Vehicle Emissions Reduction

One of the most important fuel parameters is the vapor pressure. It is extremely important that the vapor pressure be well adjusted to the climactic conditions during the year. The vapor pressure for each season must be as low as possible in order to minimize evaporation from storage terminals and vehicles but still sufficiently high to give safe cold starts.

Fuel volatility, as measured by Reid vapor pressure (RVP) has a marked effect on evaporative emissions from gasoline vehicles both with and without evaporative emission controls. Tests on vehicles without evaporative emission controls showed that increasing the fuel RVP from 9 pounds per square inch (psi) (62 kilopascals) to approximately 12 psi (82 kPa) roughly doubled evaporative emissions. The percentage effect is even greater in controlled vehicles. In going from 9 psi (62 kPa) to 12 (81 kPa) RVP fuel, the US EPA found that average diurnal emissions in vehicles with evaporative controls increased by more than 5 times, and average hot-soak emissions by 25-100%. The large increase in diurnal emissions from controlled vehicles is due to saturation of the charcoal canister, which allows subsequent vapors to escape to the air.

Vehicle refueling emissions are also strongly affected by fuel volatility. In a comparative test on the same vehicle, fuel with 11.5 psi (79 KPA) RVP produced 30% greater refueling emissions than gasoline with 10 psi (64 KPA) RVP (1.45 vs. 1.89 g/liter dispensed). In response to data such as these, the US EPA has established nationwide summertime RVP limits for gasoline.

An important advantage of gasoline volatility controls is that they can affect emissions from vehicles already produced and in-use and from the gasoline distribution system. Unlike new-vehicle emissions standards, it is not necessary to wait for the fleet to turn over before they take effect. The emissions benefits and cost-effectiveness of lower volatility are greatest where few of the vehicles in use are equipped with evaporative controls. Even where evaporative controls are in common use, as in the US, control of volatility may still be beneficial to prevent in-use volatility levels from exceeding those for which the controls were designed.

In its analysis of the RVP regulation, the US EPA (1987) estimated that the long-term refining costs of meeting a 9-psi (62 KPA) RVP limit throughout the US would be approximately US$0.0038 per liter, assuming crude oil at US$20 per barrel. These costs were largely offset by credits for improved fuel economy and reduced fuel loss through evaporation, so that the net cost to the consumer was estimated at only US$0.0012 per liter.