Knock in Spark Ignition Hydrogen Engines(Internal combustion engines)

Available online at www.sciencedirect.com

International Journal of Hydrogen Energy 29 (2004) 859865www.elsevier.com/locate/ijhydene

Knock in spark ignition hydrogen engines

Hailin Li, Ghazi A. Karim

Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Drive N.W., Calgary, Alta,Canada T2N 1N4

Accepted 25 September 2003

Abstract

In engine applications, the onset of knock remains one of the prime limitations that needs to be addressed so as to avoidits incidence and achieve superior performance. In the present contribution relating to the spark ignition hydrogen-fuelledengine, the e1ects of changes in key operating variables, such as compression ratio, intake temperature and spark timing onknock-limiting equivalence ratios are established both analytically as well as experimentally. Some other factors consideredincluded the optimization of spark timing for maximum indicated power output and/or e4ciency and for the avoidance ofknock while maintaining high thermal e4ciency values.? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Knock; Hydrogen engines; SI engines

1. Introduction

Hydrogen, a renewable fuel resource that can be producedthrough the expenditure of energy to replace increasinglythe depletion of conventional fossil fuels, has long beenrecognized as a fuel having some unique and highly desir-able properties, for application in engines [1]. These fea-tures make hydrogen an excellent fuel to potentially meetthe ever increasingly stringent environmental controls ofexhaust emissions from combustion devices, including thereduction of green house gas emissions. Discussion of thepositive features of hydrogen as a fuel and the associatedlimitations that are impeding its wide application as an en-gine fuel have been the subject of much research and pub-lications [2]. The viability of hydrogen as a fuel remainscritically dependent on the e1ective solution of a number ofremaining key limiting economic and technical problems.These are primarily related to its production, storage, porta-bility, transport and purity. For example, a hydrogen en-gine when operated stoichiometrically needs to be 4060%larger in size than for gasoline operation for the same power

Corresponding author. Tel.: +1-403-220-5775;fax: +1-403-282-8406.

E-mail address: [email protected] (G.A. Karim).

output and speed. This could impose some reduction to en-gine speed, increased mechanical and motoring losses andreduced tolerance to knocking. Some design modiCcationsare needed then.Hydrogen is a remarkably light gaseous fuel. Its heating

value on mass basis is exceedingly high but on volume basisit is the lowest among common fuels. Its energy release bycombustion per unit mass of stoichiometric mixture remainshigh. Hydrogen has some very high values of key transportproperties, such as kinematic viscosity, thermal conductiv-ity and di1usion coe4cient, in comparison to other fuels.Such di1erences together with its extremely low density andlow luminosity help to give hydrogen its unique di1usiveand heat transfer characteristics. The combustion propertiesof hydrogen have much inDuence on its performance as anengine fuel. It has a very wide Dammable mixture rangein air that permits extremely lean or rich mixtures to sup-port combustion. It requires also a remarkably low mini-mum amount of energy to e1ect ignition with extremely fastresulting Dames. However, the values of its spontaneous ig-nition temperatures are in the range of those of other fuelsand the values of its maximum adiabatic combustion tem-perature in air are only a little higher.The chemical kinetics of the combustion of hydrogen in

air are well understood at present and its oxidation reaction

0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2003.09.013

860 H. Li, G.A. Karim / International Journal of Hydrogen Energy 29 (2004) 859865

rates and the associated temporal variation of the concen-trations of the reactive species can be predicted well. Thesedi1erences from other fuels together with those of the ther-modynamic and transport properties of hydrogen, contributeto the substantially di1erent combustion characteristics ofhydrogen from those of other common fuels.The paper describes the basis of a knock-prediction

model, which we developed previously for spark ignition(SI) methane engine applications, modiCed and extendedto consider operation on hydrogen engine. The e1ect of theassociated wide changes possible in operating conditionsincluding a wide range of variations in equivalence ratio onthe onset of knock, combustion duration, power, e4ciency,

and operational limits were investigated. The predictedresults using this approach are compared to correspondingexperimental results of our own obtained in a variable com-pression ratio (CR) single-cylinder CFR engine. The e1ectsof changes in some of the key operating SI engine variables,such as CR, intake temperature, and spark timing on theonset of knock when operating on hydrogen are presentedand discussed. The role of the presence with the hydrogenof some carbon monoxide or methane on the incidenceof engine knock is also considered. Some guidelines forknock-free operation of engines on hydrogen are presented.

2. SI engine knock and its prediction

Knock in SI engines is an acknowledged barrier to thefurther improvement of e4ciency, increased power and theuse of a wider range of fuels. The onset of knock, which iscaused mainly by the auto-ignition of the unburned mixturein the end gas region of the charge which is at any mo-ment yet to be burned, involves exceedingly rapid rates ofcombustion of the fuelair mixture, increased heat transferto the cylinder walls, excessively high cylinder pressure andtemperature levels and increased emissions. These may leadto highly undesirable engine performance and the potentialdamage to engine components when allowed to persist. Ac-cordingly, it is critically important not only to avoid knock-ing but also to know the limiting conditions for its incidenceunder any set of operating and designing conditions. E1ec-tive means for extending knock-free operation need to bedeveloped.A knock-prediction model developed earlier primarily for

applications to gas-fueled engines was based on establish-ing whether auto-ignition is to take place or not within theend gas region during Dame propagation period. The modelwas modiCed for application to hydrogen-fueled engines.

Predicted results, which were veriCed experimentallyshowed that the onset of borderline knock is encounteredwhen the value of a calculated knock criterionKnjust exceeded a certain critical value. The criterion is atime-varying function deCned as the calculated tempo-ral variation in the accumulated energy released due topre-ignition chemical reaction activity within the temporallydiminishing in size end gas per instantaneous value of thecylinder volume. This energy release which is calculatedwhile employing a full accounting of the details of chem-ical reactions is normalized relative to the correspondingtotal amount of energy to be released through normal Damepropagation per unit of cylinder swept volume. This ratio ofenergy releases is expressed by the following equation [3]:

Kn =

total energy released due to end gas reactions up to time tcylinder volume|t

total energy of the whole charge to be released by Dame propagationcylinder volume|t0

: (1)

This dimensionless function would represent also the frac-tional increase in total cylinder pressure due to the pre-Dameoxidation reaction activity of the instantaneous size of thediminishing end gas relative to the mean e1ective combus-tion pressure. The latter is deCned as the average increasein pressure due to normal combustion when it is assumedto take place within the engine swept volume. Accordingly,the value of the knock criterion function Kn is an integratedfunction of the normalized current energetic consequence ofany pre-Dame-front oxidation energy release of the end gasat any instant before Dame arrival. Such a modelling ap-proach was shown to be capable of predicting successfullythe onset of knock in CH4-fuelled SI engines and was ex-tended to consider cases when H2 was present as an addi-tive [4]. However, for engine operation with pure hydrogen,knock may be encountered as shown in Fig. 1 [5], over amuch wider range of equivalence ratio values on either sideof the stoichiometric value and is associated with widelyvarying Dame propagation.To account for the contributions of these di1erences, the

value of Kn was modiCed so that the total energy releasefor any mixture is further normalized relative to that of thestoichiometric mixture. Also, the combustion duration rep-resenting the inverse of the mean rate of Dame propagationis normalized relative to a typical constant value Kc; ref ,such as that observed with the common fuel methane (e.g.50CA), as shown in the following equation [6]:

Kn =hsp htKh0

mum0

(CR 1) Kh0(Kh0)stoich

Kc; refKc; f :c:

;

(2)

where hsp and ht are the speciCc enthalpy of the unburnedend gas at spark passage and instant time t, respectively,Kh0 is the e1ective heating value of the fresh charge, mu isthe corresponding remaining mass of the end gas at the timet; m0 is the mass of the initial fresh charge.

H. Li, G.A. Karim / International Journal of Hydrogen Energy 29 (2004) 859865 861

Fig. 1. The rich and lean knock limits for unthrottled hydrogenoperation in a CFR engine at three di1erent intake temperatures at900 rev=min.

A two-zone, quasi-dimensional model using the knockcriterion approach, developed and described by Shresthaand Karim [3,4], was used to predict the performance of SIengines operating on methane. The homogeneous chargeinside the cylinder at any instant during turbulent Dame prop-agation and combustion was considered to be divided intotwo zones of burned products and of unburned reactantsthat include the end gas region while accounting for heattransfer to the surrounding walls. An appropriate combus-tion energy release rate function is employed to model thecombustion and the corresponding energy release processesafter ignition. This formulation was obtained following ex-amination of a large number of experimental data for a vari-ety of operating conditions and shown to be quite adequatefor the present modelling approach [7]. The composition ofthe burned products following Dame propagation is calcu-lated while accounting for thermodynamic dissociation. Inorder to monitor at any instant during Dame propagation thelikelihood of the onset of knock due to auto-ignition of theunburned end gas, the reactivity of the temporally chang-ing end gas region is evaluated employing a su4ciently de-tailed description of the chemical reaction kinetics of theunburned mixture. For hydrogenair combustion reactions,a 30 steps and 12 species scheme was employed. For themethane and hydrogen fuel system, 150 reaction steps wereemployed. The formation and consumption of each speciesof the reactive end gas charge are calculated as a functionof time, together with the corresponding energetic conse-quences of such reactions. These are employed in calculatingthe instantaneous value of the knock criterion, the dimen-sionless energy function deCned in Eq. (1) throughout theentire Dame propagation period. In an engine cycle calcula-tion, key variables such as composition, pressure and tem-perature values for the two regions of the cylinder charge

are calculated together with the corresponding value of Kn.Should this sequence of calculations Cnd that up to the endof the combustion period the value of Kn never exceeds thecritical value, which was found experimentally to be around1.01.5 [3], then the cycle is considered to be knock free.On the other hand if its value during combustion exceedsthat critical value then knock is encountered at that instant.The other variables are then used to calculate performanceparameters such as power and e4ciency at the borderlineknock condition. The critical value for the knock criterionis e1ectively governed by the relative mass of the end gasat the onset of auto-ignition and its relative energy releasethen ampliCed by the CR.

3. Experimental procedure

In the present work, the incidence of knock was es-tablished directly from the observation of the cylinderpressure variations with time while using a Dush-mountedpressure transducer. The onset of knock, such as when thespark timing was advanced or the CR increased from aknock-free setting could be identiCed by the appearance ofhigh-frequency pressure oscillations near the beginning ofthe expansion stroke together with the characteristic knock-ing sound. Knock would take place Crst in only a small pro-portion of the cycles. Increasing for example the CR or thespark advance would increase the proportion of knockingcycles, the intensity of pressure oscillations and the knock-ing sound. In the present work, the onset of knock boundarywas taken when clearly perceptible knocking can be seen in1020% of the cycles. Once knock was detected, the vari-ations of the pressure with crank angle for 100 consecutivecycles were recorded for performance analysis. The limitsfor knock can be established in terms of the correspondinglimited equivalence ratio, CR or spark timing [5]. The ex-perimental examination of the onset of knock with hydrogenas the fuel was carried out in a single cylinder variable CRCFR engine of 82:6 mm bore and 114 mm stroke operatedat constant speed of 900 rev=min. A schematic diagram ofthe layout of the apparatus is shown in Fig. 2.The prediction results were validated against correspond-

ing experimental results. Unless otherwise stated, the sparktiming variation with CR is that shown in Fig. 3, which isconsistent with the ASTM procedure and that we followedin earlier experiments for the knock rating of a range offuels [5].

4. Results and discussion

Figs. 4 and 5 show the predicted variation with CR of theknock-limited equivalence ratio (KLER) plotted logarithmi-cally together with the corresponding experimental valuesdisplaying relatively good agreement. The reason for the lesssatisfactory agreement between the two types of values for


Fig. 2. Schematic diagram of the experimental setup. 1. Metering valves; 2. pressure gauges; 3. choked nozzles; 4. pressure regulator; 5.charge ampliCer; 6. crank angle signal.

6

10

14

18

22

5 7 9 11 13 15Compression Ratio

Spa

rk Ti

min

g, C

A BT

DC

Fig. 3. The variation of spark timing with changes in CR employedat 900 rev=min.

the limits at the very low CRs and low intake temperatures isuncertain but it could be the result of less adequate account-ing at these conditions for the increased e1ect of residualgas and heat transfer on the onset of knock. It is possibleunder the low CR condition that the experimental detectionof knock could have been a1ected by the pressure gradi-ents produced by the very fast Dame propagation rates of

0.10

1.00

10.00

6.0 8.0 10.0 12.0 14.0

Compression Ratio

K.L.

E.R.

Fig. 4. The comparison of the predicted KLER under unthrottledoperation with those determined experimentally in a CFR engine.Triangle: experimental data, solid line: predicted data. Tin =311 K,900 rev=min.

hydrogen rather than due to exclusively auto-ignition [1,8].A higher value of Kn could have been tolerated then. How-ever, to maintain simplicity of prediction, the calculated val-ues were obtained while using the same Cxed numericalvalue for the knock criterion throughout. On this basis, thee1ects of changing some of the key operating variables on


0.10

1.00

10.00

6.0 7.0 8.0 9.0 10.0 11.0 12.0Compression Ratio

K.L.

E.R.

Fig. 5. The comparison of the predicted KLER under unthrot-tled operation with those determined experimentally in a CFR en-gine. Triangle: experimental data, solid line: model-predicted data.Tin = 345 K, 900 rev=min.

0.10

1.00

10.00


K

.L.E.

R.

298 K311 K

345 K

Fig. 6. The typical variance of the predicted KLER under unthrot-tled operation in a CFR engine, with changes in CR and intaketemperatures at 900 rev=min.

engine performance parameters could be predicted. It wasconCrmed that the incidence of knock is the most importantconsideration in hydrogen engine operation. As an exampleFig. 6 shows typically the e1ects of changing the CR onthe knock limits for a range of intake mixture temperaturevalues indicating that both the CR and intake temperatureare key variables that control the incidence of knock. How-ever, as shown in Fig. 7, the e1ects of retarding the sparktiming further than what was employed tended to producein comparison a lesser e1ect on the knock limits than otherparameters such as CR or temperature.For the operation of an SI engine on hydrogen, a

su4ciently wide operational mixture region bounded bylimiting mixtures producing knock and the correspond-ing operation limiting mixtures that produce misCre [6,9],should be aimed at especially for the lean mixture operation.However, it was found that the knocking region tends towiden and the operational region to narrow with increasingthe CRs and/or intake temperatures as shown in Figs. 8 and 9.

0.10

1.00

10.00


K.L

.E.

R.

ST=2

ST=6

ST=10

ST=14

Fig. 7. Predicted variation of the KLER with CR for a range ofspark timings (BTDC) in unthrottled CFR engine at 900 rev=min,Tin = 311 K.

0.10

1.00

10.00

6.0 8.0 10.0 12.0 14.0Compression Ratio

Equi

vale

nc

e R

atio

Knocking Region

Operational region

Operational region

Fig. 8. Predicted variation in the knocking and operational regionswith CR in unthrottled CFR engine at 311 K and 900 rev=min.

This would indicate that for a satisfactory performanceof the SI engine with hydrogen as the fuel, great care isneeded in controlling the mixture composition. Also, thisis less of a challenge when moderately low CR and intaketemperature values are employed.To achieve satisfactory engine performances while

obtaining superior power production, which is the mainconcern of the engine combustion control, a knocking bor-derline operation is desirable. Fig. 10 shows typical poweroutput and e4ciency values at knock borderline operationfor a range of values of CRs. It can be seen that the engineachieves a power production peak at moderate values of CRbut high e4ciency values are continued with higher CRs.This behavior is due to the combined e1ects of increasingthe CR that tends to increase engine power production andits e4ciency and the corresponding deliberate decrease inequivalence ratio to avoid the onset of knock which reducesthe power but increases the e4ciency due to the leanermixture operation. The spark timing also contributes to


0.10

1.00

10.00


Equ

iva

lenc

e R

atio

Knocking Region

Operational Region

Operational Region

Fig. 9. Predicted variation in the knocking and operational regionswith CR in unthrottled CFR engine at 345 K and 900 rev=min.

240.0

260.0

280.0

300.0

320.0

340.0

6.0 8.0 10.0 12.0 14.0Compression Ratio

Indi

cate

d Po

wer

Pro

duct

ion

0.25

0.3

0.35

0.4

0.45

Indi

cate

d Po

wer

Pro

duct

ion

Effic

ienc

y

Fig. 10. Variations in the predicted knock-limited indicated poweroutput and e4ciency of a CFR engine with CR at 311 K and900 rev=min.

this tendency since increasingly retarded spark timing wasadopted for high CRs, as shown in Fig. 3.Other than power production, achieving high e4ciency

values is becoming more important than ever because ofthe increasing concerns about fuel consumption. Sparktiming optimization can be another means for optimiz-ing engine performance. This optimization tends to bemore complex for hydrogen-fuelled engines because ofthe wide variations in Dame propagation rates possi-ble and the need to avoid both the onset of knock andcombustion failure. Thus, the spark timing that achievesthe highest e4ciency while avoiding the onset of knockis selected. When the engine is operated on mixturesaround the stoichiometric region where knock is oftenencountered, the spark timing is retarded signiCcantlyto avoid knock, as shown typically in Fig. 11. Fig. 12shows the indicated power output and e4ciency variationswith equivalence ratio while using the optimized sparktiming. The value of the peak indicated power productione4ciency is about 38%, achieved for a lean operation of

= 0:36.

- 20.0

- 10.0

0.0

10.0

20.0

30.0

40.0

0.20 0.40 0.60 0.80 1.00Equivalence Ratio

Optimized for Knock

Optimized for Effic iency

Fig. 11. Typical variation of optimized predicted values of sparktiming with leaning of equivalence ratio at constant CR = 8,Tin = 311 K, 900 rev=min.

0.2

0.24

0.28

0.32

0.36

0.4

0.20 0.40 0.60 0.80 1.00Equivalence Ratio

Indi

cate

d Po

wer

Ef

ficie

ncy

0

100

200

300

400

500

Indi

cate

d Po

wer

, J/

Cycl

e

Fig. 12. Variation of the predicted values of indicated power ande4ciency of a CFR engine with changes in equivalence ratioswhile optimized spark timing is employed. CR=8:0, Tin =311 K,900 rev=min.

Methane and carbon monoxide are often present with hy-drogen when it is derived from fossil fuels such as when itis manufactured through the steam reforming of fossil fu-els, notably natural gas or their partial oxidation in oxy-genated air [10]. Pure CH4 and dry pure CO show superiorknock-resisting qualities to those of H2 in air [11]. The ef-fect of the presence of CH4 or CO with H2 on the incidenceof knock was determined experimentally over a range offuel composition. Both KLER and CR were established. Asshown in Figs. 13 and 14, the presence of CH4 improvesthe knock limits of the binary fuel mixtures reDecting theincreasing inDuence of the superior knock-resisting qualityof CH4. However, the presence of CO shows a weak inDu-ence on the knock limits of the fuel mixtures, although puredry CO has superior knock-resisting qualities to those ofCH4 [11]. This reDects on the enhancing e1ect of H2 on theoxidation reaction rate of CO. Such an e1ect dramatically


2

4

6

8

10

12

0 20 40 60 80 100CH4/(CH4+H2), CO/(CO+H2), %

K.L.

C.R.

H2-CH4 Mixtures

H2-CO Mixtures

Fig. 13. Comparison of the variation of the experimentally de-termined knock limited compression ratio (KLCR) in a CFR en-gine with changes in the fuel composition of binary fuel mixturesof H2 with CH4 and CO. Equivalence ratio = 1:0, Tin = 311 K,Pin = 87 kPa, N = 900 rev=min, spark timing as in Fig. 3.

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

CH4/(CH4+H2), CO/(CO+H2), %

K.L

.E.

R.

H2-CH4 Mixtures

H2-CO Mixtures

Dry Air

Fig. 14. Comparison of the variation of the experimentally de-termined KLER in a CFR engine with changes in fuel com-position of binary fuel mixtures of H2 with CH4 and H2.CR=12, spark timing=12CA BTDC, Tin =311 K, Pin =87 kPa,N = 900 rev=min.

reduces the superior knock-resisting properties of CO. Thebinary mixtures of H2 and CO show similar knock-resistingqualities to that of H2 for a wide range of fuel compositionsuntil very high concentrations of CO are employed. Sim-ilar trends were observed for higher operational CRs withhydrogen.

5. Conclusions

The two-zone model developed earlier when modiCedwas shown to be capable of predicting hydrogen engineperformance including the incidence of knock.

The avoidance of the incidence of knock is a mostimportant consideration in H2 spark ignition engine

operation. The compression ratio and intake temperatureare the main parameters that a1ect the knock limitedequivalence ratio while the e1ect of spark timing tends tobe in comparison less.

The knock-free operational mixture region tends to nar-row signiCcantly with increasing compression ratio and/orintake temperature. This represents a practical limitationto the improvement of power and e4ciency of hydrogenengines.

The presence of methane with hydrogen improves itsknock limits while the presence of carbon monoxide incomparison tends to show a weaker e1ect.

Acknowledgements

The Cnancial support of Canadian NSERC is gratefullyacknowledged.

References

[1] King RO, Hayes SV, Allan AB, Anderson RWP, Walker EJ.The hydrogen engine: combustion knock and the related Damevelocity. Trans Eng Inst Canada (EIC) 1958;2(4):1438.

[2] Karim GA. Hydrogen as a spark ignition engine fuel. IntJ Hydrogen Energy 2003;28:56977.

[3] Shrestha SOB, Karim GA. A predictive model for gas fuelledspark ignition engine applications. SAE paper 1999-01-3482,1999.

[4] Shrestha SOB, Karim GA. Hydrogen as additive tomethane for S.I. engine applications. Int J Hydrogen Energy1999;24:57786.

[5] Karim GA, Klat SR. The knock and autoignitioncharacteristics of some gaseous fuels and their mixtures. J InstFuel 1966;39:10919.

[6] Li HL, Karim GA. Hydrogen fuelled spark ignition engines:predictive and experimental performance. Spring TechnicalConference of the ASME Internal Combustion EngineDivision, Salzburg, Austria, May 1114, 2003.

[7] Shrestha SOB, Karim GA. An experimental and analyticalexamination of the combustion period for gas-fuelled sparkignition engine applications. Proceedings of the Institution ofMechanical Engineers, vol. 215 (Part A); J Power Energy2001;215:6374.

[8] Allan AB. Hydrogen as the fuel for a spark ignition Otto cycleengine. SAE paper 821200, 1982.

[9] Shrestha SOB, Karim GA, Wierzba I. Examination ofoperational limits in gas fuelled spark ignition engines. SAEpaper 2000-01-1944, 2000.

[10] Jamal Y, Wyszynski ML. On-board generation ofhydrogen-rich gaseous fuelsa review. Int J HydrogenEnergy 1994;19:55772.

[11] Li HL, Karim GA, Sohrabi A. An experimental investigationof the knock characteristics of CH4, CO, H2 and theirmixtures. 2003 Spring Technical Meeting, CombustionInstitute Canadian Section, Vancouver, BC, Canada, May1114, 2003.

Knock in spark ignition hydrogen enginesIntroductionSI engine knock and its predictionExperimental procedureResults and discussionConclusionsAcknowledgementsReferences

Knock in Spark Ignition Hydrogen Engines(Internal combustion engines)

Documents

Transcript of Knock in Spark Ignition Hydrogen Engines(Internal combustion engines)