Two-stroke engines remain competitive today in recreational applications (boats, snowmobiles, etc.) due to cost, power density, weight, and maintenance benefits. The objective of this research was to investigate whether reactivity controlled compression ignition (RCCI) combustion could be beneficial in a two-stroke engine. Experiments were conducted at ~25% rated speed and ~40% rated load on a two-cylinder 60 HP outboard engine. RCCI combustion was implemented in one cylinder, where two gasoline direct injection (GDI) fuel injectors were implemented in a modified cylinder head, allowing in cylinder blending of low-reactivity (LR) and high-reactivity (HR) fuels without significant short circuiting. The other cylinder maintained the production direct injection spark ignition (DISI) configuration, providing a means for comparison.
Off-engine low pressure (≤135 bar) fuel spray characterization was conducted to improve the understanding of the injection process and physical implications in the engine. The octane “appetite” of the engine was investigated in the homogeneous charge compression ignition (HCCI) combustion mode to determine the overall fuel reactivity required for maintaining constant combustion phasing with changes in operating conditions. At 1500 rpm and 2.5 bar IMEP, a relatively low peak primary reference fuel (PRF) number of 42 was required, which translated to a LR fraction requirement of 0.25 using gasoline and diesel. Diesel fuel vaporization issues were encountered under RCCI, resulting in high particulate matter (PM) emissions and a failure to adequately control combustion. Replacing diesel fuel with n-heptane resulted in acceptable HR fuel vaporization. RCCI combustion was successfully demonstrated with gasoline and n-heptane, where both LR fraction and HR start of injection (SOI) were effective methods to control the heat release rate (HRR). At 1500 rpm, 2.5 bar IMEP, and a NOx emission constraint of ~1.25 g/kW-hr, RCCI combustion resulted in higher gross indicated efficiency (GIE) compared to the DISI homogeneous and stratified modes, respectively (36.3% vs 27.0 and 25.7%). At 1200 rpm, 2.0 bar IMEP, and a HC+NOx emission constraint of ~16.5 g/kW-hr, RCCI combustion resulted in higher GIE compared to the DISI stratified and homogeneous modes, respectively (32.9% vs 26.6 and 25.2%).