Knock is an undesirable combustion mode occurring in diesel engines due to longer delay period that decreases the engine performance and life. In diesel engine, knock is mainly due to the improper fuel burning or missing caused by the injection system problems. Engine miss causes rapid combustion with very high pressures generating a rumble or dull clattering sound with violent vibration called “knocking or detonation”. The method introduced in the present study using alternate fuel (biodiesel-triacetin additive) in the engine suggests a newly developed approach towards analyzing the cylinder vibration of diesel engine. This method is based on fundamental relationship between the engine vibrations pattern and relative characteristics of the combustion process in the cylinder. Vibrations generated by the engine during knock are measured by using DC-11 Fast Fourier Transform (FFT) analyzer with accelerometer. The FFT output at each load of the cylinder excitation frequencies is obtained and compared with the frequencies of diesel fuel as base line. Time waveforms on the cylinder head and derived heat release rate curves are used to analyze the modus operandi of how combustion generates the vibrations and finally to detect the engine knock.

Knock is a primary factor that limits the design of Internal Combustion (IC) engines and depends on engine design characteristics, operating conditions and fuel properties. Knocking in diesel engine always leads to severe increase in pressure pulses inside the combustion chamber, resulting in metallic noise which causes irreversible damage to the engine components and structure. In cylinder, pressure measurement is one of the techniques to detect knock and its intensity. Statistical and cross-correlation analysis between cylinders and cylinder pressure development shows that knock occurrence and intensity are random and depend solely on the conditions of firing cycle (Leppard, 1982). Cylinder imbalance was detected in medium-speed six cylinders operating diesel engine using combustion pressure variation and correlating the same with engine harmonic torque and speed by harmonic order (Gawande
et al., 2010). Knocking is analyzed in the converted diesel engine into Spark Ignition (SI) engine and fuelled natural gas with a bowl piston head. It was adapted to SI by reducing the compression ratio and fixing spark plug in the injector location. The cyclic integral modulus of pressure oscillations in the SI engine occurring under knock conditions was analyzed by Brecq and Le (2005).

Mechanical Engineering Journal, Engine vibration, Combustion knock, Fast Fourier Transform (FFT) curves, Time waveform, Triacetin, Additive.