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One of the principal issues of low-temperature combustion modes is caused by the imbalances in the distribution of air and EGR across the cylinders, which affects the combustion process. Cylinder to cylinder variations lead to imbalances in the cylinder pressure, indicated torque, exhaust gas thermodynamic conditions and emissions. In principle, a cylinder-by-cylinder control approach could compensate for air, residuals and charge temperature imbalance. However, in order to fully benefit from closed-loop combustion control, a feedback from each engine cylinder would be necessary to reconstruct the pressure trace. Therefore, cylinder imbalance is an issue that can be detected only in a laboratory environment, wherein each engine cylinder is instrumented with a dedicated pressure transducer. This paper describes the framework and preliminary results of a model-based estimation approach to predict the individual pressure traces in a multi-cylinder engine from the output of a crankshaft speed sensor. The objective of the estimator is to reconstruct the complete pressure trace during an engine cycle with sufficient accuracy to allow for detection of cylinder to cylinder imbalances. Starting from a model of the engine crankshaft dynamics, a sliding mode observer is designed to estimate the cylinder pressure from the crankshaft speed fluctuation measurement. The results obtained by the estimator are compared with experimental data obtained on a four-cylinder Diesel engine.