Optical diagnostics of isooctane and n-heptane isobaric combustion 2020-01-1126
Isobaric combustion has demonstrated a great potential to reach high thermodynamic efficiency in the advanced Double Compression Expansion Engine (DCEE) concept. It appears as one of few viable choices for applications with high-pressure combustion. At these conditions, releasing heat at a constant pressure minimizes the peak in-cylinder pressure and, hence, mitigates excessive mechanical stress on the engine. This study focus on the effect of fuels on the multiple-injection isobaric combustion. A single-cylinder heavy-duty engine was utilized to test and compare the isobaric combustion with pure isooctane and n-heptane fuels. The engine was equipped with a fully optical piston to allow studying multiple-injection interactions and combustion behavior using high-speed acquisition of chemiluminescence. Two isobaric cases has been studied with peak pressure of 50 bar (IsobaricL) and 70 bar (IsobaricH). These two cases were compared with conventional diesel combustion (CDC) case that shares similar intake conditions as the IsobaricL but with peak cylinder pressure as the IsobaricH. For cases with high soot luminosity, a short band-pass filter was used to avoid image saturation. Fuels with short ignition delay time such as n-heptane are usually used for isobaric applications as they offer good controllability of injections. However, the study herein demonstrates that isobaric combustion can be achieved even with high octane number fuels such as isooctane. The dwell time between injections is much larger for isooctane, which allows enhanced mixing of the fuel jets with air and thus a partially premixed combustion (PPC) behavior. The high-speed imaging demonstrated evidence of flame extinguishing during the sequential injection of n-heptane.
Abdullah S. Al Ramadan, Gustav Nyrenstedt, Moez Ben Houidi, Bengt Johansson
King Abdullah University of Science & Technology