The effects of the length of the gas flow path from the exhaust outlet in the cylinder head to the catalyst inlet in the exhaust line, the ignition timing and the engine idle speed on the three-way catalyst light-off behavior in an electronically controlled inlet port LPG (liquefied petroleum gas) injection SI (spark-ignition) engine during cold start were investigated experimentally. The results showed that these factors affect the catalyst light-off behavior significantly during cold start. The reduction of the gas flow path length upstream the catalyst reduces the heat loss from the exhaust gases, increases the temperature of the catalyst, and results in faster catalyst light-off. Retarding the ignition timing from 0 to 15 degrees CA ATDC decreases 22 and 8 s catalyst light-off time for HC and CO respectively. Increasing the engine idle speed from 1400 to 1800 rpm decreases 19 and 15 s the light-off time for HC and CO respectively. (C) 2010 Elsevier Ltd. All rights reserved.