Energy Release Characteristics inside a Spark-Ignition Engine with a Bowl-in-Piston Geometry

The conversion of compression ignition (CI) internal combustion engines to spark-ignition (SI) operation by adding a spark plug to ignite the mixture and fumigating the fuel inside the intake manifold can increase the use of alternative gaseous fuels (e.g., natural gas) in heavy-duty applications. This study proposed a novel, less-complex methodology based on the inflection points in the apparent rate of heat release (ROHR) that can identify and separate the fast-burning stage inside the piston bowl from the slower combustion stage inside the squish region (a characteristic of premixed combustion inside a diesel geometry). A single-cylinder 2L CI research engine converted to natural gas SI operation provided the experimental data needed to evaluate the methodology, at several spark timings, equivalence ratios, and engine speeds. The results indicated that the end of the bulk combustion traditionally defined as the location of 90% energy release was not greatly affected by the change in operating conditions. Moreover, the actual duration of the rapid-burning stage was 60-80% shorter than the crank angle interval between 10% and 90% energy release. However, the fast-burning period (i.e., the start and the end of the rapid-burning stage) was well-characterized by the crank angle duration between the first and the second ROHR inflection points. Moreover, this novel methodology to characterize the combustion process suggested that the longer time interval between the end of fast-burn (i.e., the first ROHR inflection point) and the end of combustion was due to an important fuel fraction burning slower inside the squish region, which finally affects both efficiency and emissions of such converted engines.