Fuel Contamination – Gasoline vs. Diesel Fuel

In recent news, gasoline distributed to multiple gas stations in Florida has suspected to have been contaminated with diesel fuel. The contamination could have serious consequences – at the time of this blog post, Hurricane Idalia is traveling across Florida and up the east coast. Florida officials have warned that contaminated fuel could potentially disable vehicle and generator engines when needed most.

We asked a Failure Analysis expert from ARCCA to help us understand why contaminated fuel can be a serious issue.

Why is diesel contamination in gasoline a problem?

Gasoline and diesel are chemically very similar. They are composed primarily out of hydrocarbons – materials made out of molecules composed of a chain of carbon atoms surrounded by hydrogen atoms. Both substances can be burned to release energy. When supplied with enough heat and oxygen, the hydrocarbon molecules will react with the oxygen and produce heat along with carbon dioxide gas and water vapor. Both gasoline and diesel are made from distilling crude oil, and both tend to mix readily with other hydrocarbons.

However, despite the chemical similarity, gasoline and diesel are different. Crude oil is a mix of hydrocarbons of many different sizes. The distillation process separates the molecules into “fractions” based on the boiling point – which depends on molecule size. In short, gasoline is made out of smaller, lower boiling point molecules than diesel. Diesel is made from larger, higher boiling point molecules than gasoline. As a result, diesel is more viscous (thicker) and has more energy per ounce compared to gasoline. Their flashpoints – or the temperature where they form a flammable vapor – are also very different. The flashpoint of gasoline is very low – so it is almost always able to form a flammable vapor. Diesel, on the other hand, has a relatively high flashpoint and generally requires extra heat.

Gasoline engines are not designed for diesel. With enough contamination, the higher viscosity and high flashpoint of diesel will prevent the fuel in a gasoline engine from being properly injected into the engine cylinders or ignited. A gasoline engine running with diesel fuel may fail to start entirely. Or, if the engine can start, the high flashpoint and high energy density (energy per ounce) of diesel can interfere with engine function. The fuel may burn too hot and damage the engine. Or, portions of the fuel may fail to burn, send flammable vapor down the exhaust system, and increase the risk of fire or catalytic converter damage. Incomplete combustion of the diesel may also increase carbon monoxide in the exhaust.

Can diesel contamination of gasoline be detected?

Both diesel and gasoline are made from hydrocarbon molecules derived from crude oil, which makes them very similar chemically. Because of this, it can be difficult to detect diesel in gasoline. One possible exception would be if gasoline appeared red or pinkish in color, which might indicate contamination by off-road diesel, which is dyed red. ARCCA has experts familiar with fuel testing, who suggested some ways to determine if gasoline was contaminated with diesel.

The main difference between gasoline and diesel are that they are composed of different-sized molecules, which offers some ways to distinguish them. The energy density of a fuel could be burned in a calorimeter – a device for measuring the heat of a chemical reaction – to see if the energy density was too high for gasoline. The mass density of the fuel could be measured by a device like a float hydrometer to see if it was the liquid was too dense to be gasoline. A viscometer might be used, especially at low temperatures, to detect whether the fuel viscosity was too high for gasoline. Unfortunately, these methods would only be useful for ruling out especially high levels of contamination.

However, the most direct way to detect diesel contamination is gas chromatography (GC). GC applies heat to vaporize a sample, then flows the sample through a long capillary tube, which separates different molecules from one another. On the other end of the capillary tube, a sensor like flame ionization detector (FID) or a mass spectrometer (MS) takes measurements of the molecules as they arrive. GC functions similarly to the distillation process that is used to separate gasoline and diesel fractions from crude oil – so it is very useful in separating out and detecting molecules by size. GC methods can even distinguish crude oil-based diesel from biodiesel, due to the presence of different types of hydrocarbons in biodiesel. GC-MS would provide additional information that could identify specific molecules, including fuel additives, that could be used to try to identify the source(s) of the contamination.

Call an expert

Diesel is not the only thing that can contaminate gasoline. Water, sediment, and other materials can find their way into fuel storage tanks, especially during a flood, fire, or other disaster. Detecting the contamination early can help prevent damage to engines or injuries from fires, carbon monoxide, etc.

Do you have a question about a fuel system failure or fuel contamination? Call an expert!