Emissions with HCCI Operation

Emission potential of HCCI operation relative to conventional diesel engines is illustrated in Fig 7.12. The HCCI engine employed multiple early injection strategy with compression ratio equal to 13.4:1 and high rates of EGR. The NO_x emissions were just 0.074 times (7.4%) and soot emissions were only 5.5% of the conventional CI engine operation. NO_x emissions with HCCI operation ranged from 0.01 to 0.06 g/kWh and the soot emissions were below 0.02 g/kWh.  Nearly 95% reduction in soot and NO_x emissions are obtained compared to the conventional diesel operation. However, due to quenching of combustion in excessively lean mixture HC increased to 348%. CO  increased to 487% of the conventional CI engine. As the mixture is not entirely homogeneous and some over rich zones exist more CO is produced. Although excess air is present in burnt gases but the combustion temperatures are low to promote oxidation reactions  of HC and CO in the post combustion gases.

Regimes of HCCI and Conventional CI Engine Operation

Developments in application of HCCI combustion to conventional diesel engines may be summarized as below:

• Most of the strategies for HCCI operation essentially attempt to increase the fraction of fuel burned as premixed charge.
• The premixed charge obtained with early and more so with late in-cylinder injection actually may not be fully homogeneous.
• The attempt is to keep local fuel-air ratio lean to reduce soot formation.
• Most of the highly premixed charge undergoes precombustion reactions and burns simultaneously such that no flame front is present and low temperatures are maintained.
• The production diesel engines are unable to operate on homogeneous charge through out the entire operational range. HCCI mode of operation is feasible only at part loads. At high loads, conventional diesel combustion is to be employed.

Combustion concepts likely to be employed in the different speed-load operation regimes of a future diesel engine are shown on Fig 7.13. As the HCCI operation in the entire range of engine loads and speeds is not possible , it is envisaged that the future low emission engines would operate in combination of HCCI operation at low BMEP and the conventional compression ignition mode in the high BMEP region . The technologies already developed for control of emission in the conventional CI mode such as EGR , turbocharging , high pressure injection would be employed during high engine power operation.