Research on Knock Control Method of Turbocharged Gasoline Engine

Knock is an abnormal combustion in the engine cylinder, which reduces the engine power output and fuel economy, and worsens the engine emissions. In this research, the knock of turbocharged engine was divided into conventional knock and super knock. The knock judgment was mainly based on the knock signal collected by the knock sensor. The knock detection strategy mainly adopted CIC (Cascade Integrator Comb) filter, FIR (Fine Impulse Response Filter) filter, and offset compensation, which were used to process knock signals. Obtaining the characteristic quantity r_knock was used to judge whether knock occurs under the current working condition. The value of this characteristic quantity showed the current knock level. The result described that no knock misjudgment occurred under steady-state and transient conditions, and omission judgment rate was less than 5%. This meets engineering error requirements. The knock control was accurate and no limit torque and no super knock caused by abnormal identification, under extreme working conditions. In simulated preignition working points, preignition can be identified. Delaying Spark angle, fuel enrichment, temporary reduction of air, fuel cut-off and continuous gas reduction can effectively eliminate knock.

Nowadays, turbocharging direct injection technology has been widely used in engine design and production because it could provide very flexible stratified combustion, ensure combustion adequacy, improve power and fuel economy, and achieve fuel saving and emission reduction. However, the application of this technology also increased the probability of engine detonation. [1] Knock is an abnormal combustion in the engine cylinder. The combustible mixture in the combustion chamber ignited spontaneously or was ignited by other non-sparking plug heat sources. This caused suddenly non-long lasting vibration and abnormal noise of the engine. This abnormal combustion reduces fuel economy and engine power, and worsens the engine emissions. When abnormal combustion cannot be effectively controlled, knock will have adverse effects on engine hardware, how terrible, spark plugs, valves, pistons, etc. will be damaged. [2] When the engine works at the knock critical point or has slight knock, the engine can obtain better power and economy. Therefore, the spark angle of the engine should be controlled at the critical state of knock; However, in the actual work points, the knock critical state will change with the change of engine operating conditions and fuel quality. In order to make the engine work in the critical state of knock, it is necessary to control the knock. The knock level is monitored in real time by the knock sensor installed on the cylinder block [1‐3].

In this paper, knock was divided into conventional knock and super knock. The control methods of these two types of knock were studied. It was found that the way of using the decrease spark angle could effectively eliminate the conventional knock, but this way was no longer applicable to the super knock caused by preignition. When super knock occured, it could be treated by reducing air or enriching fuel. The result described that no knock misjudgment occurred under steady-state and transient conditions, and omission judgment rate was less than 5%. The knock control was accurate and no limit torque and no super knock caused by abnormal identification, under extreme working conditions. In simulated preignition working points, preignition can be identified.