天然气/DME混合物在CI发动机中的实验研究

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"这篇研究论文‘Experimental Study of CI Natural-Gas/DME’主要探讨了在压缩点燃发动机中使用天然气和二甲醚(DME)混合物以降低NOx排放并提高热效率的可能性。研究中，通过适当比例的DME与天然气混合来控制点火时间和改善燃烧过程。实验结果显示，增加一定量的DME可以使发动机在较宽的负荷范围内运行，并通过调整混合气的稀薄程度，可以将NOx排放降低到接近零的水平。同时，这种运行方式的热效率超过了常规柴油燃料操作的效率。该研究由来自Ibaraki大学的Zhili Chen、Mitsuru Konno以及Ibaraki大学研究生院的Mitsuharu Oguma和Tadanori Yanai共同完成，并在2000年SAE世界大会上发表于‘Compression Ignition Combustion Processes’会议中。" 这篇论文的关键知识点包括： 1. **天然气与DME混合燃烧**：研究采用了一种均质充量的天然气和空气混合物，旨在利用天然气的环保特性（低NOx排放）和DME的易于点燃性质，以改善发动机性能。 2. **控制点火和燃烧**：DME被用作助燃剂，以精确控制发动机的点火时间，这在压缩点燃发动机中至关重要，因为它直接影响到燃烧效率和排放。 3. **负荷范围扩大**：通过添加DME，发动机能够适应更广泛的负荷范围，这表明混合燃料可能具有更好的灵活性和适应性。 4. **NOx排放降低**：通过优化DME与天然气的比例，可以实现非常稀薄的混合气，从而显著降低NOx排放，这是环保驾驶的一个重要指标。 5. **提高热效率**：与常规柴油操作相比，使用天然气-DME混合燃料能提高热效率，这意味着能源转化更有效，同时也可能减少燃油消耗。 6. **实验方法**：论文中描述的实验是实地进行的，通过在实际发动机上测试各种工况，收集性能数据和排气特性，以验证理论分析。 7. **SAE世界大会**：研究结果在SAE（美国汽车工程师学会）的世界大会上发布，表明这是一个受到同行评审且有影响力的科学研究。 8. **版权信息**：文章下载自SAE International，版权受法律保护，允许个人或内部使用，但对超出法定复制权限的情况需要支付版权费。这些知识点不仅展示了混合燃料技术在减少污染和提高能效方面的潜力，也体现了在汽车工程领域，特别是发动机燃烧科学和排放控制方面持续的技术探索。

2000-01-0329

Experimental Study of CI Natural-Gas/DME

Homogeneous Charge Engine

Zhili Chen and Mitsuru Konno

Ibaraki Univ.

Mitsuharu Oguma and Tadanori Yanai

Graduate School, Ibaraki Univ.

ABSTRACT

In this study, a homogeneous mixture of natural-gas and

air was used in a compression ignition engine to reduce

NOx emissions and improve thermal efficiency. In order

to control ignition timing and combustion, a small amount

of DME was mixed with the natural-gas. Engine perfor-

mance and the exhaust characteristics were investigated

experimentally. Results show the following: the engine

can run over quite a large load range if a certain amount

of DME is added into natural-gas. By optimizing the pro-

portion of DME to natural-gas, NOx emissions can be

lowered to near zero levels if the mixture is lean enough.

Thermal efficiency is higher than that obtained with nor-

mal diesel fuel operation.

INTRODUCTION

Recent research into reducing NOx emission effectively

and increasing thermal efficiency in diesel engines has

shown that the new homogeneous charge compression

ignition combustion (HCCIC) is a promising way to

accomplish these tasks. However, certain problems in its

application need to be resolved

(1-4)

. The rest of this sec-

tion will discuss some of the problems reported by differ-

ent researchers in this field.

Conventional diesel fuel is not suitable for HCCIC

because of problems arising from inherent physical and

chemical characteristics of diesel fuel. For example: in

order to achieve a homogeneous diesel/air mixture, early

injection timings are required, because the vaporization

of diesel fuel is poor. However, if the temperature in a cyl-

inder becomes higher than the self-ignition temperature

of the charge during the compression stroke, self-ignition

will occur before TDC. Since ignition timing has to be

controlled around TDC, the amount of homogeneous fuel

has to be restricted to less than a certain level

(2)

Early fuel injection tends to splash the spray at the wall of

the cylinder. This bars the formation of the homogeneous

mixture, thus, emissions of THC, CO, and PM (more than

twice) will be increased and the engine thermal efficiency

will be decreased

(3)

. To resolve these problems, Hisashi

Akagawa et al

(4)

. employed a pintle type injection nozzle

and a reduced top-land-crevice piston to increase the

homogeneity and control THC and CO emissions. Intro-

ducing a large amount of EGR is also a good way to

delay ignition timing.

On the other hand, natural-gas is a better alternative than

diesel fuel as it easily forms a uniform mixture for HCCIC.

The recoverable reserves of natural-gas are almost the

same as that of crude oil based on energy content)

(5)

Therefore it would be wiser to use the gaseous fuel rather

than any synthetic liquid fuel converted from natural-gas

because the price of the latter would likely be higher

(6)

The application of natural-gas to an internal-combustion

engine should be by way of compression ignition if high

thermal efficiency and low NOx emission are expected.

However, the self-ignition temperature of natural-gas is

much higher than that of the conventional diesel fuel.

Therefore, even if natural-gas is introduced into an exis-

tent diesel engine, compression self-ignition will not

occur

(7)

. Therefore, in order to operate the compression

ignition engine with natural-gas, a method of raising the

compression temperature must be considered. Yasuharu

Kawabata et al. used a two-stage heater with an external

supercharging unit to raise the intake air temperature by

180 °C. However, since the ignition timing cannot be

effectively controlled by this method, the excess air ratio

range was restricted within narrow limits from 4.2 to 3.22

(8)

Another idea made by Seiichi Shiga et al

(9)

. for natural-

gas compression ignition in two-stroke engines (com-

pression ratios were set to 13.5 or 14.5) was to shut

down the exhaust throttle valves to as small as 10% of

the maximum. This was done to maintain a considerable

amount of burnt gas (from a previous cycle) in the cylin-

der, in order to keep a high compression temperature.

However, the engine operating conditions were restricted

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