluid and Power Machinery, Ministry of Education, China

Received date: 2019-12-15; Accepted date: 2020-05-09

Corresponding author: CHEN Zheng, PhD, Associate Professor; Tel: +86-731-88664452; E-mail: chenzheng@hnu.edu.cn; ORCID: https://orcid.org/0000-0003-2453-4755