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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.

Single-excite dual-fluorescence PLIF was applied to a diesel spray of a two-component fuel, the components of which have different boiling points. The spray was formed by injecting fuel into a constant-volume vessel under high-temperature, high-pressure conditions. The fluorescence emitted from the two tracers for the fuel was optically separated to measure the concentration of each component. Mixture formation was investigated based on the concentration distributions of each fuel component. The fuel concentration was derived based on the change in fluorescence intensity due to temperature and the assumption of adiabatic mixing of fuel and the surrounding fluid. The variation in the mixture distribution due to differences in the vaporization characteristics was investigated, and the results revealed that the two components have similar distribution. The concentration of the high-boiling-point component increased upstream region in a spray.

Numerical calculations were performed to investigate the mixture formation, ignition, and combustion processes in a diesel spray. The spray was formed by injecting n-heptane into a constant volume vessel under high-temperature and high-pressure conditions. The fuel droplets were described by a discrete droplet model (DDM). Numerical calculations for the flow and turbulent diffusion processes were performed on the basis of large eddy simulation (LES) to describe the processes of local non-homogeneous mixture formation and heat release. The oxidation processes in the mixture were calculated by Schreiber's five-step mechanism for n-heptane. Calculations were performed for sprays formed by single-stage injection and pilot/main two-stage injection. The flame structure in a diesel spray and its temporal change were discussed using a flame index proposed by Yamashita et al.

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.

Numerical calculations based on large eddy simulation were performed in order to investigate mixture formation, ignition, and combustion processes in a diesel spray formed by fuel injection into a constant-volume vessel under high-temperature and high-pressure conditions. Fuel concentration distributions in a spray and local non-homogeneous mixture distributions were compared with experimental results to verify the accuracy of the calculations. In addition, calculations were carried out to examine the effect of injection parameters, namely, injection pressure and nozzle orifice diameter. Ignition and combustion processes were also investigated using Schreiber's model for calculating the progress of oxidation reactions.

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.

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.

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.

Auto-ignition of a non-homogeneous mixture was fundamentally investigated by means of a numerical calculation based on chemical kinetics and the stochastic approach. In the present study, the auto-ignition process of n-heptane is calculated by means of a reduced mechanism developed by Seiser et. al. The non-uniform states of turbulent mixing are statistically described using probability density functions and the stochastic method, which was originally developed from Curl's model. The results show that the starting points of the low-temperature oxidation and ignition delay period are hardly affected by the equivalence-ratio variation; however, combustion duration increases with increasing variance of equivalence ratio. Furthermore, combustion duration is mainly affected by the non-homogeneity at the ignition and not very much affected by the mixing rate.

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.