Comparison of diesel and hydrotreated vegetable oil as the high-reactivity fuel in reactivity-controlled compression ignition

Abstract

Hydrotreated vegetable oil (HVO) is becoming a widely accepted renewable drop-in alternative fuel to diesel. However, conventional diesel combustion does not fully exploit HVO’s superior physicochemical parameters. Its high cetane index should significantly improve the performance and emission of next-generation, dual-fuel, reactivity-controlled compression ignition (RCCI) engines. These have a promising future in marine and off-road sectors. This study is the first comprehensive verification of HVO’s benefits towards achieving superior RCCI combustion with natural gas. It used a sophisticated, single-cylinder research engine with a fully controllable air/fuel paths, calibrated in conventional compression ignition mode. The calibration experiments in a corresponding RCCI setpoint covered the cross-sensitivity of high-reactivity fuels (HVO and diesel) to boost pressure, excess air ratio, exhaust gas recirculation and start of injection, investigated at 85 % and 93 % energy-based blending ratios with natural gas. Extensive measurement instrumentation provided combustion and emission characterisation, enabling observations regarding both the phenomenology and applied potential of HVO-activated RCCI. Best performance was observed at the boundary of mixture dilution, restricted by the misfire or combustion variability limits. High reactivity of HVO allows for extending combustion stability limits, enabling increasing both, the local dilution (earlier injection timings) and the global dilution (higher mixture strengths or higher exhaust recirculation ratios). Calibrated along these phenomenological outcomes, HVO and diesel allow achieving efficiency over 2 percentage points superior in RCCI mode, compared to conventional diesel reference. With HVO, RCCI can be calibrated to comfortably meet EPA Tier 4 emission limits in all legislated species, without aftertreatment. Particular merits are in NO_X reduction, for which the best case HVO-RCCI tested at 0.7 g/kWh vs 2.8 g/kWh of diesel-RCCI. HVO further cuts down methane slip by more than 45 %, while PM emissions for RCCI are generally measured ultra-low. Corresponding conventional diesel reference exceeds the EPA NO_X and PM limits by respectively 1500 % and 400 %.