Gasoline Oxygenate: What Makes Our Vehicles Hum?

Guest Author: Tim Nelson, Global Marketing Manager Fuels & Chemicals

Do we take the gasoline we add to our vehicles for granted? Almost purely by habit we instinctively drive into the gas station, get our fuel, and then drive on with no thoughts about what we just put into our expensive highly sophisticated transportation system. We know it’s running, which is of course what we want, but what is happening in the engine as we drive?

There are different types of engines, however the two most common types run with petroleum fuel that are either a spark ignition type or a compression ignition type.  Compression ignition is also commonly known by the name “diesel” and spark ignition is commonly referred as “gasoline”, “petrol”, or “benzin” (depending on the region of the world).

You may have noticed that there have been an increasing number of gas stations that have labels on the pumps stating that there is ethanol as a fuel component. There are of course laws and manufacturing quality standards in place to assure that the quality of the fuel is good, but with formulations changing do we understand how it impacts our vehicle engine’s performance?  The ethanol being used in the spark ignition fuel in most cases is bioethanol, which is made by fermentation of the sugar and starch in plants.  This is a different source of hydrocarbons from the normal crude oil derived fuel and therefore we can assume that by adding this to the fuel it is altering its chemical composition that our engines were designed around.

The bioethanol industry has had its historic economic ebbs and flows as the cost of fuel fluctuates along with the price of crude oil. However, overtime even with the price of oil dropping, bioethanol has become an established institution. The business has become an integral part of our valuable national agricultural resources and it is an important part of the economy for many agricultural regions.  The reason being that bioethanol is now a valuable oxygenate additive in our fuel that helps to reduce our automobile emissions.

There is a trend for increasing levels of ethanol to be used as an oxygenate to improve the octane levels in the spark ignition fuels. Higher-octane provides the performance for the many premium vehicle engines, and it is also helpful in reducing vehicle emissions. Disadvantages to ethanol fuel blends are that when they are used in engines designed exclusively for gasoline that they will affect the fuel mileage and there is also the potential for higher water absorption. Why is the trend still then to push it forward?

On March 27th, 2019 there were two reports issued by the Alexandria, Va.-based Fuels Institute pressing the value to increase the octane level even higher for gasoline sold in the US to improve motor vehicle fuel efficiency which will then reduce emissions even further. This adjustment in octane levels would be effective with engines that are designed around the new fuel, which then can be used in a parallel emission control strategy to the slow, yet growing, electric and other alternative-fueled vehicles. Unfortunately, this circles back to the problem that the level of ethanol is limited to use as an octane enhancement because the change in the fuel composition effects the way the fuel behaves in the engine, as well as in the infrastructure of equipment to store and ship it.

Eni SPA and Fiat Chrysler Automobiles (FCA) are developing a high-alcohol vehicle fuel that can lower exhaust emissions of carbon dioxide by up to 3%.  The vehicles they are testing run “A20” gasoline, which contains 15% methanol and 5% ethanol. The A20 formula is designed to reduce direct and indirect CO2 emissions, and they state it is compatible with gasoline vehicles sold since 2001. One benefit of methanol is that it is typically made from natural gas which we have a growing ample supply and it is comparable in price to gasoline.  Currently in both Europe and the U.S., there is a limited amount of methanol in fuel, but there are discussions on increasing these levels.

These are unsettling times for spark ignition fuels with their formulations under pressure to change due to environmental improvements, combustion engine efficiency gains, and competition from alternative energy sources. To understand the need for formulation changes it is beneficial to look at what are oxygenates along with some of their historical milestones.  Oxygenates are chemical additives used to enhance the quality of fuels and boost performance of the engines. They are effective modifiers to improve acceleration of the engine which can also lower the vehicle’s fuel mileage and help keep engines clean for longer performance.

The oxygenates most commonly used are either alcohols or ethers.  The ethers are preferred because of the lower volatility and flammability. Historically the most popular ether to be used as an oxygenate additive in gasoline has been methyl tert-butyl ether (MTBE).  MTBE was chosen over other oxygenates primarily for its blending and formulation characteristics, and for economic reasons.

The octane number is a rating that is derived in a controlled test engine performed to determine the combustion characteristics of the fuel.  The fuel must combust in the engine to its controlled parameters so that the engine runs according to design. Any uncontrolled combustion will adversely impact the engine’s performance and even damage the engine components. MTBE and other oxygenates help gasoline burn efficiently in combustion to provide optimum torque. One of the other benefits is that an efficient burn lowers harmful carbon monoxide levels and other harmful emissions from vehicles.

Back in 1979 MTBE was growing in use as the primary oxygenate additive for gasoline in the United States, though in relative low levels, as it replaced tetraethyl lead as an octane enhancer. Later MTBE was used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by Congress in the 1990 Clean Air Act Amendments (CAA). Smog had become a health and environmental problem in many cities, so the CAA required the use of oxygenated gasoline in areas with unhealthy levels of air pollution. There were smog warnings and a growing “hole in our ozone” atmospheric layer which were being broadcast to us daily in the media.  In 1995, the CAA required use of reformulated gasoline (RFG) year-round in cities with the worst ground-level ozone and smog. As part of the CAA, the RFG had to have an oxygenate additive content of at least 2% by weight.

Unfortunately, MTBE is much more soluble in water than most of the other components in gasoline, and therefore when released into the ground it easily gets into groundwater. The result is that over time it has polluted many groundwater systems due to leaking gasoline station underground storage tanks that have been progressively corroding. MTBE has a very unpleasant taste and odor so when in water the MTBE is easy detectable by taste in extremely low concentrations (ppb) which makes contaminated drinking water unacceptable to the public. The EPA determined that concentrations in the range of 20 to 40 micrograms per liter (µg/L) of water or below will likely avert unpleasant taste and odor effects. This range of exposure levels is 20,000 to 100,000 times lower in which cancer or non-cancer effects were observed in rodent studies. So, MTBE isn’t necessarily unhealthy in a low dose, but it ruins the taste of our valuable water resources. The EPA has not yet set a MCL for MTBE in drinking water, though many states have set their own limits.

As a result of the ground water contamination issues, Congress passed the Energy Policy Act in 2005 which removed the oxygenate requirement for reformulated gasoline (RFG). As of 2007, 25 states had issued complete or partial bans on the use of MTBE. MTBE can still be used in some specialty fuel applications and in other parts of the world as a gasoline additive. Companies in the United States continue to export gasoline with MTBE to other countries due to the benefits of lowering emissions.

Currently ethanol and ETBE are becoming more commonly used as additives to oxygenate gasoline. ETBE is popular because it is typically made from bioethanol, and by some regulations therefore considered a biofuel additive. ETBE is not prone to absorb moisture, so it is preferred over ethanol though it is more expensive than MTBE.

Bioethanol has grown as an industry from the early days as an alternative source of fuel to an established industry and a readily available source of ethanol for fuel. E10 gasoline (10% ethanol) is readily available throughout a good part of the developed world, and as of 2011 that was more than 20 countries led by the United States.

Ethanol blends have encouraged the growth in Flexible Fuel Vehicles (FFV) that have modified spark ignition engines that are designed to run on high alcohol levels. The E85 fuel contains up to 85% ethanol and is designed to simulate the M85 methanol formulas that the FFV engines were first designed to use back in the 1970-1980’s to reduce reliance on imported oil. Unfortunately, despite the sale of thousands of FFVs, as of five years ago only 2.4% of all gasoline stations in the U.S. sell the E85 fuel. There are special supply contractual restrictions, infrastructure requirements, and some state laws that limit its availability. In general, availability does tend to be stronger near the bioethanol manufacturing regions.

Today, bioethanol has become the primary oxygenate additive that is available due to its established infrastructure, and because of the environmental failings in other oxygenates. Changes to the spark ignition fuel by adding alcohol does create a challenge since the energy content of ethanol is about 33% less than pure gasoline by equal volume. In addition, ethanol effects the volatility of the gasoline, which effects the evaporation pressure that is limited in fuel to prevent vapor lock problems in the engine. That being stated the E10 Blends of petroleum-based gasoline with 10% ethanol now account for more than 95% of the fuel consumed in spark ignition motor vehicles in the United States.

The current limitations on the availability of E15 and E85 fuels will change with the new fuel filling pumps that can custom blend right in the fuel filling stations as we pump it into our vehicles. The capital cost for these systems right now is high but as demand increases, they will hopefully drop. These new pumps are already being used by some fuel stations and so you will see these higher blends become more available. Also keep an eye out for alternative formulas like A20 in the future.

The changes in spark ignition fuel formulations can be a bit confusing and unfortunately, it will most likely continue to be.  However, the main goal for continuous changes is balance of fuel performance, environmental emissions, fuel sourcing/availability, and of course cost.  For us it is most important that our vehicles run efficiently for great mileage and minimal maintenance, and so it is critical to be mindful of what fuel we choose and to understand a little about formulation changes.

Keep in mind as the formulations do change, it will always be certain that Phenomenex has a chromatography solution as a resource for the fuel companies so that they can perform an exceptional analysis of the oxygenates in your fuel to ensure that they meet manufacturing design. 

For additional information please visit us at our Phenomenex Industry Fuels page and check out our awesome technical note on the analysis of fuel oxygenate additives at “A Fast Isothermal GC Analysis of Spark Ignition Fuel Oxygenate Additives Using an Alternative Phase”.

Gasoline Oxygenate Additives method development analysis

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