Knocking and Combustion Noise Analysis

Abstract

Knock is an abnormal and stochastic combustion phenomenon which needs detailed analysis because it governs the power density of engine, fuel consumption (engine efficiency), and engine durability, as well as noise and emission characteristics. Typically, compression ratio of spark ignition (SI) engine is limited by knock characteristics or knock propensity. This chapter discusses the knock fundamentals including modes of knock, onset of knock, characteristic knock frequencies, and super-knock. The super-knock is an extremely intense knock phenomenon which limits the engine turbocharging and downsizing proposed for improving the fuel conversion efficiency in SI engine. Accurate and repeatable measurement of engine knock is an important aspect of knock analysis and control. In-cylinder pressure-based techniques are considered as the most reliable method for knock detection; however, installation of pressure transducers in the combustion chamber is both difficult and expensive. This chapter presents the detailed cylinder pressure-based knock detection and analysis methods. Cylinder pressure- and heat release-based knock intensity indices (in time and frequency domain) along with their signal processing methods are discussed. Different methods of knock characterization/detection including statistical methods, stochastic method, and wavelets are also discussed. To fulfill the requirement of a low-cost and nonintrusive alternative method, knock detection using ion current sensors, engine vibrations, and microphones is used. Reduction of combustion noise is required as the part of engine development process due to customer demands. The multiple degrees of freedom in engine control and calibration provides more scope to influence combustion noise, which is required to be measured first, to control effectively. This chapter presents the discussion on the combustion noise estimation from the in-cylinder pressure measurements. Different combustion noise metrics are discussed along with their calculation algorithms and signal processing techniques.