Takuji Ishiyama

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.

Analysis of ignition processes in a fuel spray using an ignition model including turbulent mixing and reduced chemical kinetics

Abstract In order to clarify the mechanism of fuel spray ignition, an ignition model was developed employing a stochastic turbulent mixing model and a quasi-global chemical kinetics model. Using this model, histories of heterogeneity in temperature and equivalence ratio of fuel/air mixtures were analysed. At low initial ambient temperatures, exothermic reactions begin in lean mixtures with equivalence ratios in a very narrow range. On the other hand, at higher air temperatures, mixtures with a variety of equivalence ratios ignite. This phenomenon can be explained by the dependency of mixture reactivity on the equivalence ratio and temperature. Based on these results, a discussion is given on the similarities and differences in the temperature dependency of ignition delays between sprays and homogeneous mixtures.

Characteristics of Spontaneous Ignition and Combustion in Unsteady High-Speed Gaseous Fuel Jets

In order to obtain fundamental data to employ direct injection in gas-fueled engines, an experimental study was carried out using a constant volume vessel. Heat release rates and shadowgraph photos were acquired for natural-gas and hydrogen jets simulating the changes in engine-combustion-control factors. The results show that although a higher temperature is needed for ignition, the temperature dependencies of ignition delay and heat release rate in natural-gas jets are similar to those of diesel sprays. The ignition delay and heat release rate are sensitive to injection and ambient conditions. Hydrogen jets have shorter ignition delays compared with natural gas jets. At sufficiently high ambient temperatures, the heat release pattern shows an entire diffusion combustion. Under such conditions, the ignition delay is not greatly influenced by injection conditions and the heat release rate can be controlled by the injection rate.

Modeling and Experiments on Ignition of Fuel Sprays Considering the Interaction Between Fuel-Air Mixing and Chemical Reactions

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. Calculated ignition delays and pressure curves were compared with experimental data. Based on the comparison, explanations are made on the measured effects of injection conditions with an emphasis on the relation between fuel-air mixing and chemical reactions.

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.

Utilization of low-calorific gaseous fuel in a direct-injection diesel engine

Low-calorific gases with a small portion of hydrogen are produced in various chemical processes, such as gasification of solid wastes or biomass. The aim of this study is to clarify the efficient usage of these gases in diesel engines used for power generation. Effects of amount and composition of low-calorific gases on diesel engine performance and exhaust emissions were experimentally investigated adding hydrogen-nitrogen mixtures into the intake gas of a single-cylinder direct-injection diesel engine. The results indicate that optimal usage of low-calorific gases improves NO x and Smoke emissions with remarkable saving in diesel fuel consumption.

Computational fluid dynamics analysis of the combustion process and emission characteristics for a direct-injection-premixed charge compression ignition engine

The direct-injection-premixed charge compression ignition–based combustion process with a high exhaust gas recirculation ratio and early injection timing is simulated using a Reynolds-averaged Navier–Stokes–based commercial computational fluid dynamics code with a nonhomogeneous mixture auto-ignition combustion model. In addition, the formation processes of nitrogen oxide (NO), carbon monoxide (CO), and unburnt hydrocarbon are calculated. The calculation results are compared with the experimental data. According to the calculation results, the formation processes of NO, CO, and hydrocarbon are discussed in relation to the spray development and ignition point. Furthermore, the combustion process of two-stage injection is also calculated. The result shows that the combustion process is described well by this model except in the case where auto-ignition occurs in the squish area. Additionally, the relationship between the combustion process and the mixture distribution is clarified. The main origins of the unburnt hydrocarbon and CO emissions are located in the center region of the combustion chamber, where the mixture becomes excessively lean and low in temperature.

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.

A Study on Fluidized Bed-Type Particulate Filter for Diesel Engines

A fluidized bed-type diesel particulate filter (DPF) was applied to filter particulate matter (PM) in diesel engine exhaust gas. The effects of the fluidized bed design parameters, such as gas velocity, bed particle size, and height, on PM and smoke filtration efficiencies, and pressure drop were experimentally investigated using a single-cylinder direct injection (DI) diesel engine. High PM filtration efficiency and low pressure drop were achieved with the DPF, especially at a lower gas velocity. The PM filtration efficiency was higher with a smaller bed particle size at the lower gas velocity; however, it drastically decreased with an increase in gas velocity due to excessive fluidization of the bed particles. Increase in bed height led to higher PM filtration efficiency while causing an increase in pressure drop. The theoretical work was also conducted for further investigation of the effects of the above-mentioned parameters on PM filtration. These results indicated that diffusion filtration was the dominant mechanism for PM filtration under the conditions of this study and that the decrease in PM filtration efficiency at high gas velocity was caused by a deterioration in the diffusion filtration. The bed particle diameter and the bed height should be optimized in order to obtain a high filtration efficiency without increasing the DPF size.