In view of the declining global oil reserves and the environmental concerns associated with automotive emissions, it is imperative to improve the fuel efficiency of engines. Using higher compression ratios or boosting the specific output through turbocharging are proven strategies to accomplish this goal. However, the ability to achieve elevated peak pressures required by either mechanism to be effective is limited by knock. The lack of understanding of knock also hinders the realization of potential benefits of homogeneous charge compression ignition, a promising technology that relies on controlled autoignition. Thus, knock is one of the most serious obstacles in the development of fuel efficient engines. For this reason, the phenomenon of knock has been studied extensively, but even after more than a century of mostly experimental research, the basic mechanism governing knock remains poorly understood.
In order to develop a fundamental understanding of engine knock, detailed chemical kinetic modeling of the hydrocarbon oxidation mechanism associated with the autoignition process is conducted in CHEMKIN (a chemical kinetics software). Based on the insight gained from kinetic modeling, some of the key reactions and species that are instrumental to the autoignition of hydrocarbons are identified. The sensitivity of knock to various parameters including inlet pressure, inlet temperature, compression ratio, wall temperature, fuel-air equivalence ratio, and exhaust gas recirculation (EGR) is examined through CHEMKIN simulations. Ignition delay predictions for the autoignition of a toluene reference fuel (TRF) blend with an antiknock index of 91 (TRF 91), obtained through extensive chemical kinetic modeling in CHEMKIN for a constant volume reactor, are used to develop an improved ignition delay correlation for predicting knock in spark ignition (SI) engines. In addition to NOx control, EGR is increasingly being utilized for managing combustion phasing in SI engines to mitigate knock. Therefore, along with other operating parameters, the effects of EGR on autoignition are incorporated into the correlation to address the need for predicting ignition delay in SI engines operating with EGR. The modeling approach adopted for TRF 91 is then extended to develop an ignition delay correlation for an oxygenated surrogate fuel blend of 87 octane gasoline (with 10% ethanol). In addition, a conceptually new approach based on multiple timescales is developed to predict ignition delay for the autoignition of a primary reference fuel blend. Finally, the new ignition delay correlation for TRF 91 is implemented into the engine simulation tool GT-POWER and engine dynamometer experiments with knocking combustion are conducted to validate the knock predictions from the correlation. Comparison of knock onset predictions from GT-POWER with engine experiments illustrates the accuracy of the TRF 91 ignition delay correlation. Hence, the contributions of the present study include an enhanced understanding of the underlying physics governing knock, development of improved ignition delay correlations, and better knock predictions from engine simulations through implementation of the TRF 91 correlation in GT-POWER.