A TMI based CNG dual-fuel approach to address the soot-NOx-BSFC trade-off characteristics of a CRDI assisted diesel engine - an EPA perspective

The benefits of CRDI in registering significant reductions in soot and specific fuel consumption over conventional diesel engine operation has been well established. However, such premiums have been observed at the expense of drastic increase in NOx emissions. With emission legislations getting more stringent in order to comply with the responsibilities of environmental obligations, the focal point of contemporary diesel engine design has converged to meet the challenges of the omnipresent paradox of its soot-NOx-BSFC trade-off characteristics. CNG with its comparatively higher sustainable index than conventional fossil fuels due to its established greater R/P ratios, has been widely used as a green fuel on IC engine platforms and forms an amenable alternative resource in the immediate future of IC engine power trains. The present work attempts to study the synergetic benefits of CRDI coupled with CNG dual-fuel operation to explore an in-situ option to address the NOx penalty of CRDI operation while retaining its advantages of lower soot and BSFC characteristics. To gauge the efficacy of such dual fuel operation, an adaptive merit function was constituted to analyze the soot-NHC-BSFC trade-off characteristics with respect to EPA Tier-4 mandate perspectives. Trade-off analyses of the CRDI operations at all optimal injection durations corresponding to each operating load points were observed to be consistently higher than the corresponding baseline diesel operations owing to commendable soot and BSFC reductions. Such gains in Trade-off merit however were found to violate the EPA-Tier 4 NHC mandates owing to an increase in NOx emissions. At 25%, 50%, 75% and full load, optimal CRDI operations registered a 6.8, 29.5, 23.3 and 29 times higher trade-off merit than corresponding baseline diesel operations with 66.5%, 45%, 13.1% and 37% lower soot emissions but were penalized with 240.6%, 61.9%, 14.5% and 23.3% higher NHC emissions with respect to the desired EPA mandates as applicable to the experimental engine. Corresponding CNG operations at the optimal CRDI operations yielded commendable reductions in NHC owing to consistent reductions in NOx emissions. The best trade-off merits were scored at low CNG energy participation ratios corresponding to an energy share of 7.5%, 4.3%, 6.2% and 10.7% at 25%, 50%, 75% and full load conditions respectively which were successful in scoring 237.5%, 63.5%, 49.6% and 30.9% higher trade-off merit along with a simultaneous 63.9%, 67.95% 50.7% and 74.6% lower NHC emissions than its corresponding CRDI counterparts.