To fire a component event, the syntax is a bit different from an application event, and uses the name identifier to identify the event:

var compEvent = component.getEvent("sampleComponentEvent");
// Optional: set some data for the event (also known as event shape)
// A parameter's name must match the name attribute
// of one of the event's <aura:attribute> tags
// compEvent.setParams({"myParam" : myValue });
compEvent.fire();