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Owing to its inherent high internal residual gas rate in partial load operation, the 2-stroke engine has been the first application to take benefit of the unconventional CAI™ (Controlled Auto-Ignition) combustion process. For a long time, the objective of the different research works on 2-stroke engines optimization was to eliminate its two main drawbacks leading to high emissions of unburned hydrocarbons and a poor fuel efficiency. The first one is the unstable running operation combined with incomplete combustion, especially at light load, The second one is fuel short circuit at medium and full load. From the end of seventies, an approach developed by Onishi from Nippon Clean Engine was to take benefit of an high amount of hot internal residual gases to help auto-ignition of the fresh charge. This solution has been further developed up to the industrialization on 2-stroke engines.

The IAPAC® Direct fuel Injection (DI) system, developed by IFP, has already well proven its capability to reduce pollutants emissions and fuel consumption of 2-stroke engines for both 2-wheeler and marine outboard application. This crankcase Compressed Air Assisted Fuel Injection process allowing the introduction of the fuel separately from the scavenging air, minimizes the fuel short-circuiting and has shown its potential on various prototype demonstrators. This paper presents the development and pre-industrialization work performed to apply this concept to an SELVA Marine 2-cylinder 50 HP outboard 2-stroke engine. A standard carbureted engine has been converted to a IAPAC® prototype engine by mainly modifying the cylinder head. Then, this prototype engine has been calibrated, tested and optimized on the dyno test bench to comply with future emissions regulation while keeping similar power output than the reference carbureted engine.