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The Brazilian Fiat Siena Tetrafuel 1.4 is the first bi-fuel car that runs with natural gas (CNG) alternating automatically with any of the typical fuel blends used in flexible-fuel vehicles, pure gasoline, or gasohol E25, or just ethanol (E100). Shown below are the CNG storage tanks in the trunk.

Bi-fuel vehicles are vehicles with multifuel engines capable of running on two fuels. On internal combustion engines one fuel is gasoline or diesel, and the other is an alternate fuel such as natural gas (CNG), LPG, or hydrogen.[1] The two fuels are stored in separate tanks and the engine runs on one fuel at a time in some cases, in others both fuels are used in unison. Bi-fuel vehicles have the capability to switch back and forth from gasoline or diesel to the other fuel, manually or automatically.[2][3][4][5]

The most common technology and alternate fuel available in the market for bi-fuel gasoline cars is Autogas (LPG), followed by natural gas (CNG),[6] and it is used mainly in Europe. The Netherlands and the Baltic states have a large number of cars running with LPG. Italy currently has the largest number of CNG vehicles, followed by Sweden. They are also used in South America, where these vehicles are mainly used as taxicabs in main cities of Brazil and Argentina. Normally, standard gasoline vehicles are retrofitted in specialized shops, which involve installing the gas cylinder in the trunk and the LPG or CNG injection system and electronics.

Diesel conversionsEdit

Diesel engine is compression ignition engine and does not have spark plug. To operate a diesel engine with an alternate combustible fuel source such as Natural gas, Dual-Fuel system used. Natural gas as the main fuel while diesel fuel is used for the ignition of the gas/air mixture inside the cylinder (a portion of diesel is injected at the end of the compression stroke, thereby maintaining the original diesel operation principle)[7]

Dual fuel operation means the engine uses two fuels (gas and diesel) at the same time, as opposed to Bi Fuel which would mean the engine could have the option of using either fuel separately.

There usually two type of conversions - low speed (below 1000 RPM) and high speed (between 1200 and 1800 RPM).

Low Speed Conversion

Low and middle speed conversionEdit

Gas is injected into the cylinder inlet manifold by individual gas electromagnetic valves installed as close to the suction valves as possible. The valves are separately timed and controlled by injection control unit. This system interrupts the gas supply to the cylinder during the long overlap of the suction and exhaust valves (just typical for slow-speed and medium-speed engines – within the valve overlap cylinder scavenging is performed). This avoids substantial gas losses and prevents dangerous gas flow to the exhaust manifold.

  • This conversion is adjusted for low speed engines up to 1000 RPM.
  • System for conversion of industrial diesel engine to Bi-fuel operation by substitution of 70-90% natural gas for diesel or HFO.
  • Gas is injected directly before intake valve by high speed electromagnetic injector, one or two injector per each cylinder.

High speed conversionEdit

High Speed Conversion

Gas is mixed with air by a common mixer installed before turbocharger(s). Gas flow is controlled by a throttle valve, which is electronically operated by the special control system according to the required engine output and speed. In order to avoid knocking of the engine, knocking detector/controller is installed, thus enabling engine operation at the most efficient gas/diesel ratio.

  • Suitable for all High Speed engines, 1200-1800 RPM.
  • System for conversion of industrial diesel engine to Bi-fuel operation by substitution of 50-80% natural gas for diesel.
  • Gas and air are blended behind air filter before turbocharger by central mixer.


Common conversion featuresEdit

  • Major substantial savings on operation costs
  • Practically no engine modification required
  • Non-derated output power
  • Fuel flexibility: Possibility of bi-fuel or original pure diesel operations
  • Safe operations.
  • Lower emissions
  • Longer engine life span, longer service and maintenance intervals

Gas types usedEdit

It is common to use CNG (Compressed Natural Gas) or LNG (Liquid Natural Gas) for bi-fuel operations. Both are also mostly used for Generator sets conversions, because the engine does not lose the output power.

In recent years biogas is being used. The biogas composition and calorific value must be known in order to evaluate if the particular biogas type is suitable. Calorific value may be an issue as biogas is derived from different sources and there is low calorific value in many cases. You can imagine you have to inject sufficient volume of gas into the cylinder to substitute diesel oil (or, better to say, substitute energy delivered by diesel oil). If the calorific value (energy) of the biogas was very low, there is a need to inject really big volume of biogas into the cylinder, which might be technically impossible. Additionally, the composition of the biogas has to lean towards ignitable gases and be filtered as much as possible of non-combustible compounds such as carbon dioxide.

Associated gas is the last type of gas which is commonly used for bi-fuel conversions of generator sets. Associated gas is a natural gas found in association with oil, either dissolved in the oil or as a cap of free gas above the oil. It means it has almost the same quality as CNG or LNG. [9]

Diesel/gas ratioEdit

It depends on the technical state of the engine, especially of the injection system. The typical Diesel / Gas ratio is 40/60% for the high-speed engines. If the operating output of the engine is constant and between 70-80% of nominal output, than it is possible to reach up to 30/70% ratio.[10] If the operating output is lower (for example 50% of the nominal output) or if there are variations, the rate is about 45/55% (more of diesel is used).For Low Speed conversions it is possible to reach the Diesel/gas ratio up to 10/90%. Generally, it is not possible to guarantee an exact diesel/gas ratio without a test being done after the conversion.


Aftermarket 'bi-fuel' and even 'tri-fuel' conversions are also available.

Factory bi-fuel passenger carsEdit

Factory bi-fuel pickupsEdit

See alsoEdit


  1. ^ Diane Nassy. "Flexible Fuel Vehicles". Motopoint. Archived from the original on 23 July 2008. Retrieved 2008-08-24.
  2. ^ "Glossary" (PDF). Biofuel Marketplace Project. Archived from the original (PDF) on 2012-09-15. Retrieved 2014-12-21. See Glossary for definition of Bi-fuel Vehicle
  3. ^ Dominik Rutz and Rainer Jansen (February 2007). "BioFuel Technology Handbook" (PDF). WIP Renewable Energies. Archived (PDF) from the original on 20 August 2008. Retrieved 2008-09-01. See definition in Glossary and Abbreviations
  4. ^ "Definition of Terms". Sustainable Green Fleets. Archived from the original on 2011-07-21. Retrieved 2008-09-01.
  5. ^ "Glossary". Biofuel Marketplace. Archived from the original on December 11, 2008. Retrieved 2008-09-01. See definition of FFV
  6. ^ Alternative Fuels and Advanced Vehicles Data Center. "Natural Gas Vehicles". US Department of Energy. Archived from the original on 1 September 2008. Retrieved 2008-09-01.
  7. ^ Mukhsin, Ismail, Muammar; Hakim, Zulkifli, Abd Fathul; Fawzi, Mohd Ali, Mas; Azmir, Osman, Shahrul (April 2016). "Conversion method of a diesel engine to a CNG-diesel dual fuel engine and its financial savings". Retrieved 2018-08-20.
  8. ^
  9. ^
  10. ^
  11. ^ "Fiat website". Retrieved 2008-04-22.[permanent dead link]
  12. ^ Christine Lepisto (2006-08-27). "Fiat Siena Tetra Power: Your Choice of Four Fuels". Treehugger. Archived from the original on 19 September 2008. Retrieved 2008-08-24.
  13. ^ "Nouvelle Fiat Siena 2008: sans complexe" (in French). Caradisiac. 2007-11-01. Archived from the original on 2008-07-01. Retrieved 2008-08-31.
  14. ^ Agência AutoInforme (2006-06-19). "Siena Tetrafuel vai custar R$ 41,9 mil" (in Portuguese). WebMotor. Archived from the original on 2008-12-10. Retrieved 2008-08-14. The article argues that even though Fiat called it tetra fuel, it actually runs on three fuels: natural gas, ethanol, and gasoline.
  15. ^ "Lada Vesta CNG". Retrieved 2018-08-14.

External linksEdit