A bivalent engine is one that can use two fuel types. Examples are petroleum/CNG or petroleum/LPG which are widely offered in the European passenger vehicle aftermarket. Bivalent engines can also use hydrogen as demonstrated by the recently introduced BMW 'Clean Energy' H7 Series, featuring a V12 bivalent engine and BMW Hydrogen 7 which is more fuel efficient than the standard H7 Series engine.
Hydrogen is now being researched as a fuel for vehicles because of its ability to be regenerated. The burning of hydrogen fuel also produces no carbon dioxide emissions which makes it environmentally friendly. BMW has been the leader in consistent research to develop cars that use hydrogen. Once hydrogen fuel becomes more popular it will be a lot less expensive than other fuels. Hydrogen can be used and created in fuel cells to power electric motors or burnt directly in combustion engines. BMW has developed a bivalent internal combustion engine that can switch between petroleum and liquid hydrogen fuels. Liquid hydrogen has almost three times as much energy as gasoline in terms of density per unit volume. The liquid hydrogen can be produced in many different ways but many of these methods produce carbon dioxide in the process. The most promising method of hydrogen production is through a process known as electrolysis. There are two fuel tanks in the vehicle and it can run on either type of fuel if one is not available. Safety of the hydrogen storage tanks has been investigated in the event of an accident and various tests show that the storage tanks used do not present any problems.<ref> http://www.carlist.com/autonews/2004/autonews_36.html BMW: Hydrogen and Clean Energy Strategy</ref> The engine itself is similar to a regular gasoline combustion engine with an exception to the fuel injection system. As seen in BMW's Hydrogen 7, when the vehicle is running in gasoline mode the fuel is injected directly into the cylinders, and when the vehicle is running on hydrogen the fuel is injected into the intake manifold. Hydrogen when used as a fuel has a wide range of flammability, and has a low ignition energy. These properties allow hydrogen to be combusted over a wide range of air-fuel mixtures but problems arise with premature ignition. Crankcase ventilation is also very important in burning hydrogen because of the low ignition energy. Proper ventilation should be ensured to prevent ignition in the crankcase and water forming in the engine oil.<ref>?.0</ref> Hydrogen and natural gas are very similar as fuels, so the differences between parts needed to burn both fuels are trivial, and interoperable systems are easily made.
There are some disadvantages of using liquid hydrogen fuel, most of which have been dealt with decades ago by the LNG vehicle industry. The technology is very new and the infrastructure for liquid hydrogen filling stations is currently very limited. Also, many of the methods commonly used in creating the fuel give off greenhouse gases during the process and the hydrogen typically produced is derived from finite resources. One major problem with these vehicles is in the storage of the liquid hydrogen. Since the boiling point of liquid hydrogen is very low (-252.88 °C), it is difficult to keep the fuel cold enough to maintain its liquid form. When the liquid hydrogen warms, it evaporates and the pressure in the fuel tank increases and some of the gas must be released. There are release valves installed on these vehicles so the pressure in the tank does not get too high but a small amount of the fuel is lost in this process.
Compressed/Liquefied Natural Gas (CNG/LNG)/Petroleum
CNG is made by compressing methane which is extracted from natural gas and stored at high pressures, LNG is made and stored cryogenically, much like liquid hydrogen. The physical properties of natural gas require that the compression ratio of the engine be higher than in normal internal combustion engines, which also makes them more efficient. Natural gas also has a higher octane rating which allows the fuel to be burned at a higher temperature therefore reducing engine knock. Another advantage of these engines is that they produce nearly no carbon dioxide emissions and the fuel can be produced without complicated refinement processes. Since little carbon is produced in the combustion of natural gas, the vehicles engine and oil are kept much cleaner than if just gasoline was being burned which then increases the engine's life. There are aftermarket conversion kits available that can convert vehicles to run on LNG or CNG and gasoline. The cost of natural gas in the United States is cheaper than gasoline but CNG at typical pressures requires more frequent refueling because it contains only a quarter of energy per unit volume when compared to gasoline,<ref>http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-cng.pdf Compressed Natural Gas</ref> while LNG contains only 80%. Although natural gas is a finite resource like petroleum and its reserves can be depleted, natural gas is unique among current fuels in that it has a net positive EROEI, while other fuels such as petroleum are net negative energy sinks.
Liquefied Petroleum Gas (LPG)/Petroleum
LPG is a mixture of several hydrocarbons (mainly propane, butane and ethane). The gas mixes readily with air which allows for more complete combustion. The fuel costs less than regular gasoline but LPG has lower energy per unit volume which results in lower fuel economy and also is not that fuel efficient. The use of LPG provides a longer engine life due to its clean burning characteristics. The main difference between these vehicles and others is the fuel storage system. LPG is a gas at room temperature but is a liquid when it is pressurized (levels vary depending on composition of mixture) and is usually stored at around 10 bar. One drawback is that these fuel tanks are much heavier than conventional ones, and two tanks would be needed which would increase the vehicle's weight. There are many automobile manufactures that make vehicles that run on LPG and gasoline.<ref> http://www.whatgreencar.com/lpg.php Liquefied petroleum gas vehicles</ref> Some say that LPG is the least environmentally friendly alternative fuel because it is derived from fossil fuels and one way or another greenhouse gases will be released into the atmosphere.<ref>Liquefied Petroleum Gas (LPG)</ref>
Reduction of greenhouse gas emissions and preservation of natural resources is becoming more and more important to people around the world due to. Many countries have regulations and restrictions as to the fuel economy of newly manufactured vehicles and many governments offer incentives in the form of tax breaks for vehicle manufactures that use clean burning fuel technologies. All of these issues give vehicle manufacturers motivation for the development of new technologies in internal combustion engines. The bivalent engine allows for an easier transition from fossil fuels to alternative fuels. The future of these engines looks to be promising because of the advancing technology and higher demand for more efficient and cleaner burning engines.