While Low Temperature Combustion (LTC) strategies such as Reactivity Controlled Compression Ignition (RCCI) exhibit high thermal efficiency and produce low NOx and soot emissions, low load operation is still a significant challenge due to high unburnt hydrocarbon (UHC) and carbon monoxide (CO) emissions, which occur as a result of poor combustion efficiencies at these operating points. Furthermore, the exhaust gas temperatures are insufficient to light-off the Diesel Oxidation Catalyst (DOC), thereby resulting in poor UHC and CO conversion efficiencies by the aftertreatment system. To achieve exhaust gas temperature values sufficient for DOC light-off, combustion can be appropriately phased by changing the ratio of gasoline to diesel in the cylinder, or by burning additional fuel injected during the expansion stroke through post-injection. Alternatively, variable valve actuation (VVA) strategies such as Early Exhaust Valve Opening (EEVO) and cylinder deactivation may be implemented to raise the exhaust gas temperatures for DOC activation, and/or improve fuel economy by firing fewer cylinders at operating points with higher combustion efficiencies.
Since each of these strategies has benefits and drawbacks, it is of interest to compare them in terms of engine performance, emissions and catalyst efficiency for low load operation. In this work, coupled GT-Power and KIVA simulations of a multi-cylinder light duty engine operating on RCCI are performed at a near-idle operating point of 1 Bar BMEP at 1,500 rev/min. Five operating strategies are considered: 1. Varying combustion phasing via the gasoline-diesel ratio, 2. Using late fuel injection during the expansion stroke to activate the catalyst, 3. EEVO using a cam phaser, 4. EEVO using a fully flexible variable valvetrain, and 5. Cylinder deactivation. The cylinder-out emissions are compared and explained for each strategy. The effects of exhaust gas temperature on DOC performance, and the impact of each strategy on fuel economy are also studied to determine the most suitable strategy for low load operation.