© Strojni{ki vestnik 50(2004)1,15-21 © Journal of Mechanical Engineering 50(2004)1,15-21 ISSN 0039-2480 ISSN 0039-2480 UDK 621.43.03:621.43.056 UDC 621.43.03:621.43.056 Izvirni znanstveni ~lanek (1.01) Original scientific paper (1.01) Obravnava motorja z notranjim zgorevanjem in vbrizgavanjem plinskega goriva A Rapid-Compression-Machine Study of Gaseous Fuel Injection and Combustion . Dariusz Klimkiewicz - Tomasz Lezan´ski - Rafal Jarnicki - Tadeusz J. Rychter V prispevku so predstavljene analize sistema za dovajanje goriva pri motorjih z notranjem zgorevanjem in neposrednim vbrizgavanjem plinskega goriva. Kratki primerni časi za vbrizgavanje plinskega goriva ter slabo prodiranje goriva in mešanje le-tega z okoliškim zrakom pomenijo velike probleme pri pravilnem vžigu in nadzorovanem zgorevanju mešanice. Ena izmed rešitev za olajšanje vžiga je uporaba manjše vžigalne predkomore. Vžig mešanice se tako začne že v predkomori, vroči in kemično dejavni zgorevalni plini pa nato pripomorejo k razbitju curka goriva v glavni zgorevalni komori, kjer poteka nadaljnje zgorevanje. Predstavljeni so rezultati raziskav vpliva oblike zgorevalnih komor pri uporabi neposrednega vbrizgavanja plinskega goriva na učinkovitost in ponovljivost vžiga. Raziskave so podprte z rezultati numeričnih simulacij postopka vbrizgavanja in mešanja plinskega goriva. Prikazano je, da lahko s predlaganim sistemom obidemo težave pri doseganju ponovljivega vžiga pri motorjih z neposrednim vbrizgavanjam plinskega goriva. © 2004 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: motorji z notranjim zgorevanjem, vbrizgavanje goriva, goriva plinska, zgorevanje) Rapid-compression-machine studies of an engines combustion system with the direct injection of gaseous fuel were made. The very short time available for the injection, combined with the poor penetration and 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 ignition takes place within the prechamber and the hot, chemically active combustion gases saturate the gaseous fuel jet that enters the main chamber where the mixing and combustion processes are continued. The results of the investigations 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 the results of numerical calculations of the injection and mixing processes. We concluded that this type of combustion system has the potential to overcome the difficulties in achieving the repetitive ignition of the gaseous fuel jet. © 2004 Journal of Mechanical Engineering. All rights reserved. (Keywords: internal combustion engines, fuel injection, gaseous fuels, combustion) 0 INTRODUCTION The use of natural gas as a fuel for internal combustion piston engines has a number of advantages, which are well known to engine designers and researchers. There are many production engines where gasoline has been substituted by natural gas without major changes to the engine’s operating mode but with a change in the fuel feeding system. In all such gas engines the gaseous fuel is delivered through the induction tube, and this means that the gas occupies a certain volume of the entire charge, decreasing the amount of air that is delivered to the engine cylinder during each engine cycle. This, in Opomba uredništva: Znanstveni članki tujih avtorjev so lahko odslej samo v angleščini. turn, tends to decrease the volumetric efficiency of the engine. To improve that efficiency, engine research centers try to find another solution, e.g., to develop the idea of injecting the gaseous fuel directly into the engine’s combustion chamber. There are two ways to do this: to start the injection at an early stage of the compression stroke of the piston or to initiate the injection at the end of compression stroke. The former solution has already been applied to some production engines and it did not create major difficulties. The latter solution, however, still creates a lot of problems. The time available for the mixing of the injected gas with the air is very short, and the gaseous jet penetration in the combustion-chamber isfFIsJBJbJJIMlSlCšD I stran 15 glTMDDC . Klimkiewicz D., Lezan´ ski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression volume and its mixing with the air is weak. ignited when the injection is still in progress. The Compression-ignition of natural gas is practically injected gas is then saturated with the combustion impossible within the range of reasonable gases generated in the prechamber, which are then compression ratia; therefore, any type of forced convected to the main chamber. The hot combustion ignition has to be used. If the mentioned problems gases containing chemically active free radicals create were solved the combustion system with the late so-called multi-point ignition in the main chamber. direct injection of natural gas would take advantage of the high compression ratio (on a diesel level) and 2 EXPERIMENTAL SETUP still remain a spark-ignition system because of the necessary stabilizing role of the forced combustion The objective of this paper is to present the initiation. The advantage of this type of combustion results of the preliminary investigations of the system is the reason that engine research centers try combustion system described above. The to remove the difficulties associated with the mixing investigations were performed with the use of the rapid and repeatad ignition of the charge. It is appropriate compression machine, described elsewhere, which to mention that practically all the problems with natural makes it possible to visualize the in-cylinder gas storage, its supply to injectors and the action of phenomena [1]. The results of the experiments were the injector itself have already been solved. However, compared with the results of calculations performed the in-cylinder processes in this type of combustion with the use of the KIVA3V computer code reduced to system still wait for the right organization. planar geometry [2]. The schematics of the experimental setup and combustion-chamber geometry are shown 1 THE GENERAL IDEA OF THE INVESTIGATED in Figures 2 and 3. The compression ratio was 10.8 and SYSTEM the piston velocity corresponded to an actual engine speed of 1600 rpm. To ensure the repeated ignition of the charge A Mitsubishi GDI injector was used for the and, at the same time, to increase the mixing rate of gas injection. The injection pressure of the methane the injected gaseous fuel with air and the combustion was 25 bar. The beginning of the injection, its duration rate, the use of an ignition prechamber was proposed and the ignition timing were adjusted and automatically (Fig. 1). This prechamber is connected to the main controlled with an accuracy of 1 CA deg (crank angle chamber by an orifice. The orifice diameter is carefully degree). The reactions of the system investigated on designed to be a little bigger than the dimension of the changes to following parameters were: beginning the gas jet’s cross-section. During injection the main of the injection – (20-165 deg BTDC); injection duration volume of the injection stream passes undisturbed – (15-50 CA deg); ignition timing – (10-30 deg BTDC). to the main chamber and only a small external part of Two prechamber geometries were the jet is scrubbed off by the orifice edges. This investigated: without (Version I) and with the bypass portion of the injected gas remains in the prechamber, channels (Version II). is mixed with the air and creates the portion of the charge that is ignited by the conventional spark plug. 3 RESULTS Since the shape of the gas jet remains basically unchanged the stoichiometry of the mixture in the As a result of the experimental investigations prechamber should also be, relatively speaking, the a number of pressure profiles and the corresponding same during each consecutive engine cycle. series of framed pictures of the combustion Therefore, there is a chance for repeatable and reliable processes was obtained. First, the development of ignition. Moreover, the charge in the prechamber is the injected gaseous fuel jet was visualised to Fig. 1. Schematic of the general idea ^BSfiTTMlliC | stran 16 i Klimkiewicz D., Lezanski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression Fig. 2. Schematics of the experimental setup Fig. 3. Combustion-chamber geometry Fig. 4. Visualisation of methane injection IgfÜTMÖMMliSESI] stran 17 | ^BsfifWlOCC iMg^Ü . Klimkiewicz D., Lezan´ ski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression Fig. 5. Velocity and methane-concentration distribution ( prechamber geometry - version I ) Fig. 6. Velocity field ( prechamber geometry - version I ) grin^SfcflMISDSD ^BSfiTTMlliC | stran 18 . Klimkiewicz D., Lezan´ ski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression 2.999SG7ir*01 19233 10 deg BTDC Position at TDC Fig. 7. Velocity and methane-concentration distribution ( prechamber geometry - version II ) 10 deg BTDC Fig. 8. Velocity field ( prechamber geometry - version II ) isfFIsJBJbJJIMlSlCšD I stran 19 glTMDDC . Klimkiewicz D., Lezan´ ski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression determine its geometry and its ability to mix with the Version II. To decrease the air velocity in air. The framed pictures (obtained with the use of a the orifice generated by the compression it was schlieren technique) of the injection process are necessery to increase the overall area of the chan-presented in Figure 4. The generation of the fuel jet nels connecting both chambers. The main orifice re-was also carefully tested and the full characteristics mained unchanged but two additional discharging of the jet (jet dimensions, fuel dose etc.) were channels were made. This drastically reduced the determined. upstream air velocity in the orifice and the injected Version I. Preliminary investigations of the gaseous fuel was passing to the main chamber with-combustion process in the chamber geometry pre- out difficulties. The motion of the gas in the chamber sented in Figure 3 have shown that it is impossible to and the methane-concentration distribution in this achieve the ignition of the charge under any set of case are presented in Figures 7 and 8. This change in system parameters. To find the reason for that the the combustion-chamber geometry allowed for the numerical analysis of the injection process was per- repeatable ignition of the charge. formed. The resulting gas-velocity distribution and The framed pictures of the combustion the methane concentration in the prechamber allowed process obtained from the experiments with the use for the determination of the cause of the lack of igni- of the rapid compression machine in the same tion. The simple reason for this was that the injection combustion-chamber geometry are presented in took place during the end of compression stroke when Figure 9. First, the injected stream of fuel passes the intensive flow of the air from the main chamber to through the prechamber and its main portion enters the prechamber occurred in the orifice. The upstream the main chamber. The fuel gas scrubbed off the velocity, of the air in the orifice was much greater external part of the jet is mixed with the air in the than fuel jet velocity and therefore whole amount of prechamber where the flammable mixture is created. the fuel injected remained in the prechamber. The Then the charge in the prechamber is ignited and the mixture in the prechamber was much too rich side. combustion gases are intensively discharged to the The demonstration of the velocity and methane- main chamber due to the prechamber pressure rise concentration distribution for this case is introduced and the action of the still injected gaseous fuel. in Figures 5 and 6. Although it was observed that the overall pressure Fig. 9. Visualization of the methane-combustion process (prechamber geometry – ver. II) ^BSfiTTMlliC | stran 20 i . Klimkiewicz D., Lezan´ ski T., Jarnicki R., Rychter T.J.: Obravnava motorja - A Single-Compression rise rate in the combustion chamber is still not satisfactory, the difficulties with the ignition were removed. The first frame in Figure 9 presents the methane jet 2 ms after the beginning of the injection. The methane jet enters the main chamber and the turbulence is generated in the prechamber. Frames 2 and 3 show the subsequent stages of methane injection. The ignition takes place during methane injection and this moment is presented in the fourth frame. Unfortunately, the high level of the turbulence in the prechamber is the reason why the combustion zone is hardly visible in the schlieren pictures. Moreover, the hot combustion gases generated in the prechamber are also ejected in the main chamber through the by-pass channels. The next frame was made just before TDC and shows the flame propagation process. The combustion has already been transferred in the main chamber but methane injection continues. The last frame taken after TDC presents the final stage of flame propagation, right after the end of the methane injection. It is important to stress that the combustion-chamber geometry and dimensions were only designed for the rapid compression machine experiments and the aim of the presented investigations was to check whether of not the assumed idea of the system operation has been right. For the actual engine experiments the shape and proportions of the combustion chamber must be completely redesigned. It is worth mentioning that the attempts to decrease the volume of the prechamber or to change the size of the orifice between chambers were not successful and they again caused serious problems with ignition. 4 SUMMARY The major result of the investigation is that the proposed idea of a combustion system for engines with direct fuel-gas injection might be reasonable. The observed sensitivity of the system to its geometry and dimensions indicates that its application to the actual engine would require thorough optimization. Acknowledgement The presented investigations were supported by the State Committee for Scientific Research under the grant No. 9 T12D 018 19. 5 REFERENCES [1] Rychter, TJ., T. Ležanski (2003) Inertia-driven single compression machine for combustion study, The Archive 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 Klimkiewicz Mag. Tomasz Ležanski Dr. Rafal Jarnicki Prof.Dr. Tadeusz J. Rychter Institute of Heat Engineering Warsaw Univ. of Technology Nowowiejska 21/25 00-665 Warsaw, Poland dklim@itc.pwedu.pl lezanski@itc.pw.edu.pl rjarn@wp.pl rychter@itc.pw.edu.pl Prejeto: Received: 29.11.2003 Sprejeto: Accepted: 12.2.2004 Odprto za diskusijo: 1 leto Open for discussion: 1 year gfin^OtJJIMISCSD stran 21