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The potential of direct injection in spark-ignition engines regarding the significant reduction of fuel consumption and pollutant emission as well as advantages in the field of torque characteristic and acceleration behavior induced a remarkable development of GDI engines for automobile applications. The mentioned advantages are of special interest for motorcycle four-stroke engines as well. However, in compact cylinder units, respectively in a wide speed range as for motorcycle engines, the adaptation of direct injection gains on complexity. The paper presents the development results of a GDI four-stroke four-valve single cylinder motorcycle engine with 125 cc. The pressure pulse direct injection system developed for this application is presented from the fluid-dynamic behavior up to the obtained injection spray characteristics.

The main development targets for future motorcycle and small vehicle propulsion units are the reduction of dimensions, weight, fuel consumption and pollutant emission for a considered power output. The paper presents a concept for the improvement of the thermodynamic process stages consisting on scavenging, mixture formation and combustion - as a main support for achieving the mentioned targets. The concept is exemplified by results in terms of compared indicator diagrams, specific fuel consumption and exhaust gas emissions - in base of numerical simulation and experimental analysis at the engine - respectively at the vehicle test bench.