Heat Transfer Characteristics of Conventional and High Efficiency IC Engines Using External or Internal Exhaust Gas Dilution

摘要

The use of exhaust gases as a diluent for IC engine mixtures has been shown to be advantageous for lower emissions for certain operating conditions. These exhaust gases may be introduced via an external circuit (perhaps including cooling), or they may be retained (internal) in the cylinder by the use of certain valve timings. Three (3) types of dilution processes were examined in this work: external EGR, negative valve overlap, and delayed exhaust valve closing. The first process is based on external dilution, and the last two processes are based on internal dilution. Thermodynamic evaluations for an automotive engine were completed for a constant load (bmep = 900 kPa) and constant speed (2000 rpm). The thermal efficiencies decreased for all three dilution processes in similar fashion as the burned gas fraction increased. This was shown to be a result of increasing heat losses for increasing burned gas fraction. The calculations showed that even though the gas temperatures decrease with the use of exhaust gas dilution, the cylinder heat transfer actually increases. This was shown for six different heat transfer correlations typically used for these calculations. This is a result of the dependences of the convective heat transfer on cylinder gas pressure and temperature. For the cases examined, cylinder pressures increased and cylinder temperatures decreased as the dilution level increases. In all cases examined, the net result was an increase of the convective heat transfer coefficient which dominated the decrease of temperatures.