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A fast-FID sampling technique has been developed to study top-land crevice out-gassing from the moving piston of an SI engine. A sampling probe, housed in the piston crown, delivered gas to the FID head via a flexible transfer tube. Comparisons of the HC concentrations at the top-land location and the bulk gas above the piston crown confirm that HC material is out-gassed from the top-land region during the expansion stroke and is followed by more rapid out-gassing after EVO. The removal of wall HCs has been detected during the exhaust stroke from the probable scrolling effect produced by the rising piston scraping unburned material from the cylinder wall.

The action of crevices in an operating SI engine has been studied with a fast-FID. A single-cylinder Ricardo E6 research engine was fuelled with propane and operated at 1300 RPM. FID measurements in the exhaust port have shown that advancing the ignition timing from 30°BTDC (MBT) to 60°BTDC raises the HC concentration by 25% during the first 120°CA of the exhaust stroke and by 20% for the remainder of the stroke. A static “artificial” crevice of known volume, mounted inside the engine cylinder was used to study the differing HC outgassing characteristics at the two ignition timings. When sampling in-cylinder at the mouth of this crevice, the opposite effect of a 50% reduction in outgas HC concentration occurred when the ignition was advanced to 60°BTDC. It is argued that advancing the ignition causes earlier enflamement of the static crevice and induces burned as well as unburned gas to enter the crevice thereby diluting the HCs from this source.