Knock onset, knock intensity, superknock and preignition in spark ignition engines

Abstract

Knock is an abnormal and stochastic combustion phenomenon which limits the efficiency of spark ignition engines. It occurs because of autoignition initiated locally in hot spots in the fuel/air mixture ahead of the advancing flame front. The onset of knock is governed by chemical kinetics and is determined by the pressure and temperature history of the hot spot and the anti-knock quality of the fuel. Knock intensity is governed by the evolution of the pressure wave set off by the initial autoignition. As engine designers seek higher efficiency through downsizing and turbocharging, occasional extremely intense knock, 'superknock', is found to occur, even though operating conditions are chosen to avoid knock. Superknock is a manifestation of developing detonation which can be triggered by autoignition of the fuel/air mixture at high pressures and temperatures. Another stochastic phenomenon, preignition, is necessary but not sufficient to enable the pressures and temperatures that could cause superknock. These phenomena are discussed in this review.

Experimental analysis of super-knock occurrence based on a spark ignition engine with high compression ratio

“…Different from previous studies and known knowledge [1,13], the present work gives the distinct conclusion with respect with the cause of super-knock occurrence. In addition, the relationship between knock intensity and knock onset was investigated.…”

“…However, high loads in a high boost engine could also induce severe thermodynamic conditions in the cylinder, which could [1,12] cause extremely high-intensity knock events. These knock events are generally described as "super-knock" or "mega-knock" [1,[12][13][14]. Super-knock can lead to very high peak pressure (~30 MPa) and pressure oscillation (~10 MPa), which can significantly damage the cylinder or piston.…”

“…Significant advances have been made over the past years in understanding the mechanism of super-knock occurrences by different simulations and experiments [1,2,12,13,[15][16][17][18][19][20]. The super-knock phenomenon is significantly different from conventional knock.…”

“…In addition, Wang et al [23] proposed that the mechanism of super-knock is constituted by hotspot-induced deflagration to detonation followed by high-pressure oscillation (DDP). In recently comprehensive reviews by Wang et al [1,13] and Kalghatgi [1,13], the relationship between super-knock and pre-ignition was defined. Their research showed that pre-ignition may lead to super-knock, heavy-knock and normal combustion (non-knock).…”

Probabilistic Approach to Predict Abnormal Combustion in Spark Ignition Engines

“…Engines are operated at boosted intake pressure to account for the power loss due to downsizing of the engine [5]. Pre-ignition refers to a premature ignition of the air/fuel mixture before spark firing and through an unknown source, and the subsequent event that is characterized by high pressure peaks and oscillations is called superknock [6]. Such an event can severely damage the moving components of the engine such as valves, piston and piston rings resulting in loss of the engine [7].…”