A Study on Combustion Process in Hydrogen/Hydrotreated Vegetable Oil Dual-Fuel Operation Using Hydroxyl Radical Chemiluminescence

<div class="section abstract"><div class="htmlview paragraph">Hydrogen–diesel dual-fuel combustion processes were visualized using an optically accessible rapid compression and expansion machine (RCEM). A hydrogen-air mixture was introduced into the combustion chamber, and a pilot injection of diesel fuel was used as the ignition source. A small amount of diesel fuel was injected as the pilot fuel at injection pressures of 40, 80, and 120 MPa using a common rail injection system. The injection amounts of diesel fuel were varied as 3, 6, and 13 mm<sup>3</sup>. The amount of hydrogen was manipulated by varying the total excess air ratio (λ<sub>total</sub>) at 3 and 4. The RCEM was operated at a constant speed of 900 rpm, and the in-cylinder pressure and temperature at the top dead center (TDC) were set as 5 MPa and 700 K, respectively. The combustion processes were visualized via direct photography and hydroxyl (OH*) chemiluminescence photography using a high-speed camera and an image intensifier. The results indicated that the diesel mixture first ignited near the wall of the piston bowl, and the flame spread in the hydrogen–air mixture. It was also found that a reduced λ<sub>total</sub> shortened the combustion duration, with an increased tendency of the heat release rate to shift from a dual peak to a single peak.</div></div>

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).

<div class="section abstract"><div class="htmlview paragraph">This study investigated the combustion process in a hydrotreated vegetable oil (HVO)–hydrogen dual-fuel operation using simultaneous imaging of the OH* and CH* chemiluminescence in a rapid compression and expansion machine (RCEM). In this operation, hydrogen served as the primary fuel, ignited by a small quantity of pilot fuel. CH* chemiluminescence was primarily detected in the pilot fuel combustion regions, whereas OH* chemiluminescence was detected in both the pilot fuel and hydrogen combustion regions, enabling the separation of pilot ignition and hydrogen flame propagation. The combustion mechanism was found to proceed through four distinct stages: autoignition of the pilot fuel, combustion of the mixture in the lean pilot fuel region, propagation of the hydrogen–air premixture flame, and flame propagation toward the wall and squish area. Furthermore, the effects of the pilot injection parameters on the combustion characteristics were systematically evaluated by varying the injection quantity, injection pressure, and nozzle specifications (hole diameter and number of holes). Increasing the pilot injection quantity improved the degree of constant volume of combustion but intensified the combustion near the wall, potentially increasing the cooling loss. Reducing the injection pressure shifted the autoignition location toward the center of the piston bowl, potentially reducing cooling loss but prolonging the combustion duration. With smaller injection quantities, fewer nozzle holes resulted in a higher second heat release rate peak, owing to the increased space for hydrogen flame propagation. Conversely, with larger injection quantities, a greater number of nozzle holes led to a shorter combustion duration while maintaining the combustion away from the wall.</div></div>

<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>

<div class="htmlview paragraph">In this study, a rapid compression and expansion machine(RCEM) with a pancake combustion chamber was designed to investigate fundamentally on the knocking phenomena in spark ignition(S.I) engines. This RCEM is intended to simulate combustion in an actual engine. The homogeneous pre-mixture of n-pentane and air was charged into a quiescent atmosphere of the chamber. Then, the combustion field become simpler in this machine than it in a real S.I. engine. Also, the combustion phenomena, that is a cylinder pressure history, the behavior of flame propagation and so on, with high reproducibility are realized in this machine. The phenomena caught in this experiment were so-called low speed knocking. And, this knocking characteristics such as a knock intensity and a knock mass fraction were revealed by the cylinder pressure analysis varying the charge pressure and the equivalence ratio of the mixture, a compression ratio and an ignition timing. Then, the effects of the pressure, the temperature and the concentration of the charged mixture on the knocking characteristics were discussed. Further, the flame propagation process in the visualized combustion chamber was taken by the high-speed schlieren photography (14000f.p.s.) and also OH radical in the chamber was measured with the luminous method(6000f.p.s.). Then, an autoignition process of the endgas was revealed from the luminous images of OH radical over the chamber for several knocking phenomena.</div>

The combustion characteristics of emulsified diesel fuels are investigated in a rapid compression and expansion machine (RCEM). Among the test cases, the 40 water-oil (W/O) fuel injected at 20° before top dead center (BTDC) has shown the best performance with respect to efficiency and NOx and soot emissions. The pressure trace of the 40 W/O fuel is characterized by a longer ignition delay and a lower rate of pressure rise in premixed combustion. High-speed photographs show reduced flame luminosity and lower flame temperature with increasing W/O ratio. Microexplosions of emulsified fuel droplets, which affect the local shape and brightness of the flame, are identified in magnified flame images.

<div class="section abstract"><div class="htmlview paragraph">This work investigates the effects of ignition improvers on the ignition and combustion characteristics of hydrous ethanol with 5% by weight water and 1% by weight Lauric acid (Eh95) under simulated diesel engine conditions using the rapid compression and expansion machine (RCEM). Results indicate that hydrous ethanol with commercial additive (ED95) and hydrous ethanol with 5% by weight glycerol ethoxylate in hydrous ethanol exhibit a near identical rate-of-pressure-rise and heat release rate. Ignition delay of hydrous ethanol with 5% by weight glycerol ethoxylate is shorter, but hydrous ethanol with 1% by weight glycerol ethoxylate has longer ignition delay time and different combustion characteristics compared with hydrous ethanol with commercial additive (ED95). Hydrous ethanol with 1% by weight glycerol ethoxylate and hydrous ethanol with 5% by weight glycerol ethoxylate are considered suitable fuels for high compression-ratio diesel engines. Ignition and combustion characteristics of hydrous ethanol with 5% by weight DEE and hydrous ethanol with 5% by weight biodiesel are similar to hydrous ethanol with 5% by weight water and 1% by weight Lauric acid (Eh95), with the longest ignition delay and the highest peak rate-of-pressure-rise of tested fuels.</div></div>

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

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.

<div class="section abstract"><div class="htmlview paragraph">With the development of downsized spark ignition (SI) engines, low-speed pre-ignition (LSPI) has been observed more frequently as an abnormal combustion phenomenon, and there is a critical need to solve this issue. It has been acknowledged that LSPI is not directly triggered by autoignition of the fuel, but by some other material with a short ignition delay time. It was previously reported that LSPI can be caused by droplets of lubricant oil intermixed with the fuel.</div><div class="htmlview paragraph">In this work, the ignition behavior of lubricant component containing fuel droplets was experimentally investigated by using a constant volume chamber (CVC) and a rapid compression and expansion machine (RCEM), which enable visualization of the combustion process in the cylinder. Various combinations of fuel compositions for the ambient fuel-air mixture and fractions of base oil/metallic additives/fuel for droplets were tested.</div><div class="htmlview paragraph">CVC results confirmed the high ignitability of lubricant component containing fuel droplets in pre-heated air, which was responsible for the early timing of LSPI. RCEM results also revealed the high ignitability of lubricant component containing fuel droplets in the premixed fuel/air mixture. Stochastic occurrence of ignition induced by fuel droplets was also observed, particularly under a condition of a small lubricant oil fraction.</div><div class="htmlview paragraph">Finally, the contribution of base oil and metallic additives to the ignitability (delay times and probabilities) of droplets was separately evaluated to understand the fundamental mechanism involved. Results confirmed that the base oil itself can promote the high ignitability of fuel droplets and that metallic additives can also additionally promote or inhibit the ignitability of droplets in the premixed fuel-air mixture.</div></div>

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

Hydrogen has a potential alternative to conventional hydrocarbon fuels, because no CO2 is emitted followed by combustion and it is able to make it by electroanalysis of water. This study aims to obtain stable ignition timing and combustion processes of hydrogen when it is applied into the direct-injection diesel engine. This report, it thought about the influence of the ambient conditions on the auto-ignition and combustion behavior of the hydrogen jet was investigated with a rapid compression and expansion machine (RCEM). First, the ambient temperature in combustion chamber is raised by adding argon into the ambient gas. Second, that is raised through the oxidation process of lean DME mixture. In addition, the effect of intermediates of DME reaction on the autoignition of hydrogen jet was investigated by use of CHEMKIN III code. Moreover, this effect was experimentally clarified.

<div class="section abstract"><div class="htmlview paragraph">Pre-chamber (PC) natural gas and hydrogen (CH<sub>4</sub>-H<sub>2</sub>) combustion can improve thermal efficiency and greenhouse gas emissions from decarbonized stationary engines. However, the engine efficiency is worsened by prolonged combustion duration due to PC jet velocity extinction. This work investigates the impact of cylindrical PC internal shapes to increase its jet velocity and shorten combustion duration. A rapid compression and expansion machine (RCEM) is used to investigate the combustion characteristics of premixed CH<sub>4</sub> gas. The combustion images are recorded using a high-speed camera of 10,000 fps. The experiments are conducted using two types of long PC shapes with diameters <i>φ=</i>4 mm (hereafter, long<i>φ</i>4) and 5 mm (hereafter, long <i>φ</i>5), and their combustions are compared against a short PC shape (<i>φ</i>=12 mm). For all designs of the PC shapes, the PC holes are 6 with 2 mm in diameter. Initial recorded results using only CH<sub>4</sub> show that jet extinction does not occur using the short and long 5mm types. The combustion duration of <i>φ</i>=4 mm PC is the shortest compared to the short-type and 5 mm PC. With CH<sub>4</sub>-H<sub>2</sub> blending (0%H<sub>2</sub>, 10%H<sub>2</sub>, 20%H<sub>2</sub>) and 4 mm shape, the combustion durations of 10%H<sub>2</sub> and 20%H<sub>2</sub> can be shortened compared to the CH<sub>4</sub>-only case (0%H<sub>2</sub>). However, the jet extinction probability is not zero.</div><div class="htmlview paragraph">3D-CFD combustion simulations are performed using CONVERGE software, and a 1/6 sector-mesh, GRI-Mech 3.0, G-equation combustion, and law-of-wall models are utilized in conjunction with RNG k-epsilon turbulence model. Simulated in-cylinder pressure and burning rate are validated against the recorded data. PC jet velocity, turbulent kinetic energy (TKE) of the main chamber, PC jet temperature, total heat loss in PC, and PC heat loss rate of 12 mm, 4 mm, and 5 mm are compared. Experimental combustion images and 3D-CFD temperature distribution with and without H<sub>2</sub> blending are also reported. The results show that long PC types can accelerate the jet velocity and shorten combustion duration. The jet extinction can be prevented by designing a small PC diameter area larger than the nozzle’s cross-sectional area. With H<sub>2</sub> blending, laminar burning velocity increases, and the jet ejection timing can be advanced. The result shows that the combustion duration can be shortened by 15 degrees using CH<sub>4</sub>-20%H<sub>2</sub> against CH<sub>4</sub> only.</div></div>

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

<div class="htmlview paragraph">A new type of rapid compression and expansion machine (RCEM) has been developed, and typical knock scenes were clearly recorded with a high speed laser shadowgraph at a speed of 100,000 frames per second.</div> <div class="htmlview paragraph">The RCEM is intended to simulate combustion in an automotive engine. Its piston is driven by an electrohydraulic servo system and is allowed to execute continuous reciprocations up to five times. The combustion chamber is a simple pancake type with an ignition plug on its side and the whole inner view is observable through a glass window on the top.</div> <div class="htmlview paragraph">Knock observation was made under the following conditions; (1) the fuel was butane, (2) the charged gas was homogeneously pre-mixed and static, (3) the piston executed a single reciprocation. Other parameters were set for heavy knock to occur.</div> <div class="htmlview paragraph">The shadowgraph observation revealed that autoignition occurs at a point in the endgas far from a normal flame front and is reflected by the opposite wall. Comparison with dynamic pressure measurements suggests that the wave is a shock wave, which causes an abrupt pressure rise followed by vibration. Knock can be explained by the autoignition and the subsequent pressure wave propagation process.</div>