Cycle-To-Cycle Effects and Knock Prediction using Spark Induced Disturbances on a PFI Methanol HD SI Engine

Stoichiometric operation of a Port Fueled Injection (PFI) Spark-Ignited (SI) engine with a three-way catalytic converter offers excellent CO₂ reduction when run on renewable fuel. The main drawbacks with stoichiometric operation are the increased knock propensity, high exhaust temperature and reduced efficiency. Knock is typically mitigated with a reactive knock controller, with retarded ignition timing whenever knock is detected and the timing then slowly advanced until knock is detected again. This will cause some cycles to operate with non-ideal ignition timing. The current work evaluates the possibility to predict knock using the measured and modelled temperatures at Inlet Valve Closing (IVC) and Top Dead Center (TDC). Feedback effects are studied beyond steady state operation by using induced ignition timing disturbances. The approach is based on a deterministic controller where the timing is advanced beyond steady state knock limited operation or vastly retarded to produce warmer residuals in the following cycle. The results indicate that for the current engine there is no feedback effect. Chemical kinetics explains the lack of feedback due to lack of reactivity at TDC conditions. The chemical kinetic study in conjunction with the established auto ignition models described by Livengood-Wu reveals that the charge mixture entered a region of reactivity around the 50% burned point. It was also found that knocking and non-knocking cycles can have overlapping thermodynamic trajectories but for knocking cycles there is less dispersion. The study uses a solver which corrects the IVC temperature to minimize the error between observed knock onset and the point where the Livengood-Wu expression reaches unity for a knocking cycle. The corrections were found to have a correlation to uncaptured evaporation effects. Combined experimental and modelling results were in line with previous findings, namely that cycle-to-cycle combustion variations are plausibly explained by early flame propagation.