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The classical small two-stroke engine is mechanically simple with a high power-to-weight ratio. But direct fuel losses are high and therefore induce a high fuel consumption and an unacceptable pollution level. We have developed a simple device (named Delayed-Charging) which is able to largely reduce fuel losses by almost separating the scavenging and fuel-charging functions and, consequently, retaining the major part of delivered fuel in the cylinder. Recent improvements, by use of an air-assisted injection combined with an air curtain to deviate the fuel spray trajectory throughout the cylinder, lead to a further reduction of hydrocarbon emission level from 4% (FIDHC methane equivalent) in a standard carbureted engine down to a 1% level associated to a fuel consumption reduction within a larger speed range in our newly modified 50 cc engine.

The classical two-stroke engine is well known for its mechanical simplicity, its high power-to-weight ratio but also, unfortunately, for its poor fuel utilization effectiveness due to large losses of fuel during scavenging leading to a high specific consumption and a high hydrocarbon pollution rate. A very simple system, called Delayed-Charging (D.-C.), was proposed in earlier papers, improving fuel utilization by separating the scavenging and charging functions, the latter being retarded. In this paper, comparisons are made between a 50 cc production engine and D.-C. modified 50 cc engines equipped with a pressurized carburetter or a low-pressure injector feeding the air flow into the charging-transfer duct (i.e. various engines including parameter changes such as injector location, fuel jet angle, jet direction, injection closing angle, D.C.-transfer duct geometry, number of scavenging transfer ducts,…).

To reduce fuel losses during the gas-exchange-process in a two-stroke engine cylinder, a simple Delayed-Charging Device was developed to first scavenge burnt gases with fresh air and then introduce a fresh charge.