Many strategies exist for reducing NOx and PM emissions while simultaneously maintaining high thermal efficiencies in a light-duty (LD), multi-cylinder, turbocharged diesel engine. However, these strategies are not without drawbacks when run with a largely stock engine configuration designed for conventional diesel combustion. Experiments using various levels of port fuel injected synthetic gas (Syngas) were run at steady state and transient engine operating conditions to assess the controllability, emissions characteristics, and effect on thermal efficiency. Syngas, comprised of mainly hydrogen and carbon monoxide, can be generated through a variety of thermochemical reforming methods with numerous hydrocarbon fuels and can be used to generate two fuels with a single fuel stream. Thus mitigating the need to carry two fuels for dual-fuel combustion. Using two fuel streams of different reactivity, in this case diesel and syngas, can improve combustion performance by reducing mixing requirements, lowering in-cylinder temperature, and more efficient use of the expansion stroke due to shorter combustion duration. To simplify the analysis and more effectively isolate the effects that syngas has on combustion, bottle gas with equal parts hydrogen and carbon monoxide by volume was used in lieu of a reforming process. Two different dual fuel combustion modes, Diesel Pilot Ignition (DPI) and Reactivity Controlled Compression Ignition (RCCI), were selected to compare against conventional diesel combustion.
DPI can be characterized by a very early injection of syngas in the intake runner before intake valve closing with a near top dead center injection of direct injected fuel for combustion phasing control. The premixed charge reduces in-cylinder mixing requirements and results in lower soot, generally higher NOx, relatively high levels of combustion noise, and high controllability. Transient time-based ramp load changes with DPI combustion demonstrate that peak NO and opacity can be substantially reduced in load changes from 2 to 8 bar BMEP with moderate levels of EGR. This reduction is due to decreased mixing requirements due to substitution of direct injected fuel energy, carbon monoxide’s tendency to not form precursors to soot, and use of hydrogen fuel increasing oxidation rates. The benefits in peak opacity reduction are found with all syngas level additions, but the benefits are the most substantial at low to moderate levels.
RCCI can be characterized by a similar early injection of syngas with a direct injection well before top dead center. This strategy results in low soot, low NOx, much more modest levels of combustion noise, and high thermal efficiency. This combustion concept is achieved by an in-cylinder stratification of fuel reactivity and is controlled kinetically by varying the relative amounts of each fuel and DI injection timing. RCCI is much more sensitive to cyclic variation and cylinder-to-cylinder boundary conditions as well as hardware variance but is nonetheless demonstrated. Experimental RCCI data from the GM 1.9L at variable thermodynamic states was used with C15 2.5L experimental data to aid in the development of a scaling methodology for highly premixed combustion strategies. The methodology is based on scaling parameters given by a characteristic bore-ratio and the Livengood-Wu auto-ignition integral for predicting start of combustion. In order to match combustion phasing, the compression ratio is scaled to adjust for combustion duration. Experiments with constant manifold pressure, matched compression stroke pressure, and two simulated higher compression ratios show promising results, with highest simulated compression ratio closely matching combustion phasing at the substitution levels used in a heavy-duty (HD) engine baseline.