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To clarify the relationship between the local heat release and the velocity distribution inside the diesel spray flame, simultaneous optical diagnostics of OH* chemiluminescence and particle image velocimetry (PIV) have been applied to the diesel spray flame under the elevated in-cylinder pressure and temperature conditions formed in a rapid compression expansion machine (RCEM). The cranking speed of the RCEM was 900 rpm, and the in-cylinder pressure and temperature were 8 MPa and 800 K at the start of injection, respectively. The amount of fuel was 10.2 mg. The injection pressure was 120, 90, and 60 MPa. To minimize the disturbance of luminous flame on optical diagnostics, a solvent, with comparable combustion characteristics to diesel fuel was used as fuel. The oxygen concentration was set to 15%. Results clearly show that PIV can successfully analyze the velocity distribution in diesel spray flames.

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.

Natural gas/diesel dual fuel engines have potential for a high thermal efficiency and low NOx emissions. However, they have the disadvantages of high unburned species emissions and lower thermal efficiencies at low loads (at low equivalence ratio). A way to solve this problem is to properly distribute the pilot fuel vapor in a natural-gas premixture. The combustion chamber geometry affects the combustion process since it influences the distribution of the pilot fuel vapor. This study investigates the influence of injection conditions and the piston bowl geometry on the performance and emissions of a dual fuel engine. Experiments were carried out using two pistons with different bowl diameters, 52 mm and 58 mm, at single-and two-stage diesel-fuel injection. The results show that the larger bowl provides lower hydrocarbon emissions at a lower equivalence ratio in the case of single-stage injection.

The main objective of this study is to evaluate the characteristics of combustion that combine premixed charge compression ignition (PCCI)-based combustion with conventional mixing controlled combustion. In this type of combustion, it is supposed that the combustion duration is shortened due to the synchronization of the timing of two types of combustions. In addition, the cooling loss caused by spray impingement is expected to decrease by the reduction of the proportion of mixing controlled combustion. In this study, the effect of injection pressure, injection timing, and split injection on thermal efficiency and emissions were investigated in order to determine the appropriate injection parameters for PCCI-based combustion to realize the proposed combustion concept.

This study aims to utilize high-pressure split-main injection for improving the thermal efficiency of diesel engines. A series of experiments was conducted using a single-cylinder diesel engine under conditions of an engine speed of 2,250 rpm and a gross indicated mean effective pressure of 1.43 MPa. The injection pressure was varied in the range of 160–270 MPa. Split-main injection was applied to reduce cooling loss under the condition of high injection pressure, and the split ratio and the number of injection stages were varied. The dwell of the split main injection was set to near-zero in order to minimize the elongation of the total injection duration. As a result, thermal efficiency was improved owing to the combined increase in injection pressure, advanced injection timing, and split-main injection. According to the analysis of heat balance, a larger amount of the second part of the main injection decreased the cooling loss and increased the exhaust loss.

Multi-stage injection with pilot injection and post injection has been widely used for the noise and emissions reduction of diesel engines. Considering many parameters to be decided for optimal combustion, computer simulations such as three dimensional computational fluid dynamics (3D-CFD) and lower dimensional codes should play a role for optimal selection of intervals and quantity ratios. However, the data for the sprays are insufficient for reproducing the actual fuel-air mixture formation process related to pilot and post injection. Hence, there is a need for experimental data with a small-quantity injection. The small-quantity injection is characterized with an injection rate shape similar to a triangle rather than a rectangle. This study is mainly focused on the spray characteristics of diesel sprays in which the entire process is dominated by unsteady injection processes.

A series of experiments was conducted using a single-cylinder small-bore (85 mm) diesel engine to investigate the smoke-reduction effect of post injection by varying the number of injection nozzle orifices and the injection pressure. The experiments were performed under a constant injection quantity condition and under a fixed NOx emission condition. The results indicated that the smoke emission of six-hole, seven-hole, and eight-hole nozzles decreased for advanced post injection, except that the smoke emission of the 10-hole nozzle increased as the post injection was advanced from a moderately late timing around 17° ATDC. However, the smoke emission of the 10-hole nozzle with a higher injection pressure decreased for advanced post injection. These trends were explained considering the influence of the main-spray flames on post sprays based on CFD simulation results.

A series of experiments using a single-cylinder direct injection diesel engine was conducted to investigate the smoke reduction effect of post injection while varying numerous parameters: the post-injection quantity, post-injection timing, injection pressure, main-injection timing, intake pressure, number of injection nozzle orifices, and combustion chamber shape. The experiments were performed under a fixed NOx emission condition by selecting the total injection quantities needed to obtain the predetermined smoke emission levels without post injection. The smoke reduction effects were compared when changing the post injection timing for different settings of the above parameters, and explanations were found for the measured smoke emission trends. The results indicate that close post injection provides lower smoke emission for a combination of a reentrant combustion chamber and seven-hole nozzle.

A reduced kinetic model for n-heptane, i-octane, n-cetane and heptamethylnonane is developed based on that for the primary reference fuel (n-heptane and i-octane). The present model, which can be easily applied to a conventional CFD code, is constructed simply from 59 chemical species and 96 reactions. The ignition delay times are calculated by this kinetic model and compared with those by full kinetic models under high pressure and temperature conditions. The results indicate that the general trend of the ignition delay times for various temperatures and pressures is well described with this reduced model. Furthermore, the present model is combined with a commercial CFD code and used to simulate the ignition process of a diesel spray under a high pressure and temperature condition. The effect of the cetane number of the fuel on the ignition process is investigated.

The purpose of this study is to determine a pilot injection control strategy for the improvement of dual-fuel combustion with a lean natural gas/air mixture. Experiments were performed using a single cylinder test engine equipped with a common-rail injection system. The injection pressure, timing and quantity were varied at a fixed overall equivalence ratio of 0.5. The results of single-stage-injection experiments show that middle injection timings (−20 to −10 degATDC) produce low emissions of unburned species, because the pilot-fuel vapor spreads into the natural-gas lean mixture and raises the effective equivalence ratio, which leads to fast flame propagation. Early injection (−35degATDC) is advantageous for low NOx emission; however, increased emissions of unburned species are barriers.

The objective of this study is to obtain a strategy for adapting injection and exhaust gas recirculation (EGR) conditions to various engine speeds. An experimental study was conducted using a single-cylinder test engine and varying the injection timings of two-stage injection, the injection-quantity ratio, the EGR rate, and the swirl ratio at low (1300 rpm) and high (2300 rpm) engine speeds. When using base injection conditions, the results indicated that problems occurred for the high maximum pressure rise rate at low engine speed and the low thermal efficiency at high engine speed. At low engine speed, retarding the injection timings and increasing the first-injection quantity ratio reduced the maximum pressure rise rate without sacrificing engine performance. At high engine speed, advancing the injection timings improved the thermal efficiency but increased smoke emission.

A diesel combustion model for CFD simulation is established taking into account the auto-ignition process of a non-homogeneous mixture. In a previous paper, the authors revealed that the non-homogeneity of a fuel-air mixture has a more significant effect on the auto-ignition process with respect to, for example, ignition delay or combustion duration, as compared to the turbulent mixing rate. Based on these results, a novel diesel combustion model is proposed in the present study. The transport calculation for the local variation of the fuel-air PDF is introduced, and the chemical reaction rate is obtained based on the local non-homogeneity. Furthermore, this model incorporates RANS-based CFD simulation of the spray combustion in a constant-volume vessel under a high-temperature, high-pressure condition. The results show that the combustion process is well described for a wide range of temperature and pressure conditions.

Diesel combustion model for CFD simulation is established taking account of an auto-ignition process of non-homogeneous mixture. Authors revealed in their previous paper that the non-homogeneity of fuel-air mixture affected more on auto-ignition process such as its ignition delay or combustion duration than the turbulent mixing rate. Based on these results, novel diesel combustion model is proposed in this study. The transport calculation for local variation of fuel-air PDF is introduced and the chemical reaction rate is provided by the local non-homogeneity. Furthermore, this model is applied the RANS based CFD simulation of the spray combustion in a Diesel engine condition. The results show that the combustion process is well described for several engine operations.

This study aims to determine strategies for improving the relations between the pressure rise rate and emissions of nitrogen oxide (NOx), hydrocarbons (HC), and carbon monoxide (CO) in low temperature combustion (LTC) operation of a diesel engine. For this purpose, an analysis was conducted on data from experiments carried out using a single-cylinder direct-injection diesel engine with variation in the injection quantity, injection timing, exhaust-gas recirculation (EGR) rate, injection pressure, injection nozzle specification and combustion chamber geometry. The results reveal that the pressure rise rate and NOx exhibit similar tendencies when varying injection timing and EGR rate, which is opposite to CO and total HC (THC) emissions, regardless of injection quantity. When the injection quantity is increased, smoke emission becomes problematic in the selection of the injection timing.

The flow and mixing process of a high-speed unsteady jet are analyzed by using computational fluid dynamics for incompressible flow with the k-ε turbulence model and a stochastic approach. The pseudo-nozzle concept is applied to the inlet condition with a large pressure gradient. The non-uniform states of turbulent mixing are statistically described using probability density functions (PDFs). The results show that the time history of the jet development agrees with experimental data for methane and hydrogen fuels. In addition, the effect of the injection condition on the development of the jet tip is well described with this model. Furthermore, the micro-mixing process is successfully described with this PDF model.

The effects of fuel injection conditions (injection pressure, nozzle orifice diameter and fuel injection quantity) on NOx formation in direct-injection Premixed Charge Compression Ignition (DI-PCCI) combustion were investigated using a constant-volume vessel and a total gas-sampling device. The results show that promotion of fuel-air mixing reduces final NOx mass accompanying a delayed hot flame. In particular, under low oxygen mole fraction conditions, in addition to the hot flame delay, the promotion of fuel-air mixing results in a lower heat release rate. In this case, the final NOx mass is further reduced. For a fixed nozzle orifice diameter, the final NOx mass is reduced with increasing injection pressure. This effect is remarkable for smaller nozzle orifice diameters. Regardless of the oxygen mole fraction, under the low injection fuel quantity condition, enhancement of fuel-air mixing reduces the final NOx mass per released heat.

Aim of this study is to clarify the NOx formation mechanism in diesel combustion under high-supercharged condition. Effects of ambient conditions and fuel injection parameters on diesel combustion were investigated using a constant volume chamber. NOx formation process was investigated using a total gas-sampling device. The results indicate that by using the above experimental setup it is possible to realize entirely diffusion combustion like what seen in the highly supercharged condition. Increasing ambient pressure up to 8MPa with high injection pressure shortens the ignition delay and offers a heat release rate proportional to the fuel injection rate with a short combustion duration. Increasing ambient pressure gives a higher NOx formation rate and final NOx concentration. This is due to enhancement in the fuel-air mixing which promotes the heat release.

The aim of this study is to find strategies for fully utilizing the advantage of diesel-ethanol blend fuel in recent diesel engines. For this purpose, experiments were performed using a single-cylinder direct injection diesel engine equipped with a high-pressure common rail injection and a cold EGR system. The results indicate that significant PM reduction at high engine loads can be achieved using 15% ethanol-diesel blend fuel. Increasing injection pressure promotes PM reduction. However, poor ignitability of ethanol blended fuel results in higher rate of pressure rise at high engine loads and unstable and incomplete combustion at lower engine loads. Using pilot injection with proper amount and timing solves above problems. NOx increase due to the high injection pressure can be controlled employing cold EGR. Weak sooting tendency of ethanol-blend fuel enables to use high EGR rates for significant NOx reduction.

The combustion process of natural gas in a premixed charged compression ignition (PCCI) engine is analyzed using computational fluid dynamics via stochastic approach. The nonuniform states of turbulent mixing and the ignition process are statistically described using probability density function (PDF). The results show that the course of in-cylinder pressure is good agreement with experimental data, and the effect of mixture heterogeneity on the ignition delay and the rate of heat release is revealed.

This study aimed to elucidate the ignition processes in transient fuel-sprays over a wide range of ambient conditions corresponding to PCCI combustion, as well as diesel combustion. Ignition of n-heptane sprays was experimentally investigated by using a constant-volume vessel. The well-known temperature dependencies of ignition delays were observed at a high ambient pressure. On the other hand, a negative temperature coefficient (NTC) accompanying a two-stage pressure rise was detected for lower ambient pressures. High-speed shadowgraph images indicated that the temperature rise begins in the highly homogenous mixture along the combustion chamber wall. Enhancement of fuel-air mixing with elevated injection pressure and a reduced nozzle orifice delays the appearance of hot flame in the NTC condition. To better understand these phenomena, ignition processes were predicted using an ignition model including a stochastic turbulent mixing model and a reduced chemical reaction scheme.