Study of Combustion Behavior and Combustion Stability of HCCI-DI Combustion for a Wide Operating Range using a Low Cost Novel Experimental Technique 2014-01-2661

An innovative method is developed for achieving HCCI-DI combustion without any major engine geometry modification. Many control strategies have been reported in literature, in spite of that even today no HCCI combustion engine is available on commercial basis. Pilot and main fuel injection strategy is used as a control strategy in this work. Our developed technique is a low cost alternative to conventional CRDI pumps which can be implemented readily at least for rural application engines (where cost of the system is more important than any other aspects) to reduce emissions. Using this new technique a stable HCCI-DI combustion was achieved for a wide operating range. To realize the effectiveness of this developed experimentation technique, a detailed combustion study at various operating conditions were investigated using commercial diesel as fuel. Pilot injection timing was fixed at 270 degrees bTDC allowing sufficient time for homogeneous mixture preparation and main injection was fixed at 26 degrees bTDC to trigger the combustion. Split ratio was varied from zero to 95% at various operating load (2 bar Net IMEP to 6.5 bar Net IMEP) conditions. This study is limited up to 95% split ratio since the engine went beyond drivability limit (COV of IMEP <10% as defined in Stone, 2012 [10]) with 100% split ratio in most of the operating conditions. The behavior of all combustion parameters (cylinder pressure, pressure rise rate, heat release rate and start of combustion) at various operating conditions are analyzed and presented in this paper. It was evident from the results that with increasing split ratio, all combustion parameters start advancing. Similarly, at any constant split ratio all combustion parameters started advancing with increasing load. Cycle by cycle statistical analysis of peak cycle pressure and net IMEP for 100 consecutive cycles is also analyzed to understand the combustion stability achieved using this technique. In HCCI combustion, low load limit is widely defined by high value of COV of IMEP and High load limit is defined by knocking combustion. This study shows, even at a 95% split ratio and at very low intake air temperature, cycle by cycle variation was found within drivability limit and therefore, No misfire region exists even at idling (2 bar IMEP) condition. Therefore, extreme Low load range can be achieved using a combination of pilot and main injection strategy. Combustion stability at various split ratio and load were also compared with conventional diesel combustion. Ringing Index (RI) is calculated to find the high load limit of HCCI-DI combustion of the modified engine. It is found that RI monotonically increased with increasing split ratio and increasing load. At higher split ratio, RI is extremely sensitive to engine load. At 80% split ratio and at 6.5 bar IMEP condition, RI was found to be 8 times higher than the corresponding baseline combustion. At higher split ratio, high load range narrows down. To increase load range, split ratio should be reduced if no other control strategy is used.