Near-wall PLIF Imaging of Formaldehyde in an HCCI Engine

This project studied the characteristics of Homogeneous-Charge Compression Ignition (HCCI) combustion near the combustion chamber surfaces using Planar Laser- Induced Fluorescence (PLIF) of the intermediate combustion species formaldehyde. Motivation for this study is a more complete understanding of the thermochemistry associated with HCCI combustion in the thermal boundary layer near the chamber surfaces. Current multi-zone models are based on a mass distribution defined by predictions of the thermal boundary layers. The engine used for this study was a single-cylinder optically accessible research engine, operating with a compression ratio of 9.4:1, at 600 RPM, with equivalence ratios ranging from 0.06 to 0.26 and an intake temperature of 90�C. Formaldehyde fluorescence was excited by 355 nm light from a pulsed Nd:YAG laser using an innovative through-the-wall approach to avoid vignetting effects. Images were acquired with an intensified CCD camera and post-processing was performed to remove background signal interference. This technique allowed spatial resolution both across the field of view and into the combustion chamber with a resolution of 0.5 mm at the wall. The data indicate the absence of a strong global thermal stratification effect near the chamber surfaces, with no preferred location for formaldehyde formation or consumption. In all cases, images show a small thermal boundary layer, counter to model predictions. Trends in formaldehyde concentration as a function of crank timing and equivalence ratio were consistent with the literature