Impact of Spark Assistance and Multiple Injections on Gasoline PPC Light Load

Abstract

Along the last years, engine researchers are more and more focusing their efforts on the advanced low temperature combustion (LTC) concepts with the aim of achieving the stringent limits of the current emission legislations. In this regard, several studies based on highly premixed combustion concepts such as HCCI has been confirmed as a promising way to decrease drastically the most relevant CI diesel engineout emissions, NOx and soot. However, the major HCCI drawbacks are the narrow load range, bounded by either misfiring (low load, low speed) or hardware limitations (higher load, higher speeds) and the combustion control (cycle-tocylce control and combustion phasing). Although several techniques have been widely investigated in order to overcome these drawbacks, the high chemical reactivity of the diesel fuel remains as the main limitation for the combustion control.The attempts of the researchers to overcome these disadvantages are shifting to the use of fuels with different reactivity. In this sense, gasoline PPC has been able to reduce emissions and improve efficiency simultaneously, but some drawbacks regarding controllability and stability at low load operating conditions still need solution. In this field, previous researches have been demonstrate the multiple injection strategy as an appropriate technique to enhance the combustion stability. However, PPC combustion has been found limited to engine loads higher than 5 bar BMEP when using fuels with octane number greater than 90. In this regard, previous work from the authors showed the capability of the spark plug to provide combustion control in engine loads below this limit even using 98 ON gasoline.The main objective of the present work is to couple the control capability of the spark assistance together with an appropriate mixture distribution by using double injection strategies with the aim of evaluating performance and engine-out emissions at low load PPC range using a high octane number gasoline. For this purpose the optical and metal version of a compression ignition single-cylinder engine, to allow high compression ratio, has been used during the research. A common rail injection system enabling high injection pressures has been utilized to supply the 98 octane number gasoline. An analysis of the incylinder pressure signal derived parameters, hydroxyl radical (OH*) and natural luminosity images acquired from the transparent engine as well as a detailed analysis of the air/fuel mixing process by means of a 1-D in-house developed spray model (DICOM) has been conducted. Results from both analysis methods, suggest the spark assistance as a proper technique to improve the spatial and temporal control over the low load gasoline PPC combustion process. A noticeable increase in the cycle to cycle repeatability (5% versus 15.1% CoV IMEP at 2 bar load) as well as a reduction in the knocking level (20.5 versus 33.6 MW/m 2 at 7 bar load) is observed. In addition, the combination of the spark assistance with the use of the double injection strategy ...

Achieving clean and efficient engine operation up to full load by combining optimized RCCI and dual-fuel diesel-gasoline combustion strategies

“…Trying to solve this problem, some researchers investigated the use of a spark plug to assist the low load PPC operation. It was found that the spark discharge improves the control of the PPC combustion process, temporal [17] and spatially [18], over the low load gasoline PPC combustion process, but led to excessive NOx and soot emissions [19], even using double injection strategies [20] [21].…”

Effects of direct injection timing and blending ratio on RCCI combustion with different low reactivity fuels

“…With the aim of improving the PPC cycle-to-cycle control at low loads using high ON gasolines, PPC spark assisted concept has been studied [12] [13]. It has been demonstrated that the spark assistance provides temporal and spatial control over the combustion process [14], however the high local reactivity required between the spark plug electrodes at the start of spark timing and the flame propagation process result in an unacceptable NOx and soot emissions [15]. In this sense, the double injection strategy applied to the PPC spark assisted concept has been confirmed as a suitable strategy to improve the unburned HC and CO emissions, but still do not solve the unacceptable NOx and soot emission levels [16] [17].…”

An assessment of the dual-mode reactivity controlled compression ignition/conventional diesel combustion capabilities in a EURO VI medium-duty diesel engine fueled with an intermediate ethanol-gasoline blend and biodiesel

“…However, the concept demonstrated difficulties at low load conditions using gasoline with octane number (ON) greater than 90 [5]. At these conditions, the spark assistance provided temporal [6] and spatial control over the gasoline PPC combustion process [7], but resulted in unacceptable NOx and soot emissions [8], even using double injection strategies [9]. Thus, experiments performed by Bessonette et al [10] suggested that, to achieve proper operation, the fuel characteristics must vary depending on the engine operating conditions.…”