Tracer laser induced fluorescence (LIF) imaging of piston-top fuel films has been performed within the combustion chamber of an optically-accessible, single cylinder Diesel engine. The first objective of the study was to adapt the tracer LIF technique so as to perform in-cylinder imaging of the fuel films under reacting (i.e. combustion) conditions. The results obtained in a wall-guided, combustion chamber operating under highly dilute, Diesel low temperature combustion (LTC) conditions reveal the significant presence of late-cycle piston-top fuel films. Furthermore, it is believed that these fuel films contribute to engine-out hydrocarbon (HC) emissions via a mechanism of flash boiling.
An attempt was also made to evaluate the role of fuel volatility on fuel film lifetimes. This was achieved by using a 50/50 fuel mixture of two single component fuels whose boiling points correspond to moderately high and low volatility components of standard Diesel fuel. Several fuel tracers were considered such as 5-nonanone, tetramethyl-p-phenylendiamine (TMPD) and naphthalene. Finally, the tracer naphthalene with a boiling point of 218°C, coupled with dodecane (boiling point 216°C) was used to represent the higher volatility component of Diesel fuel. An attempt to trace the lower volatility fuel components using the tracer TMPD (boiling point 260°C) proved unsuccessful due to excessive oxygen-induced quenching of the TMPD fluorescence signal. However, experiments performed with the single component fuel n-tetradecane (boiling point 253°C) revealed significant ‘natural fluorescence’, probably due to the presence of trace impurities within the fuel. These characteristics were exploited so as to allow qualitative studies of the evolution of the lower volatility component of Diesel fuels. The results presented here reveal that late cycle piston-top fuel films are preferentially due to the presence of the lower volatility (i.e. heavier) components of the fuel whilst the LIF signal corresponding to the higher volatility tracer/fuel combination is detected until about mid-way through the expansion stroke clearly suggesting that these lighter components undergo faster evaporation.
On a broader note, the present study highlights the problems/challenges of performing tracer-LIF studies of Diesel-type fuels due to an apparent lack of appropriate high boiling point fuel tracers.