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End-gas auto-ignition leading to knocking combustion is one of the major barriers to achieving higher thermal efficiencies in downsized boosted spark-ignition engines. Despite the available framework addressing hotspot-induced ignition (detonation peninsula), a quantitative investigation on hotspot-induced auto-ignition of gasoline surrogates is yet to be done. In particular, the effect of negative temperature coefficient (NTC) chemistry on the distribution of the ignition modes in the detonation peninsula is still missing. Using the established one-dimensional (1D) theoretical and computational framework, the effect of average temperature (including NTC range), initial pressure, and ethanol addition are investigated. Moreover, appearance of NTC chemistry-related events i.e. coolspots, secondary ignition kernels, and off-centered ignition are analyzed using 1D simulations. The results are as follows. 1) NTC chemistry affects the distribution of ignition regimes in detonation peninsula and the dynamics of the front propagation via altering the reactivity gradient. 2) NTC chemistry increases the temperature gradient range associated with the detonation regime. 3) NTC may inhibit detonation development by simultaneously promoting the spontaneous/supersonic ignition modes. 4) An ethanol blend decreases the knock propensity; however, lower ignitability may promote detonation development and the appearance of strong shock waves. 5) Finally, detonation may result in a normal knock at lower initial pressures (20 bar). However, at elevated initial pressures (50 bar), detonation is noted to yield pressure intensities resembling super-knock. [ABSTRACT FROM AUTHOR]

Thermal stratification may switch the combustion mode from deflagration to spontaneous (auto-ignition) in spark-ignition engines leading to knock. Despite the available numerical and experimental works, an analytical and systematic method on the role of local thermal stratification on the combustion mode is still missing. Particularly, the effects of heat diffusion before ignition in negative temperature coefficient (NTC) chemistry of gasoline surrogates are typically ignored. In this study, an a-priori diagnostics tool is provided to separate the deflagration and auto-ignition combustion modes by considering the diffusion effects, based on two parameters: stratification wavelength ( λ) and amplitude ( δ). The diagnostics tool is an extension of Zeldovich's theory to transient problems by solving the diffusion equation and considering the flame and ignition timescales. It is found that 1) The theory is valid against one-dimensional numerical simulations under different average temperatures and pressures. 2) NTC chemistry promotes spontaneous ignition at both low and high pressures. 3) In the presence of NTC chemistry, the transition region between the two combustion modes is broadened. A third blended mode is observed (spontaneous ignition assisted flame) with front speeds ≈ 6 S l . 4) Finally, estimation of the knock propensity from the generated maps is related to the surrogates' octane sensitivities. [ABSTRACT FROM AUTHOR]

Several countries have set ambitious targets for the transport sector that mandate a gradual increase in advanced biofuel content in the coming years. The current work addresses this transition and indicates two promising gasoline bio-blendstocks: Anisole and isobutanol. The whole value chains of these bio-components were considered, focusing on end-use performance, but also analyzing feedstock and its conversion, well-to wheel (WTW) greenhouse gas (GHG) emissions and costs. Three alternative fuels, namely a ternary blend (15% anisole, 15% isobutanol, 70% fossil gasoline on an energy basis) and two binary blends (15% anisole with fossil gasoline and 30% isobutanol with fossil gasoline), were tested, focusing on their drop-in applicability in spark ignition (SI) engines. The formulated liquid fuels performed well and showed the potential to increase brake thermal efficiency (BTE) by 1.4% on average. Measured unburned hydrocarbons (HC) and carbon monoxide (CO) emissions were increased on average by 12–29% and 17–51%, respectively. However, HC and CO concentrations and exhaust temperatures were at acceptable levels for proper catalyst operation. The studied blends were estimated to bring 11–22% of WTW GHG emission reductions compared to base gasoline. Additionally, the fleet performance and benefits of flexi-fuel vehicles (FFV) were modeled for ternary blends. [ABSTRACT FROM AUTHOR]