Ulf Aronsson

Analysis of Smokeless Spray Combustion in a Heavy-Duty Diesel Engine by Combined Simultaneous Optical Diagnostics

A heavy-duty diesel engine operating case producing no engine-out smoke was studied using combined simultaneous optical diagnostics. The case was close to a typical low-load modern diesel operating point without EGR. Parallels were drawn to the conceptual model by Dec and results from high-pressure combustion vessels. Optical results revealed that no soot was present in the upstream part of the jet cross-section. Soot was only observed in the recirculation zones close to the bowl perimeter. This indicated very slow soot formation and was explained by a significantly higher air entrainment rate than in Decs study. The local fuel-air equivalence ratio, Φ, at the lift-off length was estimated to be 40% of the value in Decs study. The lower Φ in the jet produced a different Φ-T history, explaining the soot results. The increased air entrainment rate was mainly due to smaller nozzle holes and increased TDC density. Furthermore, increased injection pressure was believed to reduce the residence time in the jet, thus reducing the soot formation. OH was detected at the periphery of the jet, upstream of the location where fuel started to react on the jet centerline. The OH region extended relatively far into the jet, further supporting the conclusion of a less fuel-rich jet in the current case. Partially oxidized fuel (POF) was found at the center of the jet, downstream of the lift-off position. This indicated that the temperature needed to start chemical reactions inside the jet had not been obtained at the lift-off position. The high-temperature reaction zone at the periphery thus added heat over a distance before POF was observed on the centerline.

UHC and CO Emissions Sources From a Light-Duty Diesel Engine Undergoing Late-Injection Low-Temperature Combustion

Low load carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions sources are examined in an optically accessible, light-duty diesel engine employing a late-injection, low-temperature combustion strategy. The study focus is to identify the cause of the rapid degradation in emissions and efficiency as injection timing is retarded. The in-cylinder progression of mixing and combustion processes is examined through ultraviolet planar laser-induced fluorescence (UV PLIF) imaging of hydrocarbon spatial distributions. Spectrally-resolved, deep-UV LIF measurements are also used to construct late-cycle spatial distributions of CO, C2 , and polycyclic aromatic hydrocarbons within the clearance volume. Engine-out emissions measurements and numerical results from both detailed chemistry homogeneous reactor and multidimensional simulations complement the measurements. The measured spatial distributions show that while most fuel accumulates on the bowl-pip during high-temperature heat-release, much of it is transported into the squish-volume by the reverse squish flow. Homogeneous reactor simulations further show that expansion cooling quenches reactions, preventing the transition to high-temperature heat-release for mixtures with an equivalence ratio below 0.6. Lean squish-volume mixtures, coupled with wall heat losses, severely inhibit squish volume fuel oxidation. Further retarding injection timing exacerbates quenching, resulting in a two-fold increase in UHC emissions and a 33% increase in CO, primarily from the squish-volume.Copyright