Numerical Evaluation of the Effects of Compression Ratio and Diesel Fuel Injection Timing on the Performance and Emissions of a Fumigated Natural Gas–Diesel Dual-Fuel Engine

Abstract

Various solutions have been proposed for reducing the exhaust emissions and improve the well-known soot and nitrogen oxide (NO) trade-off in diesel engines, without making serious modifications to the engine, one of which is the use of natural gas as supplement to liquid diesel fuel. In these types of engines, referred to as fumigated, natural gas–diesel dual-fuel compression ignition (CI) engines, gaseous fuel is fumigated and premixed with the aspirated air during the induction stroke. Natural gas is a clean-burning fuel with a relatively high auto-ignition temperature, which is a serious advantage in dual-fuel combustion. Previous research studies have shown that the natural gas–diesel fuel dual-fuel combustion in a CI engine environment, compared with conventional diesel fuel operation, suffers from high specific fuel consumption and high carbon monoxide (CO) and unburned hydrocarbon (HC) emissions. Compression ratio (CR) and diesel fuel injection timing (IT) are two engine parameters that can influence considerably the combustion process inside the combustion chamber of a dual-fuel CI engine. For examining the effect of these parameters on the performance and exhaust emissions of a light-duty diesel engine fueled with methane, which is the basic constituent of natural gas, a theoretical investigation is conducted in this work by using a numerical simulation. The results concern engine performance characteristics, NO, CO, and soot emissions for various methane-diesel mass ratios at engine partial-load conditions and constant speed, by using a comprehensive two-zone phenomenological model. Model predictions were tested and validated against experimental measurements obtained from a fumigated methane–diesel dual-fuel CI engine, operating with constant CR and constant diesel fuel IT under constant engine speed and partial-load conditions for various methane-diesel fuel mass ratios. The main objective of the comparative assessment shown in this work is to record and comparatively evaluate the relative impact of CR and diesel fuel IT on the engine performance characteristics and emitted pollutants. The conclusions from the present investigation may be very valuable for the use of natural gas as a supplementary fuel in conventional dual-fuel engines. From the comparative evaluation of the theoretical findings presented in this work, it is generally revealed that for a turbocharged dual-fuel (i.e., methane–diesel) CI engine, operating at partial-loading conditions and high gaseous fuel mass ratios, the increase of CR (accompanied with advanced diesel fuel IT) could be a promising solution for improving engine power output and efficiency while simultaneously reducing CO emissions. Conversely, at lower gaseous fuel mass ratios, the simultaneous increase of both parameters could cause problems to the engine structure because the maximum cylinder pressure becomes considerably higher compared with that observed under normal engine operating (NEO) mode, which may prove to be harmful for the engine’s structural integrity.