Petrochemical Fuels, Their Uses, Facts and Myths.

There is an email that has been circulating around the web that implores readers to fill their gas tanks early in the morning while using the slowest setting on the pump. The reasons that it gives for doing so are false. As an informed worker in the transportation field, it is incumbent upon me to set the record straight regarding these myths.

This email has been circulating for some time. While the temperature component of delivered gasoline or other aliphatic hydrocarbon fuels via a closed pipeline system is true, the low vapor pressure and high volatility of gasoline does not account for significant losses when fueling your car at the pump regardless of pumping speed. Moreover, most pumps are calibrated with a temperature compensation component.

Not all pumps have vapor collection and return on them. Only newer pumps in certain areas that mandate them do. The largest contributor of gasoline vapors and other unburnt hydrocarbons in the atmosphere does not come from fueling or operating your vehicle, it actually comes from tanker truck accidents that occur while delivering fuel to gas stations or leaking underground storage tanks!

Gasoline is highly volatile but only burns within a very narrow range of vapor concentrations. Gasoline has a lower explosion limit of 1.4% by volume and an upper explosion limit of 7.6% by volume. This means that gasoline vapors mixed with atmospheric air at concentrations below 1.4% by volume will fail to ignite because the mixture is too lean, and gasoline vapors mixed with atmospheric air at concentrations above 7.6% by volume will fail to ignite because the mixture is too rich.

Gasoline is more volatile than diesel oil, Jet-A or kerosene, not only because of the base constituents, but because of the additives that are put into it. The final control of volatility is often achieved by blending with butane. The Reid Vapor Pressure (RVP) test is used to measure the volatility of gasoline. The desired volatility depends on the ambient temperature. In hot weather, gasoline components of higher molecular weight and thus lower volatility are used. In cold weather, too little volatility results in cars failing to start. This is the reason behind differing blends of gasoline for each season.

In hot weather, excessive volatility results in what is known as "vapor lock", where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines, rendering the fuel pump ineffective and starving the engine of fuel. This effect mainly applies to (older) camshaft-driven (engine mounted) fuel pumps, which lack a fuel return line. Vehicles with fuel injection require the fuel to be highly pressurized, to within a set range (usually an extremely narrow range). Because camshaft speed is nearly zero before the engine is started, an electric pump is used. It is located in the fuel tank so that the fuel may also cool the high-pressure pump. Pressure regulation is achieved by use of a pressure modulator that returns unused fuel to the tank. Therefore, vapor lock is almost never a problem in a (newer) vehicle with fuel injection.

In the United States, volatility is regulated in large cities to reduce the emission of unburned hydrocarbons. In many of those large cities, so-called reformulated gasoline that is less prone to evaporation, among other properties, is required. Most countries simply have a summer, winter, and perhaps intermediate limit.

Volatility standards may be relaxed (allowing more gasoline components into the atmosphere) during gasoline shortages. For example, on 31 August 2005, in response to Hurricane Katrina, the United States permitted the sale of non-reformulated gasoline in some urban areas, effectively permitting an early switch from summer to winter-grade gasoline. As mandated by EPA administrator Stephen L. Johnson, this "fuel waiver" was made effective until 15 September 2005.^[9]

Modern automobiles are also equipped with an evaporative emissions control system (called an EVAP system in automotive jargon), which collects evaporated fuel from the fuel tank in a charcoal-filled canister that is usually located inside the engine compartment while the engine is stopped and then releases the collected vapors into the engine intake for burning when the engine is running (usually only after it has reached normal operating temperature.) The evaporative emissions control system also includes a sealed gas cap to prevent vapors from escaping via the fuel filler tube. There is also a myth around "stuffing" your car with gasoline which says that the extra gasoline just evaporates into the atmosphere. As you can see, that would be an impossible thing in such a closed system. Modern vehicles with OBD-II emissions control systems will illuminate the malfunction indicator light, (MIL) "check engine" or “Service Engine Soon” light if the leak detection pump (LDP) detects a leak in the EVAP system. If the Electronic Control Unit (ECU) or Powertrain Control Module (PCM) detects a leak, it will store an OBD-II code representing either a small or a large leak, thus illuminating the MIL to indicate a failure. Some vehicles can detect whether the gas cap is incorrectly fitted, and will indicate this by illuminating a gas cap symbol on the dash.

Energy content (High and low heating value)

Gasoline contains about 35 MJ/L (9.7 kW·h/L, 132 MJ/US gal, 36.6 kWh/US gal) (higher heating value) or 13 kWh/kg. This is an average; gasoline blends differ, and therefore actual energy content varies from season to season and from batch to batch, by up to 4% more or less than the average, according to the US EPA. On average, about 19.5 US gallons (16.2 imp gal; 74 L) of gasoline are available from a 42-US-gallon (35 imp gal; 160 L) barrel of crude oil (about 46% by volume), varying due to quality of crude and grade of gasoline. The remaining residue comes off as products ranging from tar to naptha.^[10]

Below is a chart that compares the volumetric and mass energy density of some popular fuels compared with gasoline (in the rows with gross and net, they are from ^[11]):

        

Fuel type	Gross MJ/L	MJ/kg	Gross BTU/Gal (imp)	Gross BTU/Gal (U.S.)	Net BTU/Gal (U.S.)	RON
Conventional Gasoline	34.8	44.4	150,100	125,000	115,400
Autogas (LPG) (60% Propane + 40% Butane)	26.8	46				108
Ethanol	21.2	26.8	101,600	84,600	75,700	113
Methanol	17.9	19.9	77,600	64,600	56,600	123
Butanol	29.2	36.6				91-99
Gasahol	31.2		145,200	120,900	112,400	93-94
Diesel(*)	38.6	45.4	166,600	138,700	128,700	25
Biodiesel	33.3-35.7			126,200	117,100
Avgas (High-Octane Aviation Gasoline)	33.5	46.8	144,400	120,200	112,000
Jet Fuel (Kerosene Based)	35.1	43.8	151,242	125,935
Jet Fuel (Naphtha)				127,500	118,700
LNG (Liquefied Natural Gas)	25.3	~55	109,000	90,800
LPG (Liquefied Petroleum Gas)				91,300	83,500
Hydrogen	10.1(at 20 Kelvin)	142			130

(*) Diesel is not used in gasoline engines, so its low octane rating is not an issue; the relevant metric used for diesel engines is the cetane number because it is burned by a different process called detonation instead of the deflagration process that occurs in other internal combustion engines.

As you can see from the above chart, Ethanol (Cornahol) and Methanol (wood alcohol) have a higher octane rating (RON) than conventional gasoline (which only means that it burns hotter and cleaner) but has a significantly lower energy density as measured by both mass and volume. This lower energy density coupled with the highly corrosive effects on internal combustion engine parts makes its use as a large-scale replacement fuel for conventional gasoline very impractical without massive government subsidies, which is a hidden tax on all taxpayers. The highly corrosive effects of both Ethanol and Methanol require that it cannot be pumped through a pipeline structure; instead, it must be trucked to market further increasing costs to bring it to market. The highest energy density based on mass in order goes Hydrogen, Liquefied Natural Gas, Aviation Gas, Autogas (LPG), Diesel, and then Conventional Gasoline. What makes Hydrogen impractical is that it must be stored cryogenically to achieve the highest energy density by mass (note the temperature of 20 Kelvin in the chart).

The real tricks to adopt are maximizing your driven mile efficiencies by combining your trips and carefully planning your route for the least miles driven. Avoid excessive idling. Alter your driving habits to avoid jack-rabbit starts or excessive (hard) acceleration. Slow down gradually well in advance of intersections with traffic control devices or signs and avoid hard braking situations; this means no tailgating! Proper tire inflation and vehicle maintenance including regular oil changes will help you to control your expenditures on gasoline. Carpool when possible, and simply avoid all unnecessary driving. Beyond that, the prices we pay at the pump are primarily driven by environmental, geopolitical, and market forces that are largely beyond our individual control. How or when you fuel your vehicle is of very little consequence to how much we pay for that fuel or the efficiencies that we can achieve in the grand scheme of things.