Analysis of Dual-Fuel CNG-Diesel Combustion Modes Towards High Efficiency and Low Emissions at Part Load

Previous research carried out by the authors described the role of dual-fuel combustion efficiency on overall engine efficiency at high diesel substitution ratio. At low loads, high diesel substitution ratios result in excessive total unburned hydrocarbon emissions due to bulk flame quenching, and consequently lower substitution ratios in combination with intake air heating, throttling and increased turbulence should be adopted (or even full diesel operation) in order to meet emission legislation targets. At those operating points where combustion efficiency is not a limiting factor, engine operation can be really flexible and different options can be implemented. Conventional dual-fuel (CDF) pilot-ignition and reactivity controlled compression ignition (RCCI) combustion have been evaluated and compared based on emissions, heat transfer and efficiency. The results presented in this paper show that CDF is a really robust combustion mode but efficiency and NOx emissions are not optimal when substitution ratio is not maximized. Diesel injection close to TDC promotes fuel rich areas and NOx is formed in near-stoichiometric regions surrounding the diesel jets, so diesel fuel amounts should be kept to a minimum in order to reduce NOX emissions and heat losses. However, excessively low fuel quantities turn out in combustion phasing control and ignition quality problems and consequently high hydrocarbon emissions. Efficiency at low substitution ratio is reduced due to the effect of fuel trapped in crevices and excessive combustion temperatures, resulting in high heat losses through combustion chamber walls. For these reasons, RCCI combustion mode allows a more efficient use of CNG at high substitution ratios than CDF because of high reactivity fuel dispersed over the entire combustion chamber, including squish volume, while NOX emissions are sharply reduced compared to less premixed combustion modes.