Homogeneous Charge Compression Ignition (HCCI) is considered a very promising concept to achieve low NOx and Particulate Matter emissions in traditional spark ignition and Diesel engines. However, controlling the complex mechanisms which govern the combustion process and finding a proper method for the fuel introduction for Diesel HCCI engines have proven to still be a challenge. In addition, the well known IMEP limitations of HCCI combustion restrict the benefits on emissions to low engine load conditions.
The current work attempts to extend the benefits of HCCI combustion to a broader range of engine operating conditions by blending the conventional Direct Injection (DI) with the external fuel atomization. A dual combustion system could potentially overcome the limits of low-load operations and allow for a gradual transition between the conventional DI mode at high load and the HCCI external mixture formation at idle and low load.
Using an automotive 2.5l common rail Diesel engine, a rapid prototyping ECU is used to control the direct injection system, varying the number of injections, rail pressure, timings and fuel quantities. The ECU also controls the quantity of fuel atomized in the intake manifold, as well as the EGR dilution and charge temperature (with an EGR bypass cooling loop).
The results included in the paper show a characterization of the mixed-mode HCCI/DI combustion based on the analysis of heat release, IMEP, torque, specific fuel consumption and emissions. Comparisons with results in pure DI mode at the same speed and load conditions are provided for comparison.