08 Fuel metering for SI engine

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Fuel metering for SI engines The task of fuel metering is to control the mass and physical conditions of fuel, added to the new air that is induced in the cylinder at the beginning of each working cycle. An IC engine requires a certain air/fuel ratio to work properly at any operating condition. Air/fuel mixing in IC engines is influenced by: Injection system Fuel spray evolution Fuel properties Charge motions Introduction Spark-ignition engines use fuels which are sufficiently volatile to be easily vaporized and mixed with air, before combustion is started with a spark-plug (premixed charge). As a consequence, the task of fuel metering is mainly reduced to the control of the fuel mass, necessary to obtain an air fuel/ratio required by the engine in each point of its operating map. Spark-ignition engines use fuels which are sufficiently volatile to be easily vaporized and mixed with air, before combustion is started with a spark-plug (premixed charge). As a consequence, the task of fuel metering is mainly reduced to the control of the fuel mass, necessary to obtain an air fuel ratio α required by the engine in each point of its operating map. Mixture Requirements The optimum air/fuel ratio for SI engines depends on the engine performance which has to be optimized. At full load conditions, with a given filling coefficient: 1) The maximum power is obtained with a slightly rich mixture: φ= αstoich/α≈1.1 2) The maximum efficiency (or minimum specific fuel consumption) requires a lean mixture:φ= αstoich/α≈0.9 3) To reduce the pollutant emissions and to allow a correct operation of the three way catalyst, the engine should run with stoichiometric mixtures (φ= 1). In this way, conversion efficiencies for CO, NOx and HC are very high. For each operating regime, the best…