High Efficiency by Miller Valve Timing and Stoichiometric Combustion for a Naturally Aspirated Single Cylinder Gas Engine

Abstract

Small-scale cogeneration units (Pel < 50 kW) frequently use lean mixture and late ignition timing to comply with current NOx emission limits. Future tightened NOx limits might still be met by means of increased dilution, though both indicated and brake efficiency drop due to further retarded combustion phasing and reduced brake power. As an alternative, when changing the combustion process from lean burn to stoichiometric, a three-way-catalyst allows for a significant reduction of NOx emissions. Combustion timing can be advanced, resulting in enhanced heat release and thus increased engine efficiency.Based on this approach, this work presents the development of a stoichiometric combustion process for a small naturally aspirated single cylinder gas engine (Pel = 5.5 kW) originally operated with lean mixture. To ensure low NOx emissions, a three-way-catalyst is used. In order to achieve high engine efficiency, measures implemented include Miller valve timing, optimized intake system, reduced engine speed and increased compression ratio. In the first step, a detailed 1D engine cycle simulation model was used to investigate the efficiency benefit of Miller valve timing and increased compression ratio. Within the numerical study, inlet valve closing timing and intake pipe length were varied, yet a closed-loop control was implemented to maintain a constant effective compression ratio of 14.66 by adjusting geometrical compression ratio for each configuration. Subsequently, the most expedient valve timing was designed using multi-body simulation of the inlet valve train, while increased compression ratio was achieved by modifying the series piston bowl geometry.Engine trials agree with simulation results and show highest efficiency for a Miller valve timing closing +15 °CA later to the series valve timing and geometrical compression ratio of 15.36. Compared to the series lean burn engine, indicated and brake efficiency increase by 3.2 %-points to 39.0 % and by 3.9 %-points to 34.4 %, respectively, while maintaining original brake power of Pe = 6.1 kW. Finally, an experimental study accompanied by 3D-CFD simulations was conducted to investigate the potential of optimized piston geometry to further increase efficiency. However, results reveal only minor effect of piston geometry on efficiency, what is likely stemming from interrelation of combustion efficiency, wall heat losses and heat release rate.