Limits on Charge Dilution, Fuel and Air Proportions for Stable Combustion in Spark Ignition Engines 2004-01-1533

Experimental studies have been carried out on homogeneous charge, spark ignition engines running on European pump-grade, 95 RON gasoline. The aim was to determine the limits on dilution and mixture proportions imposed by a deterioration in engine cycle-by-cycle combustion stability. The stable limit was taken as corresponding to a COV_imep of 10%. Residual gas fraction was raised through engine load and valve timing changes, and determined by cylinder gas sampling. The results define a bounded region of stable operation on two diagrams, with axes of air/burned gas and fuel/burned gas for the first and burned gas fraction and air/fuel ratio for the second. For stable operation, gas/fuel ratio is limited to around 25:1 and is the constraint on upper limits of dilution and air/fuel ratio. For fuel-rich mixture operation, the limiting value of air/fuel ratio rises from around 9:1 to stoichiometric as burned gas fraction increases. The limiting value of burned gas fraction was less than or around 0.3.