Simultaneous PLIF measurements for visualization of formaldehyde- and fuel-distributions in a di HCCI engine

Simultaneous laser induced fluorescence (LIF) imaging of formaldehyde and a fuel-tracer have been performed in a direct-injection HCCI engine. A mix of N-heptane and iso-octane was used as fuel and Toluene as fluorescent tracer. The experimental setup involves two pulsed Nd:YAG lasers and two ICCD cameras. Frequency quadrupled laser radiation at 266 nm from one of the Nd:YAG lasers was used for excitation of the fuel tracer. The resulting fluorescence was detected with one of the ICCD cameras in the spectral region 270-320 nm. The second laser system provided frequency tripled radiation at 355 nm for excitation of Formaldehyde. Detection in the range 395-500 nm was achieved with the second ICCD. The aim of the presented work is to investigate the applicability of utilizing formaldehyde as a naturally occurring fuel marker. Formaldehyde is formed in the low temperature reactions (LTR) prior to the main combustion and should thus be present were fuel is located until it is consumed. Measurements were performed when injecting fuel early and late in the compression stroke. Early injection timing results in a homogeneous charge at the time of auto-ignition, while late timing gives a more stratified charge. The crank angle position at which measurements were performed was altered to cover the entire combustion cycle. The measurement images show instantaneous distributions of toluene and formaldehyde respectively. Images from both early and late injection and at all crank angle degrees show good spatial resemblance between toluene signal area and formaldehyde signal area. The work presented in this paper show that formaldehyde is a feasible alternative to traditional fuel tracers for visualizing fuels featuring low temperature reactions in HCCI combustion.