LAUSR

Abstract Here, internal combustion engine operating speed effects on combustion cycle-to-cycle variations (CCV) are numerically investigated. The recent study by Ghaderi Masouleh et al. (2018) is extended to higher engine speeds including 560, 800 and 1000 RPM. The 3D scale-resolving simulations are carried out in a spark ignited simplified engine geometry under fuel lean condition. The numerical results include the following main findings. (1) Flow velocity and turbulence levels are noted to increase with RPM. (2) For a fixed spark timing, the combustion duration in CAD time increases with RPM contrasting the respective trend in physical time. (3) The link between early flow conditions around the spark position and the whole cycle combustion rate is demonstrated and explained for all the RPM for the investigated three example cycles. (4) On average, the moderate increase of turbulent flame speed with RPM is not able to compensate the reduced physical time for combustion. Hence, the higher RPM cycles burn typically slower in CAD time. (5) On average, the increased combustion duration in CAD time for higher RPM increases the CAD period, where the spark kernel is highly prone to local turbulence fluctuations. (6) A noted effect of RPM on CCV is the stretched combustion duration in CAD time so that the effect of the initial fluctuations can persist for a longer CAD period. (7) In the present model, the velocity magnitude near the spark largely explains cycle-to-cycle variations in the investigated low RPM range.