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 be able to 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 in RRCI combustion. 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. These were investigated at different 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. The results showed that in RCCI, unlike in conventional diesel combustion, HVO advances combustion significantly when compared with diesel fuel. HVO injected 15 crank angle degrees earlier than diesel pilot fuel provided correct combustion timing, well before the threshold of incomplete or instable combustion. HVO’s superior reactivity cut methane slip by 50%, with near-zero NOX and PM emissions. HVO-activated RCCI also accepted higher mixture dilution by air or recirculated exhaust than when using diesel high-reactivity fuel. These results bring new fundamental-level knowledge of RCCI combustion with HVO, together with immediate applied recommendations towards improving performance and emissions.

Keywords: RCCI, HVO, natural gas, fuel reactivity, engine calibration