Investigation and Optimization of CAM actuation of an Over-Expanded Atkinson Cycle Spark-Ignited Engine 2019-01-0250
An over-expanded engine was investigated via engine simulation in this work with a design constrained Atkinson mechanically actuated cycle mechanism. A conventional 4-stroke spark-ignited turbo-charged engine with a compression ratio of 9.2 and peak BMEP of 22 bar was selected as the baseline. With geometry and design constraints including bore, stroke, compression ratio, clearance volumes at TDC firing and gas exchange, and packaging, two over-expanded engine mechanisms with stroke ratios of 1.3 and 1.5 were designed. Starting with a validated 1D engine simulation model which included calibration of the in-cylinder heat transfer model, SI turbulent combustion model, and combustion knock model, several investigations of the two Atkinson variants including cam optimization were studied. This included investigation of the effects of offset piston TDC locations and different durations of the 4-strokes due to the mechanism. Incremental effects of adjusted combustion phasing, scaled valve durations, to a fully optimized cam duration and phasing are found, and the impacts of each discussed. Significant improvements in net indicated fuel conversion efficiency were found at medium to high loads including a 39.8% to 44.4% increase at 1750rpm 10.3 bar IMEPnet and from 32.6% to 38.5% at 1500rpm 16.0 bar IMEPnet. Further with valve optimization, light load over expansion was avoided and improvement of indicated fuel efficiency was still observed.