Time frequency analysis for the evaluation of ignition delay in conventional and PCCI combustion modes

In-cylinder pressure signal has been widely adopted to obtain simple offline and online diagnosis of the combustion process through one-zone analysis, which provides the apparent heat release of the mixture. The present paper also starts from the in-cylinder pressure measurement to apply time frequency analysis for an alternative evaluation of the combustion events. A comparison with traditional one-zone analysis is then performed. In particular, time frequency analysis is here applied for the investigation of all the characteristic events that define the fuel ignition delay, i.e. hydraulic start of injection, first, along with the corresponding start of combustion, next. The study here reported is relative to single-injection patterns and has been applied to both conventional and premixed low temperature diesel combustion modes. For the considered injection pattern, time frequency analysis, compared to single-zone combustion model supplemented by measurements of injection rate at the hydraulic rig and by engine measurements of fuel injection pressure in the injector supply pipe, provided precise evaluation of the relevant combustion events.