Quantification of knock benefits from reformate and cooled exhaust gas recirculation using a Livengood–Wu approach with detailed chemical kinetics

Abstract

In this work, a simple methodology was implemented to predict the onset of knock in spark-ignition engines and quantify the benefits of two practical knock mitigation strategies: cooled exhaust gas recirculation and syngas blending. Based on the results of this study, both cooled exhaust gas recirculation and the presence of syngas constituents in the end-gas substantially improved the knock-limited compression ratio of the engine. At constant load, 25% exhaust gas recirculation increased the knock-limited compression ratio from 9.0 to 10.8:1 (0.07 compression ratio per 1% exhaust gas recirculation) due to lower end-gas temperature and reactant (fuel and oxygen) concentrations. At exhaust gas recirculation rates above 43%, higher intake temperature outweighed the benefits of lower end-gas reactant concentration. At constant intake temperature, cooled exhaust gas recirculation was significantly more effective at all exhaust gas recirculation rates (0.10 compression ratio per 1% exhaust gas recirculation), and no diminishing returns or optimum was observed. Both hydrogen and carbon monoxide were also predicted to improve knock by reducing end-gas reactivity, likely through the conversion of high-reactivity hydroxy-radicals to less reactive peroxy-radicals. Hydrogen increased the knock-limited compression ratio by 1.1 per volume percent added at constant energy content. Carbon monoxide was less effective, increasing the knock-limited compression ratio by 0.38 per volume percent added. Combining 25% cooled exhaust gas recirculation with reformate produced from rich combustion at an equivalence ratio of 1.3 resulted in a predicted increase in the knock-limited compression ratio of 3.5, which agreed well with the published experimental engine data. The results show the extent to which syngas blending and cooled exhaust gas recirculation each contribute separately to knock mitigation and demonstrate that both can be effective knock mitigation strategies. Together, these solutions have the potential to increase the compression ratio and efficiency of spark-ignition engines.

Optimization of a Spark Ignition Engine Knock and Performance Using the Epsilon-Constrained Differential Evolution Algorithm and Multi-Objective Differential Evolution Algorithm

“… 46 , 47 The overall induction time of this model comprises three different induction times. 48 − 50 Each induction time is defined by eq 5 . where M 1 is the induction time multiplier, a i through f i are the constant parameters of the model, ON represents the octane number of the fuel used to run the engine, [Diluent] means the mass fraction of the residuals in the unburned zone, mainly including N 2 , CO 2 , and H 2 O, and M 2 is the activation energy multiplier.…”

“…In order to capture the different chemistry of knock lover a wide range of temperatures, the kinetics-fit model uses three different induction times to describe the knock in the low-, intermediate-, and high-temperature regions, respectively. , The overall induction time of this model comprises three different induction times. − Each induction time is defined by eq . where M 1 is the induction time multiplier, a i through f i are the constant parameters of the model, ON represents the octane number of the fuel used to run the engine, [Diluent] means the mass fraction of the residuals in the unburned zone, mainly including N 2 , CO 2 , and H 2 O, and M 2 is the activation energy multiplier …”

Evaluation of ASTM D6424 standard for knock analysis using unleaded fuel candidates on a six cylinder aircraft engine

“…Another simplified approach is to implement an empirical Arrhenius model coupled with Livengood-Wu integral for knock detection. 60,61 For this purpose, Arrhenius model coefficients need to be tuned based on experimental measurements from rapid compression machines or shock tubes. Another technique is to tune the Arrhenius coefficients with the help of detailed chemical kinetics for different pressures, temperatures, and equivalence ratios if the reaction mechanism is known.…”