Near-wall PLIF Imaging of Formaldehyde in an HCCI Engine
Abstract
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.