The　auxiliary　power　unit　of　hybrid　electric　vehicle　(HEV)　is　an　engine　with　small　displacement,　which　could　charge　the　battery　and　directly　drive　the　air-conditioning　system.　In　order　to　reduce　the　noise　of　engine,　two　silencers　are　used.　The　front　muffler　is　designed　as　a　reactive　silencer　with　expansion　chambers.　The　rear　muffler　is　just　the　primary　one　of　the　engine.　After　the　noise　experiments,　it　can　be　concluded　that　the　two　silencers　have　especially　much　more　effects　at　both　high　and　medium　frequencies　than　at　low　frequency.　The　two　silencers　can　lower　engine　noise　about　5　dB　during　the　whole　engine　speed　range　without　reducing　the　engine　power　too　much.