Development of a Model for Predicting the Knock Boundary in Consideration of Cooled Exhaust Gas Recirculation at Full Load

Engine knock limits the efficiency of turbocharged SI engines at high loads. The occurrence of this phenomenon can be inhibited by deploying recirculation of cooled exhaust gas (EGR) at full load. However, the development of full load EGR combustion systems cannot be per-formed in the 0D/1D engine simulation, as no meaningful models for the reliable prediction of the knock limit under the influence of EGR exist.

Measurements of ignition delay times in a shock tube and a rapid compression machine under the influence of exhaust gas have been carried out. The addition of 25% EGR prolonged the ignition delay time up to 100%. A detailed reaction mechanism for gasoline surrogates was defined and validated against the measurement results. Furthermore, the effects of EGR on combustion, knock behavior and emissions were investigated on a single-cylinder re-search engine. The center of combustion could be advanced by up to 9° CA with the addition of EGR leading to a four percenter higher indicated efficiency. The influence of catalytically treated exhaust gas was examined as well. At high EGR rates of 25% catalytically treated exhaust gas allowed a 2° CA earlier center of combustion. Furthermore, the influence of nitric oxide on the knocking tendency was investigated. It has been found that a total cylinder NO concentration of about 100 ppm leads to the highest knocking tendency. At NO concentrations below 40 ppm NO the knocking tendency was decreased. Higher concentration than 100 ppm of NO in the cylinder decreased knocking tendency as well.

The pressure trace analysis of the measured single working cycles shows that the pre-reaction state of the unburned mixture at knock onset calculated with commonly used knock models decreases with rising EGR rate and engine speed, although by definition it must be constant at the time of auto-ignition. Consequently, reaction kinetics simulations at in-cylinder conditions proved that, under specific boundary conditions, the auto-ignition of the unburnt mixture resulting in knock happens in two stages. In this case, low-temperature ignition occurs in the unburnt mixture while the combustion is taking place. This phenomenon significantly influences the ignition delay of the mixture, which severely impairs the prediction capabilities of commonly used knock models.

Based on these findings, a new knock modeling approach capable of predicting the low-temperature ignition occurrence as well as reproducing its influence on the mixture’s auto-ignition was developed. The results from 3D-CFD simulations accompanying the model development supported all model assumptions made. The developed knock model was successfully validated against measurement data at various boundary conditions, such as different inlet temperatures and mixture compositions as well as EGR rate and engine speed variations. It can predict the knock limit very accurately with errors in center of combustion below 2° CA and thus contributes to an efficient development process of full load EGR combustion systems in the 0D/1D engine simulation.