Impact of Gasoline Octane Rating on Engine Knock using Detailed Chemistry and a Quasi-dimensional Stochastic Reactor Model

Fuel properties have a major impact on engine knock and therefore need to be considered in detail in spark ignition engine simulations. In our method, the auto-ignition and emissions are calculated based on a new reaction scheme for mixtures of iso-octane, n-heptane, toluene and ethanol (ETRF). Surrogates can be formulated with close agreement to the physical and chemical properties of the commercial gasoline. The engine simulations are carried out using a quasi-dimensional stochastic reactor model, which allows to study cycle-to-cycle variations efficiently. Effects of the surrogate composition on the knock tendency are evaluated using the detonation theory by Bradley et al. Introduction One of the main challenges in spark ignition (SI) engine development is the trade-off between thermal efficiency and the avoidance of engine knock. Knocking combustion results from auto-ignition of the unburned gases that releases chemical energy spontaneously. This released energy may lead to a pressure wave that hits the cylinder wall and causes the typical sharp metallic noise [1]. The auto-ignition and pressure wave can lead to thermal and mechanical damage of the engine. This irregular combustion phenomenon appears in direct consequence of too high temperatures and pressures in the end gas and cause auto-ignitions. The avoidance of these conditions limits the operation range and therefore the efficiency of SI engines. Engine knock depends on several variables: engine design and operation parameters can avoid or promote the auto-ignition in the end gas. In general, an increasing knock tendency with higher load and compression ratio and a decreasing tendency with combustor head walls cooling and engine speed is observed. Further, long combustion duration, e.g. caused by low engine speeds or low tumble level, can support auto-ignition [2]. Engine knock occurs randomly due to the interaction of the different influencing parameters. At a knocking operating point, both smoothly burning undisturbed cycles and knocking cycles alternate. Additionally, the knock events of these cycles can vary from light to heavy knock [1] or develop over several cycles: an auto-ignition in a prior cycles can lead to higher temperatures and prone a subsequent auto-ignition in following cycle and so on [2]. Besides the engine design and operating mode, the fuel quality has a major impact on engine knock [1]. Frequently primary reference fuel (PRF) blends are applied. In these blends, the amount of f n-heptane and iso-octane defines research octane number (RON) and the motored octane number (MON). Even though the desired RON and MON can be adjusted, the rather simple two components surrogate does not capture