Naoto Horibe

Improvement of premixed charge compression ignition-based combustion by two-stage injection

Abstract The objective of this study is to find strategies for extending the load range of premixed charge compression ignition-based combustion while improving thermal efficiency and reducing combustion noise and exhaust emission levels. Experiments were performed using a single-cylinder direct-injection diesel engine equipped with a common-rail injection system and a cooled EGR system. First, experiments were carried out with single-stage injection. The results indicated a notable improvement of NO x and smoke emissions by selecting lower EGR rates and later injection timings according to the increase in injection quantity. However, the problems of high pressure rise rates and levels of unburned species emissions developed. To solve these problems, two-stage injection was applied. These additional experiments started with injection and EGR conditions that were based on the results of the single-stage injection tests, and modifications were made to mitigate the increased emissions and decreased thermal efficiency. As a result, judicious selection of injection and EGR conditions for two-stage injection provided a drastic improvement in exhaust emissions with a sufficiently low pressure rise rate to be equivalent to pilot-diesel operation.

An Optimal Usage of Recent Combustion Control Technologies for DI Diesel Engine Operating on Ethanol Blended Fuels

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. Above finding indicates that low level of PM and NOx emission with no fuel consumption penalty is achievable when diesel-ethanol blend is used with combination of modern combustion control methods.

Combustion modelling for a diesel engine with multi-stage injection using a stochastic combustion model

This study presents the development of a phenomenological combustion model to simulate the combustion processes in diesel engines with multi-stage fuel injection. A newly developed zero-dimensional spray propagation model and a model of spray-to-spray interaction were combined with a stochastic combustion model, which had been developed for the calculation of diesel combustion in the case of single-stage injection. In this model, the combustion chamber is divided into an ambient air zone and several spray zones, where the spray formed by each injection is treated as a spray zone. The turbulent mixing, the fuel evaporation, the heat loss and the chemical reactions are calculated in each spray zone separately. A zero-dimensional spray propagation model including the spray evolution after the end of injection and a model of interaction between the sprays from sequential injections are developed to describe the spray behaviour for the case of multi-stage injection. Then the developed combustion model is validated against the experimental data from a single-cylinder direct-injection diesel engine with two-stage pilot–main injection, in which the pilot injection conditions are varied with a fixed main-injection timing. Based on the analysis of the heat release rate, the entrainment rate and the microscopic information inside the spray, such as the probability density function of the equivalence ratio, the effects of the wall impingement and the interaction between adjacent sprays on the fuel–air mixing rate and the entrainment rate are formularized and employed to reproduce the measured histories of the heat release rate. The reduction in the fuel–air mixing rate is considered when the spray flows into the squish region after wall impingement, which is effective in obtaining the measured decrease in the heat release of the pilot spray with advancing pilot injection timing. The effects of the wall impingement of the main spray and the interaction between adjacent sprays are modelled to reproduce the heat release rate during the initial part and later part of the mixing-controlled combustion. After these improvements, the heat release rates of the test engine when varying the pilot injection conditions were successfully predicted.

An Experimental Study on Smoke Reduction Effect of Post Injection in Combination With Pilot Injection for a Diesel Engine

With the universal utilization of the common-rail injection system in automotive diesel engines, the multistage injection strategies have become typical approaches to satisfy the increasingly stringent emission regulations, and especially the post injection has received considerable attention as an effective way for reducing the smoke emissions. Normally the post injection is applied in combination with the pilot injection to restrain the NOx emissions, smoke emissions, and combustion noise simultaneously, and the pilot injection condition affects the combustion process of the main injection and might affect the smoke reduction effect of the post injection. Thus this study aims at obtaining the post injection strategy to reduce smoke emissions in a diesel engine, where post injection is employed in combination with pilot injection. The experiments were performed using a single-cylinder diesel engine under various conditions of pilot and post injection with a constant load at an IMEP of 1.01 MPa, fixed speed of 1500 rpm, and NOx emissions concentration of 150 ± 5 ppm that was maintained by adjusting the EGR ratio. The injection pressure was set at 90 MPa at first, and then it was varied to 125 MPa to evaluate the effects of post injection on the smoke reduction in the case of higher injection pressure. The experimental results show that small post injection quantity with a short interval from the end of main injection causes less smoke emissions. And larger pilot injection quantity and later pilot injection timing lead to higher smoke emissions. And then, to explore and interpret the smoke emissions tendencies with varying pilot and post injection conditions, the experimental results of three-stage injection conditions were compared to those of two reference cases, which only included the pilot and main injection, and the interaction between main spray flames and post sprays was applied for analysis. Based on the comparative analysis, the larger smoke reduction effect of post injection was observed with the larger pilot injection quantity, while it is not greatly influenced by pilot injection timing. In addition, the smoke emissions can be reduced considerably by increasing the injection pressure, however the smoke reduction effect of post injection was attenuated. And all of these tendencies were able to be interpreted by considering the intensity variation of the interaction between main spray flames and post sprays.

Selection of Injection Parameters for Various Engine Speeds in PCCI-Based Diesel Combustion with Multiple Injection

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. In addition to advancing the injection timings, a decrease of the first-injection quantity ratio reduced smoke emission.

Improvement of Thermal Efficiency in a Diesel Engine with High-Pressure Split Main Injection

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. Computational fluid dynamics calculations were performed to reveal the causes of the lower cooling loss; however, the results could not capture the experimental trend when using an ordinary spray cone angle. While using a wider spray angle for the second part of main injection, the calculated trend improved. The total cooling loss depends on the balance between the cooling losses by the first and second main sprays.

Phenomenological modeling of diesel spray with varying injection profile

Accurate and quick prediction of spray characteristics such as spray penetration is paramount for the understanding and quantitative analysis of the combustion process in diesel engines, in order to perform parametric study on advanced combustion process in diesel engines, zero-dimensional diesel spray model is often used for the prediction of the spray evolution. In this study, a previous zero-dimensional diesel spray model applied for the spray penetration prediction including the part after the end of injection with a constant injection rate was extended to the cases with varying injection rate. The effective injection velocity was introduced into the previous spray model, which is defined as the ratio of the momentum flux and fuel mass flow rate over the spray tip cross-sectional area. Combined with this definition, the analysis of effective injection rate and its response time was performed during and after the end of injection. After that, the fuel mass flow rate and momentum flux over the spray tip cross-sectional area were derived for varying injection rate even after the end of injection based on the momentum and fuel mass conservation along the spray axis, and further the spray penetration. Finally, the developed model was validated by comparing with the experimental data.