An exploratory study of knock intensity in a large-bore heavy-duty methanol engine

With current interest from the heavy-duty industry in high -octane renewable fuels like methanol, the currently used compression ignition engines will have to be adapted to dual -fuel (DF) type combustion systems or replaced by spark -ignition (SI) engines. Since both DF and SI engines are characterized by a premixed air-fuel mixture, there is a risk that part of the unburned mixture auto-ignites before the flame front can reach it, leading to engine knock. Accurate control of this abnormal combustion phenomenon is necessary and can be done by means of the in -cylinder pressure signal. The purpose of this work is to provide a framework for predicting and evaluating the intensity of knock in large -bore methanol fueled engines based on the knowledge of an automotive sized counterpart. At first, the band -pass filter, needed to focus on the knock-induced pressure oscillations, has to be greatly adjusted for engine size. Second, an appropriate knock metric and associate threshold value is needed. The performance of five conventional knock intensity metrics (MAPO, IMPO, AE, MVTD and HRR) was investigated at the 50 %, 75 %, and 87.5 % load points of a 256 mm bore methanol-diesel medium speed engine. With knock threshold values being derived from a small -bore (B = 82.55 mm), methanol fueled SI engine, the calculated knock intensities were well above the thresholds even for the non-knocking cycles. To counter this, a set of five normalized knock intensity metrics were tested as well. From these, the signal -to -noise ratio metric MAPOnorm showed the most promising results. The R2 values agreed with the findings of the conventional MAPO method, and, with a normalized knock threshold value of 8.07, it could distinguish knocking cycles in both the small bore and the large -bore engine, although additional engine-specific calibration would be recommended.