Tatsuya Kuboyama

Extension of an operating load range of a four-cylinder gasoline homogeneous charge compression ignition engine via a blowdown supercharge system, aiming at in-cylinder thermal stratification

In order to extend the operational range with a gasoline fueled multi-cylinder homogeneous charge compression ignition engine, a blowdown supercharge system and an exhaust gas recirculation guide was developed. The concept was to provide a large amount of diluted mixture and a strong in-cylinder thermal stratification for decreasing nitrogen oxide emissions and pressure rise rate during high-load homogeneous charge compression ignition operation. Secondary air injection was also proposed to reduce a cylinder-to-cylinder variation in ignition timing, which is one of the limiting factors of multi-cylinder homogeneous charge compression ignition operation. In advance of experimental works, the blowdown supercharge system and the exhaust gas recirculation guide were proved to be effective to reduce pressure rise rate for high-load operation using three-dimensional in-cylinder flow simulations and zero-dimensional multi-zone simulations with detailed chemical kinetics. Based on the simulation results, experiments were conducted using a slightly modified production four-cylinder gasoline engine. At first, the effects of the proposed techniques were experimentally investigated by focusing on one cylinder out of four. Then, a four-cylinder homogeneous charge compression ignition operation test using the new techniques was carried out. The one-cylinder measurements revealed that the blowdown supercharge system with an exhaust gas recirculation guide is capable of extending the homogeneous charge compression ignition operating range up to a net indicated mean effective pressure of 590 kPa for naturally aspirated conditions. In addition, secondary air injection was experimentally demonstrated as a technique for reducing a cylinder-to-cylinder variation in ignition timing. Reducing the cylinder-to-cylinder variation in ignition timing, four-cylinder homogeneous charge compression ignition operation with a net indicated mean effective pressure of 570 kPa was successfully achieved with the blowdown supercharge system and the exhaust gas recirculation guide.

Improvement of thermal efficiency of a four-cylinder gasoline homogeneous charge compression ignition engine via blowdown supercharging

The objective of this study is to develop a practical technique to achieve homogeneous charge compression ignition operation with a wide operating range using a blowdown supercharge system, which has been previously demonstrated as an effective technique to extend the upper load limit of acceptable homogeneous charge compression ignition operation. The valve actuation strategy to attain acceptable homogeneous charge compression ignition operation in a wide operating range has been newly developed and experimentally examined. The proposed strategy provides high in-cylinder temperature and a relatively small amount of in-cylinder mixture during low-load operations to improve the combustion stability while providing a large amount of diluted mixture for high-load operations to keep the in-cylinder pressure rise rate and nitrogen oxide emissions low. In addition, thermal efficiency and exhaust emissions for various homogeneous charge compression ignition operating loads using the blowdown supercharge system were experimentally investigated. Experimental results showed that the proposed valve actuation strategy, in which early intake valve closing and relatively late exhaust gas recirculation valve opening occurred, was effective to increase combustion stability during low-load conditions, and a stable homogeneous charge compression ignition operation at a net indicated mean effective pressure of 140 kPa was achieved. Compared to conventional spark-ignition operation, 14% to 35% improvement in brake specific fuel consumption rate was attained with more than 99% reduction in nitrogen oxide emissions for homogeneous charge compression ignition operation using the blowdown supercharge system.

Diesel flame imaging and quantitative analysis of in-cylinder soot oxidation

This study concerns a quantitative analysis of late-cycle soot oxidation in diesel engines that focuses on two-dimensional KL factor images obtained by the two-color method. The spatially integrated KL factor was converted into the in-cylinder soot mass using a new formula of diesel soot emissivity. This methodology was applied to two combustion systems: a heavy-duty optical engine which was tuned for a higher fuel–air mixing capability and a rapid compression and expansion machine which had a lower mixing performance. The in-cylinder soot mass history during the last stage of soot oxidation phase was converted into a normalized soot mass history and was used for comparison with simulated soot mass history. A model calculation of in-cylinder soot mass history which was based on oxidation of a primary soot particle was performed with the surface-specific soot oxidation rate as a parameter. A value of the surface-specific soot oxidation rate was specified from the curve fitting approach between the experimental and simulated in-cylinder soot mass traces. The resultant soot oxidation rates plotted on the Arrhenius diagram were found to lie in domains with different oxidation mechanisms. The reason for the scattered plots was discussed referring to model predictions of soot oxidation in the literature, and it was concluded that the higher oxidation rates could be attributed to well-mixed soot oxidizer structure.