The propulsion of future motorcycles and small vehicles will be determined by the reduction of dimensions, weight, fuel consumption and pollutant emission for a considered power output, implicating an improved control of the internal process stages consisting on scavenging, mixture formation and combustion.
A main support of such process improvement is the internal mixture formation by gasoline direct injection. However, the compactness and the high-speed range of a small engine for two-wheelers, marine or garden equipment make the application of direct injection more difficult than for automotive engines.
On the other hand, after the initially tested wall- and air- guided techniques, it is generally recognized that the only way for a successful large scale utilization of gasoline direct injection is the spray-guided mixture formation. The paper presents a new spray-guided direct injection concept, successfully applied on a single- cylinder four- stroke four- valves motorcycle engine with a swept volume of 125 cm2, which does not require any modification of the usual combustion chamber design. A fuel coat with air core is formed in the vicinity of the spark plug by using two micro- injectors, actuated by a fuel injection system with high- pressure modulation. This allows the control of the fuel distribution within the air at every load and speed in combination with sufficient droplet atomization, but avoiding droplet impacts on chamber walls.