Bivalent (engine)

A bivalent engine is an engine that can use two different types of fuel. Examples are petroleum/CNG and petroleum/LPG engines, which are widely available in the European passenger vehicle aftermarket.

Engines that can use either alcohol (often produced as a biofuel) or standard gasoline are variants of flex fuel vehicles. Such vehicles are in production and commonly available for sale in the United States and other countries.

Compressed natural gas (CNG) is made by compressing methane to store it high pressures. Liquefied Natural Gas (LNG) is made and stored cryogenically, much like liquid hydrogen. The physical properties of natural gas require the compression ratio of the engine to be higher than in normal internal combustion engines, and the higher compression makes for greater efficiency. Natural gas also has a higher octane rating, so it can be burned at a higher temperature, reducing engine knock, and the fuel can be produced without complicated refinement processes. Since little unburned carbon is produced in the combustion of natural gas, the engine and oil are kept much cleaner than would be the case if gasoline alone was being burned, and the engine's life is thus increased. Aftermarket kits are available to convert vehicles to run on LNG or CNG and gasoline. In the United States, natural gas is cheaper than gasoline, but CNG at typical pressures requires more frequent refueling, because it contains only a quarter of the energy per unit volume of gasoline, whereas LNG contains only 80%. Although natural gas is a finite resource and its reserves can be depleted, it is unique among current fuels in having a net positive EROEI (energy returned on energy invested), while petroleum and other fuels are net energy sinks.

Liquefied petroleum gas (LPG) is a mixture of several hydrocarbons, mainly propane, butane and ethane. The gas mixes readily with air, allowing for more complete combustion. The fuel costs less than regular gasoline, but LPG has lower energy per unit volume, so its fuel economy and efficiency are lower. LPG gives a longer engine life due to its clean burning characteristics. The main difference between these vehicles and others is in their fuel storage systems. LPG is a gas at room temperature, but a liquid when pressurized (the required pressure varies according to the composition of the mixture). It is usually stored at around 10 bar. One drawback is that LPG fuel tanks are much heavier than conventional ones, hence two tanks would be needed, which would increase the vehicle's weight. Many automobile manufacturers make vehicles that run on LPG and gasoline. Some say that LPG is the least environmentally friendly alternative fuel, because it is derived from fossil fuels, so that greenhouse gases will inevitably be released into the atmosphere.

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