Enhancing Low Temperature Lean Combustion of CH4-H2 Blends through a Prechamber Equipped Engine

<div class="section abstract"><div class="htmlview paragraph">The use of hydrogen as a sustainable fuel in the short term is hampered by the impossibility of large scale use due low availability. In order to promote decarbonization, complementary solution for a smooth transition is to dilute it in a mixture with methane, in a current Port Fuel Injection (PFI) internal combustion engine (ICE). This can be done as a retrofit after limited structural modifications, such as the introduction of a passive prechamber. Such a solution allows a reduction of the carbon footprint of traditional ICEs through more efficient combustion (both the prechamber technology and the hydrogen fuel properties promote an increase in combustion speed) and a reduced carbon content in the fuel.</div><div class="htmlview paragraph">The present research activity has been carried out through numerical investigation based on three-dimensional CFD analyses to simulate the behavior of a natural gas engine fueled with CH4-H2 blends. The combustion mechanism for the fuel blend was validated against measurements of the flame front propagation carried out on an optical engine.</div><div class="htmlview paragraph">The focus of the work is to evaluate the effect of both the hydrogen share and the charge dilution. The introduction of a passive prechamber was necessary to stabilize the mixture ignition and increase the combustion speed. In particular, simulations were carried out for increasing hydrogen content, from 0% to 30% in volume, and increasing excess air from stoichiometric (λ=1.0) to ultra-lean (λ=1.8), in order to achieve a Low Temperature Combustion (LTC). For each operating condition investigated, the ignition advance was optimized in order to maximize the gross indicated mean pressure (GMEP). The results show that, at λ = 1.4, 20% H2 is sufficient to obtain a slightly higher GMEP than using pure CH4, while at the same time guaranteeing a 40% reduction of raw NOx emissions. When further increasing the value of λ up to 1.8, slow combustion is obtained even with 30% share of H2. In this case, hot EGR is needed to increase the reactivity of the mixture and allow achieving a complete combustion.</div></div>