Investigating Unburned Hydrocarbon (UHC) Emissions in a GDI Engine (Homogeneous and Stratified Modes) Using Formaldehyde LIF and Fast-FID Measurements in the Exhaust Port

An experimental method was set up in a single cylinder optical engine in GDI configuration to study UHC origins in both homogeneous and stratified operation. On the one hand, UHC were observed in the combustion chamber by formaldehyde LIF (excitation at 355 nm and collection from 400 to 470 nm). Formaldehyde is a natural UHC tracer since it results from partly decomposed fuel that has not been fully oxidized during combustion. A quartz cylinder liner was used in order to benefit from a large optical access. On the other hand, UHC emissions measurement were simultaneously performed with a fast FID analyzer whose sampling probe was located 2 cm downstream the exhaust valve, in one of the two exhaust ports. An instantaneous exhaust mass flow model was also developed using 0-D simulation. The computed mass flow rate was coupled to fast FID measurements to estimate instantaneous and cycle to cycle UHC emissions. Temporal form of the fast FID signals was correlated to the UHC visualizations and was found to provide valuable information to investigate UHC sources. In case of stratified combustion, UHC were located in the whole volume of in-cylinder gases, with significant local heterogeneities, UHC were mostly attributed to flame quenching in over mixed (too lean) regions and partial oxidation in under mixed (too rich) zones. In homogeneous mode, UHC were rather observed close to the walls, with very low UHC concentration in the in-cylinder bulk gases. UHC appeared to be mostly due to flame quenching along the walls and in crevices.