A new procedure for the real-time estimation of the EGR rate and charge oxygen concentration has been developed, assessed and applied to a low-compression ratio GMPT-E EURO V diesel engine.High EGR rates are usually employed in modern diesel engines to reduce combustion temperatures and NOx emissions, especially at medium-low load and speed conditions. The EGR rate is usually calibrated in steady-state conditions, but, under transient conditions, it can be responsible for misfire occurrence or non optimal combustion cycles, if not properly controlled. In other words, combustion instabilities can occur, especially during tip-in maneuvers, which imply transition from high EGR (low load) to low EGR (high load) rates. Misfire is determined by a temporary reduction in the intake charge oxygen concentration during the closure of the EGR valve.Therefore, a model-based approach for real-time estimation of the EGR rate and intake charge oxygen concentration is a powerful tool that could allow the engine ECU to prevent misfire occurrence.In this paper, a semi-empirical correlation has been developed to estimate the EGR rate under steady-state and transient operating conditions, on the basis of the measured pressure in the intake manifold, of the measured pressure and temperature upstream from the EGR valve, and of the duty cycle signal of the EGR valve. The intake charge oxygen concentration has been estimated on the basis of the measurements of the air mass-flow rate and injected fuel mass.The proposed technique has been applied to a modern EURO V diesel engine, in order to analyze two different engine transients: a severe tip-in maneuver with misfire occurrence and an acceleration ramp during the ECE cycle. The methodology has proved to be effective in the real-time monitoring of the EGR rate and intake charge oxygen concentration and to be simple enough to be implemented in the engine ECU in order to diagnose misfire occurrence in advance.