Simultaneous Formaldehyde and Fuel-Tracer LIF Imaging in a High-Speed Diesel Engine With Optically Accessible Realistic Combustion Chamber

Simultaneous laser-induced fluorescence (LIF) imaging of formaldehyde and a fuel-tracer have been performed in a high-speed diesel engine. N-heptane and isooctane were used as fuel and toluene was used as a tracer. This arrangement made it possible to make simultaneous measurements of toluene by exciting at 266 nm and detecting at 270-320 nm while exciting formaldehyde at 355 nm and detecting at 400-500 nm.

The aim of this study is to investigate how traditional fuel tracer and natural-occurring formaldehyde formed in the cool chemistry are transported in the piston bowl. A range of ignition delays were created by running the engine with different amounts of EGR. During this sweep the area where the low-temperature reactions take place were studied.

The measurements were performed in a 0.5-l, single-cylinder optical engine running under conditions simulating a cruise-point, i.e., about 2.2 bar imep. The ignition delay was elongated compared to the normal mapping and the engine-out emissions of soot and NOx were ultra-low.

It was found that the spatial location of LTR's does not shift significantly for different EGR levels. The formaldehyde signal overlaps the fuel signal in most cases before the onset of the main heat release.