柴油燃料性质对内燃机排放的影响及机动车燃料的发展趋势

任福民,舒 星,尹 静,王 政,姚思雨,门正宇

(1.北京交通大学土木建筑工程学院,北京100044;2.北京海光仪器有限责任公司,北京100015; 3.天津理工大学环境与安全工程学院,天津300384)



柴油燃料性质对内燃机排放的影响及机动车燃料的发展趋势

任福民1,舒 星2,尹 静3,王 政1,姚思雨1,门正宇1

(1.北京交通大学土木建筑工程学院,北京100044;2.北京海光仪器有限责任公司,北京100015; 3.天津理工大学环境与安全工程学院,天津300384)

摘 要:对显著影响内燃机排放的柴油燃料的性质进行表征,通过分析燃料化学组成对机动车污染排放物的影响,提出降低柴油中硫和芳香烃含量、改进十六烷值可满足排放标准的要求.从几个关键技术指标对机动车柴油燃料发展趋势进行总结,提出降低改进内燃机排放的技术措施.

关键词:机动车燃料;性质;排放;内燃机;标准

1 Effect of Diesel Fuel Properties on Diesel Engine Exhaust Emission

The wider usage of diesel-powered engines or vehicles all over the world is due to their advantages of higher power output,cheaper fuel price and lower emission of traditional pollutants including carbon monoxide(CO),hydrocarbons(HCs),and nitrogen oxides(NOx)than gasoline-powered engine[1].Specific attention has been devoted to hydrocarbons such as polycyclic aromatic hydrocarbons(PAHs)from diesel exhaust emissions,which have been reported linking with the adverse mutagenic and/or carcinogenic activity to publichealth[2].Both of the diesel fuel structures and the engine operation conditions affect the chemical composition and potential biological activity of diesel emission,so co-operation between engine designers and fuel technologists is needed to determine the optimum solutions.

Diesel vehicles face a serious challenge from stringent legislation in reducing exhaust emissions.The harmful chemicals emitted by diesel fuel are generated mainly in the combustion process,depending on the engine technology,engine operating conditions and the diesel fuel properties such as fuel aromatics content,cetane number,back end volatility(T9595% boiling poing temperature),density and sulfur content.Particulate matter(PM)and soot are significantly influenced by factors such as cetane number,distillation range,density and aromatics content,and soluble organic fraction(SOF)is related to cetane number,sulfur and other factors.The PAH in PM mainly comes from the fuel,which could be greatly reduced by introducing oxygen containing components and adding combustion smoke eliminating agent.The key fuel property influencing particulates is density,especially the fuel density above the engine calibration range(above approximately 0.86 kg/L),resulting in an apparent over-fuelling phenomena with a sharp increase in particulate and hydrocarbon emissions[3].

Publications show that decreasing fuel aromatic content,sulfur,and volatility are generally associated with decrease in regulated emissions[4].The emission of diesel engine has a direct relationship with the content of aromatic in diesel fuel[5].Many countries and areas have put the aromatic content into the quality control index of diesel fuel.Diesel particles consist of an element carbon core and a large number of absorbed chemicals,including 3-rings and higher PAHs in the gas phase and the diesel particles[6].Both particle-associated compounds and vapor-associated compounds from diesel emission are mutagenic[7].Correlation has been confirmed between the level of poly-aromatics in the fuel and PAH emission[8].To limit the aromatic content in diesel fuel seems a feasible way to reduce the PAH emission under the controlling strategy of refining technology and regulated laws.

2 Analysis of Source and Chemical Properties in Pollutants Emitted by Diesel Vehicles

Organic pollutants are detected in the diesel exhaust particles,resulting from incomplete combustion of linear paraffins and olefins,with chain carbon number range from C12-C24.Polycyclic aromatic hydrocarbons and heterocyclic compounds are also included.The polycyclic aromatic hydrocarbons in diesel exhaust contain 17 species,and the heterocyclic compounds 6 kinds,including anthracene,phenanthrene,pyrene,etc.,followed by benzene,aldehydes,ketones,alcohols,acids and phenols compounds.

Paraffin and aromatic fuels display different thermal cracking and condensation polymerization processes[9].The paraffin fuels are thermal cracked to form lower boiling point hydrocarbons,followed by condensation polymerization and production of benzene ring compounds.The formation of particulate matter takes place via polycyclic aromatic hydrocarbons.However,the benzene ring in the aromatic fuels is not thermally cracked to form polycyclic aromatic hydrocarbons.

Most of the researches show that the composition of the SOF in terms of n-alkane and PAH is predominantly unburnt fuel compounds[10].Search for pyrosynthesised compounds show that Benzo(a)pyrene,Benzo(c)pyrene,Chrysene,Carbozole etc.are all formed in the engine at this higher temperature condition as the fuel contained none of these compounds.A higher fuel PAH content would result in higher PAH emissions.But the PAH and n-alkane emissions are very low at the high speed maximum power condition,due to the utilization of the VCO(valve covered orifice)nozzle which eliminates the main source of the unburnt fuel.

Comparing the effect of mono-aromatic and poly-aromatic content in surrogate diesel fuel on PAH emission,decreasing the poly-aromatic content in diesel fuel are of great important for the re-duction of PAH emission from an heavy-duty diesel(HDD)engine[11].To regulate the content of poly-aromatic content in diesel fuel,in contrast to the total aromatic content,will be more suitable for the management of PAH emission.

For an HDD engine,at a constant cruising speed,a higher engine load results in a higher PAH emission;while at a constant engine load,a higher cruising speed results in a lower PAH emission.The majority of PAH mass is oxidized during the combustion process in the HDD engine.At the condition of a higher speed with a high load,the HDD engine has a higher temperature to reduce the PAH amount.

The decrease in cetane number causes an increase in NOxand a decrease in PM for DI(Direct Injection)diesel engine because of the long ignition delay[12],however,a decrease in cetane number seldom causes an increase in PM emission for DI impingement diffusion combustion diesel engines.Although NOxand PM from aromatic fuel are higher than paraffinic fuel,the oxygen in fuel can reduce emission for both engine with and without EGR,so fuel with oxygenate is a possible choice for low exhaust emission.

Researches show that increasing cetane number can reduce a regulated diesel emissions species[13-14],and decrease in diesel aromatics content can reduce NOxand particulate emissions.Increasing cetane number using cetane improver will be far more cost-effective in controlling emissions than reducing aromatic content,which will meet post emission standards.

Fuel additives can significantly reduce the particulate emissions,especially when use high aromatic content of fuel oil and poor combustion conditions in part load conditions.The experimental results also show that the additive has little influence on the generation of PAH.

Oxygenated fuels[15],especially the methanol are typically deploying the fuel emission to control pollution,but the emission of unburnt methanol and formaldehyde still remains unresolved.

In conclusion,the techniques and measures for reducing the harmful fuel emission of the diesel engine are proposed as follows:1)further reducing the sulfur content in fuel from the current level of 0.05%to 0.01%;2)reducing the aromatic components in the fuel;3)improving the cetane number in fuel;4)controlling the fuel distillation curve;5)fuel oxidation,which increases the oxygen content in the fuel.

3 Current Situation and Trend of Automobile Diesel Standards

Three main indicators could be used to represent the quality of diesel fuel.1)physical characteristics,i.e.,the density of diesel fuel;2)the total aromatic content of diesel fuel which represents the chemical properties;3)the cetane number,which to some extent could represent the integration of the former two factors.

Sulfur contents:According to ASTM D975,sulfur content in USA was changed from no more than 500 mg/kg(2000-2004)to no more than 15 mg/kg(2005-2013).According to JISK 2204 standards,Sulfur contents mass fraction in automobile diesel fuels in Japan was changed from no more than 50 mg/kg to no more than 10 mg/kg(2004-2007).Major parameters of the 4 categories of automobile diesel fuel by WWFC(World Wide Fuel Charter),from the third to the fourth categories of automobile diesel fuel products,sulfur contents were changed from no more than 50 mg/kg to 10 mg/kg.

Aromatic content:Major parameters of the 4 categories of automobile diesel fuel by WWFC,from the third to the fourth categories of automobile diesel fuel products,PAHs(2 or 3 ring)contents mass fraction in automobile diesel fuels was changed from no more than 3% to no more than 2%.However,the light diesel standard in China hasn’t been specified before 2011.Poly-aromatic content mass fraction in automobile diesel fuels was changed to no more than 11%in 2011 according to China's standards GB 19147—2009.EPA regulates the total aromatics’volume fraction in light diesel fuel can’t exceed 35%;the EU regulates of polycyclic aromatic hydrocarbons content in diesel fuel is no more than 11% from 1999 to2009.

Cetane number:The cold starting performance and reducing the fuel consumption and emissions can be improved by increasing the cetane number.According to China present development of economy and actual requirement for environment protection,China's standards GB 19147—2009“Automobile Diesel Fuels III”stringent legislation enacted in 2011,China's standards GB 19147—2013“Automobile Diesel Fuels IV”stringent legislation will enact in 2015,According to the two standards,automobile diesel fuels cetane number is required to be larger than 49.China's standards GB 19147—2013“Automobile Diesel Fuels V”stringent legislation will enact on 2018,which requires cetane number to be larger than 51.According to EN 590 standards,from 1999 to 2009,Automotive diesel fuel in Europe cetane number is larger than 50.According to JISK 2204 standards,from 1992 to 2007,cetane number of automotive diesel fuel in Japan must be larger than 50.According to ASTM D975,from 2000 to 2013,automotive diesel fuel in USA cetane number is larger than 40.

There’s still a gap between China and the WWFC standards in fuel composition and content:

1)Density and viscosity:The requirement of density is 0.820-0.850 kg/L at 15℃and the viscosity is 2-4 mm2/s at 40℃.

2)Sulfur content:According to China light diesel oil standard before 2011,the superior grade sulfur content of should be less than 0.2%,Grade A less than 0.5%,and the qualified products less than 1.0%.The new standard GB 19147—2013 “Automobile Diesel Fuels IV”is a revision of the original GB 19147—2009“Automobile Diesel Fuels III”on items such as sulfur content,referred to automobile diesel fuel standard developed by European Union during its execution of Stage IV emission.Mass fraction of sulfur content in automobile diesel fuels was changed from no more than 350 mg/kg to no more than 50 mg/kg.According to GB 19147—2013“Automobile Diesel Fuels V”,mass fraction of sulfur content in automobile diesel fuels will be changed from no more than 50 mg/kg to no more than 10 mg/kg before 2018.

3)Distillation range:Both T90below 340℃and T95below 355℃are permitted,and the final boiling point should be lower than 365℃.

As can be seen from the evolution of automotive diesel standards in China,automotive diesel standards have been developing straight forward to cleaner oil.Compared with standard of EU,USA and Japan,automotive diesel standards in China relatively lag behind,it is imperative to improve the quality of the fuel.

The sulfur content has maintained relatively stable for a long time from the standard evolution of the Chinese light diesel.Although it was decreased in the last two years,the gap still remains comparing with that in the developed countries.As the sulfur and aromatics content in diesel fuel have an enormous impact on the environment,in order to improve the vehicle emissions and clean the air,it is very necessary to reduce sulfur and aromatics content,particularly PAHs in diesel fuel.

Fuel quality is of great importance to vehicles emissions and air pollution,and in turn it is directly linked to the effective implementation of stricter vehicle emission control standards,as well as the emission reduction of all in-use vehicles.It is urgent to find out which is the important diesel fuel technical indexes and control parameters work out the control approaches,in order to carry out the joint prevention and effective control of air pollution.The new standard will have impact on technological improvement in oil refineries,as well as mitigating air pollution caused by vehicle exhaust and reducing hazy weathers.

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Effects of diesel fuel properties on diesel engine emissions and development trend of automobile fuel standards

REN Fumin1,SHU Xing2,YIN Jing3,WANG Zheng1,YAO Siyu1,MEN Zhengyu1
(1.School of Civil Engineering,Beijing Jiaotong University,Beijing 100044,China; 2.Beijing Haiguang Instrument Co.,Ltd.,Beijing 100015,China; 3.School of Environmental Science and Safety Engineering,Tianjin University of Technology,Tianjin 300384,China)

Abstract:Properties of several kind of diesel fuel have been identified with their significant effects on diesel engine emission.Analysis of fuel source with chemical properties in pollutants emitted by diesel vehicles revealed that reducing the sulfur content and the aromatic content,improving the cetane number in fuel will meet post emission standards.In this review,the development trend of automobile diesel fuel with a few important technical indexes is summarized,and the proposed technologies and measures for reducing the emission of the diesel engine are also presented.

Key words:automobile diesel fuel;property;emission;diesel engine;standard

作者简介:任福民(1966—),男,河南灵宝人,副教授,博士.研究方向为环境工程.email:renfumin2004@sina.com.cn.

基金项目:铁道部科技发展计划项目资助(2012Z02-F)

收稿日期:2015-06-02

DOI:10.11860/j.issn.1673-0291.2016.01.009

文章编号:1673-0291(2016)01-0056-05

中图分类号:X502

文献标志码:A