Vapor pressure

Vapor pressure is a common measure of gasoline volatility. It indicates how gasoline will perform under different operating conditions.

For easy engine starting, gasoline with a high vapor pressure is preferred. However, it also influences the engine’s evaporative emissions. Volatile organic compounds emitted when gasoline vaporizes will create ozone when they react with nitrogen oxides.

For these reasons, gasoline specifications set a maximum vapor pressure limit to help reduce evaporative emissions and thereby reduce ozone formation. The specifications also set a minimum limit to ensure that engines start easily.

New Zealand’s Engine Fuel Specifications Regulations 2011 set a minimum vapor pressure limit of 45 kPa for all grades sold across the country all year round.

However,