Gasoline Vs Diesel: Vapor Pressure Differences Explained

Vapor pressure is a measure of the tendency of a material to turn into vapour, and Reid vapor pressure (RVP) is a common measure of and generic term for gasoline volatility. Gasoline has a higher vapor pressure than diesel fuel, which is why gasoline engines are more susceptible to vapor lock. Vapor lock is a problem caused by liquid fuel turning into vapour while still in the fuel delivery system, disrupting the operation of the fuel pump and causing a loss of power or even complete stalling of the engine.

Gasoline's higher Reid vapor pressure

Gasoline has a higher Reid vapour pressure (RVP) than diesel fuel. RVP is a common measure of and generic term for gasoline volatility. It is defined as the pressure at which a hydrocarbon liquid will begin to flash to vapour under specific conditions. In other words, it is the measure of the tendency of a material to change into a gaseous or vapour state.

The RVP of gasoline is 8-14 psi, whereas diesel fuel is far less volatile and has a lower RVP. The higher volatility of gasoline means it has a greater tendency to vaporize and, thus, a higher vapour pressure than diesel fuel.

The U.S. Environmental Protection Agency (EPA) regulates the RVP of gasoline sold at retail stations during the summer high ozone season to reduce evaporative emissions of volatile organic compounds from gasoline, which contribute to ground-level ozone (smog) and associated health problems. During this period, the RVP of gasoline must not exceed 9.0 pounds per square inch. Some areas are subject to a more stringent RVP standard of 7.8 psi.

The RVP of a fuel is important for engine performance and drivability. Higher values of RVP generally result in better cold-start performance, whereas lower values are preferable for preventing vapour lock and other hot fuel handling problems. Gasoline is designed to balance these considerations.

Diesel engines' infrequent vapor lock

Diesel engines rarely suffer from vapor lock. This infrequent occurrence is due to diesel fuel's low volatility. Diesel is far less volatile than gasoline, which has a higher vapor pressure.

Vapor lock is a problem caused by liquid fuel turning into vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the fuel pump, causing a loss of pressure to the carburetor or fuel injection system, resulting in a transient loss of power or complete stalling of the engine. The vapour can be caused by the engine's heat, the local climate, or a lower boiling point at high altitudes.

Vapor lock is more common in older vehicles with carbureted engines, which operate at low pressure and locate the fuel pump higher than the fuel tank. The fuel pump is directly heated by the engine and feeds fuel directly to the float bowl inside the carburetor. This setup increases the risk of a vapor lock developing between the tank and the pump.

Modern engines, on the other hand, are equipped with fuel injection systems and have an electric submersible fuel pump located inside the fuel tank. This design keeps the pump cooler and the entire fuel system under positive pressure, reducing the risk of vapor lock.

While diesel engines rarely experience vapor lock, they are more susceptible to air locks in their fuel lines due to the non-compressibility of diesel fuel. Air locks can be caused by air leaking into the fuel delivery line or entering from the tank when it runs dry, during a fuel filter change, or because of leaky fuel lines. Modern diesel injection systems have self-bleeding electric pumps that eliminate this issue.

Volatility and vapor pressure

Volatility is the property of a liquid fuel that defines its evaporation characteristics. Reid vapour pressure (RVP) is a common measure of and generic term for gasoline volatility. It is defined as the pressure at which a hydrocarbon liquid will begin to flash to vapour under specific conditions. RVP is the measure of the absolute pressure exerted by a liquid at 37.8°C (100°F), at a vapour-to-liquid ratio of 4:1. The test procedure for gasolines and naphthas involves combining one part of chilled liquid sample at 3.6°C with four parts of laboratory air at 21.4°C in a bomb that has a pressure gauge attached to the lid. The bomb is placed in a constant temperature bath at 37.8°C, and when the pressure reaches a constant value, this is recorded as the RVP.

The RVP of gasoline is 8–14 psi, and the percentage of butanes in the stream is the same as the RVP. Butane, one of the many compounds found in pump gasoline, is used to tailor gasoline's vapour pressure to the EPA specs for each US state based on the typical climate throughout the year. Adding about 10% more butane in the winter—thus raising the vapour pressure—helps engines start in colder weather. However, butane contains about 20% less energy than gasoline, so the fuel efficiency will be slightly lower.

The EPA regulates the RVP of gasoline sold at retail stations during the summer high ozone season to reduce evaporative emissions of volatile organic compounds from gasoline that are a major contributor to ground-level ozone (i.e. smog) and diminish the effects of ozone-related health problems. Under the Clean Air Act, gasoline RVP may not exceed 9.0 pounds per square inch during the summer season. However, some areas of the country are subject to a more stringent RVP standard of 7.8 psi.

Vapour lock is a problem caused by liquid fuel changing state to vapour while still in the fuel delivery system of gasoline-fuelled internal combustion engines. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburettor or fuel injection system, resulting in transient loss of power or complete stalling. Vapour lock was far more common in older gasoline-fuel systems. Most modern engines are equipped with fuel injection and have an electric submersible fuel pump in the fuel tank, which helps prevent vapour lock. Diesel fuel is far less volatile than gasoline, so diesel engines almost never suffer from vapour lock.

Fuel viscosity and vapor pressure

The viscosity of a fuel refers to its resistance to flow, or how easily it can be poured. Vapor pressure, on the other hand, is a measure of a fuel's volatility, or how easily it evaporates. These two properties are important factors in the performance and suitability of a fuel for different applications.

Gasoline vs Diesel

Gasoline has a lower viscosity than diesel. This means that gasoline is less resistant to flow and pours more easily than diesel. Gasoline also has a higher vapor pressure than diesel, which means it has a higher volatility and evaporates more readily.

The Impact of Climate

The difference in viscosity between gasoline and diesel can be particularly notable in cold climates. Diesel fuel tends to have a higher viscosity, which means it can become thicker and more difficult to pour in colder temperatures. To address this issue, diesel engines are often equipped with heating systems that warm the fuel, reducing its viscosity and ensuring proper combustion.

In warmer climates, the higher vapor pressure of gasoline can lead to a problem known as "vapor lock." Vapor lock occurs when gasoline changes from a liquid to a vapor state within the fuel delivery system, disrupting the operation of the fuel pump and causing a loss of power or even complete engine stalling. This issue is less common in modern fuel injection systems, but it can still occur in certain conditions, such as high temperatures or high altitudes.

Viscosity and Vapor Pressure in Aviation Fuels

The viscosity of aviation fuels is a critical factor in their performance. For example, jet fuel must have the right viscosity to be properly atomized during combustion. If the viscosity is too high, the fuel may not atomize finely enough, leading to incomplete combustion and reduced engine performance.

Aviation fuel is also manufactured with lower vapor pressure than automotive gasoline to prevent vapor lock at high altitudes. This is a significant safety consideration, as vapor lock has been known to cause forced landings in aircraft.

In summary, fuel viscosity and vapor pressure are important characteristics that influence the performance and suitability of different fuels. Gasoline has a lower viscosity and higher vapor pressure than diesel, which can impact their behavior in different climatic conditions. In aviation, fuel viscosity and vapor pressure are critical factors that must be carefully controlled to ensure safe and efficient operation.

The EPA's seasonal RVP regulations

Gasoline has a higher vapour pressure than diesel fuel due to its volatility, which is the property of a liquid fuel that defines its evaporation characteristics. Reid vapour pressure (RVP) is a common measure of and generic term for gasoline volatility.

The Environmental Protection Agency (EPA) regulates the RVP of gasoline sold at retail stations during the summer high ozone season (from May 1 to September 15 for most regulated parties, and from June 1 to September 15 for retailers and wholesale purchaser-consumers). This regulation aims to reduce evaporative emissions of volatile organic compounds from gasoline, which are a major contributor to ground-level ozone (smog) and the associated health problems.

Under the Clean Air Act (CAA) section 211(h), the RVP of gasoline must not exceed 9.0 pounds per square inch during the summer season. However, some areas are subject to a more stringent RVP standard of 7.8 psi. The CAA also provides a 1.0 psi RVP allowance for gasoline containing 10% ethanol.

The EPA has the authority to relax the more stringent 7.8 psi RVP standard to the 9.0 psi standard if a state submits a revision to its attainment demonstration or maintenance plan, demonstrating timely attainment or continued attainment of the ozone National Ambient Air Quality Standard.

On December 4, 2020, the EPA revised and relocated the federal RVP regulations to 40 CFR 1090.215. The current regulatory text sets a maximum per-gallon RVP limit of 9.0 psi for summer gasoline (June 1 to September 15) unless a lower RVP limit is required. Lower RVP limits may be applicable in areas that are subject to the federal 7.8 psi RVP standard, required to sell federal reformulated gasoline, or subject to a state fuel rule in an approved State Implementation Plan that mandates an RVP of less than 9.0 psi.

The EPA has approved relaxation of the summer gasoline RVP standards for several states and areas since 2014, and the specific designations and boundaries of areas subject to the federal 7.8 psi RVP standard can be found in the regulatory text. Alaska, Hawaii, and U.S. territories are generally exempted from federal volatility regulations.

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