Abstract
Partially Premixed Combustion (PPC) is a combustion concept that aims to provide combustion with low smoke and NOx emissions and a high thermal efficiency. Extending the ignition delay to enhance premixing, avoiding spray-driven combustion, and controlling temperature at an optimum level through use of suitable dilution levels has been recognized as a key factor to achieve such a concept. Fuels with high auto-ignition resistance do extend ignition delay. In this work three ternary blends of an alcohol (ethanol or n-butanol), n-heptane and iso-octane with a target research octane number (RON) of 70 are studied. RON70 was earlier found to be close to optimal for PPC over a large load range. The objective of this research is to analyze the sensitivity of the combustion parameters to changes in air-excess ratio when using these three blends. The engine was operated at 1250 rpm and 1000 bar injection pressure with a single injection strategy. Results revealed that efficiency was increased from rich to lean combustion, and these three blends show distinct premixed combustion even in lean PPC operation. The premixed fraction of combustion however reduces with the increase of air-excess ratio, which is especially apparent for PRF70 which consists of n-heptane and iso-octane alone.
Highlights
Diesel engines are widely used in agriculture and transportation sectors due to their high efficiencies which result from their relatively high compression ratios and lack of throttlingFlow Turbulence Combust (2016) 96:309–326 losses
In premixed combustion, injected fuel has longer time to mix with oxidizer beforehand which results in faster heat release, strong gradients in rate of heat release (RoHR)
At lower boost conditions (1.3 bar absolute intake pressure), each blended fuel demonstrated a low temperature heat release (LTHR) event followed by the main combustion, or high-temperature heat release (HTHR)
Summary
Diesel engines are widely used in agriculture and transportation sectors due to their high efficiencies which result from their relatively high compression ratios and lack of throttlingFlow Turbulence Combust (2016) 96:309–326 losses. Diesel engines are widely used in agriculture and transportation sectors due to their high efficiencies which result from their relatively high compression ratios and lack of throttling. Conventional diesel combustion suffers from high Nitrogen Oxides (NOx) and soot emissions [1, 2]. The emission standards for diesel engines become increasingly stringent, which forced the introduction of after-treatment systems for NOx and soot emissions. Advanced combustion strategies combined with the optimal fuels may find an in-cylinder approach to meet these emission standards fully, and avoid the need to use after-treatment or at the very least. Homogeneous charge compression ignition (HCCI) engines have the advantage of both low NOx emission and high thermal efficiency. Soot emissions decrease in HCCI engines due to leaner mixtures. HCCI combustion is kinetically controlled, due to the self-accelerating nature of chemical-kinetic rates as the temperature rises [3], HCCI combustion tends to release heat instantaneously, which can result in unacceptable pressure rise rate (PRR) and even damage the engine
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