Investigation and Optimization of Cam Actuation of an Over-Expanded Atkinson Cycle Spark-Ignited Engine 2019-01-0250

An over-expanded spark ignited engine was investigated in this work via engine simulation with a design constrained, mechanically actuated Atkinson cycle mechanism. A conventional 4-stroke spark-ignited turbo-charged engine with a compression ratio of 9.2 and peak brake mean effective pressure of 22 bar was selected for the baseline engine. With geometry and design constraints including bore, stroke, compression ratio, clearance volume at top dead center (TDC) firing, and packaging, one over-expanded engine mechanism with over expansion ratio (OER) of 1.5 was designed. Starting with a validated 1D engine simulation model which included calibration of the in-cylinder heat transfer model and SI turbulent combustion model, investigations of the Atkinson engine including cam optimization was studied. The engine simulation study included the effects of offset of piston TDC locations as well as different durations of the 4-strokes due to the mechanism design. Incremental effects of adjusted combustion phasing, scaled valve durations, to a fully optimized cam duration and phasing are determined, and the impacts of each discussed. A constant speed load sweep was conducted to compare the net indicated fuel conversion efficiency difference between baseline and Atkinson cycle engine. Besides, two speed load conditions (1300rpm, 3.3bar IMEPnet and 1750rpm and 10.3bar IMEPnet) with valve timing optimization were also investigated.

Results from the study indicate that with an increase in the load from 4bar to 12bar IMEPnet at 2500rpm, over expansion contributed to an increase of 2.3% in net indicated efficiency. Furthermore, with valve optimization, negative work was avoided in the Atkinson cycle engine at 3.3bar IMEPnet, 1300rpm with an increase of 2% in the net indicated fuel conversion efficiency. At 10.3bar IMEPnet and 1750rpm, net indicated fuel conversion efficiency increased by 4.6%.