Thermodynamic Advantages of Low Temperature Combustion (LTC) Engines Using Low Heat Rejection (LHR) Concepts
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Low temperature combustion (LTC) modes for reciprocating engines have been demonstrated with relatively high thermal efficiencies. These new combustion modes involve various combinations of stratification, lean mixtures, high levels of exhaust gas recirculation (EGR), multiple injections, variable valve timings, two fuels, and other such features. LTC engines may be attractive in combination with low heat rejection (LHR) engine concepts. The current work is aimed at evaluating the thermodynamic advantages of such a LTC-LHR engine. A thermodynamic cycle simulation was used to evaluate the effect of cylinder wall temperature on the engine performance, emissions and second law characteristics. An automotive engine at 2000 rpm with a bmep of 900 kPa was considered. Both a conventional and a LTC design were compared. The LTC engine realized small gains in efficiency whereas the conventional engine did not realize any significant gains as the cylinder wall temperature was increased. One of the major reasons for this result was the higher values for the ratio of specific heats for the LTC engine which results in a more effective conversion of thermal energy to work. For the cases examined here, the thermodynamic cycle simulation work shows a maximum potential net indicated thermal efficiency of about 50% and a brake thermal efficiency of about 43% for the case of no net heat transfer (LHR configuration). For increasing cylinder wall temperatures, the exergy destruction decreased for both engines. The nitric oxide emissions increased for the conventional engine as the wall temperature increased, but the nitric oxide emissions remained near zero for all cases for the LTC engine. Copyright 2011 SAE International.
