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A LPG fuelled, spark ignition engine for heavy duty vehicles has been developed from a Diesel DI engine which is currently in production. The development concept was based on the targets of obtaining output performances by LPG fuelling comparable with those of the original Diesel engine under full load conditions to be achieved with relatively simple technologies. A conventional mixer system with closed-loop control was consequently used for the LPG fuel supply system, operating at λ = 1.0. A systematic optimization was applied in the areas of the compression ratio, combustion chamber configuration, intake swirl ratio and plenum configuration, spark timing, in view of reaching high output performances and knock phenomena control. The results of optimization were higher torque and equal fuel conversion efficiency (at full load) in comparison with the original Diesel version of the engine.

Knock characteristics of commercial LPG, with 83…87% propane and 9…15% alkenes have been investigated in a spark ignition engine for heavy duty vehicles. While knock characteristics of propane have been already studied, it was considered that there is at present a lack of information concerning a possible synergistic interaction in the end gas of propane with the alkenes, the second important component of LPG. Different conclusions existing on some issues, like the correlation between knock intensity and knock onset time on an individual cycle basis, emphasizing our insufficiencies in information, were also suggestive for our study. At last, special problems resulting from the need to make best use of the recorded pressure data, like the best way to isolating the high frequency component, were also addressed.

Cycle-to-cycle combustion variations in a spark ignition engine, lean operated, were approached with symbolic sequence statistics and return maps. The engine was operated with a gaseous fuel (LPG) and with a strong swirl induction generated, to give relatively advanced mixture homogeneity. A transition to nonlinear deterministic behavior was identified when the equivalence ratio was decreased to very lean conditions. It was thus found that the effect of residual gas fraction on communication between successive combustion events is weaker with an improved mixing. The deterministic effects, which are not controllable, can thus be removed to lower equivalence ratios and higher levels of residuals by an improved mixing within the cylinder.