Gasoline still evaporates in cold weather, but the rate is significantly reduced compared to warmer temperatures. Gasoline is a complex liquid mixture of various hydrocarbon molecules, each with a different boiling point and tendency to vaporize. This process of a liquid turning into a gas, known as evaporation, is continuous as long as the molecules possess sufficient energy to escape the liquid surface. The evaporation characteristics of a fuel are defined by its volatility.
The Fundamental Science of Evaporation
Evaporation occurs when high-energy molecules at the surface of a liquid overcome the forces holding them together and transition into the gas phase. This molecular escape happens across a wide range of temperatures. Gasoline is a highly volatile liquid because its constituent hydrocarbons have relatively low boiling points, meaning they require less energy to become a vapor.
The tendency of a liquid to vaporize is measured by its vapor pressure. This is the pressure exerted by the gas molecules in equilibrium with the liquid. A substance with higher volatility will have a higher vapor pressure at a given temperature, meaning more molecules are present in the gaseous state. Even in cold conditions, gasoline retains a measurable vapor pressure because the mixture contains lighter hydrocarbons.
How Temperature Affects Vapor Pressure
The temperature of the gasoline directly influences the kinetic energy of its molecules, which determines the vapor pressure. As the temperature drops, the molecules move slower and possess less energy, making it more difficult for them to break free from the liquid. This decrease in molecular energy leads to an exponential drop in the fuel’s vapor pressure, significantly slowing the rate of evaporation.
This relationship creates a challenge for internal combustion engines, which require a specific ratio of fuel vapor mixed with air to ignite properly. To counteract the natural reduction in vapor pressure caused by cold air, the petroleum industry adjusts the fuel composition seasonally by creating a “winter blend” gasoline. This winter formulation is engineered to be more volatile, meaning it has a higher vapor pressure than the summer blend.
The industry uses Reid Vapor Pressure (RVP) to measure and control this volatility. Winter blends are rated higher to ensure adequate vaporization in cold conditions. Refiners achieve this by adding highly volatile components like butane, which vaporizes easily at lower temperatures. The higher RVP of the winter blend helps compensate for the temperature drop, allowing the engine to start correctly.
Practical Effects of Gasoline Volatility in Winter
The formulation of gasoline directly impacts a vehicle’s performance in cold weather, particularly during startup. If the fuel’s vapor pressure is too low, such as using summer-blend gasoline in freezing temperatures, there will not be enough fuel vapor in the cylinder for the spark plug to ignite the mixture. This lack of sufficient vapor is the primary reason why engines can be difficult to start or may run rough until they warm up.
The increased volatility of winter gasoline, achieved by blending in light hydrocarbons, provides the necessary vapor for reliable cold-start performance. These lighter components contain less energy per gallon than the heavier hydrocarbons used in summer blends. As a result, vehicles may experience a decrease in fuel economy during the colder months due to the change in the fuel’s energy density.
Stored gasoline is also affected by volatility adjustments, which is an issue for small engines like those in lawnmowers or snowblowers. If winter-blend fuel is stored until the warmer months, its high volatility can lead to excessive evaporation and pressure buildup in unvented containers. Conversely, using summer-blend gasoline (low RVP) in a small engine during winter can make it almost impossible to start due to insufficient vapor production.