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

Abstract

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.