Nitrogen-based emissions of a heavy-duty diesel engine converted to neat ammonia spark ignition lean operation

Neat ammonia (NH 3 ) as fuel for heavy-duty compression ignition internal combustion engines converted to spark ignition was recently demonstrated. However, experimental emissions data from such an engine platform and their analysis are scarce. This study presents an original comparison of nitrogen-based emissions from neat NH 3 operation relative to a baseline methane (CH 4 ) operation at equivalence ratios (ϕ) and engine speeds ranging from 0.7 to 1.0 and from 800 rpm to 1200 rpm, respectively. Neat NH 3 increased unburned fuel emissions at comparable engine performance compared to the baseline. In addition, nitrogen oxides (NO x ) emissions increased as ϕ decreased, peaking at ϕ = 0.7. Numerical simulations suggested that contrary to conventional hydrocarbon fuels both the Zeldovich and the non-Zeldovich NO formation pathways played a significant role with neat NH 3 as fuel. The reason was the fuel-bound nitrogen promoting NO generation under lean combustion conditions while contributing to NO consumption under rich conditions. Additionally, numerical results revealed substantial chemical activity during in-cylinder post-combustion phase, with NH 3 radicals reducing NO emissions up to a maximum of 56% at stoichiometric conditions. The NH 3 released from engine crevices produced nitrous oxide (N 2 O) via selective non-catalytic reduction, with rates increasing at leaner operation. Higher engine speed increased NH 3 emissions, while also increasing NO x and N 2 O almost linearly due to higher in-cylinder temperatures, and higher unburned NH 3 , respectively. But results were encouraging because both NO x emissions and the global warming effect of emitted N 2 O were always lower than those from CH 4 operation at similar conditions.