Experimental and Numerical Investigation of Performance and Emissions in Compression Ignition Engines with Alternative Fuels.

Abstract

The experimental investigation in this work concerns the compression-ignition (CI) engine combustion process both in normal operation and dual-fuel operation. There is a bulk of literature reporting thermal efficiencies, brake specific fuel consumption (BSFC) and emissions under single and dual fueling conditions in CI engines. Most of the studies lack the full implications of changing load (power output) and speed on these performance indicators. The studies are either restricted to various loads/powers at one engine speed (neglecting the effect of engine speed) or one or two load/power conditions at various speeds (neglecting load variations). There is a scarcity of full engine maps in the open literature (these are the full contours of thermal efficiency or BSFC plotted throughout the power versus speed range of the engine, or the torque versus speed range of the engine). This thesis provides performance and emissions maps for a CI engine using two different fuels (diesel and rapeseed methyl ester used as single fuels) and two gaseous fuels (natural gas and hydrogen) used with two different pilot fuels (diesel and rapeseed methyl ester ) under what is termed dual fueling mode. A novel approach is used to present the performance and emissions over the entire engines operational range. The results are presented as iso- contours of thermal efficiency, volumetric efficiency and brake specific NOX, specific HC and specific CO2 on a power-speed graph throughout the operating range of the engine. Many studies conclude that the emissions, particularly NOX during dual fueling are expected to form in the spatial region around the pilot spray. This region is expected to be subjected to high localised temperatures as the equivalence ratio is close to stoichiometric, thus maximising heat release from combustion. The effect of changing the pilot fuel quantity on performance and emissions is rarely reported. This study addresses this scarcity in the literature and investigates the effect of changing the pilot fuel quantity and type on various combustion and emission parameters. Diesel and rapeseed methyl ester (RME) have been used as pilot fuels for both the natural gas as well as hydrogen and three different pilot fuel settings have been employed for each of the gaseous fuels. The effect of using a different pilot fuel quantity to achieve the same brake mean effective pressure (BMEP) for the two gaseous fuels has been analysed and compared. This thesis also includes a chapter on the computational modeling of the engine esmissions. This study uses combinations of different spray and combustion models to predict in-cylinder pressure, rate of heat release and emissions. The approach employs two combustion models: Unsteady Flamelet Model (UFM) with PDF method and Finite Rate Chemistry (FRC) with stiff chemistry solver implemented through In-Situ Adaptive Tabulation (ISAT) algorithm. Two spray models used includeWAVE and Kelvin Helmohltz Rayleigh Taylor (KHRT) spray models. The UFM coupled with KHRT spray model has been used to predict NOX, CO and CO2 emissions. The model captures the emissions trends well. In-cylinder contours of O2, NO and mass average temperature have also been presented. A chemical mechanism of n-heptane with 29 species and 52 reactions has been used.