The requirement of reducing HC emissions during cold start and improving transient performance has prompted a study of the fuel injection process. Port-fuel-injection with the Intake-valve open using small droplets is a potentially feasible option to achieve the goals. To gain a better understanding of the injection process, the effects of droplet size, injection timing, and coolant temperature on the total and speciated HC emissions were tested In a Single-cylinder engine. It was found that droplet size plays an important role in the total HC emission increase during open-valve injection, especially with cold operation. Large droplets (300 μm SMD) produced a substantial HC increase while small droplets (14 μm SMD) produced no observable increase. Increase In the total HC emissions was always accompanied by an increase in the heavy fuel components in the exhaust gases. Fuel loss during open-valve injection with cold operation was also observed when using large droplets, while no fuel loss was present with smaller droplets. The test results imply that cylinder-liner-wetting by large fuel droplets during open-valve injection is the main reason for the HC emission increase, the change in the HC species distribution in the exhaust gases, and the fuel losses. The effects of droplet size on wall-wetting were also studied through modeling using particle dynamics. The modeling results confirmed that large droplets can result in cylinder-liner-wetting, and small droplets can substantially avoid both Intake-port-wetting and cylinder-liner-wetting.