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Lift-Off Length and KL Extinction Measurements of Biodiesel and Fischer-Tropsch Fuels under Quasi-Steady Diesel Engine Conditions
Abstract:
The relationship between ignition, lift-off length and soot formation was investigated for a collection of fuels in an optically-accessible modified 2-stroke engine under a set of typical quasi-steady state Diesel DI conditions. Five fuels including biodiesel blends and Fischer-Tropsch fuels have been selected for their potential to substitute conventional diesel with no major modifications on the engine hardware, and were previously characterized under ambient pressure following ASTM standards.
Fuels were injected into a large volume through a single-hole nozzle at three levels of injection pressure, by sweeping ambient temperatures at constant density, and ambient densities at constant temperature. The 8 ms single-shot injections were long enough to reach the stabilization of a free diffusion flame. The OH-chemiluminescence was imaged and lift-off length was measured via image post-processing. Simultaneously, flame opacity was measured along the flame axis using the laser extinction technique. The pressure trace analysis quantified the ignition delay.
In a first step, the averaged lift-off length measurements are presented and contrasted with both ignition delay and previous liquid length measurements performed in similar thermodynamic conditions but with an inert spray configuration. Statistical analysis permitted assessment of the influence of engine parameters such as air density, air temperature and injection pressure independently of fuel origin. The strong effect of air temperature and the more moderate ones of both air density and injection pressure presented by other authors were confirmed. Then, fuel stoichiometric mixture fraction and ignition delay have been introduced in the analysis in order to investigate their implication in processes of lift-off stabilization.
In a second analysis, values of both lift-off length and KL were compared with a cycle-to-cycle approach. Results showed that, at fixed engine operating conditions, the scattering of lift-off length measurements is directly linked to that of KL. This result was confirmed when using average values and when comparing different parameters that were tested in the study. Different sensitivities were observed from one fuel to another.