Effects of charge reactivity on NOx–hydrocarbon trade-off in low load RCCI operation

Low load reactivity controlled compression ignition (RCCI) operation results in high engine-out total hydrocarbon (THC) emissions, and low combustion efficiency with increased fraction of the low reactivity fuel. Methods to reduce THC emissions generally lead to increased nitrogen oxides (NO x ) emissions resulting in a NO x -THC trade-off in low load RCCI operation. High THC emissions is attributed to reduced reactivity of the in-cylinder charge. Three strategies, such as: (1) increased coolant temperature, (2) use of higher reactivity (compared to diesel) paraffinic synthetic gas to liquid (GTL) fuel and (3) a small quantity of diesel injection in the intake stroke, are employed in this work to enhance the reactivity of the in-cylinder charge prior to the start of combustion and their effects on reducing THC emissions and improving combustion efficiency are investigated. With an increased coolant temperature (100°C), THC emissions reduce by ∼18% compared to baseline RCCI operation with 85°C coolant temperature. GTL, when used as an HRF at similar start of injection timing (SOI) as baseline RCCI, results in ∼34% reduction in THC emissions, albeit with an increase in NO x emissions, which, however, is reduced significantly with advancing the SOI further. A small quantity of diesel injection in the intake stroke (17% of the total diesel quantity) results in 22.5% reduction in THC emissions, and 3.2 and 4.7 percentage points increase in thermal efficiency and combustion efficiency, respectively, compared to the baseline RCCI strategy. In all these strategies, THC reduction is achieved while keeping NO x emissions below 15 ppm (<0.3 g/kWh (indicated) ).