Influence of Mixture Quality on Homogeneous Charge Compression Ignition

The major advantages with Homogeneous Charge Compression Ignition, HCCI, is high efficiency in combination with low NOx-emissions. The major drawback with HCCI is the problem to control the ignition timing over a wide load and speed range. Other drawbacks are the limitation in attainable IMEP and relativly high emissions of unburned hydrocarbons. But the use of Exhaust Gas Recycling (EGR) instead of only air, slows down the rate of combustion and makes it possible to use lower air/fuel ratio, which increases the attainable upper load limit. The influence of mixture quality was therefore experimentally investigated. The effects of different EGR rates, air/fuel ratios and inlet mixture temperatures were studied. The compression ratio was set to 18:1. The fuels used were iso-octane, ethanol and commercially available natural gas. The engine was operated naturally aspirated mode for all tests. Stable operation could be achieved with ethanol at about 5 bar of IMEP with a gross indicated efficiency close to 50 %. Stable operation of the engine was achieved with over 60 % EGR.