One of the attractive alternatives to traditional spark ignition engines is the gasoline compression ignition (GCI) engine technology. Fuels with octane numbers lower than those of market gasolines have been identified as a viable option for GCI engine applications. Their longer ignition delay time characteristics compared to diesel fuel and their similar volatility features compared to gasoline fuels make them interesting to be explored. In this study, we have numerically investigated the effect of different injection timings at part-load conditions using a research octane number (RON) 75 fuel in gasoline compression ignition single cylinder engine. Full cycle GCI computational fluid dynamics (CFD) engine simulations have been successfully performed while changing the start of injection (SOI) timing from −60° to −10° CAD aTDC at 5bar net indicated mean effective pressure (IMEPn). The effect of SOI on mixing, combustion phasing and engine-out emissions is investigated using detailed equivalence ratio-temperature maps. Also, the effects of different rates of exhaust gas recirculation (EGR) on the combustion and emissions characteristics are investigated. Rebreathing valves profiles along with double injection strategies are also examined in the current study. Fuel consumption, soot, nitric oxides (NOx), hydrocarbon (HC) emissions and combustion phasing (CA50) are the targeted parameters throughout this study. Optimum engine parameters to obtain the best combination of the targeted properties were identified.
The work presented here seeks to compare different means of providing uniflow scavenging for a 2-stroke engine suitable to power a US light-duty truck. The three different configurations which could utilize this type of scavenging system that were investigated are (1) the opposed-piston engine, which has been applied to aircraft propulsion as well as engines for power generation and rail traction, (2) the poppet-valve uniflow configuration, as exemplified by the Detroit Diesel 2-stroke engine, and (3) the sleeve-valve uniflow engine, the unusual arrangement of which was used in the Rolls-Royce Crecy, intended for high-speed interceptor aircraft application. All of these concepts were compared in terms of indicated fuel consumption for the same cylinder swept volume, and a new methodology for optimization was developed using a one-dimensional engine simulation package which also took into account charging system work. As a result of this work it was found that the opposed-piston configuration provides the best attributes since it allows maximum expansion and minimum heat transfer. It was found that existing experiential guidelines for port angle-area specification for loop-scavenged, piston ported engines using crankcase compression could also be applied to all of the other scavenging types. This has not been demonstrated before.
Knock is a major challenge for high load operation of spark ignited gasoline engines with higher compression ratios, since the end-gas undergoes higher temperature and pressure trajectories during combustion. Pre-chamber combustion creates long-reach ignition jets that have the potential to mitigate knock due to their rapid consumption of end-gas. However, conventional pressure oscillation-based knock metrics may not accurately capture the end-gas autoignition severity in pre-chamber systems due to differences in ignition and combustion behavior. This work investigates the knock behavior of both traditional spark ignition and pre-chamber combustion (including different nozzle designs) in a high compression ratio engine fueled with regular octane certification gasoline. The data was analyzed using statistical methods to show the random nature of knock events. Detailed analysis was used to explain the pressure oscillations of both knocking and non-knocking cycles of pre-chamber jet combustion and show that conventional pressure oscillation-based knock metrics may not adequately quantify end-gas autoignition severity. A novel knock metric is introduced to avoid consideration of the non-knock related pressure oscillation and better quantify the end-gas autoignition severity. The new metric was used to explain the knock mitigation mechanism for pre-chamber jet combustion and demonstrate an additional pre-chamber jet ignition benefit of reduced combustion variability during engine operation with cooled exhaust gas circulation within its dilution limit.
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