Combustion Cycle-To-Cycle Variation Analysis in Diesel Baseline Hydrogen-Fueled Spark-Ignition Engines

Abstract

<div class="section abstract"><div class="htmlview paragraph">In the search for zero-carbon emissions and energy supply security, hydrogen is one of the fuels considered for internal combustion engines. The state-of-the-art studies show that a good strategy to mitigate NOx emissions in hydrogen-fueled spark-ignition engines (H2ICE) is burning ultra-lean hydrogen-air mixtures in current diesel architectures, due to their capability of standing high in-cylinder pressures. However, it is well-known that decreasing equivalence ratio leads to higher engine instability and greater cycle-to-cycle variations (CCVs). Nevertheless, hydrogen flames, especially at low equivalence ratios and high pressures, present thermodiffusive instabilities that speed up combustion, changing significantly the flame development and possibly its variability. This work evaluates the hydrogen combustion and their CCVs in two single-cylinder diesel baseline H2ICEs (light-duty and medium-duty) and their influence on performance parameters. The analysis is done using three CCV indicators (for flame initiation, propagation, and end-flame periods) in four main strategies: varying fuel-air equivalence ratio (from 0.2 to 0.8), swirl intensity, spark timing, and spark plug type. The cyclic variations are higher at low loads and leaner mixtures. While, at high loads, the engine presents low combustion CCVs, around 10 % in all combustion phases, at idle they can go up to 20 % in the flame propagation phase (10 to 50 % of mass fraction burned - MFB). The fluctuations of the flame propagation duration are highly impacted by the equivalence ratio. Furthermore, the behavior of the combustion duration at the initiation (0 to 10 % MFB) and propagation phases suggests that other phenomena play an important role in hydrogen combustion in engines besides the laminar burning velocity property. For this, a flame speed enhancement model which considers hydrogen’s intrinsic instabilities is applied to evaluate the flames at the operating conditions.</div></div>