Numerical Investigations of Overexpanded Cycle and Exhaust Gas Recirculation for a Naturally Aspirated Lean Burn Engine 2013-32-9081
A large number of small size gas-fired cogeneration engines operate with homogenous lean air-fuel mixture. It allows for engine operation at high efficiency and low NOx emissions. As a result of the rising amount of installed cogeneration units, however, a tightening of the governmental emission limits regarding NOx is expected. While engine operation with further diluted mixture reduces NOx emissions, it also decreases engine efficiency. This leads to lower mean effective pressure, in particular for naturally aspirated engines. In order to improve the trade-off between engine efficiency, NOx emissions and mean effective pressure, numerical investigations of an alternative combustion process for a series small cogeneration engine were carried out.
In a first step, Miller and Atkinson cycles were implemented by advanced or retarded inlet valve closing timings, respectively. By keeping the effective compression ratio constant, the expansion ratio is increased, resulting in higher engine efficiency. The modified valve timing, however, leads to a reduced in-cylinder charge. Conventionally, this is compensated by the use of a turbocharger. In this work, however, power compensation for the naturally aspirated engine was realised by an exhaust gas recirculation (EGR) strategy. Due to its higher specific heat capacity, EGR allows for engine operation at lower mixture dilution but same absolute heat capacity of the in-cylinder charge. In simplified terms, this can be used to increase engine power, while maintaining low combustion temperature and NOx emissions.
Based on a 1D-CFD-model of the series single-cylinder natural gas engine, numerical studies of an engine configuration including Miller/Atkinson valve timings, EGR and homogeneous lean burn operation were conducted. For constant engine power and NOx emissions, brake specific fuel consumption (BSFC) was reduced by 1.1 %.