Mechanisms of NO _x Production and Heat Loss in a Dual-Fuel Hydrogen Compression Ignition Engine

The combustion process of a homogeneous hydrogen charge in a small-bore compression ignition engine with diesel-pilot ignition was simulated using the CONVERGE computational fluid dynamics code. Analysis of the simulation results aimed to understand the processes leading to NO_x production and heat loss in this combustion strategy, and their dependence on the hydrogen fuel energy fraction. Previous experimental results demonstrated promising performance, but this comes with a penalty in increased NO_x emissions and potentially higher heat losses. The present study aims to enhance understanding of the mechanisms governing these phenomena. The simulated engine was initialised with a lean homogeneous hydrogen-air mixture at BDC and n-dodecane was injected as a diesel surrogate fuel near TDC. The simulations were validated based on experimental results for up to 50% hydrogen energy fraction, followed by an exploratory study with variation of the energy fraction from 0% to 90%. The addition of hydrogen increased the ignition delay and changed the combustion mode from typical diesel combustion to a premixed burn involving flame propagation. The causes of NO_x emissions and heat loss trends in the simulations are investigated through an analysis of the temperature and equivalence ratio distributions in the engine. The results show that NO_x production peaks at approximately 50% hydrogen, before decreasing as combustion becomes more premixed, which is shown to result in lower peak temperature. Heat loss is significant at all hydrogen energy fractions, but highest at intermediate values. Differences in the wall heat transfer are driven by the near-wall equivalence ratio, turbulence, and combustion phasing.