The critical emissions component for hydrogen internal
combustion engines are oxides of nitrogen (NOX).
Unlike hydrocarbon (HC) and carbon monoxide (CO)
emissions which are the result of incomplete combustion
with conventional fuels, NOX emissions are a result of
using air as the oxidizer (containing nitrogen, besides
oxygen) and high combustion temperatures. Fig. 5 shows
the well-established trend [14]–[16] for NOX emissions as
a function of fuel-to-air equivalence ratio, with homogeneous
hydrogen operation.