Rapid Compression And Expansion Machine Research Articles

Experimental study on diesel spray flame and wall heat transfer of two-dimensional combustion chamber: effect of common rail pressure

Reducing the heat transfer occurring during the spray combustion of diesel engines remains a challenging task. This study investigated the spray flame behaviour, wall heat flux, and heat transfer phenomena of diesel engines at varying common rail pressures. A rapid compression and expansion machine (RCEM) was employed to simulate a single cycle of diesel spray combustion using split injection sprays. The two-dimensional (2D) stepped piston cavity is a uniquely shaped model used to describe the spray flame behaviour in the combustion chamber. To investigate the heat transfer phenomena, we installed wall heat flux sensors in the cylinder and cavity side. The results indicated that higher common rail pressures increased the peak value of the wall heat flux and reduced the combustion duration and the luminous flame. The higher common rail pressure led to an increase in spray velocity, thereby enhancing the turbulence level. This increase in turbulence to higher peak value of the wall heat flux. The highest peak value of the wall heat flux was reached by the cylinder head owing to the intense combustion region, followed by the cavity bottom owing to the highest flame temperature occurring around this location. Additionally, the heat transfer phenomena in diesel engines were successfully represented by local Nu-Re correlations. Our results contribute to the discussion on heat transfer phenomena that influence the thermal efficiency of diesel combustion engines.

Combustion Characteristic Prediction of Dual Direct Injection Fuel (Diesel-Propane) on RCEM Based on an Artificial Neural Network Approach

Studies from around the world show that engines using biofuel, LPG, and CNG emit fewer pollutants than those using conventional fuels. Experimental research has focused on a rapid compression and expansion machine (RCEM) that resembles a compression ignition (CI) engine. It uses dual direct injection fuel, diesel and propane (DP), with propane injection timing varying from 0 to 40 before top dead center (BTDC) and diesel injection timing remaining at 10 BTDC. The compression ratio was changed at points 17 and 19 by adjusting the RCEM connecting rod. A converge simulation program was used to run the simulation model, which was used to examine how the fire and inflow inside the chamber developed. The ANN method was used to predict pressure, temperature, power, TKE, and ITE data output based on propane energy fraction, compression ratio, and SOI of propane as input data parameters. It was noticed that the ANN prediction on experimental data has a higher accuracy compared to the simulation prediction. The R and MSE values were used to identify the accuracy of the prediction on output parameter data. ANN generalization capability is comparatively high when trained with large enough databases. The highest accuracy of prediction was produced on TKE, which had an MSE of 0.003715 and R value of 0.99981 from 287900 sample data. This shows that the ANN model is quite accurate in forecasting output experimental data.

The Influence of Ozone on the Flame Propagation Behavior of n-Heptane/air and i-Octane/air Premixed Flame in an Engine-Relevant Environment

ABSTRACT The promotional effect of ozone on lean premixed spherical flame propagation in an engine-relevant, high-temperature, high-pressure environment was investigated using an optically accessible rapid compression and expansion machine (RCEM). The laminar burning velocity, burned gas Markstein length, and initial flame development time based on mass fraction burned (MFB), were examined for n-heptane/air and i-octane/air premixture (φ = 0.55) with 15 [ppm] ozone, through self-emission observation of a propagating flame with a high-speed camera and combustion pressure analysis. The results showed that ozone reduced the initial flame development time and increased the laminar burning velocity for n-heptane/air premixture under temperature conditions in which the low-temperature oxidation (LTO) reaction was more clearly pronounced; but ozone did not enhance either the initial flame development or the laminar burning velocity in the case of i-octane/air premixture, where the LTO reaction was weak owing to the fuel molecular structure, even under the same temperature conditions. These results suggest that the enhancement of the LTO reaction by ozone, and the subsequent generation of thermal energy and CO radicals through an H2O2 loop reaction in the pre-flame region, were responsible for the increase in the laminar burning velocity. Furthermore, ozone increased the burned gas Markstein length, flame thickness, and Zeldovich number of the n-heptane/air mixture under the temperature conditions in which the laminar burning velocity was enhanced by ozone. This suggests that the chemical reaction in the reaction zone was more active in response to the increase in flame temperature.

Analysis of the relationship between near-wall velocity distribution and wall heat flux under diesel spray flame impingement in an RCEM

To clarify the relationship between the near-wall flow and the heat flux through the wall, the velocity distribution of the diesel spray flame and the heat flux at the combustion chamber wall were measured in a rapid compression and expansion machine (RCEM), which has a twodimensional combustion chamber. The velocity distribution of the diesel spray flame near the wall was measured by the particle image velocimetry (PIV), and the heat flux through the wall where the diesel spray flame impinges was measured using a newly developed multi-point heat flux sensor. Furthermore, the velocity gradient tensor was calculated based on the measured velocity distribution results, and the effect of the shear flow characteristics on wall heat transfer was analyzed. As a result, the higher the injection pressure leads the higher the peak value of the heat flux. A fluctuation of the heat flux exhibits the same trend as the fluctuations of the velocity, and a high heat flux appeared in the high-velocity gradient tensor region, which suggests that the characteristics of shear flow near the wall can affect the heat flux.

Effect of Blending Dimethyl Carbonate and Ethanol with Gasoline on Combustion Characteristics

We investigated the effects of blending dimethyl carbonate (DMC) and ethanol with commercial gasoline on combustion characteristics. Our experimental approach involved using a rapid compression and expansion machine (RCEM) to achieve elevated temperatures and pressures. The fuels containing different volumes of oxygenated hydrocarbons were burned at equivalence ratios of 1.0 or 0.7, an initial temperature of 340 K, and initial pressures of 0.10 or 0.05 MPa. To simulate knocking phenomena, we installed a rectangular channel in the combustion chamber of the RCEM and measured the pressure history inside the chamber. By analyzing the pressure history resulting from the end-gas autoignition, we evaluated the combustion duration and maximum pressure amplitude. Blending both oxygenated fuels with gasoline effectively reduced the maximum-pressure amplitude in the end-gas autoignition, with ethanol exhibiting a more pronounced suppression effect compared to DMC in the same volumetric mixing ratio. At an initial pressure of 0.10 MPa, the combustion durations of DMC/gasoline blends showed non-linear behavior, being shorter than those of pure gasoline and DMC and comparable to those of the ethanol/gasoline blends. However, the blending effect of DMC on combustion durations was greatly mitigated when the initial pressure was reduced to 0.05 MPa. Conversely, the combustion durations for ethanol/gasoline blends showed a nearly monotonic reduction with an increase in the ethanol blending ratio at both initial pressures of 0.10 and 0.05 MPa. Finally, we discussed the differential impact of the blending effect of oxygenated hydrocarbons on combustion characteristics.

Ignition Characteristics of Dielectric Barrier Discharge Ignition System under Elevated Pressure and Temperature in Rapid Compression and Expansion Machine

<div>A rapid compression and expansion machine (RCEM) was used to experimentally investigate the ignition phenomena of dielectric-barrier discharge (DBD) in engine conditions. The effect of elevated pressure and temperature on ignition phenomena of a methane/air premixed mixture was investigated using a DBD igniter. The equivalence ratio was changed to elucidate the impact of DBD on flame kernel development. High-speed imaging of natural light and OH* chemiluminescence enabled visualization of discharges and flame kernel. According to experimental findings, the discharges become concentrated and the intensity increases as the pressure and temperature rise. Under different equivalence ratios, the spark ignition (SI) system has a shorter flame development time (FDT) as compared with the DBD ignition system.</div>

Experimental study on wall heat transfer from diesel spray flame in two-dimensional combustion chamber operated with rapid compression and expansion machine (RCEM)

Heat transfer phenomenon in internal combustion diesel engine is one of problems to be solved. This study was conducted to investigate the relationship between diesel spray impingement, combustion processes, and heat loss using a rapid compression and expansion machine (RCEM). Furthermore, spray combustion and heat transfer processes were investigated using multiple injection strategies with a 6-holes injector. The amounts of diesel fuel used in the pilot, pre-injection, and main injection sprays were 3.964, 0.747, and 11.205 mg, respectively. Variations in common rail pressure conditions (30, 120, and 180 MPa) were applied to properly investigate spray evaporation, air–fuel mixture formation, and heat transfer in the combustion chamber. In addition, a local wall heat flux sensor was installed in the cylinder (head and liner) and piston cavity (upper lip, lip, and bottom) to measure the heat flux distribution, and a diffused backlight illumination method was used to visualize the spray and flame behavior in the combustion chamber and 2D piston cavity. The results showed that the impingement flame surrounding the cylinder head contributed to the highest magnitude and longest duration of the heat flux on the cylinder side owing to the spray flame velocity, impingement, and residence time. Meanwhile, the highest different temperature at cavity bottom induced more heat flux on the cavity side than that on the squish side. In addition, the higher injection pressure increased the spray momentum and spray tip penetration which led to a better air–fuel mixture, better evaporation rate, and less flame residence that contributed to reduce the accumulated heat loss in combustion chamber. Furthermore, a low common rail pressure was found to induce a greater difference in the accumulated heat flux ratio than a high common rail pressure which is owing to longer flame residence time in the combustion chamber. They were approximately 14% and 6% greater than that compared to the baseline common rail pressure which mean that a high common rail pressure contributes for the improving thermal efficiency in spray combustion.

Spark discharge energy effect on in-cylinder characteristics performance of rapid compression and expansion machine with spark ignition direct injection strategy

A spark ignition strategy in compression ignition has been considered to achieve high performance of gasoline direct injection engines. The spark ignition energy depends on the discharge current and the spark discharge duration. However, the respective roles of both on the rapid compression and expansion machine (RCEM) running with spark ignition and gasoline direct injection in-cylinder pressure have not been fully investigated. Both experiments and simulations were carried out in this study to investigate the spark discharge energy’s effect on in-cylinder characteristics performance. Three spark ignition strategy ranging from 50 mA up to 200 mA is applied to enhance the ignition discharge energy according to six selected cases of ignition timing variation (0.7 ms, 1.0 ms, 2.0 ms, 3.0 ms, 4.0 ms, and 5.0 ms). The results show the total released energy at the simultaneous ignition strategy was the largest (around 190 mJ/s), followed in order by the pair ignition strategy and individual ignition strategy. The highest turbulent kinetic energy of gasoline RCEM with spark ignition strategy is 1.43 m2/s2 (an increase 27.96 % from RCEM fueled with diesel). When the start of injection (SOI) of direct injection gasoline was set the same as the SOI of direct injection diesel at 10 oCA BTDC resulted in the thermal efficiency of gasoline RCEM with spark application slightly increased when spark time duration is prolonged at the average of 15.96 %. Finally, the establishment of a compression ignition engine combined with spark energy discharge might contribute to faster plasma formation, as well as expose the kernel development to a wider range of spark duration.

Computational study of the Premixed Charge Compression Ignition combustion in a Rapid Compression Expansion Machine: Impact of multiple injection strategy on mixing, ignition and combustion processes

Combustion processes operating under the low-temperature (LTC) are a promising alternative for internal combustion engines, achieving high efficiency and low emissions within the legislative framework. Under this field of study, the concept of Premixed Charge Compression Ignition (PCCI) was introduced to control the combustion phase by varying the injection strategy. The present research, based on the aforementioned combustion strategy, employs Computational Fluid Dynamics (CFD) to study the mixing, ignition and initial combustion processes in a Rapid Compression and Expansion Machine (RCEM). The operating conditions of the PCCI strategy are achieved through the split injection technique, which separate the injection into several shots. In this case, the injection strategy consists of a long base injection and two very short, consecutive post injections delivered when the piston is close to the top dead center. Three different operating conditions have been considered which differ mainly in the duration and the starting time of the first injection. Large Eddy Simulations (LES) are selected to account for the effects of turbulence. Results are validated against experimental data, showing good agreement between both approaches. The effect of the first injection on the second one in multiple injection strategy is appreciated as the penetration of the subsequent fuel sprays is faster. Differences in the local mixture field and ignition dynamics are observed as a function of varying operating conditions. The ignition process is highly influenced by the operating strategies affecting the distribution of the mixture and thus the location of the low temperature oxidation. In the current computational study, in addition to the evaporating fuel scalar, the corresponding source term due to evaporation has also been provided to three passive transported scalars, namely three mixture fractions, one for each injection event. This allows for discerning the fuel stemming from each one of the pulses in the overall distribution, thus offering new insights into the effect of the injection strategy.

Effect of Jet-Jet Angle on Combustion Process of Diesel Spray in an RCEM

<div class="section abstract"><div class="htmlview paragraph">The effects of jet-jet angle on the combustion process were investigated in an optical accessible rapid compression and expansion machine (RCEM) under various injection conditions and intake oxygen concentrations. The RCEM was equipped with an asymmetric six-hole nozzle having jet-jet angles of 30° and 45°. High-speed OH* chemiluminescence imaging and direct photo imaging using the Mie scattering method captured the transient evolution of the spray flame, characterized by lift-off length and liquid length. The RCEM operated at 1200 rpm. The injection timing was -5°ATDC, and the in-cylinder pressure and temperature were 6.1 MPa and 780 K at the injection timing, respectively, which achieved a short ignition delay. The effects of injection pressure, nozzle hole diameter, and oxygen concentration were investigated. The results show that the liquid and lift-off length of the jet-jet angle of 30° were shorter than those of the jet-jet angle of 45°, irrespective of injection conditions and oxygen concentration. Particularly, when the injection rate was high with high injection pressure and a large nozzle hole, the liquid and lift-off length rapidly and simultaneously decreased, but when the injection pressure was low or the nozzle hole diameter was small, the lift-off length decreased first, and then the liquid length decreased.</div></div>

A Study of Combustion Characteristics of Two Gasoline–Biodiesel Mixtures on RCEM Using Various Fuel Injection Pressures

Experimental research was conducted on a rapid compression and expansion machine (RCEM) that has characteristics similar to a gasoline compression ignition (GCI) engine, using two gasoline–biodiesel (GB) blends—10% and 20% volume—with fuel injection pressures varying from 800 to 1400 bar. Biodiesel content lower than GB10 will result in misfires at fuel injection pressures of 800 bar and 1000 bar due to long ignition delays; this is why GB10 was the lowest biodiesel blend used in this experiment. The engine compression ratio was set at 16, with 1000 µs of injection duration and 12.5 degree before top dead center (BTDC). The results show that the GB20 had a shorter ignition delay than the GB10, and that increasing the injection pressure expedited the autoignition. The rate of heat release for both fuel mixes increased with increasing fuel injection pressure, although there was a degradation of heat release rate for the GB20 at the 1400-bar fuel injection rate due to retarded in-cylinder peak pressure at 0.24 degree BTDC. As the ignition delay decreased, the brake thermal efficiency (BTE) decreased and the fuel consumption increased due to the lack of air–fuel mixture homogeneity caused by the short ignition delay. At the fuel injection rate of 800 bar, the GB10 showed the worst efficiency due to the late start of combustion at 3.5 degree after top dead center (ATDC).

Demonstration of knock intensity mitigation through dielectric barrier discharge reformation in an RCEM

A knock intensity mitigation effect resulting from the application of dielectric barrier discharge (DBD) was experimentally demonstrated. The DBD was utilized to reform fuel–air premixtures. A rapid compression and expansion machine (RCEM) was used for the demonstration experiment. A rectangular combustion channel was installed in the RCEM's cylinder to observe flame propagation and end-gas auto-ignition behavior. The effect of the DBD was investigated by installing a plug-shaped DBD reactor in the combustion chamber. Part of the fuel-air mixture was reformed by the DBD and diffused in the chamber, and the combustion behavior was observed by a color and a monochrome high-speed camera with several different interference filters. In ordinary end-gas auto-ignition, a hot flame rapidly appears throughout the end-gas region, and generates strong pressure oscillation; whereas, in the present study, when the DBD was applied, the magnitude of the pressure oscillation decreased and a blue flame was generated in the end gas before full end-gas auto-ignition. The onset time of the blue flame, and the interval between the onset and the hot flame's appearance, depended on the fuel and initial temperature. The effect was investigated in the case of a primary reference fuel, surrogate gasoline, and n-butane lean mixture; however, though the magnitude of the effect varied, the mitigation effect was demonstrated for every fuel-air mixture. The proposed method is therefore expected to mitigate knocking in internal combustion engines and contribute to greater thermal efficiency.

Effect of injection pressure and ambient density on spray characteristics of diesel and biodiesel surrogate fuels

This paper presents novel experimental investigations on the influence of varying injection pressure and ambient density on evaporating sprays of diesel, tetradecane (as a diesel surrogate) and methyl oleate (as a biodiesel surrogate) fuels. Evidently, adapting high injection pressure and ambient density would remarkably enhance the atomization process, and in return improve the engine combustion, performance, and emission characteristics. Measurements were carried out in a rapid compression and expansion machine (RCEM) working under diesel engine-like conditions. Furthermore, a shadowgraph optical technique was applied, using a high-speed camera to capture the spray development at 20,000 frames per second (fps) to obtain the vapor penetration length, liquid penetration length and spray cone angle. The tested fuel was injected into a pressurized nitrogen gas in a diesel engine-like condition without combustion where the injection pressure and ambient density were varied from 50 MPa to 150 MPa, and from15 kg/m3 to 25 kg/m3, respectively. Results indicated that injection pressure and ambient density have a significant effect on the spray characteristics. Increasing the injection pressure from 50 MPa to 150 MPa results in the increase of vapor penetration by up to 33.2%, 21.5% and 24.4% for diesel, tetradecane and methyl oleate, respectively. Besides, increasing the ambient density from 15 kg/m3 to 25 kg/m3 results in a wider spray cone angle by up to 9.6%, 11.8% and 13.8%, for diesel, tetradecane and methyl oleate, respectively and a shorter vapor penetration by up to 9.2%, 11% and 13.1%, respectively. It is noticed that high injection pressure causes the least reduction in spray cone angle for biodiesel. The present measurements are considered bench-mark for validation of high-fidelity numerical simulation of diesel and biodiesel sprays.

In-Cylinder GDI Soot via Visualization and Time-Resolved Total Cylinder Sampling

<div class="section abstract"><div class="htmlview paragraph">For better understanding, model development and its validation of in-cylinder soot formation processes of Gasoline Direct Injection (GDI) engines, crank-angle-resolved mass and size distribution of in-cylinder soot during a GDI combustion cycle were investigated via optical measurements and total cylinder sampling technique in an optically accessible Rapid Compression and Expansion Machine (RCEM). A direct-injection, spark-ignited and single-shot combustion event was achieved in the RCEM operated with engine speed 600 rpm, compression ratio 9.0, equivalence ratio 0.9 and natural aspiration. A three-component (iso-octane 65%, n-heptane 10%, toluene 25%) gasoline surrogate fuel and a multi-hole injector shared within the Japanese SIP Innovative Combustion Technology research program were used. As for the optical measurements, two-color method and laser/LED-based Diffused Back Illumination (DBI) high-speed imaging through sapphire windows on the cylinder head and the flat-top piston provided time-sequential in-cylinder soot mass. As for the total cylinder sampling, filter gravimetry and Portable Aerosol Mobility Spectrometer (PAMS) measurements of total-cylinder soot-laden gas provided crank-angle-resolved in-cylinder soot mass and size distribution. The total cylinder sampling was realized by replacing the cylinder head window with a stainless-steel diaphragm, rupturing the diaphragm at an arbitrary crank angle during combustion and rapidly expanding the total-cylinder soot-laden gas to effectively freeze secondary reactions and agglomeration of soot particles. The in-cylinder soot mass obtained by the above mentioned four different methods showed reasonable agreement each other both in increasing trend during combustion and quantitative soot mass. Measured variation of soot size distribution during combustion indicates that formation and agglomeration of soot are simultaneously occurring during combustion. Observation and morphology analysis of sampled soot via High-Resolution Transmission Electron Microscopy (HR-TEM) are also in progress.</div></div>

Flame Propagation Enhancement by Dielectric Barrier Discharge-Generated Intermediate Species

ABSTRACT The interaction between nonthermal plasma (NTP) and fuel-air mixtures was studied both experimentally and by means of simulations, to elucidate their promotional effect on flame propagation. A promotional effect on flame propagation velocity was experimentally observed by generating a dielectric barrier discharge (DBD), prior to spark plug ignition of an n-heptane mixture in a high-pressure and -temperature environment, using a rapid compression and expansion machine (RCEM). A novel method of concentration calibration for mass-spectral analysis was conducted, to determine the intermediate species in the fuel-air mixtures passing through the DBD plasma. The effect of the intermediate chemical species on flame propagation was examined numerically using the PREMIX code, with the observed species being considered as initial additives in the calculations. The promotional effect was reproduced numerically with the addition of alkyl hydroperoxide to an n-heptane/air mixture; and the higher the temperature and leaner the mixture in the parameter region, the more pronounced was the effect. The promotional behavior was examined using reaction path analysis, on the basis of which enhanced low-temperature oxidation reactions, due to the dissociation of the alkyl hydroperoxide, were determined in pre-flame region.

Turbulent flame propagation enhancement by application of dielectric barrier discharge to fuel-air mixtures

The promotional effect of non-thermal plasma (NTP) on lean flame propagation in turbulent fuel-air mixtures in a reciprocating engine-like high-temperature and -pressure environment was investigated. A turbulent flow was created by installing a perforated plate with oblique holes in the combustion chamber of a rapid compression and expansion machine (RCEM). Ignition was conducted by conventional spark plug. The NTP was generated by a dielectric-barrier discharge (DBD) device installed in the combustion chamber near the spark plug. A portion of the lean fuel-air mixture (ϕ = 0.5) in the chamber passed through the NTP and diffused throughout the chamber before spark ignition. To elucidate whether the effect persisted even when the plasma-affected volume was diffused by the flow, two types of experiments with temporal delay were conducted. The fuels evaluated were n-heptane as a representative fuel with a strong low-temperature oxidation reaction, i-octane as a representative fuel with a weak low-temperature reaction, and a primary reference fuel consisting of a mixture of these two fuels. Temporal growth of the flame was observed using a high-speed camera with an image intensifier. The evolution of in-cylinder pressure was also monitored and the characteristic time of the mass fraction burned was evaluated accordingly. It was found that flame propagation was promoted by DBD for n-heptane-containing mixtures at a certain initial temperature while the i-octane-air mixture did not exhibit such enhancement. The results obtained suggest that long-lived intermediate chemical species formed by the plasma diffuse into the cylinder, affecting flame propagation through promotion of a low-temperature oxidation reaction. In addition to these findings, the effect of NTP on burning periods were evaluated.

Study of prechamber combustion characteristics in a rapid compression and expansion machine

The prechamber combustion characteristics were studied using a rapid compression and expansion machine (RCEM) to improve the efficiency of cogeneration natural gas engines. The torch flames generated by a prechamber were used to investigate the effect that a prechamber has on the main combustion. In this study, we focused on observing the correlation between the torch flame and the main flame (which is a so-called “prechamber combustion”) as well as the knocking phenomena for various prechamber configurations.

Effect of glycerol ethoxylate as an ignition improver on injection and combustion characteristics of hydrous ethanol under CI engine condition

This paper investigates the effects of glycerol ethoxylate as an ignition improver on injection and combustion characteristics of hydrous ethanol under a CI engine condition. Injection characteristics were investigated by an in-house injection rate measurement device based on the Zeuch method, while spray combustion has been performed in the rapid compression and expansion machine (RCEM). The CI engine condition indicated as density, pressure and temperature of compressed synthetic gas, consisting of 80% argon and 20% oxygen, at fuel injection timing in RCEM of 21kg/m3, 4.4MPa and 900K, respectively. This condition is equivalent to the isentropic compression of air of the actual CI engine with compression ratio of 22. Hydrous ethanol without ignition improver (Eh95) and the ethanol dedicated for heavy duty vehicles (ED95: composed of hydrous ethanol with the commercial additive for ED95) are reference fuels representing low and high quality ethanol fuel for CI engines, respectively. All test fuels are injected at constant heat input. The results indicate that the additional ignition improvers change injection characteristics, i.e. injection delay, injection rate and discharge coefficient of hydrous ethanol. The maximum injection rates at fully opened needle for the ethanol dedicated for heavy duty vehicles (ED95) and hydrous ethanol with 5% glycerol ethoxylate (5%GE) are lower than that of hydrous ethanol without ignition improver (Eh95) by approximately 10%. Additional injection duration is required for ED95 and 5%GE to maintain a constant energy input. Ignition delay and heat release rate of Eh95 are significantly improved by ignition improvers. The effect of ignition improvers on flame temperature is minor. KL factor, which is approximately proportional to soot amount in the optical path, and the measured soot emission, are affected by these improvers.

Control of pressure increase rate in compression ignition by pulsed plasma irradiation

Homogeneous charge compression ignition (HCCI) engines are potentially high-efficiency, low-polluting alternatives to conventional internal combustion engines. However, the usable operation range of these engines is limited by too rapid combustion for high-load operation. The combustion is brought by the simultaneous ignition over the cylinder volume. An attempt to suppress it has been tried by realizing stratified temperature or fuel distribution in the cylinder. In this paper, the possibility of pulsed dielectric-barrier discharge (DBD) irradiating the air/fuel mixture is discussed as the new stratified ignition method. A rapid compression and expansion machine (RCEM), which can simulate a single engine cycle, was used to investigate it. DBD was generated in the intake tube of the RCEM. The stratified distribution of the irradiated premixture was obtained by the discharge formation at the end of the induction period. By the pulsed DBD method, the rate of pressure increase was decreased and the characteristic oscillation at the top of the pressure waveform was suppressed. These behaviors were confirmed by observation using a high-speed camera.

雰囲気の空間的不均一性がディーゼル燃焼に与える影響

In order to clarify the effect of heterogeneity of distributions of temperature and oxygen concentration in a combustion chamber on the soot and NO emissions from the diesel engine, the combustion characteristics and emissions of diesel spray flame achieved in a rapid compression and expansion machine (RCEM) at various patterns of temperature and oxygen concentration distributions in the chamber are investigated. The single-hole nozzle and multi-hole nozzle are used for the experiments. Results indicate that the NO and soot emissions from diesel combustion with single-hole nozzle are governed mainly by the amounts of oxygen and enthalpy entrained into the spray upstream the ignition region, when the temperature distribution and oxygen concentration distribution in a chamber are changed. In the case of multi-hole nozzle, NO emission is affected strongly by the amounts of oxygen and enthalpy entrained into the spray upstream the ignition region similarly to the results of the case of single-hole nozzle. However, the soot emission from the diesel flame with multi-hole nozzle is not affected by the heterogeneity of surrounding gas because the soot oxidation during late phase of combustion is governed mainly by the efficiency of utilization of air in a chamber which is affected significantly by the number of holes of the nozzle.