Inducing factors and frequency of combustion knock in hydrogen internal combustion engines

Hydrogen is a promising energy carrier, and the port fuel injection (PFI) is a fuel-flexible, durable, and relatively cheap method of energy conversion. However, combustion knock as an abnormal combustion phenomenon does not only limit the brake torque and thermal efficiency, but also breaks the piston or engines. This paper uses a four-stoke cycle, displacement of 2.0 L PFI hydrogen internal combustion engine and a calculated model to study the inducing factors and frequency of combustion knock. Results showed that combustion knock occurs at relatively higher engine speed (more than 3000 r/min) than the engine speed occurring knock of gasoline engine. The calculated average temperatures of air–fuel mixture at the end of combustion using thermodynamics dual zone model fall in the range of 1000–1100 K for hydrogen engines, which are higher than gasoline ones (about 200 K). Knock and the other abnormal combustion phenomena (backfire and pre-ignite) interact with each other. When the backfire generates, the components in the cylinder will be heated. In the next cycle, the components of the cylinder will release heat to the intake, which can increase the initial temperature at ignition. The high initial temperature will lead to the combustion knock. Otherwise, because of the combustion knock, the temperatures of cylinder components will increase, which generates hot spots and ultimately causes pre-ignite and backfire. Through the figures of Fast Fourier Transform (FFT) amplitude, the frequency of hydrogen engines is higher than gasoline ones for every kind of mode. The pressure waves of combustion knock spread with radial direction for light combustion knock and with circumferential direction for heavy combustion knock. These conclusions can be used to explore the working conditions close to combustion knock to achieve higher thermal efficiency and provide a guidance to detect the knock in hydrogen engine.