The present disclosure provides an improved system and method of operating a fuel injector of an engine to provide at least two different types of fuel spray in a combustion chamber of the engine by application of ultrasonic pulses to fuel in the fuel injector during an injection event. A first type of spray includes larger droplets that reduce the effective diffusion combustion area around the droplets and a second type of spray includes relatively small droplets that increase the effective diffusion combustion area around the droplets.