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The ignition phenomena of stoichiometric methanol/oxygen/argon mixture are comprehensively investigated at p = 12–24 bar, T = 840–1000 K, using a rapid compression machine (RCM). A strong tendency of stochastic ignition followed by non-forcible flame propagation at a speed of ∼11 m/s is demonstrated. Such an event may be responsible for the pre-ignition or super-knock issue in methanol engines. Under engine relevant spark ignition conditions, different ignition regimes are observed in the end-gas, including thermal explosion, supersonic deflagration, and detonation, characterized by the Chapman–Jouguet velocity criterion. All modes originate from a similar, early auto-ignition ahead of the spark-triggered flame. The auto-ignition process is proved to be dominated by chemical kinetics, where the Livengood–Wu correlation is applicable. In addition, the transition mechanism of different ignition regimes is thoroughly validated against previous ignition theories. Basically, the detonation onset is closely related to the initial thermodynamic condition of the mixture. At a given temperature, the decrease of pressure induces the gradual substitution of detonation by supersonic deflagration and thermal explosion due to a smaller reactivity gradient in the end-gas. The ε − ξ diagram proposed by Bradley is adopted to interpret the transition mechanism of methanol, which turns out to be different from previous results of isooctane. A moderate burned mass fraction range of 0.35–0.45 is found when detonation is initiated. [ABSTRACT FROM AUTHOR]

Spark ignition induced super-knock was generated in a rapid compression machine using stoichiometric iso-octane/O 2 /N 2 mixture. In addition to the pressure traces, the combustion processes were also recorded using high-speed photography, from which the characteristics of the end-gas auto-ignition were analyzed. The effect of negative temperature coefficient (NTC) on the end-gas auto-ignition was investigated using detailed and reduced reaction mechanisms. The end-gas auto-ignition of diluted methanol/O 2 /Ar mixture without NTC behavior was also tested for comparison. The results showed that the end-gas auto-ignition of iso-octane/O 2 /N 2 mixture exhibited a two-stage ignition process. During the second ignition stage of the end-gas, two auto-ignition events with very short time interval were sequentially observed in the end-gas region. The first auto-ignition event generated a weak shock wave, and the second one initiated detonation. Both the two auto-ignition events occurred near the wall but at different sites. Due to the heat loss to the wall, the near-wall region is generally considered to be colder than the adiabatic core region, and thus the near-wall auto-ignition of end-gas was usually considered as a result of fuel's NTC behavior. However, in this study the chemical kinetic calculation showed that the evolution of the end-gas almost bypassed the NTC region in the ignition delay-temperature diagram. Furthermore, for the methanol/O 2 /Ar mixture the end-gas auto-ignition also started at the very near-wall region. Considering that methanol fuel does not has an NTC behavior, the near-wall auto-ignition of methanol/O 2 /Ar mixture should be a result of other factors than NTC. Therefore, it was concluded that NTC might not play a dominant role in the near-wall auto-ignition of the end-gas. [ABSTRACT FROM AUTHOR]

Super-knock is the main obstacle to improve power density and engine efficiency of modern gasoline engines. To understand the mechanism of super-knock, this study presents an investigation on the end gas combustion process of stoichiometric isooctane/oxygen/nitrogen mixture using a rapid compression machine (RCM), under the thermodynamic conditions close to those of production engines. The combustion process was captured by simultaneous high speed direct photography and pressure acquisition in the RCM. Three end gas combustion modes: no-auto-ignition, sequential auto-ignition, and detonation under different initial conditions were identified and characterized. The super-knock in engine was confirmed to be the result of detonation by comparing the pressure oscillation, thermodynamic state, and pressure rise relative to isochoric combustion with those of detonation observed in the RCM. The experimental results also indicate that the possibility of detonation occurrence increases with increasing initial pressure under the same compression ratio. However, comparing to the pressure, temperature has less effect on detonation formation. It was found that the end gas combustion mode is closely related to the mixture energy density. Generally, as the mixture energy density increases, the end gas combustion mode gradually transits from no-auto-ignition to sequential auto-ignition, and then to detonation. The first auto-ignition spots commonly appear in the mixture near the cylinder wall. The detonation was initiated by near-wall auto-ignition. [ABSTRACT FROM AUTHOR]

High boost and direct injection are the main tendency of gasoline engine technology. However, pre-ignition/super-knock tends to occur at low-speed high-load conditions, which is the main obstacle for improving power density and fuel economy. This work distinguished the relationship between super-knock and pre-ignition by experimental investigation and numerical simulation. The experiment was conducted on a turbocharged gasoline direct injection engine with compression ratio of 10. The engine was operated at an engine speed of 1750 r/min and the brake mean effective pressure of 2.0 MPa under stoichiometric conditions. Super-knock is the severe engine knock triggered by pre-ignition. Pre-ignition may lead to super-knock, heavy-knock, slight-knock, and non-knock. Significantly advancing spark timing can only simulate pre-ignition, not super-knock. Although knock intensity tends to increase with earlier pre-ignition timing, higher unburned mixture fraction at start of knock, and higher temperature and pressure of the unburned mixture at start of knock, knock intensity cannot be simply correlated to any of the parameters above. A one-dimensional model is set up to numerically simulate the possible combustion process of the end-gas after pre-ignition. Two distinct end-gas combustion modes are identified depending on the pressure and temperature of the mixture: deflagration and detonation. Hot-spot in the mixture at typical near top dead center pressure and temperature condition can only induce deflagration. Hot-spot in the unburned end-gas mixture at temperature and pressure conditions above ’’deto-curve’’ may induce detonation. The mechanism of deto-knock may be described as hot-spot-triggered pre-ignition followed by hotspot- induced deflagration to detonation. [ABSTRACT FROM PUBLISHER]

Super-knock has been the main obstacle to improve power density and engine efficiency of modern highly turbocharged gasoline engines. Previous researches show that pre-ignition is the inducement of super-knock, while detonation is the root reason of how super-knock could damage engines dramatically. Lots of studies have been conducted to eliminate pre-ignition for suppressing super-knock indirectly. This work applies the fuel of methane to suppress detonation and then to suppress super-knock directly using a rapid compression machine (RCM). Furthermore, 1-D simulation model was set up to investigate the mechanism why methane could suppress detonation and super-knock. Finally, through single cylinder engine tests, this suppression strategy in engine practical usage was validated. The experiment and simulation results show that replacing the fuel from iso-octane or gasoline to methane while keeping other conditions identically could transfer detonation combustion mode to flame propagation. The peak pressure could be reduced dramatically and there is no pressure oscillation. Therefore, methane mixture could suppress detonation and then to suppress super-knock effectively, even if pre-ignition already exists. It could be an effective and practical control strategy to protect modern highly turbocharged engines. • First study concerning using methane to eliminate detonation for suppressing super-knock directly. • Replacing the fuel from iso-octane to methane could transfer the combustion mode from detonation to flame propagation. • 1-D direct simulation, RCM and engine experiments show that methane is an effective super-knock suppression strategy. [ABSTRACT FROM AUTHOR]

Abstract Super-knock is the main obstacle to improve power density and fuel efficiency of highly boosted gasoline engines. Previous investigations show that pre-ignition and detonation are the two key combustion processes of super-knock. The former is the inducement and the latter is the root reason of how super-knock could damage engines dramatically. Lots of studies have been conducted for suppressing super-knock through eliminating pre-ignition. Using a rapid compression machine, this study explores the stoichiometric propane or methanol mixture to suppress super-knock by eliminating detonation. Under the same pressure at the end of compression, the same fuel energy density and the same charged fresh air, the peak pressure of propane mixture could be reduced dramatically and the pressure oscillation could be eliminated, when compared to iso-octane mixture. The combustion could be transferred from detonation to flame propagation. For methanol mixture, the combustion process could be transferred from detonation to weak auto-ignition with low peak pressure and negligible pressure oscillation. These indicate that both propane and methanol mixture could suppress detonation and thus suppress super-knock effectively, even if pre-ignition occurs. It could be an effective and practical control strategy to protect modern highly turbocharged spark ignition engines. Highlights • First study using propane or methanol mixture to eliminate detonation for suppressing super-knock directly. • Using propane mixture could transfer the combustion mode from detonation to flame propagation. • Using methanol mixture could transfer the combustion mode from detonation to weak sequential auto-ignition. • Eliminating detonation with the propane or methanol mixture is conducted by preventing the generation of shock wave. [ABSTRACT FROM AUTHOR]