The super-knock poses new challenges for further increasing the power density of spark ignition (SI) engines. The critical factors and mechanism connecting regarding the occurrence of super-knock are still unclear. Misfire is a common phenomenon in SI engines that the mixture in cylinder is not ignited normally, which is often caused by spark plug failure. However, the effect of misfire on engine combustion has not been paid enough attention to, particularly regarding connection to super-knock. The paper presents the results of experimental investigation into the relationship between super-knock and misfires at low speed and full load conditions. In this work, a boosted gasoline direct injection (GDI) engine with an exhaust manifold integrated in the cylinder head was employed. Four piezoelectric pressure transducers were used to acquire the data of a pressure trace in cylinder. The spark plugs of four cylinders were controlled manually, of which the ignition system could be cut off as demanded. In particular, a piezoelectric pressure transducer was installed at the exhaust pipe before the turbocharger to capture the pressure traces in the exhaust pipe. The results illustrated that misfires in one cylinder would cause super-knock in the other cylinders as well as the cylinder of itself. After one cylinder misfired, the unburned mixture would burn in the exhaust pipe to produce oscillating waves. The abnormal pressure fluctuation in the exhaust pipe was strongly correlated with the occurrence of super-knock. The sharper the pressure fluctuation, the greater the intensity of knock in the power cylinder. The cylinder whose exhaust valve overlapped with the exhaust valve of the misfired cylinder was prone to super-knock.

The opposed rotary piston (ORP) engine, distinguished by its exceptional power-to-weight ratio and uncomplicated design, serves as an optimal power system for Unmanned Aerial Vehicles (UAVs). Based on the three-dimensional simulation platform, the engine performance, combustion, and emission characteristics of the ORP engine at different speeds and ignition timings are clearly clarified. A larger angle of the spark plug position corresponds to a wider ignition timing range and higher power output. However, this increases the likelihood of engine knock. The optimal position of the spark plug is 18 deg before top dead center 2 (TDC2). As the ignition timing is advanced, both the pressure and temperature within the cylinder rise, and the crank angle associated with the peak values shifts nearer to TDC2. As the ignition timing shifts from −13.4 °CA to −22.8 °CA, the maximum in-cylinder pressure rises from 35.5 bar to 59.6 bar at 3000 r/min. The delayed ignition at a given ignition timing range accelerates flame formation due to a higher in-cylinder pressure at ignition. Advanced ignition can significantly enhance engine power and lower fuel consumption, substantially improving the endurance of UAVs. At 3000 r/min, the peak power, 36.3 kW, and minimal ISFC, 231.1 g/kWh, are achieved at an ignition timing of −22.8 °CA. Advanced ignition results in a wider flame propagation region, effectively avoiding incomplete combustion in the combustion chamber corners under high-speed engine conditions. The distribution of NO_x closely follows the high-temperature region, with more accumulation observed in the opposite direction of rotation. Advanced ignition contributes substantially to HC emission reduction in the combustion chamber.