Agricultural & municipal waste and wood residues can be easily converted to biogas or producer gas and used for producing heat and power. The main problem with these fuels is their low energy content. This is due to the presence of certain non-combustible gases like CO2 and N2 in these fuels. The use of these gases in SI engines is associated with problems like unstable operation and high levels of HC and CO emissions. Gaseous fuel can be easily used with good efficiencies and low emissions in diesel engines running in the dual-fuel mode. In dual-fuel engines, these gaseous fuels are inducted along with air and ignited after compression by a small spray of diesel called the pilot.

The presence of these gases alters the thermodynamic properties of the intake charge and significantly influence the ignition delay of the pilot diesel fuel and hence the performance of the engine.

The aim of this paper is to modify an existing correlation for ignition delay in a dual-fuel engine to incorporate the effects of the gaseous fuel concentration and composition on the polytropic index. An ignition delay correlation of a biogas dual-fuel engine was modified so that it can be used with any primary fuel. The polytropic index was assumed to be a function of the ratio of specific heats. Further, the effect of injection timing on ignition delay was included. The adapted model was introduced in a simulation program and the results of ignition delay were compared with those given in the literature for a dual-fuel engine. In addition, the correlation was used to predict the ignition delay of the pilot fuel when biogas, LPG, natural gas and producer gas were treated as primary fuels. The results obtained with the new correlation have been compared with experimental values from a LPG-diesel dual fuel engine. The comparison was also made for a biogas dual fuel engine. Errors less than 10% were obtained for both of the fuels between the experimental measurements and simulation results.

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