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Departures from optimum stoichiometry during transients (acceleration and deceleration) and cold start can lead to significant degradation in driveability and emissions control. Such departures occur as a result of a complex interplay between fuel transport mechanisms and the fuelling strategy. The relative contributions of several of these mechanisms are affected by fuel composition. To help understand these effects an open-path differential infra-red absorption technique has been used to monitor the transient evolution of the fuel vapour phase directly within the combustion chamber. The sensor projected a narrow infra-red beam which traversed the cylinder of an optical access engine along an open path under the head, and measured the path-integrated attenuation caused by absorption of the infra-red radiation by the fuel vapour. It operated in the near infrared (NIR) spectral region around 2.3 μm, an absorption band in hydrocarbon species containing methyl groups.

The evaporation of fuel droplets is an important factor in determining the quality of mixture preparation in lean-burn engines, particularly when using open-valve injection timing, which results in large in-cylinder droplet populations. Interferometric Laser Imaging for Droplet Sizing (ILIDS) has been used to measure the in-cylinder droplet size distribution and fuel volume in the droplet phase as a function of crank angle for a typical full boiling range gasoline and three single component fuels with different boiling points: iso-pentane, iso-octane, and xylene, representing the volatilities of the light end, mid-range and heavy ends of a typical gasoline. The measurements were made under firing conditions, with both open and closed valve injection timing, and at two different coolant temperatures, representing simulated cold start and fully warmed up conditions.

The transport of fuel droplets into the combustion chamber of an SI engine and their subsequent evaporation has been studied, using a new optical diagnostic technique, Interferometric Laser Imaging for Droplet Sizing (ILIDS), which allows temporally and spatially resolved measurements of droplet size distributions. The measurement technique and its application to in-cylinder engine measurements are described. Measurements were made under warmed-up conditions, with open valve injection timing, in a Ricardo Hydra single cylinder engine. The results showed differences in the evolution of the droplet size distribution in cylinder with variations in load and speed. At 1200 rpm under full load, droplets arrived quickly into the cylinder, and were small, the Sauter Mean Diameter (SMD) being in the region 10-12 μm on arrival, so that mixture preparation was good.