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A portable PC-based heat release model for diesel engines with an emphasis on cold starting was developed to be used on an IBM PC or compatible. The model features normalized variables, the inclusion of blowby and heat transfer models, smoothing using a combination of error detection methods and low pass FFT. The derivative of the pressure data was calculated using cubical splines, from which the analytical first and second derivatives were derived. This method had the advantage of no data shifting, the ability to calculate the derivatives at any point (intermediate or original data point) and the error of both the first and second derivatives of the same order. The inclusion of the blowby model during starting was essential as losses due to blowby were found to be just as significant as other major loss factors. Smoothing angular velocity and the pressure signal using polynomial and cubical splines did not prove to be as efficient or as accurate as using the combination of error detection methods and FFT. Combustion during cold starting is mostly of the premixed type.