The Influence of Fuel Cetane Number on Catalyst Light-Off Operation in a Modern Diesel Engine

The design of modern diesel-powered vehicles involves optimization and balancing of trade-offs for fuel efficiency, emissions, and noise. To meet increasingly stringent emission regulations, diesel powertrains employ aftertreatment devices to control nitrogen oxides, hydrocarbons, carbon monoxide, and particulate matter emissions and use active exhaust warm-up strategies to ensure those devices are active as quickly as possible. A typical strategy for exhaust warm-up is to operate with retarded combustion phasing, limited by combustion stability and HC emissions. The amount of exhaust enthalpy available for catalyst light-off is limited by the extent to which combustion phasing can be retarded. Diesel cetane number (CN), a measure of fuel ignition quality, has an influence on combustion stability at retarded combustion phasing. Diesel fuel in the United States tends to have a lower CN (both minimum required and average in market) than other countries. It is unclear from existing literature to what extent low CN influences the ability to retard combustion phasing while limiting engine-out hydrocarbon emissions during the catalyst light-off phase. To begin to address this question, a single-cylinder engine study was conducted to compare the ability to generate exhaust temperature and enthalpy with low emissions with fuels of 46 and 53 CN. The fuels were found to produce comparable exhaust temperature, enthalpy and emissions when the engine was operated at the same calibration. However, when the calibrations were optimized separately for each fuel to maximize exhaust temperature and enthalpy while controlling emissions at or below a target level, the higher CN fuel tolerated significantly greater post injection retard within the hydrocarbon and particulate matter emissions constraints, thus enabling increased exhaust enthalpy and temperature. This study suggests that while a change in market CN would have only a slight impact on catalyst light-off and tailpipe emissions in legacy vehicles, future vehicles may be limited by low cetane fuel in their ability to achieve ultra-low emissions such as those required for California LEV III and Federal Tier 3. The study also highlights the importance of the assessment methodology when making conclusions about the effect of fuels on legacy and future vehicles.Cross Reference: