NOx is one of the main compositions in the modern engine emissions and the reduction requirements of NOx have turned to be more stringent. To control NO emissions better, the technologies of NOx sensors are forced to achieve much faster response and higher accuracy. In this paper, the correlation between ion current signals and NOx emissions is studied by both experiments and simulations in a direct-injection controlled auto-ignition (CAI) engine. The investigation provides the possibility of a novel method of cycle-by-cycle NOx emissions detection. The simulation results present this positive correlation based on the chemical kinetics theory, and also directly reflect the formation order of the chemical products and the influence of temperature on the rates of main ionization and NOx generated reactions. Furthermore, the distributions of both ions and NOx products are shown with the CFD results, illustrating their in-cylinder space correlation. Combined with the simulation results, the experimental results not only validate the positive correlation between two different fuel types, but also provide the evidences of linear fitting function. Based on the fitting results, the cycle-based NOx emissions could be estimated. (C) 2015 Elsevier Ltd. All rights reserved.