1997-10-01

Heat Transfer and Mixture Vaporization in Intake Port of Spark-Ignition Engine 972983

Time-resolved heat flux and gas temperature measurements in the intake port of a spark ignition engine are presented. Experiments were pursued for motored, propane fired, and liquid fuel operation. Heat transfer coefficients were built from the dry data. Also, heat transfer rates in the port and off the back of the intake valve were integrated over the main flow phases. For a typical low-load propane-fired operating condition, heat transfer in the port caused a mean intake air temperature increase of approximately 10°C. The main different intake flow phases, induction or forward flow, displacement backflow, and valve overlap backflow, contributed approximately 10°C, 3°C, and negative 3°C, respectively. These mixture temperature changes are expected to be also applicable for liquid fuel injected cases.
While the heat flux instrumentation was primarily intended for dry operation of the engine, liquid fuel experiments were also pursued. Liquid fuel vaporization was assessed for isooctane and indolene fuels at thermal steady state and for engine warm-up transients. Spray arrival caused a strong heat flux. For closed valve injection, magnitude of arrival signal was consistent with static spray targeting. Open valve injection deflected the spray upwards and away from the floor of the port. Concurrently, spray impingement onto the valve increased.
For low wall temperatures, vaporization rates inferred from the heat flux measurements compared well to the mixture vaporization potential predicted by analogy to heat transfer. For high wall temperatures, the analogy between heat and mass transfer indicated that all injected fuel could vaporize within one cycle if the fuel film evenly covered the spray targeting area. However, this was neither consistent with the data nor with known engine behavior for injection rate transients, in which a time of several cycles is required for mixture preparation to adjust to changing fueling rates. It is thought that the fuel film does not spread out in a homogenous manner. Rather, discrete fuel droplets or rivulets built up until sufficient surface area allows vaporization at a rate equal to the fuel deposition.

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