What are biofuels, and how are they used in flex fuel engines?

The first combustion engines ran on gasoline and diesel. Today, the options available to power a vehicle are more varied than ever – along with electric propulsion, combustion power can be fuelled by a variety of alternative fuels. Some of the most popular alt-fuels worldwide are biofuels, combinations of which can be run on flex fuel engines.

With such a diverse selection available, it can be easy to get lost in the different fuel terminology. In this article, we break down what biofuels are, how flex fuels engines work, and how the two relate to each other.

What are biofuels?

In short, biofuels are fuels made from organic material, typically plants such as sugarcane and rapeseed grown specifically to be processed into fuel – however, it is also possible to create biofuels via waste products from food or cooking.

These organic materials are either fermented or processed to create fuels that have similar properties to gasoline or diesel, allowing them to be used as alternative fuel sources for engines.

In many cases, these biofuel blends are mixed with gasoline or diesel for sale at the fuel pump. At small concentrations, biofuels mixes do not change the behavior of the overall fuel blend, and are labelled as conventional fuels. At large concentrations, biofuel mixes do change the behavior of the overall fuel blend. This means that these fuel types cannot be run in traditional engines, and instead the vehicle must be run using a specialised flex fuel engine that is capable of tolerating the higher ethanol concentrations these fuels contain.

Read more about alternative fuels with Brazil's technology neutral policies.

How are biofuels made?

There are two common types of biofuels: bioethanol and biodiesel.

Bioethanol is created by fermenting a sugar‑ or starch‑based crop, much like producing an alcoholic spirit. A range of alcohols are produced by fermentation, although ethanol is the most abundant alcohol and is what is usually used for fuel. Ethanol is a high‑octane renewable component that enhances gasoline quality by improving knock resistance and supporting cleaner, more efficient engine operation.

With further treatment, the methane waste gas produced by the fermentation process can also be processed into biomethanol to be used alongside bioethanol.

Biodiesel is produced from vegetable oils or animal fats using a process called transesterification, wherein they are mixed with alcoholic catalysts such as ethanol or methanol. The resulting fatty acid esters are true diesel fuels, capable of ignition in a diesel engine solely through compression without the use of a spark.

Globally, Brazil is one of the largest producers of bioethanol, while the EU is a major producer of biodiesel.

The process of creating biofuels starts with agriculture

Read more about why India is adopting bioethanol.

How do flex fuel engines differ from traditional engines?

Mixing biofuels into conventional fuels is an easy way to reduce the lifecycle emissions of a nation’s vehicle parc. At small concentrations, biofuels do not significantly change the behavior of fuel in an engine, and are simply sold as conventional fuels at the pump.

Biofuel mixtures are usually labelled with a letter and a number – the letter signalling the biofuel type, and the number signalling its share of the fuel blend. Conventional fuel blends that use biofuels include:

• Bioethanol-gasoline at E5 or E10, with some regions using E15 mixes (commonly used in the USA)
• Biodiesel-diesel at B5 or B7, with some fleets and heavy vehicles using B20 mixes

These fuels can be run in most standard gasoline and diesel engines without any modifications.

By contrast, more concentrated ethanol biofuel blends, in which the biofuel makes up a much larger share, require a specialised flex fuel engine. This allows the fuel system to take advantage of the affordability and fuel security afforded by fossil fuels, while still making use of the net-zero emissions offered by biofuels. Most of these fuel blends are bioethanol-gasoline mixtures, with the most common mixes being E85, E75, and E70.

Flex fuel vehicles must be designed specifically for these higher bioethanol concentrations, since biofuels have different properties to their fossil fuel equivalents. While biogasoline can seamlessly be used in a standard gasoline engine, ethanol is a more corrosive liquid and contains about 33% less energy per gallon than gasoline. This means that flex fuel engines must be larger and consume more fuel to achieve the same ignition as a standard engine.

Many modern flex‑fuel vehicles are not limited to one fuel ratio and can run on varying ethanol–gasoline concentrations, adjusting automatically as needed.

Read more about alternative fuels and why hybrids will stay central to global mobility.

What fuels do flex fuel engines run on?

Flex fuel vehicles almost always run on biofuel blends. Biofuels fit into a broad group of fuels derived from biological sources, such as ethanol, biodiesel, and biogas, and they may be used either in pure form or blended with conventional fuels.

Flex fuel engines often use a mixture of fuel types and concentrations but are predominantly characterized by their use of standardized high-biofuel blends. Typically, these are high bioethanol-gasoline blends designed for use in flex-fuel vehicles capable of operating on a wide range of ethanol–gasoline mixtures.

While the fuels used in a flex fuel engine are chemically defined by their composition rather than the origin of the ethanol, in practice the ethanol component is almost always bioethanol, as flex-fuel systems are primarily intended to increase the use of renewable fuels rather than synthetic or fossil-derived alcohols.

Read more about alternative fuels and why the EU must adopt a technology-neutral approach.

What are the use cases for pure and blended biofuels?

One of the main benefits of biofuels is that they are produced from renewable sources. Crops can be grown, consumed, and replanted, with each new crop recapturing the carbon released from a combustion cycle. In this way, the tailpipe emissions of biofuel vehicles will be offset by the production of further fuel.

The blended biofuels that are used in flex fuel engines, with their combination of bioethanol and gasoline, offer a compromise. The biofuel content helps reduce emissions, while the gasoline component increases efficiency and offsets some challenges facing ethanol – in particular, its difficulty as a fuel in colder climates. Ethanol has a lower vapor pressure, meaning that a predominantly ethanol‑based mix can struggle to ignite in low temperatures if not blended with gasoline.

These fuels also offer economic benefits. As petroleum and gasoline prices fluctuate and remain finite resources, biofuels provide a renewable alternative that can often be cheaper and more locally produced - though crop growth is not possible in all regions.

Read more about alternative fuels and how hybrids can speed up decarbonization.

The state of biofuels

Today, blended biofuels are used to power either large commercial fleets that are particularly emissions-intensive, or the fuel systems of regions with strong bioethanol supplies – usually, markets with large crop surpluses.

The latter makes bioethanol particularly attractive for developing or middle-income economies located near the equator, such as Brazil or India. As a result, data from the OECD shows that biofuel adoption is increasing at an average of 3% every year.

At Horse Powertrain, we develop engine technologies that use many of these fuel types to power low-emission mobility. To find out more about the engines we produce that leverage biofuels and their blends, take a look at our products.

To learn about our products, read more here: spotlight solutions.