HCCI DIESEL ENGINES

The objective of application of HCCI concept to the diesel engines is control of NO_x and particulates simultaneously which otherwise is difficult to achieve in the conventional CI engines. For HCCI combustion the efforts are directed mainly towards:

• Creation of premixed, lean homogeneous mixture, and
• Compression ignition and control of rate of combustion to obtain low emissions and optimum engine performance.

The basic objective is to increase the formation of premixed charge prior to ignition so that a large fraction of the fuel per cycle burns as premixed. Subsequent to ignition the rate of combustion is to be controlled to keep it within acceptable limits. The different methods which may be adopted to achieve the above two objectives are:

Premixed Charge Formation:

• Fuel introduction in Intake manifold or port
• Early (much before the conventional injection timing)  and multiple direct  fuel injection in the cylinder
• Late direct fuel injection (at TDC or later) in the cylinder

The different fuel injection timings that may be adopted in a diesel engine to obtain a long ignition delay and consequently a high fraction of premixed charge before start of combustion are shown schematically in Fig 7.10. For HCCI mode of operation a high fraction of premixed charge is essentially required before start of compression ignition .

Ignition and Combustion Control:

• Multiple/split injection
• High amount of EGR
• Reduction in compression ratio