Investigation of the effects of alternative fuel use on performance and emissions in a compression ignition (CI) diesel engine

Abstract

Undoubtedly, one of the most crucial options under investigation for enhancing performance and emission characteristics in internal combustion engines (ICEs) is the adoption of alternative fuels. It is of utmost importance that these alternative fuels are compatible with engine operation and do not lead to substantial structural alterations in the engine. These fuels need to be thermodynamically compatible with the engine and environmentally friendly. Key characteristics to look for in alternative fuels include having high reserves, high energy density, and causing minimal emissions. Due to possessing all these features, natural gas and hydrogen fuels hold significant importance among alternative fuels. Natural gas and hydrogen fuels are thermodynamically compatible with engines that operate on both Otto and Diesel cycles. Their ability to be used in internal combustion engines as alternative fuels without causing significant structural changes paves the way for their application. Furthermore, their lower emissions contribute to their appeal. In a compression-ignition (CI) engine, natural gas and hydrogen fuels, along with diesel fuel, can be used by mixing the gaseous fuel with intake air through a low-pressure injector in the manifold. Subsequently, ignition is achieved by injecting pilot diesel fuel onto the compressed charge during the compression process. The combustion technology that occurs in CI engines through this method is called "Dual-Fuel" combustion. The combination of the high energy density of natural gas and its environmentally friendly nature, coupled with the high thermal efficiency of the diesel engine, results in both efficient and environmentally friendly operating conditions. In high compression ratio CI (Compression Ignition) engines, natural gas with a high octane number can be used directly in a premixed form without causing knock up to a certain compression ratio. When it comes to hydrogen fuel, it is known that its use in compression-ignition engines is highly suitable, just like natural gas, and it does not require any structural changes in the engine. The advantages of natural gas, such as its low carbon-to-hydrogen ratio, and the absence of this ratio in hydrogen, combined with their high energy densities, have increased the interest in these gaseous fuels. This situation has motivated the current thesis work. The impact of CI engines on air pollution worldwide, along with the economic and high-energy capacity characteristics of natural gas and hydrogen, strengthens the case for the alternative use of these gas fuels. The environmentally friendly and high-performance attributes of natural gas, combined with hydrogen's rapid and extensive ignition range, along with its high mass-specific energy density, have shed light on the application of these two gas fuels together in CI engines. Utilizing these advantages, the current thesis work has not only improved performance values but has also met the increasingly stringent emission limit values. The thesis work involved the numerical modeling of an experimental study based on the literature, utilizing the ANSYS Forte CFD program. In the experimental part, the energy in a partially loaded (25% - 4.05 BMEP) compression-ignition diesel engine was supplied by 25% diesel fuel and 75% natural gas fuel. Different diesel fuel injection advances (10°, 14°, 18°, 22°, 26°, 30°, 34°, 38°, 42°, 46°, and 50° CA BTDC) were investigated at a constant engine speed (910 rpm) to explore their effects on performance and emission values. The test results were compared with the obtained numerical results to validate the established model. Subsequently, using the validated numerical model, parametric studies involving hydrogen in combustion were conducted. Parametric studies were conducted in two stages. In the first stage, the effects of energy fractions of natural gas and hydrogen, as well as diesel fuel injection advances, were examined. Two fundamental principles guided the energy fractionation. In the first principle, the total fuel input energy under test conditions remained constant, while a portion of the natural gas energy was gradually transferred to hydrogen fuel. The diesel-derived energy fraction was kept constant at 25%, while the remaining 75% was allocated between natural gas and hydrogen fuels. These operating points were named Mode 1, also known as the energy-sharing mode for gas fuels. For Mode 1, the energy sharing ratios were established as D25NG75H00 (test case), D25NG65H10, D25NG50H25, D25NG25H50, and D25NG00H75. The second principle involved maintaining the diesel and natural gas energy ratios (25% diesel and 75% natural gas) constant, while providing extra energy input to the system through the inclusion of hydrogen. These operating points were labeled as Mode 2, also referred to as the hydrogen enrichment mode. For Mode 2, the operating points resulting from hydrogen enrichment were designated as D25NG75H00 (test case), D25NG75H05, D25NG75H10, D25NG75H15, D25NG75H20, and D25NG75H25. Various fuel energy fractions for both Mode 1 and Mode 2, combined with different diesel fuel injection timings (10°, 14°, 18°, 22°, 26°, 30°, 34°, and 38° CA BTDC), were examined to analyze their effects on performance and emission values. Among the examined values, it was observed that the operating points D25NG50H25 (14° CA BTDC for SOI) for Mode 1 and D25NG75H15 (10° CA BTDC for SOI) for Mode 2 produced more reasonable results in terms of both engine performance and exhaust gas emissions compared to other operating points. The optimal condition obtained for Mode 1 resulted in improvements of 21%, 29%, 88%, 86%, and 77% for power, BSFC, HC, CO, and SOOT (PM), respectively. For Mode 2, the optimal condition yielded improvements of 36%, 22%, 76%, 80%, and 83% for the same parameters. However, when comparing both Mode 1 and Mode 2 test conditions, the higher cylinder combustion temperatures due to hydrogen led to higher NOx and MPRR values. While there was a 12% increase in NOx for Mode 1, Mode 2 showed an increase of 11%.