Experimental investigation of compression ratio and boost pressure influence on RG blended CNG — HCCI combustion engine

This paper investigates the compression ratio and boost pressure influence on performance of RG blended CNG - HCCI combustion over a range of load and effect on exhaust emission. HCCI engines operates on the principle of having a dilute, premixed charge that reacts and burns volumetrically throughout the cylinder as it is compressed by the piston. In some regards, HCCI incorporates the best features of both spark ignition (SI) and compression ignition (CI) engine, the charge is well mixed, which minimizes particulate emissions, and as in a CIDI engine, the charge is compression ignited and has no throttling losses, which leads to high efficiency. However, unlike either of these conventional engines, the combustion occurs simultaneously throughout the volume rather than in a flame front. This important attribute of HCCI allows combustion to occur at much lower temperatures, dramatically reducing engine-out emissions of NO_x. But, the pure CNG fueled HCCI engine successfully operates only in a narrow range of conditions bounded by severe knock on the fuel-rich side and lean misfire on the lean side and high NO_x emission. It was found that RG blending with base CNG fuel in HCCI engine produces wider operating range towards lean burn boundary. Replacing CNG with RG increases measured combustion efficiency, particularly for leaner mixture. Boost pressure increases air/fuel ratio and thus increases the thermal mass to be heated by the combustion heat release resulting in expanding operating region towards high load range and reduction in exhaust emission.