Based　on　a　retrofitted　Wankel　engine,　fundamental　experiments　and　numerical　simulations　were　conducted　to　investigate　the　role　of　the　hydrogen　addition　under　stoichiometric　and　lean　combustion　conditions.　The　testbench　recording　of　in-cylinder　pressure,　heat　release,　cycle-to-cycle　fluctuation,　and　exhaust　emissions　were　used　to　characterize　fuel　combustion.　The　verified　CFD　model　clarified　the　inherent　mechanisms　behind　experimental　data.　Results　showed　that　the　introduction　of　hydrogen　addition　caused　combustion　temperature　to　increase,　the　timing　taken　for　active　radical　formations　was　significantly　advanced,　and　the　peak　concentrations　increased.　This　contributed　to　the　increased　pressure　and　the　shortened　combustion　phasing　in　experiments.　There　was　a　proportional　correspondence　of　the　early　flame　growth　and　cyclic　variation.　Hydrogen　addition　has　a　minimal　effect　on　the　turbulent　intensity　after　spark　timing.　However,　adopting　a　highly　diluted　mixture　caused　the　enhanced　turbulent　intensity,　which　was　responsible　for　improving　the　stability　of　the　engine.　Due　to　the　synergistic　effects　of　the　work　delivery,　cyclic　fluctuation,　and　exhaust　temperature,　a　larger　hydrogen　addition　with　stoichiometric　operation　showed　lower　higher　brake　thermal　efficiency.　Nitrogen　oxide　emissions　increased　with　hydrogen　enrichment　and　reduced　with　a　higher　degree　of　mixture　dilution　owing　to　a　close　positive　correlation　between　the　mass　elevated-temperature　and　NOx　formation.　Benefits　from　sufficient　oxygen　concentration　and　elevated　combustion　temperature,　the　chain　transfer　process　can　be　accelerated.　This　explained　why　CO　emissions　decreased　at　a　larger　hydrogen-enriched　and　leaner　condition　in　the　experimental　investigation.　Increasing　the　hydrogen　content　provided　improvement　in　the　desired　HC　emissions　for　all　operating　points.