This paper presents an outline of autoignition phenomena and their role in internal combustion engines. First, the importance of autoignition control in the development of highly efficient next-generation internal combustion engines is described. The spectroscopic characteristics and chemical reactions of the autoignition process are then briefly explained. After that, specific examples of autoignited combustion in internal combustion engines are given, such as knocking in a spark-ignited (SI) engine, spectroscopic measurements of combustion in a supercharged Homogeneous Charge Compression Ignition (HCCI) engine, the chemical reactions occurring in HCCI combustion, and flame development behavior in HCCI knocking.
Combustion of a gasoline spark-ignition engine is initiated with the spark discharge formed at a spark-plug. High temperature plasma formed at the spark-plug develops and forms initial flame kernel with very thin reaction zone. Key features of spark discharge are stable ignition of super-lean, highly diluted mixture and durability to higher turbulent flow. This report explains the ignition process through formation of spark discharge at the spark-plug to initiation of initial flame kernel. The formation process of spark discharge is explained from the point of view of plasma physics. Visualization of spark discharge and initial flame kernel were carried out in a compression-expansion machine (CEM) using two high-speed cameras. Shroud attached to intake valve was used to enhance the flow field in cylinder. The initial flame and spark discharge were obtained from these images under different flow field conditions. Moreover, several spark ignition models were explained for 3D CFD of gasoline spark-ignition engine.
Laser ignition has been receiving special interests recently because of the requirement of alternative strong ignition systems for internal combustion engine in next generation. The merits of the laser ignition over the conventional spark ignition are absence of wall or electrodes which quench an initial frame kernel, capability of multi-point ignition through single hole and characteristics that favors higher pressure. This third feature is noteworthy especially for recent supercharged engines. Recent development of new laser devices such as ceramics laser and VCSEL are also very important progress for the laser ignition. In this paper, laser ignition studies from fundamental to recent activity were introduced.
Recently “plasma-assisted combustion”, especially for the use of non-equilibrium plasma for ignition has garnered interest as a new combustion technology for high efficiency and low emissions in combustors such as internal combustion engines. In this paper the focus is placed on the review of non-thermal plasma ignition and introduction of authors' recent results of this field. The ignition characteristics of non-thermal plasma are examined and compared with those of a conventional spark ignition. It is shown that a streamer discharge characterized by non-thermal plasma can ignite combustible mixtures as well as conventional thermal plasma and there are also some advantages, such as volumetric ignition and less heat loss. Through OH LIF measurement, a number of OH radical is shown to accumulate from pulse to pulse during a train of repetitive pulses, and the created radicals can initiate chemical chain reaction, which results in ignition finally.
At present, the electric power generated in coal-fired power stations accounts for about 25% of the total electric demand in Japan. The demand for steam coal is increasing all over the world. Almost all Japanese coal fired power stations utilize bituminous coal as fuel. From the viewpoint of preserving steam coal, it is important to diversify the type of coal used. In this article, coal combustion technology for pulverized low quality fuels, which are sub-bituminous coal with high moisture, low grindability coal and biomass to reduce CO2 emission, is explained.
An experimental study was conducted to investigate the patterns of the temperature distribution in a rapid compression machine, inside the cylinder during the auto-ignition process, with a stratified fuel distribution. In this paper, n-heptane was used as the fuel. Carbon dioxide was added in the charge mixture for measuring the temperature distribution using the infrared emission method. The fuel concentration gradient (FCG) was created by injecting the fuel from the bottom part of the cylinder followed by a time delay. The overall equivalence ratio was set to 0.6 and the compression ratio was 10. The relation between the difference of the equivalence ratios at the top and the bottom cylinder sides, ΔØ, and the waiting time from the injection, tw, was investigated in advance. The data obtained showed that the low temperature oxidation process started to occur from the leaner ΔØ region. On the other hand, the hot flame began at the top side with a small ΔØ value, in contrast to the bottom side where the hot flame started with a large ΔØ value. The knock intensity (KI) could be weakened when a proper FCG is applied, but in the cases where the FCG is too steep or inadequate, lower combustion efficiency or a large KI would respectively occur.
Homogeneous Charge Compression Ignition (HCCI) engines are being widely researched today, having attracted considerable interest for their low emissions and high efficiency. However, HCCI engines have a narrow range of stable operation owing to the occurrence of extremely rapid combustion at high loads and misfiring at low loads. Extremely rapid combustion at high loads is an especially large factor preventing expansion of the stable operating region. It is also known that abnormal combustion accompanied by in-cylinder pressure oscillations resembling those of knocking in a spark-ignition engine occurs in HCCI engines as well depending on the operating conditions. The purpose of this study was to identify the characteristics of pressure oscillations due to knocking in an HCCI combustion system. Using a two-stroke single-cylinder engine, pressure oscillations were investigated in detail on the basis of in-cylinder visualization/imaging of the combustion flame across the entire bore area, frequency analysis of the in-cylinder pressure waveform, and spectroscopic measurements. The results revealed that the maximum pressure rise rate, dP/dtmax, increased as the ignition timing advanced and the equivalence ratio increased and that in-cylinder pressure oscillations occurred under a condition of dP/dtmax higher than approximately 7 MPa/ms. The visualization results showed that HCCI combustion accompanied by in-cylinder pressure oscillations occurred in the latter stage of the combustion process due to rapid autoignition of the unburned end gas. Moreover, under a condition of a high equivalence ratio, a highly brilliant autoignited flame occurred over a wider area of the combustion chamber. The power spectrum of the in-cylinder pressure oscillations indicated that they possessed unique frequency components in high-frequency bands, in addition to the frequency components around 7 kHz.