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The goal of this work is to begin to understand and characterize the break-up of liquid fuel as it is torn from intake valve and port surfaces during the start-up period of a spark ignition engine. The lack of vaporization from warm engine surfaces causes the fuel to enter the combustion chamber as large droplets. Atomization results from the shearing effect of the intake air as it is pulled into the combustion chamber. Droplet sizes, air velocities, and break-up formations are studied using a high-resolution CCD camera and strobe. Indolene and iso-octane fuels are used to consider the effect of fuel properties on the break-up. The atomization processes that occur are characterized through the use of dimensionless groups. Results show that the fuel break-up follows the same processes seen for many other atomizing devices under the influence of co-flowing air. The role of valve gap, liquid fuel flowrate, air flowrate, and valve dimensions on the break-up process are discussed.