Ammonia-methanol and ammonia-ethanol dual-fuel combustion in an optical spark-ignition engine: A multiple flame generation approach

Abstract

• Compared ammonia-methanol and -ethanol dual-fuel systems. • Investigated multi-ignition effects on ammonia-alcohol blends. • Analyzed flame characteristics using high-speed imaging. • Extended lean combustion limit with multiple sparks and methanol. • Assessed pollutant emissions for various blends and conditions. The utilization of ammonia as a fuel has received considerable attention within combustion research communities in recent years, primarily due to its absence of carbon content. Various research initiatives are currently addressing the challenges associated with ammonia combustion, which include ignition difficulties, slow combustion rates, and poor combustion stability. This study investigated the incorporation of more reactive fuels, specifically methanol and ethanol, into ammonia to enhance its reactivity. It compared engine performance across ammonia-methanol and ammonia-ethanol blends at varying energy fractions. Additionally, the study examined the influence of multiple spark ignition sites on combustion performance across different ammonia-alcohol blending scenarios. Experimental evaluations were carried out utilizing a four-stroke, single-cylinder optical spark-ignition engine. The findings indicated that combustion of pure ammonia at lower compression ratio resulted in unstable combustion and lower engine performance when employing a conventional single spark plug. In contrast, the introduction of alcoholic fuels in conjunction with ammonia resulted in a faster combustion rate and higher in-cylinder pressure, thereby improving combustion stability and overall engine efficiency. In addition, the research demonstrated that the addition of methanol to ammonia yielded superior performance in comparison to ethanol blends, attributed to methanol’s higher flame speed. It was also observed that employing multiple spark plugs alongside alcohol blends led to increased heat release and shortened combustion duration, resulting in enhanced engine power output and efficiency. Furthermore, the study used a high-speed natural-flame-luminosity imaging technique to observe the flame front propagation for various combustion cases. The results showed that methanol addition in ammonia resulted in higher flame area proportion, flame intensity, and flame propagation speed than ethanol addition. Moreover, this research work further detailed the emissions produced under various combustion conditions. Notably, methanol addition to ammonia resulted in higher carbon monoxide (CO) and carbon dioxide (CO 2 ) emissions relative to ethanol addition. Conversely, ethanol blends exhibited elevated levels of nitrogen oxides (NOx) and total unburned hydrocarbons (THC) compared to those with methanol. Additionally, the research analyzed the effects of three distinct air–fuel equivalence ratios (λ) of 1.0, 1.2, and 1.4 on the combustion characteristics of ammonia-methanol blends. It was concluded that leaner combustion conditions yielded higher NOx emissions while resulting in lower CO and CO 2 emissions when compared to stoichiometric mixture conditions.