accelerator has 9 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a valve that regulates the supply of fuel to the engine
(chemistry) a substance that initiates or accelerates a chemical reaction without itself being affected
a scientific instrument that increases the kinetic energy of charged particles
Synonyms, antonyms and the broader and narrower relations for accelerator come from Princeton University’s WordNet. Full sources and licences.