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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 mm3. The amount of hydrogen was manipulated by varying the total excess air ratio (λtotal) 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.

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.

For the measurements of flow rate, pressure and/or temperature in an engine exhaust pipe, probes are often inserted into the exhaust pipe depending on the application. These measurement probes differ a lot in terms of their size and shape. The flow around the probes become further complicated due to the pulsation of engine exhaust flow. In this study, computational fluid dynamics (CFD) simulations were carried out and a zero-dimensional (0D) model was constructed to analyze the flow field around the probe and flow rate of a pulsating flow. The simulations and the measurements of the flow rate and pressure were performed on flows around a hexagonal prism inserted in a circular pipe which is intended to be a differential pressure flow meter. The velocity field was also measured using the particle image velocimetry (PIV) technique. The CFD simulation results were validated with the experiments for both steady and pulsating flows.

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.

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.

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.

The objective of the present study is to elucidate the combustion process of partial premixed charge compression ignition (PCCI) combustion using multiple injections in diesel engines. The effects of the ratio of the quantity of fuel used in the first and second injections, and the injection dwell time on heat release rate, soot and nitrogen oxide (NOx) formations are investigated in simulated partial PCCI combustion using a constant-volume vessel. N-heptane is used as fuel. The experiments are carried out under an ambient condition of 2 MPa and 900 K, which simulates a PCCI-like heat release rate with long ignition delays. The oxygen concentration is set to 21 and 15% to simulate conditions without and with exhaust-gas recirculation (EGR), respectively. The fuel quantity in the first injection is varied between 10 to 40% of the total fuel quantity, and the injection dwell is varied between 0.5 to 2.0 ms.

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 present study aims to obtain a strategy for optimizing the combination of injection conditions and combustion chamber geometry to achieve low carbon monoxide (CO), nitrogen oxides (NOx) and smoke emissions with high thermal efficiency at low loads in direct-injection premixed charge compression ignition (DI-PCCI) operation in a diesel engine. To this end, experiments were performed using a naturally-aspirated single-cylinder DI diesel engine equipped with a common-rail injection system and a cooled exhaust gas recirculation (EGR) system under various injection conditions, including injection timing, injection angle and injection quantity, and combustion chamber geometry. The results indicate that CO emission was reduced at injection timings that provide high peak heat release rates. To improve the NOx-CO trade-off relation, the spray angle should be properly selected depending on the combustion chamber geometry.

Formation of nitrogen oxides (NOx) in direct-injection premixed charge compression ignition (DI-PCCI) combustion simulated in a constant volume vessel was investigated using an ignition-combustion model that combines a stochastic mixing model with a reduced chemical reaction scheme. Several improvements were made to the model in order to predict the combustion processes in DI-PCCI. Calculations were carried out for the injection and ambient conditions equivalent to the measurements using the constant volume vessel. Analysis of the calculated results clarified the effects of mixture heterogeneity on NO concentrations and the mechanisms are discussed. The results show that the model successfully represents the experimental tendency for NO concentration when the injection conditions and ambient oxygen mole fraction are varied.

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.