Simulation Analysis on Power Performanceand NOx Emission of Engine UnderDifferent Oxygen-Air Ratios

LI Dongmin， ZHANG Hui， LI Yushan

(1. Department of Mechanical and Electronic Engineering， Shandong University of Science and Technology， Tai′an 271019， China；2. State Key Laboratory of Fluid Power and Mechatronic System， Zhejiang University， Hangzhou 310058， China；3. Department of Mechanical and Electrical Technology， Ningxia Polytechnic， Yinchuan 750021， China；4. College of Transportation， Shandong University of Science and Technology， Qingdao 266590， China)



Simulation Analysis on Power Performanceand NOxEmission of Engine UnderDifferent Oxygen-Air Ratios

LI Dongmin1，2， ZHANG Hui3， LI Yushan4

(1. Department of Mechanical and Electronic Engineering， Shandong University of Science and Technology， Tai′an 271019， China；2. State Key Laboratory of Fluid Power and Mechatronic System， Zhejiang University， Hangzhou 310058， China；3. Department of Mechanical and Electrical Technology， Ningxia Polytechnic， Yinchuan 750021， China；4. College of Transportation， Shandong University of Science and Technology， Qingdao 266590， China)

Abstract：In order to improve the dynamic performance and reduce NOx emission from the engine， the whole model for a certain type of engine is built with GT-Power， and the main components are displayed in the model. The corresponding properties and parameters for the components are set according to the geometry structure parameters of the engine， and the simulation on dynamic performance and NOx emission under oxygen-air ratios of normal value， 20%， 24%， 25% and 30% for the engine intake is achieved. The simulation results show that the rated power and torque of the engine increase with the increasing of oxygen-air ratio， but the NOx emission increases sharply when oxygen-air ratio is over 25%.

Keywords：oxygen-air ratio； NOx emission； simulation； oxygen enrichment； engine

Automobile is the essential tool for transportation in the modern life， however a lot of exhaust gases are generated and emitted into the outside atmosphere during the process of moving the automobile. There are a few poisonous substances in the exhaust gas according to the corresponding documents， e.g.， NOx， HC， CO， SOxand other particles， and some of them are the main components of PM2.5， so they are harmful for the human[1-2]. On the other hand， most of the exhaust gases are harmful for the plant too， furthermore the secondary hazard is generated after the exhaust gases are absorbed into the contaminative vegetables. Besides， the exhaust gases do much harm to the ecological environment， e.g.， greenhouse effect. Therefore， some ways must be taken to decrease the damage from the exhaust gases， even eliminate it.

A way is studied and simulated based on a special gasoline engine in this paper， and NOxemission is measured by adjusting oxygen content in the air， finally the improvement solution is proposed according to the simulation results. The whole contents are arranged as follows: the current studies on combustion with oxygen enrichment are introduced and analyzed， then the special diesel engine is modeled with GT-Power according to its parameters of structure and performance， next NOxemission is got according its working conditions， finally the simulation results are got and the solution is proposed.

1Current Studies on Combustion With Oxygen Enrichment

The studies on emission from the engine was started in 1980s′ abroad. The documents[3-4]propose that particle filter must be adopted in order to meet American emission regulation in 1994. Many studies have been done in the corresponding institutes to decrease emissions from the engine in order to deal with severer and severer emission regulation. Johnson T V summarized the ways on decreasing NOxemission， e.g.， selective catalytic reduction， adsorption reduction， and introduced the development of particle filter of the engine， texture of materials and management for coal smoke etc[5-6]. Document[7]discovers the following results via the theoretical analysis: under the two conditions of constant intake air pressure and enhanced intake air pressure， and the stable air-fuel ratio， the coal smoke of the engine and NOxemission can be decreased. Besides， under the conditions of oxygen enrichment and exhaust gas recirculation (EGR)， the coal smoke and NOxemission from heavy-duty diesel engine can be decreased.

The studies on emission from the engine and oxygen enrichment was started in 1990s′ at home. The feasibility of controlling NOxemission and coal smoke with the roles of EGR and oxygen enrichment intake is studied in document[8]. And the experimental results show that NOxand coal smoke emission can be controlled effectively with the roles of EGR and oxygen enrichment intake， however the effective fuel consumption rate deteriorates a little.

Combustion and emission control of the engine is studied deeply for a long time in some research institutions. Among that， a research group studies the effect of the shutdown time and the angle of the valve on the homogeneous combustion. Under the rule of negative valve overlap， intake gas front backflow with different levels is formed to regulate the ignition time through opening intake valve ahead of time. The research group led by professor Wanhua Su studies the influence regularity of different combustion control parameters on heavy-duty diesel engine during the process of low temperature combustion[2]. Another research group studies the influence regularity of high pressure and low pressure EGR based on two-level pressurization system， and exhaust energy distribution between highlevel and lowlevel turbine based on low pressure EGR on the combustion process， performance and emission of diesel engine， based on common rail heavy-duty diesel engine， aiming at improving the capacity of EGR cycle. The study on the influencing mechanism of EGR on combustion in low temperature shows that NOxemission depends mainly on the concentration of oxygen. Besides， the influence of the composition of the intake gas on combustion performance and emission characteristics of the diesel engine in low temperature is studied with the experiment and simulation， by contrasting the influence of different composition of the intake gas on the combustion and emission of the engine. The value of coal smoke and the corresponding EGR rate increase with the increase of pressure of intake gas. And the higher EGR rate can decrease NOxemission further under the condition of the same coal smoke emission[9]. The research elaborates and summarizes the homogeneous compression ignition， combustion theory in low temperature， and extends it to homogeneous compression ignition and combustion in low temperature for gasoline fuel， and homogeneous compression ignition and combustion in low temperature for diesel fuel， and homogeneous compression ignition and combustion in low temperature for fuel characteristics etc[10].

The emission and combustion of the engine is studied and some theories are proposed， e.g.， homogeneous compression ignition and combustion in low temperature， however the influence of the diesel fuel with different oxygen-air ratio on NOxemission and performance of the diesel engine is undefined， so it is urgent to study the above issue in this paper.

2Modeling for Engine

There are many ways to model engine， e.g.， top to down or down to top， the main modeling tools include AVL FIRE， Star-Ccm， AMESim and GT-Power， however they are different in performance. AVL FIRE is a powerful multi-purpose thermo-fluid dynamics software with a particular focus on handling fluid flow applications related to internal combustion engines and powertrains. Star-CCM is an entire engineering process for solving problems involving flow (of fluids or solids)， heat transfer and stress. AMESim offers a complete 1D simulation suite to model and analyze multi-domain， intelligent systems and to predict their multi- disciplinary performance. GT-Power is a tool to simulate the working process of engine， it has powerful auxiliary modeling pretreatment tool， rich combustion model and control function besides optimization function with it can optimize the sound attenuation components in intake and exhaust system. It represents the flow and heat transfer in the piping of the engine system based on one-dimensional gas dynamics[11]， therefore， GT-Power meets the requirement of modeling and be adopted to model the engine in this paper.

2.1Modeling for Engine

The fundamental parameters of the engine detected are shown as follows： 4-cylinder inline engine； displacement 1 390 cm3； cylinder bore 76.5 mm； piston stroke 75.6 mm； compression ratio is 10 to 1. There are many indexes to evaluate the main performance of engine， i.e.， dynamic， environment， economy and reliability.

Among that， the dynamic indexes are applied to indicate the capability of power characterization of engine， which includes brake torque， brake power and rotation speed of engine. Environmental indexes are applied to indicate the quality of exhaust and noise of engine， and emission indexes mainly refer to the amount of harmful emissions contained in gases from the fuel tank and crankcase tank， and the exhaust from cylinder.

Fig.1　Simulation model of the engine图1　发动机仿真模型

Economic indexes are indicated with effective fuel consumption. Reliability indexes are applied to indicate the ability of normal continuous working for engine under the specified conditions and time.The engine is modeled according to the structure， performance parameters and fundamental working principle of the gasoline engine， which is shown in figure 1.

2.2Oxygen-Air Ratio

As we all know， air-fuel-ratio affects the performance of engine greatly. Among that， air intake， above all oxygen intake， has substantial influence on power， torque and speed of engine. Therefore， oxygen-air ratio (abbr. OAR) is focused to improve the performance and reduce NOxemission of engine in this paper.The normal volume ratio of oxygen to air is about 21%， and the working status of engine differs as OAR varies. Some experiments show that rich oxygen and poor nitrogen intake can improve the power and torque of engine qualitatively[12]， however the influence on NOxemission with variable OAR is not definite.

2.3Simulation Setup

The geometry structure and working parameters of the engine are set before simulation， among that， the air intake is set as a reference variable， meanwhile the value of OAR is set as 5 kinds of working conditions， i.e.， normal value， 20%， 24%， 25% and 30%， to achieve the complete relation curve， and the air-fuel-ratio based on OAR is set as follows: attribute， injector delivery rate， fuel ratio specification and fuel ratio are set as objective value， 6， air-to-fuel and 12.5 respectively. Among that， the object value air-to-fuel for fuel ratio specification is an alternative variable. And the geometry structure of the engine is set as follows: attribute， bore， stroke， connecting rod length， compression ratio and TDC clearance height are set as objective value， 76.5， 75.6， 147， 10 and 0.5 respectively.， and other fundamental parameters of the engine i.e.， attribute， engine type， speed or load specification， engine friction object or FMEP and start of cycle， are set as object value， 4-stroke， speed， r·min-1， friction and -95 respectively. Among that， the object value friction for engine friction object or FMEP is a reference variable.

After finishing the setup for the components of the engine， simulation parameters with GT-Power are set. The speed of the engine is set between 4 600 r·min-1and 1 800 r·min-1with 400 r·min-1increments， so 8 cases are set in the simulation process.

3Simulation Results and Analysis

According to geometry structure， working principle of the gasoline engine detected and the above setup， the simulation for the engine is done with GT-Power， and the relation curves for speed-power， speed-torque and speed-NOxare achieved under the value of OAR is set as normal value， 20%， 24%， 25% and 30% respectively. Among that， the three curves under normal OAR， i.e.， 21%， are shown in figure 2， 3， 4. In figure 2, 3 and 4, the symbolP,T，ρ(NOx) andnindicate brake power, brake torque， mass concentration of NOxand speed respectively.

Fig.2　Speed-rated power of 　　　　Fig.3　Speed-rated torque of 　　　Fig.4　Speed-NOx emission of the　the engine under normal OAR　　　　the engine under normal OAR　　　　　engine under normal OAR　　图2　自然氧空比条件下　　　　　　图3　自然氧空比条件下　　　　　　图4　自然氧空比条件下　发动机转速-额定功率关系曲线　　　　发动机转速-额定扭矩关系曲线　　　发动机转速-NOx排放关系曲线

Figure 2 and 3 indicate that the brake power and torque both increase with the increase of the rotation speed of the engine under normal OAR， which are consistent with the characteristic curve of gasoline engine. Meanwhile figure 4 shows that NOxemission decreases gradually with the increase of the rotation speed under the same OAR.

In order to achieve the complete variation tendency of performance-OAR and NOx-OAR， the relation curves of speed-brake power under the value of OAR is set as normal value， 20%， 24%， 25% and 30% respectively are achieved and drawn in figure 5. Among that， the line decorated with rhombus shape， the line decorated with square shape， the line decorated with triangle shape， the line decorated with cross shape and the line decorated with asterisk shape represent the relation curves of speed-brake power under 20%， normal， 24%， 25% and 30% OAR respectively. It can be inferred that the brake power improves with the increase of OAR obviously.

The relation curves of speed-brake torque under 5 kinds of OAR got in the same way are shown in figure 6， and the brake torque improves with the increase of OAR too.The relation curves of speed-NOxunder 5 kinds of OAR got in the same way are shown in figure 7. In figure 5, 6 and 7, the symbolφ(O2) indicates the volumetric oxygen-air ratio.Among that， the decorated shape of the line in the figure represents the relation of speed-NOxin the same OAR as the figure 5 and 6. NOxemission increases with the increase of OAR， however NOxemission is approximately close to 0 when OAR is between 20% and 25%， so the relation curves closely coincide. The relation curve of speed-NOxunder 25% OAR shown in figure 8 shows that NOxemission increases sharply. Therefore a deduction is made that a huge increase of NOxemission arises under 30% OAR according to the relation of speed-NOxunder different OAR.

Fig.5　Speed-rated power of the engine Fig.6　Speed-rated torque of the engine under different OAR under different OAR　图5　不同氧空比条件下发动机 图6　不同氧空比条件下发动机　转速-额定功率关系对比曲线 转速-额定扭矩关系对比曲线

Fig.7　Speed-NOx emission of the engine Fig.8　Speed-NOx emission of the engine under different OAR under 25% OAR　图7　不同氧空比条件下发动机 图8　氧空比为25%时发动机　转速-NOx排放关系对比曲线 转速-NOx排放关系曲线

4Conclusions and Prospect

In order to improve the performance and reduce the emissions of the engine， a special gasoline engine is modeled with GT-Power according to its geometry structure and working parameters， and the relation curves of speed-brake power， torque and NOxof engine under 20%， normal， 24%， 25% and 30% OAR are got.

The simulation curves show that the brake power and the brake torque improve with the increase of OAR. Meanwhile NOxemission is close to 0 when OAR is between 20% and 25%， however it increases sharply under over 25% OAR， and sharp increase arises under 30% OAR.

According to the simulation results， the optimal way considering both the dynamic performance improvement and NOxemission reduction of the engine will be studied in future works.

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(责任编辑： 黄仲一英文审校： 杨建红)

不同氧空比下发动机动力性能与 NOx排放仿真分析

李东民1，2， 张慧3， 李玉善4

(1. 山东科技大学 机电工程系， 山东 泰安 271019；

2. 浙江大学 流体动力与机电系统国家重点实验室， 浙江 杭州 310058；

3. 宁夏职业技术学院 机械与电气技术系， 宁夏 银川 750021；

4. 山东科技大学 交通学院， 山东 青岛 266590)

摘要：为提高发动机动力性并减少 NOx排放，采用GT-Power对某型发动机进行整机建模，并列出主要组件.根据发动机几何结构参数设置各组件的对应属性和参数，在发动机进气氧空比分别为正常值，以及20%，24%，25%及30%条件下进行了动力性能与 NOx排放仿真实验.实验结果表明：随着氧空比增大，发动机额定功率与扭矩增大；当进气氧空比超过25%时，NOx排放急剧增大.

关键词：氧空比； NOx排放； 仿真； 富氧； 发动机

Document Code：CLC Namber：TK 401 A

通信作者：李东民(1977-)，男，讲师，博士，主要从事动力系统优化，机电系统设计与控制的研究.E-mail:ldm753@163.com.

收稿日期：2015-10-27

doi:10.11830/ISSN.1000-5013.2016.03.0281

文章编号：1000-5013(2016)03-0281-06

基金项目：山东省自然科学基金资助项目(ZR2015EM042)； 山东省高等学校科技计划项目(J14LB11)； 山东科技大学国内访学资助项目(2014-2016年度)