Heat transfer and fuel transport in the intake port of a spark ignition engine
Surface-mounted heat flux sensors have been used in the intake port of a fuel injected, spark ignition engine to investigate heat transfer between the surface, the gas flows through the port, and fuel deposited in surface films. This investigation has been carried out with a single-cylinder engine on which the cylinder head is from a production four valve per cylinder engine with a bifurcated intake port. The objective has been to establish how engine operating conditions affect trends in surface heat transfer rates both with and without fuel deposition on the surfaces, and to relate these to the mechanisms involved in the transport of fuel into the engine. The effects on these mechanisms of injector type and fuel characteristics have also been studied. Fuel transport has been characterised using the τ and X parameters, and experimental studies have been carried out to examine these for fully-warm and warm-up engine operating conditions, with a range of injector types representative of those currently used in service. This data has been compared to the results of a photographic study of the fuel distribution pattern produced by each injector type, and these combined results used to decide upon suitable positions within the inlet port for the heat flux sensors. The dynamic response characteristics of the surface-mounted heat flux sensors have been determined, and measured heat flux data corrected accordingly to account for these characteristics. Details of the model and data processing technique used, are described. Corrected intra-cycle variations of heat transfer to fuel deposited have been derived for engine operating conditions at 1000 RPM covering a range of manifold pressures, fuel supply rates, port surface temperatures, and fuel injection timings. Both pump-grade gasoline and isooctane fuel have been used. The influence on heat transfer rates of the deposited fuel and its subsequent behaviour has been examined by comparing fuel-wetted and dry-surface heat transfer measurements. With both fuel types, the heat transfer rate to the fuel reaches peak values up to around 50 kW/m2 during the engine cycle, and is typically 5 kW/m2 on average in regions of heavy fuel deposition. The effects of operating conditions on the magnitude and features of the heat flux variations are described Integration of this heat flux data has provided values of heat transfer per cycle, allowing direct comparisons of operating condition and injector type effects to be made. For dry-port conditions heat transfer per cycle varies between 0 and 300 J/m2 depending on location, towards the surface at low temperatures and away from the surface at fully-warm conditions. During warm-ups with fuel deposition, as coolant temperature increases from 0 to 90°C, values of heat transfer to the fuel typically increase from 300 J/m2 to 1000 J/m2. For a given coolant temperature, heat transfer values generally increase as manifold absolute pressure (MAP) is lowered or fuel flow rate increases. The effect of fuel deposition on heat transfer has been characterised by a function of MAP, fuel flow rate and coolant temperature. When running on isooctane fuel the heat transfer measurements were made using a heat flux gauge bonded to the intake port surface in the region where highest rates of fuel deposition occur. Heat transfer changes are consistent with trends predicted by convective mass transfer over much of the range of surface temperatures from 20°C to 100°C. Towards the upper temperature limit, heat transfer reaches a maximum limited by the rate and distribution of fuel deposition. The inferences drawn from the isooctane results are discussed and related to characteristics observed when gasoline is used.