Jingding - Formation and Restraint of Toxic Emissions in Hydrogen-gasoline Mixture Fueled Engines

8/8/2019 Jingding - Formation and Restraint of Toxic Emissions in Hydrogen-gasoline Mixture Fueled Engines

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hr. J. Hydrogen Energy, Vol. 23,No. 10,pp. 971-975, 998F) 1998 nternationalAssociation or HydrogenEnergyElsevierScience tdergamonPII: so360-3199(97)00141-9 All rights eserved. rinted n Great Britain0X%3199,/98 $19.00+0.00

FORMATION AND RESTRAINT OF TOXIC EMISSIONS INHYDROGEN-GASOLINE MIXTURE FUELED ENGINESLI JINGDING,* GUO LINSONGT and DU TIANSHEN*

* Department o f Energy Engineering, Zhejiang University, Hangzhou, 310027,P.R. China. 7 Department of ManufacturingEngineering, City University of Hong Kong, Kowloon, Hong Kong

Abstract-A little amount of hydrogen supplemented to the gasoline-air mixture can extend the flammability of themixture, increase the rate of flame propagation, accelerate the burning velocity of the lean mixture, thus improvingthe economy and emissions of engines, and enhancing thermal efficiency.In this paper, the mechanism of forming toxic emissions in spark ignition engines is expounded on basis of thetheory o f chemical dynamics of combustion. And the mechanism of which toxic emissions are restrained in the courseof the combustion of hydrogen-gasoline mixture is discussed. And last, the experimental investigation results ofrestraining toxic emissions are introduced. 0 1998 nternational Association for Hydrogen Energy

INTRODUCTIONToxic emissions from automobiles have become themain factors which deteriorate the earths environment,thus more strict emission regulation is mandated in manyareas of the world, especially in some large cities whichpossess a great number of automobiles. At present, asolar car, a battery car and a hydrogen-powered car areregarded as the most promising substitutes for a pet-roleum car in order that oil shortages and environmentalpollution can be overcome. But output power of a solarcar is too small for practical use, overweight of a batterycar restricts its application, storage and carriage of hydro-gen for a vehicle are also difficult items. Therefore, a

hybrid car will be a new tendency of developing carsin future. This kind of car has two fuel systems.When acar is driven in urban districts, its non-pollutants fuelsystem will be working. But while it is traveling a longdistance in rural districts, its petroleum fuel systemwill beworking. A car with a hydrogen-gasoline mixture fueledengines can be considered as an embryonic form of thehybrid car.Toxic emissions in spark ignition engines can berestrained by means of accelerating the burning velocityof the lean mixture. This sort of combustion processcanbe obtained by adding hydrogen to gasoline-air mixture.This is because hydrogen possesses uch characteristicsas wide flammable range, large diffuse coefficient, quickburning velocity and low ignition energy.

In this paper, the mechanism of forming toxic emis-sions in spark ignition engines is expounded on basis ofthe theory of chemical dynamics of combustion. And themechanism of which toxic emissions are restrained in thecourse of the combustion of hydrogen-gasoline mixtureis discussed. And last, the experimental investigationresults of restraining toxic emissions are introduced.

FORMATION MECHANISM OF TOXICEMISSIONS IN SPARK 1GNITlON ENGINESNO,

NO, includes NO and NO,, but mostly NO, NO2 onlyoccupies under 10% of NO,. They originate from theoxidization of N2 in the air under high temperature. For-mation of NO submits to the extended Zeldovich Prin-ciple. The reactions are as follows:

02eo-t0O+N ,zNO+Nk;

(1)(2)

N+02gNO+0ii (3)Where reaction rate constants are:

971


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912 LI JINGDING et al.k: = 7.6 x lOI exp (-3800/T)(Temperature range: 200&5000 K)k; = 1.6x IO (Temperature range: 300-5000 K)k: =6.4x 109T*exp(-3150/T)(Temperature range: 300-3000 K)k; = 1.5x 109T*exp(-19500/T)(Temperature range: 100&3000 K)

kf is much larger than k2+, his indicates that activationenergy of reaction (2) is much higher than that of reaction(3). Because N which reaction (3) needs is supplied byreaction (2), reaction (2) plays a leading role in formingNO. They are usually the main measuresof restrainingNO declining combustion temperature, controlling 0,concentration and shortening reaction period becausereaction (2) requires very high temperature up to 2000-5000 K.CO

In the premixed hydrogen-air flame, CO formationcan be described as follows:RH-+R-+RO,-+RCHO+RCO-+CO (4)

where R is a hydrocarbon radical.After having been formed in the combustion process,CO is slowly oxided into CO*, the reaction is:CO+OH$COz+H (5)

where reaction rate constant is:k& = 6.76x 100exp(-1102/T)

Reaction rate of (5) is mainly affected by the con-centration of OH and H. If gasoline-air mixture is fueledin spark ignition engines, formation of OH depends onthe reactions:co+o,eco+o (6)

O+H,O+20H k+ = 8.3 x 10i3exp(- 181OO/RT)(7)

CO+OH+C02+H (8)H+O ,+OH+O k+ = 1.8x lOexp(-8900/RT)

(9)It can be found that the rate at which CO is oxidedinto CO, is mainly affected by the concentration of 0,.CO will increase sharp while lacking in O2 (air-excessfactor GI> 1)

HCIn gasoline engines, unburned HC is formed becauseof the flame sharp cool on the surface of cylinders, the

gap effect, incomplete combustion and unburned HCoxided.The flame sharp cool includes single surface sharp cooland double surface sharp cool. The former means thatthe flame goes out and the burning charge in boundinglayer in front of the flame is charged into unburned HCbecause heat energy of scattering through the cylinderwall is more than that of the exoergic flame when theflame is near to the surface of cylinder. The latter meansthat the burning charge in circle gap between the top ofpiston and the cylinder wall escapesburning of the firsttime flame propagation.The gap effect means that the burning charge in circlevolume between the piston or piston rings and the cyl-inder wall and in other gaps goes out because he flameis sharp cooled in compression stroke, and in expansionstroke, most of gas n the gap volume flows back cylinder,50% of it is unburned HC.

The incomplete combustion occurs under operatingconditions of low load and slow speed. Because of theslow burning velocity, the flame has not spread to roundthe whole chamber even in exhaust stroke. So a greatdeal of burning charge is cooled sharp and HC is largelyincreased.Mechanism of HC oxidation is very complex, its reac-tion rate is approximately as follows:WC1- = -6.7 x lOexp(- 18735/T)X,cXoz@/RT)dt

(10)where [ ] is concentration (mol/cm3); t is time (s); X,cXo, are respectively molar numbers of HC and 0,; T isabsolute temperature (K); p/RT is item of density(mol/cm3).The characteristic reaction time of complete com-bustion is:

1 1 d[HC]-=--HC [HC] dt (11)The higher temperature, the faster oxidation reactionof HC. Actually, Oxidation of HC still continues inexhaust system, which requires a rich 0, environment,above 600 temperature and excess50 ms stay time.

EXPERIMENTAn experimental apparatus

The tes t engine is 165F model gasoline engine made nChina, whose stroke volume is 0.1261and effective poweris 4 PS at 3000 rpm. The test apparatus is shown in Fig.1. In order to change components of hydrogen-gasolinemixture within a wide range, two parallel fuel systems(gasoline-air with a carburetor and hydrogen-air with amixed device fuel system)are equipped. When the thi-ottlevalve of the mixed device is closed, the performance ofthe original engine can be tested, When hydrogen in ahigh pressure gas cylinder flows into the mixed device


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FORMATION AND RESTRAINT OF TOXIC EMISSIONS 973

ElectricdynamometerWatereparator I I Exhaust ascooler

Air-P-

war Flow metersanalyzersFig. 1. Schematic iagramof experimental pparatus.

through a pressure eduction valve and a regulating valve,various hydrogen-gasoline mixture components underdifferent vacuity of air inlet p ipe can be obtained bymeans of regulating the throttle valve of the carburetorand the mixed device.Fuel mixture consists of hydrogen, gasoline and air,the component part of gasoline in the mixed device canbe indicated by air-excess actor c(~:as = $,/(G, * -L-o) (12)where G, is the consumption of air (kg/h); G, is theconsumption of gasoline (kg/h); L,, is theoretical airquantity of completely burning 1 kg gasoline, 14.9.The component part of hydrogen can be indicated byair-excess actor an:

c[H = G/(G,*Gd (13)where GH s the consumption of hydrogen; LHOs theor-etical air quantity of completely burning 1 kg hydrogen,34.42.The total air-excess actor c+ is:

~1:= GAG, * L,, + G * -b,,)or

(14)

ax = (@, EH)/@, a) (15)Toxic exhaust emissions are measured by means of anexhaust gas analyzer made in China. Fig. 1 also showsan exhaust gas sampling and measuring device.

Experimental resultsAt first, the original gasoline engine is tested so as toobtain comparable data, and its emission characteristicson load at 3000 rpm are shown in Fig. 2.When testing a hydrogen-gasoline mixture fueledengine, change of output power is controlled by means

NOwm40003000znffo

0.6 0. I.0 I. I axFig. 2. Emission haracteristics ith gasoline uel.of regulating supplied gasoline quantity, and hydrogenflow is kept stable. Relative quantity of hydrogen in mix-ture is increased and the flammable range of mixture iswidened with decline of load. Emission characteristics onload at 3000 rpm, hydrogen Row 70 g/h is shown in Fig.3. Figure 4 shows that emission concentration varies withload while the engine is respectively fueled by gasoline-air and hydrogen-gasoline mixture at hydrogen flow 70g/h.

ANALYSIS OF RESULTSRestraint of NO

In chain reactions of H oxidation, there is a reaction:H,+O=H+OH (16)where reaction rate constant is:


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974 LI JINGDING ef al.co.

mm NOwm- 3000

Fig. 3 . Emission characteristics with hydrogen-gasoline mixture(GH = 70 g/h).

1 1 1 -\I 1 1 II HC. b tm

t30 40 37 60 70 80 90 100 .----- gasoline-hydrogen-gasoline mixtun@~=70g/h)

Fig. 4. Comparison of emission characteristics with gasoline andhydrogen-gasoline mixture.

k+ = 1.8 x 10 i*exp(-8900/RT)The activation energy of reaction (16) is much lowerthan that of reaction (2), thus after hydrogensupplemented, 0 mainly participates H2 oxidation reac-tion (16) so that concentration of NO produced in N,oxidation reaction (2) is erectly reduced. At the sametime, hydrogen supplementation also restrains reactionof N oxided by OH becauseof the following reaction:

N+OHgNO+H (17)k-where reaction rate constants are:

k+ = 1.0x 104k- = 2 .0 x 10 exp (-23650/T)

Becausehydrogen supplementation makes N productof reaction (2) largely reduced, it is difficult that reaction(17) moves towards right.The experimental results show that, in the spark ignition engine fueled by gasoline, maximum NO concen-tration occurs near air-excess factor CI= 1, amounts to4080 ppm (seeFig. 4). But in the engine fueled by hydro-gen-gasoline mixture, NO concentration is about 1 timelower than the former under all load conditions becausethe engine operates in the lean mixture, a, = 1.07-1.72(see Fig. 3), the burning is accelerated, but combustiontemperature does not go up highly. It can be deemedthat the rate of producing NO is mainly affected by 0concentration after hydrogen supplemented. NO con-centration gradually nears that of the engine fueled bygasoline with increasing load and decreasing quantity ofhydrogen supplementation.Restraint of CO

Chain reactions of H oxidation are as follows:H1+M*$2H+M (18)H+O,$OH+O (19)

O+H,+ OHfH k+ = 3.3 x 102exp(-8000/RT)(20)

OH+H,e H20+H k+ = 6.2x 103exp(-6000/RT)(21)

where M* is an activation radical, M is a radical.After hydrogen supplementation, chain reactions ofCO oxidation are as follows:CO+OH$C02+H

H+O 2eOH+OO+H,+OH+H

CO is quickly oxided because OH concentration isgreatly increased after hydrogen supplementation.On the other hand, the engine fueled by hydrogen-gasoline mixture can operate in the lean mixture underall load conditions so that CO concentration goes argelydown. The experimental results show that, under 30%rated power, the mixture is very lean, a, = 1.72,CX~ 2.6,aH = 5, CO concentration is only 950 ppm, which is 20times lower than that of the engine fueled by gasoline atthe same oad condition. Therefore, it has very notableeffect on restraining CO emission adding hydrogen togasoline-air mixture.


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FORMATION AND RESTRAINT OF TOXIC EMISSIONS 915

HC emissionFigure 4 shows that whether or not hydrogen is sup-plemented does not affect the rule of HC concentration (3)change. HC concentration is minimum under near 70%

rated power. If load is smaller or larger than 70% ratedpower, the mixture will become thinner or thicker, anddecline of burning temperature or occurrence of incom-plete combustion will lead HC increased. After hydrogenis added, HC concentration somewhat comes down (4)(about l/5-1/3) under all load conditions because theengine operates n the lean mixture, the burning is acceler-ated and high 0, concentration promotes completecombustion and HC oxidation.

tion is about one time lower than when no hydrogenis supplemented.Hydrogen-supplemented leads OH concentration lar-gely increased so that CO oxidation reaction is accel-erated, and the engine can operate n the lean mixture,thus CO concentration is about twenty times lowerat the highest point than when no hydrogen is sup-plemented.After hydrogen supplementation, the combustion ofthe lean mixture speedsup so that HC concentrationgoes down l/5-1/3.

Acknolvlr~~menfs-This research was supported by NationalNatural Science Foundation of China. Sincere appreciation isexpressed to NSFC.

CONCLUSIONS(1) Becauseof the characteristics of hydrogen, the leanmixture can rapidly burn in hydrogen-gasoline mix-ture fueled engines, thus toxic emissions arerestrained.(2) When hydrogen is added to gasoline-air mixture,0 mainly participates H, oxidation reaction so thatconcentration of NO produced by N, oxidation reac-

REFERENCES1. Jingding, Li, et al., An Experimental .StucJvof Combustion ofHydrogen/Gasoline Mixtures, SAE-China, 1984, No. 2, l-9.2. Newell, H. K., Kinetics of engine-gnerated nitrogen oxidesand carbon monoxide. Proceedings of 12th International S.vm-posium on Combustion, 1968, 603613.3. Zaw Hein. Kyaw, et a/., Application of the HAJI jet systemfor near nitrogen oxide (NO,) elimination in the SI engine,IMechE Semin., 1993, 5. Worldwide engine emission stan-dards and how to meet them, 49-55 (Eng.).

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Jingding - Formation and Restraint of Toxic Emissions in Hydrogen-gasoline Mixture Fueled Engines