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Abstract
Control of harmful pollutants such as oxides of nitrogen (NOx), particulate matter (PM)/ soot, carbon monoxide (CO), and hydrocarbons (HC) emitted by diesel engines is essential in order to protect the environment and human health. Different versions of low temperature combustion (LTC) concepts have shown their potential in controlling these pollutants, while achieving thermal efficiencies similar to conventional compression ignition (CI) engines. In this study, a diesel fueled partially premixed homogeneous charge compression ignition (PHCCI) engine was investigated to explore the feasibility of LTC combustion. All experiments were performed on a constant speed two cylinder engine prototype, in which one cylinder was modified to operate in PHCCI combustion mode and the other cylinder operated in conventional CI combustion mode. An external mixing device, called a “fuel vaporizer”, was developed for partially homogeneous fuel-air mixing. For a better understanding of the effect of intake charge temperature (Ti) and exhaust gas recirculation (EGR), experiments were performed at three different Ti’s (160, 180, and 200 °C) and three different EGR rates (0, 10, and 20%) at six engine loads with a relative fuel-air ratio (λ) ranging from 1.5 (rich limit) to 5.25 (lean limit). Experimental results showed that fuel-air mixing was significantly affected by Ti. At higher engine loads, higher heat release rate (HRR) of PHCCI combustion led to excessive knocking. Combustion phasing was found to be the most important parameter in PHCCI combustion, which affected combustion, performance, and emission characteristics. Increasing Ti improved fuel-air mixing, leading to superior combustion as well as lower HC and CO emissions. However, at 200 °C, excessive knocking deteriorated PHCCI engine performance and led to higher NOx emissions. Increasing the EGR rate effectively controlled PHCCI combustion, leading to lower HRR and NOx emissions. However, too high EGR rate increased the CO, HC, and PM emissions, primarily due to relatively lower combustion chamber temperatures. This study showed that selection of suitable control parameters (such as Ti and EGR rate) led to superior PHCCI combustion, which could possibly extend the operating load range and engine operating window and may be useful in developing a practical and efficient LTC engine.
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