压缩比对双燃料发动机中高负荷影响的试验和模拟

doi:10.3969/j.issn.1674-8484.2020.04.012

摘要/Abstract

摘要：

为了解决多缸内燃机双燃料燃烧在中高负荷粗暴燃烧的问题,该文先后采用试验和模拟的方法研究了降低压缩比（CR）对双燃料燃烧中高负荷特性的影响。结果表明：试验中随着压缩比从17.4降低至14.5,均质混合引燃（HCII）燃烧能够达到的最高汽油比例从33%左右上升至54 %左右。NO_x、碳烟（Soot）和CO排放均能够进一步降低,而碳氢化合物总量（THC）排放则有所增加。模拟中,压缩比降低后汽油组分局部温度降低可达到750 K左右; 双燃料燃烧的NO_x和Soot主要产生于柴油扩散燃烧区域,随着汽油比例的升高NO_x和Soot排放可得到降低;汽油自燃可以有效消除燃烧室狭缝中残留的THC以及汽油不完全燃烧产生的CO排放。总之,通过适当降低压缩比能够在一定程度上改善双燃料燃烧在中高负荷的粗暴燃烧问题,并改善其燃烧排放特性。

关键词: 内燃机多缸机, 双燃料燃烧, 压缩比, 均质混合引燃（HCII）, 汽油比例, 数值模拟

Abstract:

The rough combustion at the medium and high load has always been a challenge for the dual-fuel combustion. The effects of compression ratio (CR) on the dual-fuel combustion at medium and high load were investigated experimentally and numerically. The experimental results showed that the highest gasoline ratio in homogeneous charge induced ignition (HCII) combustion increased from about 33% to about 54% by reducing the compression ratio from 17.4 to 14.4. As a result, the NO_x, soot and CO emissions were further reduced but the total hydrocarbons (THC) emissions got increased. The simulation results showed that the temperature of gasoline could be lowered by about 750 K; The NO_x and soot emissions were mainly distributed in the diesel combustion area, and they could be reduced with the increasing gasoline ratio; The THC emissions in the cylinder crevice and the CO emissions generated from the incomplete combustion of gasoline can be eliminated by the gasoline auto-ignition. In conclusion, the rough combustion at the medium and high load can be alleviated and the performance of the dual-fuel combustion can be improved by reducing the compression ratio.

Key words: multi-cylinder engine dual-fuel combustion, compression ratio, homogeneous charge induced ignition (HCII), gasoline ratio, simulation

中图分类号:

TK421.2

任烁今, 张明, 郭勇, 颜燕, 王志, 王建昕. 压缩比对双燃料发动机中高负荷影响的试验和模拟[J]. 汽车安全与节能学报, 2020, 11(4): 518-528.

REN Shuojin, ZHANG Ming, GUO Yong, YAN Yan, WANG Zhi, WANG Jianxin. Experimental and numerical investigation of effects of compression ratio on dual-fuel combustion at medium and high load[J]. Journal of Automotive Safety and Energy, 2020, 11(4): 518-528.

图/表 23

图1 发动机台架试验系统示意图

汽缸数	6
缸径	108 mm
冲程	136 mm
连杆长度	209.7 mm
排量	7.47 L
压缩比(原机)	17.4
额定功率	220 kW
额定转速	2 300 r/min
最大转矩	1.250 kNm
最大转矩工况转速	1 700 r/min

表1 发动机技术参数

图2 不同压缩比的燃烧室形状

参数	汽油	柴油
十六烷值CN	-	56.5
辛烷值RON	95.1	-
密度(20 ℃) /（kg·L^-1）	0.753	0.830
热值 /（MJ·kg^-1）	42.84	42.68
运动粘度 (20 ℃) /（mm²·s^-1）	0.75	4.13
硫质量分数含量 / 10^-6	21.0	4.3

表2 燃油特性

图3 数值模拟的计算域

	区域	级别	时段
自适应	整个计算域	3	永久
嵌入式	气缸盖表面	1	永久
	气缸壁表面	1	永久
	活塞表面	1	永久
	喷束圆锥	2	每次喷油开始到结束后去曲柄转角CA = 5°

表3 计算网格加密的设置

图4 自适应和嵌入式网格加密（CA = 5°时）

化学机理	汽柴油多组分机理^[14]
状态方程	Redlich-Kwong方程
燃烧模型	SAGE-Multizone模型
喷雾模型	破碎模型,KH-RT修正模型
	蒸发模型,Frossling校正模型
	碰撞模型,NTC模型
湍流模型	RANS（RNG k-ε）模型
NO_x模型	PRF-PODE3机理中的NO_x子机理
Soot模型	Hiroyasu-NSU模型

表4 模拟计算所采用的数理模型

	组分	摩尔比 / %
95#汽油	iC₈H₁₈	65.8
95#汽油	nC₇H₁₆	8.7
	C₇H₈	25.5
0#柴油	nC₁₆H₃₄	90.0
	C₇H₈	10.0

表5 汽油和柴油多组分表征燃料构成

图5 工况B75下压缩比对中高负荷缸压和放热率的影响

图6 工况B75下压缩比对中高负荷滞燃期和燃烧持续期的影响

图7 工况B75下压缩比对中高负荷最大压升率和最高缸内压力的影响

图8 工况B75下压缩比对中高负荷NOx和Soot排放的影响

图9 工况B75下压缩比对中高负荷THC和CO排放的影响

图10 工况B75下压缩比对中高负荷燃烧效率和有效效率的影响

图11 A50工况下缸压和放热率的试验和模拟结果对比

	试验	模拟-多组分	模拟- PRF
e[NO_x] / [g·(kWh)^-1]	2.088	1.942	2.216
e[Soot] / [g·(kWh)^-1]	0.022	0.024	0.019

表6 NOx和Soot排放的试验和模拟结果对比

图12 A50工况下压缩比对缸压和放热率以及能量分布的影响

图13 压缩比对燃烧和排放特性的影响

图14 不同压缩比时温度和当量比的缸内分布

图15 不同压缩比时缸内单点温度曲线对比

图16 不同压缩比时NOx和Soot排放的缸内分布

图17 不同当量比时THC和CO排放的缸内分布

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