<div>This study demonstrates the defossilized operation of a heavy-duty
port-fuel-injected dual-fuel engine and highlights its potential benefits with
minimal retrofitting effort. The investigation focuses on the optical
characterization of the in-cylinder processes, ranging from mixture formation,
ignition, and combustion, on a fully optically accessible single-cylinder
research engine. The article revisits selected operating conditions in a
thermodynamic configuration combined with Fourier transform infrared
spectroscopy.</div>
<div>One approach is to quickly diminish fossil fuel use by retrofitting present
engines with decarbonized or defossilized alternatives. As both fuels are
oxygenated, a considerable change in the overall ignition limits, air–fuel
equivalence ratio, burning rate, and resistance against undesired pre-ignition
or knocking is expected, with dire need of characterization.</div>
<div>Two simultaneous high-speed recording channels granted cycle-resolved access to
the natural flame luminosity, which was recorded in red/green/blue and OH
chemiluminescence.</div>
<div>Selected conditions were investigated in more detail with the simultaneous
application of planar laser-induced fluorescence of OH and HCHO and recording
natural flame luminescence in a cycle-averaged manner.</div>
<div>Poly oxymethylene dimethyl ether was used as pilot fuel, building on prior
investigations. The mixture of 65 vol% Dimethyl Carbonate and 35 vol% Methyl
Formate with prior verification on a passenger-car-sized engine substitutes
synthetic natural gas in this study.</div>
<div>Thermodynamically, the increased compression ratio up to 17.6 resulted in
feasible operation and increased indicated efficiency. On the lower compression
ratio of 15.48, a more comprehensive range of applicable air–fuel equivalence
ratios and increased degrees of freedom regarding the pilot’s total energy share
are observed compared to the base configuration with natural gas and EN590 as
pilot fuel.</div>
<div>The air–fuel equivalence ratio sweep from λ = 1.0–2.0 revealed predominantly
premixed and high-temperature heat release via OH*. The temporal and spatial
evolution shifts while leaning out the mixture with increasing gradients on the
radial distribution and decouples for lean mixtures from the initial spray
trajectory.</div>
