The aim of the present work is to evaluate the influence of the pilot injection on combustion of a TDI Diesel engine for different engine torque and speed conditions. For this investigation, pilot injection timing and duration were varied on a wide range of values, and their effects on combustion pressure, rate of heat release, pilot and main combustion delay, combustion process and exhaust emissions in terms of NOx and smoke were analyzed.
An in-line, four-cylinder, turbocharged FIAT 1930 cm3 TDI Diesel engine, equipped with Common Rail injection system, was tested.
A piezoelectric sensor was located in the combustion chamber in order to acquire combustion pressure; from these signals, gross heat release rate was derived in order to analyze the combustion behavior. Pollutant emission levels have been measured by means of a gas analyzer, while for smoke an opacimeter was used.
It was found that both timing and duration of pilot injection strongly influence its autoignition tendency; in particular, pilot injection autoignition mainly occurs with low values of timing and high durations. Moreover, pilot injection timing effect is more evident on pilot ignition delay than pilot duration, whereas the effect of both variables on the main ignition delay depends on the engine operating conditions. The effect of pilot injection parameters is evident on engine emission behavior too; in particular, NOx emissions levels are mainly influenced by the pilot duration, whereas pilot timing effect is more evident at lower engine load and speed. Smoke emission seems to be influenced by both variables, but their effect is stronger especially at medium and high load.