Gasoline, the fuel that provides combustion for the vast majority of road vehicles, is a widely taken for granted. More than simply flammable liquid, the complexities of formulation and the detriment of combustion efficiency as fuel ages are widely ignored by the public. With a better understanding of the fuel source for our internal combustion engines, the technician is better armed when diagnosing hard-start, power loss or detonation issues.
Unless the “low fuel” light is glowing, few people think about gasoline. We tend to have a favorite brand or gas station, but most of us know nothing about what we’re putting in the tank. It’s not often you’re faced with a car that won’t run right because there’s something wrong with the gasoline, which is exactly way the problems caused by bad gas are can be difficult to recognize. To understand how gasoline works and what happens when it doesn’t work properly, we need to know more about how it’s made and how it works in an engine.
Gasoline is one of the most highly-developed consumer products ever made. It’s a liquid fuel, but liquid can’t burn until it becomes a vapor. The first carburetor was just a heated plate that vaporized liquid fuel dripping onto its flat surface. Air flowing over the plate carried fuel vapors into the engine, and if the mixture was just right, it ignited in the cylinder to drive the piston. Early engines could run on almost any flammable liquid: the most common fuel was kerosene.
As engines improved, better fuels were needed. Today, instead of sucking fuel vapor into the combustion chamber, we inject finely-atomized liquid fuel directly at the intake valve or at the piston, relying on heat inside the chamber to vaporize the fuel. For this to work correctly, the fuel must vaporize consistently over a wide range of conditions. Fuel that vaporizes at the wrong time or place can be hard to use, and it can even damage an engine. To understand how gasoline vaporizes, we need to understand volatility and vapor pressure.
Gasoline is a distilled product. The most basic distillation apparatus is just a kettle with a lid that has a small hole at the top. A tube leads from that hole to a collection vat. When liquid is heated in the kettle, it evaporates and the warm vapors rise up and flow out through the hole and into the tube. There the vapor cools and condenses back to a liquid that flows down into the collection vat.
If the liquid is water, then purified water flows to the collection vat. But if another liquid with a lower boiling temperature is mixed in with the water, it can be separated from the water by heating the vat to just above that temperature and keeping it there until no more liquid flows from the tube. For instance, alcohol boils at 173 degrees Fahrenheit, so...
Gasoline is distilled from crude oil, a mixture of dozens of different liquids that boil or “distill out” at different temperatures. The best crude oil can yield almost 50% gasoline by volume, but raw gasoline is actually several different liquids that boil at different temperatures. Some boil at less than 100 degrees F while others require more heat, up to 450 degrees F. The liquid that distills at each different temperature has specific characteristics that affect its performance as a motor fuel.
Gasoline that boils at a lower temperature evaporates more easily because it’s a smaller, lighter molecule, while the heavier molecules boil at higher temperatures. Modern oil refineries extract each different “weight” of gasoline into its own collection vat and then blend them in just the right proportion to produce a motor fuel with specific qualities. Other chemicals are added later, but the fuel that leaves the refinery is a specific mixture of all these different flavors of pure gasoline.
All gasoline from every oil refinery is made to ASTM Standard D4814, which provides the official, worldwide definition of gasoline. The standard specifies how the fuel must perform in certain lab tests. The most critical spec is volatility, which determines how easily the fuel evaporates (remember, liquid fuel won’t burn). The standard includes provisions for adjusting volatility.
A highly volatile gasoline contains more of the lighter molecules, and it’s needed for cold-starts when there is no heat in the engine to vaporize the fuel. However, high volatility also promotes vapor lock in a hot engine, and it increases evaporative emissions. So gasoline is blended at the refinery to adjust its volatility for the engines and conditions where it will be used. We measure the volatility of gasoline using the Reid Vapor Pressure Test.
When two different liquids are mixed together without creating a chemical reaction, they don’t form a new molecule. Some may form a weak chemical bond, like water and alcohol, and this makes the mixture more stable. But each is still a distinct molecule, and the liquids can be separated from each other through distillation. Given enough time, some liquid mixtures, like gasoline, will separate all by themselves at ambient temperature and pressure.
Reid Vapor Pressure
Fill a container part way with liquid, put a lid on it and add heat. Some of the liquid will evaporate (vaporize), increasing the pressure in the sealed container. When the pressure is stabilized at 100 degrees F, the pressure inside the container is the Reid Vapor Pressure (RVP) of that liquid. A liquid with higher RVP evaporates more easily; it’s more volatile. The RVP of water is just under 1 psi, while the RVP of gasoline can be as high as 15 psi. In other words, gasoline evaporates more easily than water.
Gasoline is a mixture of liquids that evaporate at different temperatures, so they have different vapor pressures. Consider a tank of gasoline in a moving car in summer time. As the fuel is being heated and shaken, the lighter molecules evaporate while the heavier ones are left behind in liquid form. The vapor pressure of the remaining fuel, and therefore its ability to vaporize, will decrease.
Usually a tank of fuel is consumed long before its volatility changes enough to affect engine performance. But anyone reading this magazine knows something about the problems caused by old, stale gas. If enough of the lighter molecules (light ends) evaporate out, it won’t vaporize in a cold engine. Even if you get it running with starter fluid, the engine still might not run properly until fresh gas is added to the tank. In a gas tank, gasoline has a shelf life of about 100 days.
Highly aromatic fuels that vaporize easily cause higher evaporative emissions in hot weather, especially at higher altitudes where RVP and ambient pressure are closer to each other. Also, on carbureted engines with low fuel pump pressure, vapor lock is more likely. So gasoline is blended to provide the appropriate RVP for where and how the fuel will be used. RVP is also ‘fine-tuned’ with additives after the gasoline leaves the refinery.
Octane and additives
In a spark ignition engine, the sparkplug fires just a few degrees before the piston reaches top dead center (TDC), and combustion spreads outward from that point in what’s called a ‘flame front.’ Pressure in the cylinder rises sharply, peaking just a few degrees after TDC to drive the piston. In order for this to work correctly, combustion must start at the sparkplug and spread evenly from there.
As cylinder pressure rises, it’s possible for the fuel to ignite at a second location in the chamber, causing an early and uncontrolled rise in cylinder pressure as the two flame fronts meet. This is called detonation, and we can hear it as a knocking sound coming from inside the engine. One way to prevent this is to retard ignition timing, but that reduces power and fuel efficiency. A better way is to use a higher-octane fuel that resists uncontrolled ignition.
That’s all high-octane fuel does. It doesn’t deliver more power; it allows the engine to operate as it was designed to operate, with controlled combustion at all speeds and loads. Peak cylinder pressures are lower in low-compression engines, so they can use low-octane fuel. High-octane fuel won’t help it run better. High-compression engines and engines with forced induction need high-octane fuel because their peak cylinder pressures are higher.
There are two different lab tests for measuring octane. One test measures the fuel’s Research Octane Number (RON), which describes the fuel’s ability to resist knocking at low speeds and normal operating conditions. The other test measures its Motor Octane Number (MON), which describes the fuel’s anti-knock qualities at high speeds and more extreme conditions. Auto manufacturers specify the octane requirement of their engines, but it can be difficult to know the true octane of the fuel you’re buying. In the U.S., the number we usually see on the pump is the Anti Knock Index (AKI), which is calculated using RON and MON to approximate the fuel’s real-world performance.
Remember we talked about all the different gasolines that distill out at different temperatures? Some have high octane ratings and some have very low ratings. They are blended at the refinery to create a fuel with a base octane rating, and then the fuel’s AKI is adjusted upward with additives. In most of the U.S., the automotive fuel that leaves the refinery is all the same octane. An octane booster is mixed in when it’s loaded into a tank truck for delivery to a gas station, along with other additives like anti-oxidants and corrosion inhibitors. Many different octane boosters have been used over the years. Most have been discontinued because they present environmental and/or health hazards. Today the most common octane booster is ethanol, which also affects a fuel’s RVP.
Regulation, RVP and RFG
In the U.S., the RVP of gasoline for road vehicles is regulated by the EPA. During winter months, RVP cannot exceed 9.0 psi. Between June 1 and September 15, RVP is regulated to 7.8 psi in what the EPA calls “non-attainment areas” where ground-level ozone is higher than the allowable limit. However, if the gasoline contains 9% to 10% ethanol, the “summer gas” RVP is allowed to be 1.0 psi higher.
The EPA regulates RVP to reduce ground-level ozone, which causes serious health problems. Gasoline fumes are a major contributor to ground-level ozone, and that’s why cars have an EVAP system and an On-board refueling Vapor Recovery (OVR) system. But some fuel vapors still escape into the air, and reducing the fuel’s volatility helps. Several factors affect local ozone levels, and they can change over time, so the EPA publishes an annual list of non-attainment areas. The RVP of gasoline sold in those areas is adjusted as needed.
Some states have earned waivers from these regulations by developing their own plan for reducing ozone levels. Some of those plans include Reformulated Gasoline, also known as RFG, and also known as oxygenated fuel.
Pure gasoline is a hydro-carbon molecule that has no oxygen atoms. Gasoline is oxygenated with additives that are blended into the fuel after it leaves the refinery. Ethanol has oxygen molecules, and since it’s also an octane booster, it’s used everywhere in the U.S., not just in non-attainment areas. The maximum allowable content is limited to 10% by volume. If it contains more alcohol than that, it’s not called gasoline.
Ethanol works well as an engine fuel. It’s highly volatile, has a high octane rating and is a very clean fuel. However it carries about one-third less energy than gasoline, and the way it’s produced in the U.S. makes it a net-negative-energy fuel, meaning we use more energy to produce ethanol than we get back from using it as fuel.
Ethanol has a higher vapor pressure, so when using it to oxygenate gasoline, the base fuel is blended to a lower RVP to meet EPA regulations. Extra oxygen in the fuel means more oxygen in engine-out exhaust gas. The engine control unit interprets this as a lean air/fuel ratio, so it fattens the mix to compensate. This can reduce fuel mileage, and some drivers notice a reduction in performance with summer gasoline.
Fuel oxygenated with ethanol can hold ten times more water than pure gasoline, and it can absorb water from humid air. In some climates this can cause problems. Alcohol mixes more easily with water than with gasoline, especially at lower temperatures. If the temperature drops quickly, the ethanol will be drawn out of the gasoline and into the water, and this mixture will settle to the bottom of the tank. This is known as phase separation. The remaining gasoline will be less volatile and have a lower octane, so even if the water is not picked up by the fuel pump, the engine will be harder to start and it might knock.
When an engine is shut down, the injectors and intake valves heat soak because there’s no fuel flowing through them to carry the heat away. Liquid fuel on the valves or in the injectors will cook and form a hard varnish deposit. These deposits can increase emissions, so the EPA requires gasoline to contain a minimum amount of a certified deposit control additive. This is basically a detergent that prevents deposits from forming in the engine and fuel system.
Eight auto manufacturers — BMW, GM, Mercedes-Benz, Fiat Chrysler, Honda, Toyota, VW and Audi — have shown that gasoline made with only the required amount of deposit control additive doesn’t control deposits well enough to preserve the driveability of an engine. They have tested and identified several brands of gasoline that they call top tier detergent gasoline, which meets their tougher standards for deposit control. The list of retail brands can be found at http://www.toptiergas.com/licensedbrands/.
By the time it gets to the pump, there are real differences in gasoline, because octane boosters, detergents and other additives are put in at the distribution center. But except for local adjustments to RVP, all automotive gasoline is the same when it leaves the refinery. It’s complex and fragile, and it’s one of the most important designer products in the world. ●
Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
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