The Influence and Characterization of Transient Operation on Knock-Limited Spark-Ignition Engines

Paper #:
  • 2017-01-2234

  • 2017-10-08
Fundamental engine research is primarily conducted under steady state conditions, in order to better describe boundary conditions which influence the studied phenomena. However, light-duty automobiles are operated, and tested, under heavily transient conditions. This mismatch between studied conditions and in-use conditions is deemed acceptable due to the fundamental knowledge gained from steady-state experiments. Nonetheless, it may be of use to characterize the conditions encountered during transient operation and determine if the controlled phenomena are unduly influence by the differences between steady-state and transient operation, and further, whether transient behavior can be reasonably extrapolated from steady state behavior. This study investigates the effect of transient operation on Knock-Limited Combustion Phasing (KL-CA50) compared to steady-state operation. Three compositionally dissimilar matched Research Octane Number (RON) = 98 fuels are used in this study, allowing for the assessment of fuel-specific effects on differences between steady-state and transient operation. First, the chosen transient operation mode, in which 20 cycles are fired followed by 80 motored cycles (20/80), is characterized. Next, the effect of transient operation on KL-CA50 is assessed relative to steady-state measurements. Then, the KL-CA50 is measured over a range of intake pressure conditions, ranging from 80 kPa to 160 kPa. Finally, detailed analysis of the results is used to understand the effects of transient operation on end-gas autoignition, both relative to steady state operation, as well as across fuels. The results of this study indicate the significant effect that transient operation has on KL-CA50 behavior of a fuel. This is both universal, in that all fuels show responses to the differences in compression temperatures of the charge, as well as fuel specific, in that the fuels show varying effects based on the sensitivities to temperature, [O2], and trace species. All fuels showed a significant load extension under transient operation, based on tolerance of higher intake pressures. However, transient operation moved operating conditions to "beyond RON" (Octane Index K < 0) conditions, which favored higher-sensitivity fuels. Based on the analysis of system time constants (e.g. cylinder head temperature, exhaust gas temperature), it is expected that transient operation, and the benefits for knock-limited operation, are highly influential on drive-cycle performance.
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