Obravnava motorja z notranjim zgorevanjem in vbrizgavanjem

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Klimkiewicz D., Lezanski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression. ´

V prispevku so predstavljene analize sistema za dovajanje goriva pri motorjih z notranjem zgorevanjemin neposrednim vbrizgavanjem plinskega goriva. Kratki primerni èasi za vbrizgavanje plinskega goriva terslabo prodiranje goriva in me�anje le-tega z okoli�kim zrakom pomenijo velike probleme pri pravilnem v�iguin nadzorovanem zgorevanju me�anice. Ena izmed re�itev za olaj�anje v�iga je uporaba manj�e v�igalnepredkomore. V�ig me�anice se tako zaène �e v predkomori, vroèi in kemièno dejavni zgorevalni plini pa natopripomorejo k razbitju curka goriva v glavni zgorevalni komori, kjer poteka nadaljnje zgorevanje. Predstavljeniso rezultati raziskav vpliva oblike zgorevalnih komor pri uporabi neposrednega vbrizgavanja plinskegagoriva na uèinkovitost in ponovljivost v�iga. Raziskave so podprte z rezultati numeriènih simulacij postopkavbrizgavanja in me�anja plinskega goriva. Prikazano je, da lahko s predlaganim sistemom obidemo te�avepri doseganju ponovljivega v�iga pri motorjih z neposrednim vbrizgavanjam plinskega goriva.© 2004 Strojni�ki vestnik. Vse pravice pridr�ane.(Kljuène besede: motorji z notranjim zgorevanjem, vbrizgavanje goriva, goriva plinska, zgorevanje)

Rapid-compression-machine studies of an engine�s combustion system with the direct injection ofgaseous fuel were made. The very short time available for the injection, combined with the poor penetrationand mixing of the gas jet with the surrounding air, caused the serious problems with combustion initiation.One of the solutions to facilitate the ignition seems to be the use of a small ignition prechamber. The ignitiontakes place within the prechamber and the hot, chemically active combustion gases saturate the gaseous fueljet that enters the main chamber where the mixing and combustion processes are continued. The results of theinvestigations aimed at obtaining an efficient and repetitive ignition of the gaseous fuel jet are presented.Various versions of the combustion chamber were investigated. The investigations were supported by theresults of numerical calculations of the injection and mixing processes. We concluded that this type ofcombustion system has the potential to overcome the difficulties in achieving the repetitive ignition of thegaseous fuel jet.© 2004 Journal of Mechanical Engineering. All rights reserved.(Keywords: internal combustion engines, fuel injection, gaseous fuels, combustion)

A Rapid-Compression-Machine Study of Gaseous FuelInjection and Combustion

© Strojni{ki vestnik 50(2004)1,15-21ISSN 0039-2480UDK 621.43.03:621.43.056Izvirni znanstveni ~lanek (1.01)

© Journal of Mechanical Engineering 50(2004)1,15-21 ISSN 0039-2480

UDC 621.43.03:621.43.056Original scientific paper (1.01)

Obravnava motorja z notranjimzgorevanjem in vbrizgavanjemplinskega goriva

0 INTRODUCTION

The use of natural gas as a fuel for internalcombustion piston engines has a number ofadvantages, which are well known to engine designersand researchers. There are many production engineswhere gasoline has been substituted by natural gaswithout major changes to the engine�s operating modebut with a change in the fuel feeding system. In allsuch gas engines the gaseous fuel is deliveredthrough the induction tube, and this means that thegas occupies a certain volume of the entire charge,decreasing the amount of air that is delivered to theengine cylinder during each engine cycle. This, in

turn, tends to decrease the volumetric efficiency ofthe engine. To improve that efficiency, engineresearch centers try to find another solution, e.g., todevelop the idea of injecting the gaseous fuel directlyinto the engine�s combustion chamber. There are twoways to do this: to start the injection at an early stageof the compression stroke of the piston or to initiatethe injection at the end of compression stroke. Theformer solution has already been applied to someproduction engines and it did not create majordifficulties. The latter solution, however, still createsa lot of problems. The time available for the mixing ofthe injected gas with the air is very short, and thegaseous jet penetration in the combustion-chamber

Opomba uredni�tva: Znanstveni èlanki tujih avtorjev so lahko odslej samo v angle�èini.

Dariusz Klimkiewicz - Tomasz Lezanski - Rafal Jarnicki -Tadeusz J. Rychter

. ´

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volume and its mixing with the air is weak.Compression-ignition of natural gas is practicallyimpossible within the range of reasonablecompression ratia; therefore, any type of forcedignition has to be used. If the mentioned problemswere solved the combustion system with the latedirect injection of natural gas would take advantageof the high compression ratio (on a diesel level) andstill remain a spark-ignition system because of thenecessary stabilizing role of the forced combustioninitiation. The advantage of this type of combustionsystem is the reason that engine research centers tryto remove the difficulties associated with the mixingand repeatad ignition of the charge. It is appropriateto mention that practically all the problems with naturalgas storage, its supply to injectors and the action ofthe injector itself have already been solved. However,the in-cylinder processes in this type of combustionsystem still wait for the right organization.

1 THE GENERAL IDEA OF THE INVESTIGATEDSYSTEM

To ensure the repeated ignition of the chargeand, at the same time, to increase the mixing rate ofthe injected gaseous fuel with air and the combustionrate, the use of an ignition prechamber was proposed(Fig. 1). This prechamber is connected to the mainchamber by an orifice. The orifice diameter is carefullydesigned to be a little bigger than the dimension ofthe gas jet�s cross-section. During injection the mainvolume of the injection stream passes undisturbedto the main chamber and only a small external part ofthe jet is scrubbed off by the orifice edges. Thisportion of the injected gas remains in the prechamber,is mixed with the air and creates the portion of thecharge that is ignited by the conventional spark plug.Since the shape of the gas jet remains basicallyunchanged the stoichiometry of the mixture in theprechamber should also be, relatively speaking, thesame during each consecutive engine cycle.Therefore, there is a chance for repeatable and reliableignition. Moreover, the charge in the prechamber is

ignited when the injection is still in progress. Theinjected gas is then saturated with the combustiongases generated in the prechamber, which are thenconvected to the main chamber. The hot combustiongases containing chemically active free radicals createso-called multi-point ignition in the main chamber.

2 EXPERIMENTAL SETUP

The objective of this paper is to present theresults of the preliminary investigations of thecombustion system described above. Theinvestigations were performed with the use of the rapidcompression machine, described elsewhere, whichmakes it possible to visualize the in-cylinderphenomena [1]. The results of the experiments werecompared with the results of calculations performedwith the use of the KIVA3V computer code reduced toplanar geometry [2]. The schematics of the experimentalsetup and combustion-chamber geometry are shownin Figures 2 and 3. The compression ratio was 10.8 andthe piston velocity corresponded to an actual enginespeed of 1600 rpm.

A Mitsubishi GDI injector was used for thegas injection. The injection pressure of the methanewas 25 bar. The beginning of the injection, its durationand the ignition timing were adjusted and automaticallycontrolled with an accuracy of 1 CA deg (crank angledegree). The reactions of the system investigated onthe changes to following parameters were: beginningof the injection � (20-165 deg BTDC); injection duration� (15-50 CA deg); ignition timing � (10-30 deg BTDC).

Two prechamber geometries wereinvestigated: without (Version I) and with the bypasschannels (Version II).

3 RESULTS

As a result of the experimental investigationsa number of pressure profiles and the correspondingseries of framed pictures of the combustionprocesses was obtained. First, the development ofthe injected gaseous fuel jet was visualised to

Fig. 1. Schematic of the general idea

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Fig. 2. Schematics of the experimental setup

Fig. 3. Combustion-chamber geometry

Fig. 4. Visualisation of methane injection

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Fig. 5. Velocity and methane-concentration distribution ( prechamber geometry - version I )

Fig. 6. Velocity field ( prechamber geometry - version I )

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Fig. 7. Velocity and methane-concentration distribution ( prechamber geometry � version II )

Fig. 8. Velocity field ( prechamber geometry � version II )

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determine its geometry and its ability to mix with theair. The framed pictures (obtained with the use of aschlieren technique) of the injection process arepresented in Figure 4. The generation of the fuel jetwas also carefully tested and the full characteristicsof the jet (jet dimensions, fuel dose etc.) weredetermined.

Version I. Preliminary investigations of thecombustion process in the chamber geometry pre-sented in Figure 3 have shown that it is impossible toachieve the ignition of the charge under any set ofsystem parameters. To find the reason for that thenumerical analysis of the injection process was per-formed. The resulting gas-velocity distribution andthe methane concentration in the prechamber allowedfor the determination of the cause of the lack of igni-tion. The simple reason for this was that the injectiontook place during the end of compression stroke whenthe intensive flow of the air from the main chamber tothe prechamber occurred in the orifice. The upstreamvelocity, of the air in the orifice was much greaterthan fuel jet velocity and therefore whole amount ofthe fuel injected remained in the prechamber. Themixture in the prechamber was much too rich side.The demonstration of the velocity and methane-concentration distribution for this case is introducedin Figures 5 and 6.

Version II. To decrease the air velocity inthe orifice generated by the compression it wasnecessery to increase the overall area of the chan-nels connecting both chambers. The main orifice re-mained unchanged but two additional dischargingchannels were made. This drastically reduced theupstream air velocity in the orifice and the injectedgaseous fuel was passing to the main chamber with-out difficulties. The motion of the gas in the chamberand the methane-concentration distribution in thiscase are presented in Figures 7 and 8. This change inthe combustion-chamber geometry allowed for therepeatable ignition of the charge.

The framed pictures of the combustionprocess obtained from the experiments with the useof the rapid compression machine in the samecombustion-chamber geometry are presented inFigure 9. First, the injected stream of fuel passesthrough the prechamber and its main portion entersthe main chamber. The fuel gas scrubbed off theexternal part of the jet is mixed with the air in theprechamber where the flammable mixture is created.Then the charge in the prechamber is ignited and thecombustion gases are intensively discharged to themain chamber due to the prechamber pressure riseand the action of the still injected gaseous fuel.Although it was observed that the overall pressure

Fig. 9. Visualization of the methane-combustion process (prechamber geometry � ver. II)

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rise rate in the combustion chamber is still notsatisfactory, the difficulties with the ignition wereremoved.

The first frame in Figure 9 presents themethane jet 2 ms after the beginning of the injection.The methane jet enters the main chamber and theturbulence is generated in the prechamber. Frames 2and 3 show the subsequent stages of methaneinjection. The ignition takes place during methaneinjection and this moment is presented in the fourthframe. Unfortunately, the high level of the turbulencein the prechamber is the reason why the combustionzone is hardly visible in the schlieren pictures.Moreover, the hot combustion gases generated inthe prechamber are also ejected in the main chamberthrough the by-pass channels. The next frame wasmade just before TDC and shows the flamepropagation process. The combustion has alreadybeen transferred in the main chamber but methaneinjection continues. The last frame taken after TDCpresents the final stage of flame propagation, rightafter the end of the methane injection.

It is important to stress that the combustion-chamber geometry and dimensions were onlydesigned for the rapid compression machineexperiments and the aim of the presented

investigations was to check whether of not theassumed idea of the system operation has been right.For the actual engine experiments the shape andproportions of the combustion chamber must becompletely redesigned.

It is worth mentioning that the attempts todecrease the volume of the prechamber or to changethe size of the orifice between chambers were notsuccessful and they again caused serious problemswith ignition.

4 SUMMARY

The major result of the investigation is thatthe proposed idea of a combustion system for engineswith direct fuel-gas injection might be reasonable.The observed sensitivity of the system to itsgeometry and dimensions indicates that itsapplication to the actual engine would requirethorough optimization.

Acknowledgement

The presented investigations were supported by theState Committee for Scientific Research under thegrant No. 9 T12D 018 19.

5 REFERENCES

[1] Rychter, T.J., T. Le¿añski (2003) Inertia-driven single compression machine for combustion study, TheArchive of Mechanical Engineering.

[2] Amsden, A.A. (1997) KIVA-3V: A Block-structured KIVA Program for Engines with Vertical or Canted Valves,LA-13313-MS.

Authors� Address: Mag. Dariusz KlimkiewiczMag. Tomasz Le¿añskiDr. Rafal JarnickiProf.Dr. Tadeusz J. RychterInstitute of Heat EngineeringWarsaw Univ. of TechnologyNowowiejska 21/2500-665 Warsaw, [email protected]@[email protected]@itc.pw.edu.pl

Prejeto: 29.11.2003

Sprejeto: 12.2.2004

Odprto za diskusijo: 1 letoReceived: Accepted: Open for discussion: 1 year

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Obravnava motorja z notranjim zgorevanjem in vbrizgavanjem