Engine performance and emission characteristics of a dual fuel diesel/liquefied natural gas marine engine

Abstract

The International Maritime Organization implemented the MARPOL convention to control global shipping sulphur oxide emissions. Effective January 2020, the maximum allowable sulphur content in marine heavy fuel oil was reduced from 3.5% to 0.5%. This stringent regulation prompted the shipping industry to seek alternatives to minimize sulphur emissions. Natural gas emerged as a favourable alternative fuel due to its negligible sulphur content. Conventional diesel engines could be retrofitted to operate in natural gas/diesel dual-fuel mode, with natural gas serving as the primary fuel. Dual-fuel technology offers a convenient and cost-effective solution for marine vessels transitioning to low-emission fuels, effectively tackling the sulphur oxides emission problem, along with reducing greenhouse gases and other harmful pollutants.

This thesis focused on a retrofitted marine engine operating in natural gas/diesel dual-fuel mode. A liquefied natural gas storage and supply system was developed to deliver natural gas as the primary fuel for engine operation. Natural gas was injected into the engine intake manifold and premixed with air before entering the cylinder. Diesel fuel acted as the ignition source to initiate the combustion process. The thesis comprises three main contributing parts:

The first part of the thesis involved a series of experimental studies on the dual-fuel marine engine. These parametric studies examined the effects of various factors such as engine speed, injection timing, fuel mass, dual-fuel substitution ratio, and pilot and post-injection strategies on the engine's combustion, performance, and emission characteristics. Each factor was isolated from the complex engine operations and analysed in detail to gain a comprehensive understanding of its impact. The experimental study identified the optimal engine speed and main injection timing that provided the highest power output, stable operation, and maximum brake thermal efficiency, while pilot injection reduced engine knock and post injection lowered total hydrocarbon emissions.

The second part consisted of a three-dimensional computational fluid dynamics study, aimed at validating and supplementing the experimental findings. Realistic engine geometry and settings were used for the numerical modelling, alongside accurately measured experimental data from extensive engine tests. The simulated numerical results were then compared and analysed against the experimental data to gain deeper insights into the combustion mechanisms within the engine cylinder. These calculated solutions also served as valuable supplements in scenarios where certain parameters were difficult to obtain experimentally. The simulation method accurately predicted engine performance, closely capturing power output and thermal efficiency, revealing a temperature reduction in dual-fuel operation and more uniform and rapid combustion.

The third part of the thesis was an environmental and economic viability study, examining the dual-fuel engine test bed as a complete system. This study aimed to construct a proper thermo-economic model of the engine system. A thorough evaluation of the liquefied natural gas cold-utilized dual-fuel engine system was conducted to investigate its economic and thermoeconomic attributes. Thermoeconomic analysis showed that poly-generation units outperformed single propulsion units in thermal efficiency and exergoeconomic factors, with LNG dual-fuel operation reducing exergy loss, lowering energy costs, and significantly impacting carbon emissions at low throttle and high speed.