Combustion system optimization for dimethyl ether using a genetic algorithm

Abstract

Dimethyl ether is a gaseous fuel which can easily be liquefied under moderate pressures. Its high reactivity makes it suitable for combustion in a compression ignition engine, and due to the high oxygen content, its combustion is virtually free of soot. The high oxygen content and low density of dimethyl ether lead to a lower volumetric heating value compared to Diesel fuel. Therefore, the hydraulic flow rates of the injectors have to be increased with larger nozzle holes. The influence of larger nozzle holes on the dimethyl ether spray formation and ignition are presented in this article. Experimental investigations were conducted at a constant-pressure vessel with optical access and with a single-cylinder research engine. Subsequently, a numerical optimization of the piston bowl and injector nozzle has been carried out. A very fast air/fuel mixture formation with dimethyl ether was observed, which leads to a lean combustion with small nozzle diameters. With increasing nozzle diameters, the combustion moves toward stoichiometric conditions and with very large diameters to rich combustion conditions. The ignition delay for small diameters is mostly dominated by the lean mixture, and for large diameters, the ignition delay is strongly influenced by cooling effects. For the optimization, the oxidation potential number was maximized, which proved suitable to simultaneously increase efficiency and reduce emissions. A conventional ω-shaped bowl and a step bowl have been optimized, and large bowl diameters were found to be beneficial for dimethyl ether combustion. Furthermore, nozzle diameters around 150 µm showed the most promising results. Compared to the dimethyl ether reference, the simulations with the optimized ω-shaped bowl showed a power increase of 2.7%. Experimentally, the optimized ω-shaped bowl in combination with the reference injector showed an efficiency increase by more than 1% at 2000 r/min full load compared to the dimethyl ether reference.

Impact of increased injector nozzle hole diameters on engine performance, exhaust particle distribution and methane and formaldehyde emissions during dimethyl ether operation

“…This can be explained with the superior mixture formation of DME due to its volatility. Previous investigations 30 in an HP chamber showed that for DME the mixing time scale is much shorter compared to the chemical ignition delay time, which leads to a very lean combustion. This was especially observed for small nozzle hole diameters.…”

“…Previous studies, however, showed very fast mixing of DME and therefore more premixed mixture is available before ignition, leading to high HRR despite the higher cetane number of DME. 28 The higher burning rates during DME operation compared to diesel fuel operation also lead to reduced combustion durations, indicated by the GHR. Especially, the burnout behavior of DME is improved compared to diesel fuel, which also leads to the reduced HC and CO emissions.…”

“…The high load point is representative of the current Worldwide Harmonized Light Duty Test Procedure (WLTP) cycle. For higher load points, the reader is referred to other publication by Zubel et al 28 The location of these points within the engine load map is shown in Figure 2.…”

Ignition Process and Flame Lift-Off Characteristics of dimethyl ether (DME) Reacting Spray

“…Compression ignition application using DME (engine and combustion vessel experiments) as fuel have been researched in several studies (Mitsugi et al, 2015;Lim and Lee, 2016;Benajes et al, 2018;Zubel et al, 2019;Han et al, 2020), which found that DME combustion behaves similarly to diesel. This is also in agreement with the findings of the current study.…”

Optimization of diesel fuel injection strategies through applications of cooperative particle swarm optimization and artificial bee colony algorithms

“…Small populations, or micro-genetic algorithms, have been experimentally applied to optimize diesel engine combustion in the literature. 16 GA has also been used for optimizing diesel engine performance using alternative fuels 17 and various strategies for efficiency improvement and noise reduction. 18,19 Artificial bee colony (ABC) algorithm, developed by Karaboga, 20 mimics the way bees find, develop, and eventually abandon food sources.…”