Tadayoshi Ihara

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.