A bivalent engine is an engine that can use two different types of fuel. Examples are
petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
/
CNG
Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in cy ...
and petroleum/
LPG engines, which are widely available in the European passenger vehicle aftermarket.
Types
Alcohol and Petroleum
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 or Liquefied Natural Gas and Petroleum
Compressed natural gas (CNG) is made by compressing
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
to store it high pressures.
Liquefied natural gas
Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the vol ...
(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
In spark ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignite ...
, 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
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
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
In energy economics and ecological energetics, energy return on investment (EROI), also sometimes called energy returned on energy invested (ERoEI), is the ratio of the amount of usable energy (the ''exergy'') delivered from a particular energy re ...
(energy returned on energy invested), while petroleum and other fuels are net energy sinks.
Liquefied Petroleum Gas and Petroleum
Liquefied petroleum gas (LPG) is a mixture of several
hydrocarbons
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...
, mainly
propane,
butane and
ethane
Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petroc ...
. 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
Alternative fuel, known as non-conventional and advanced fuels, are any materials or substances that can be used as fuels, other than conventional fuels like; ''fossil fuels'' (petroleum (oil), coal, and natural gas), as well as nuclear materi ...
, because it is derived from
fossil fuels, so that greenhouse gases will inevitably be released into the atmosphere.
Hydrogen and Petroleum
Bivalent engines can also use
hydrogen fuel
Hydrogen fuel refers to hydrogen which is burned as fuel with oxygen. It is zero-carbon, provided that it is created in a process that does not involve carbon. It can be used in fuel cells or internal combustion engines (see HICEV). Regarding hydr ...
, as demonstrated by the
BMW Hydrogen 7 using a bivalent V12 H7 Series engine. The engine itself is similar to a regular gasoline combustion engine, except for the
fuel injection system. When a BMW Hydrogen 7 is running in gasoline mode, the fuel is injected directly into the cylinders, but when the vehicle is running on hydrogen, the fuel is injected into the intake manifold. BMW claims the Hydrogen 7 is the "world's first production-ready hydrogen vehicle", although only 100 total vehicles have been produced, and no more are planned to be produced.
Future
Reduction of greenhouse gas emissions and preservation of natural resources are becoming increasingly important to people around the world. Many countries have regulations on the fuel economy of newly manufactured vehicles, and many governments offer tax breaks for vehicle manufactures that use clean-burning fuels. Vehicle manufacturers are thus motivated to develop new internal combustion engine technologies. The bivalent engine allows for an easier transition from fossil fuels to alternative fuels. The technology is advancing and there is increasing demand for more efficient and cleaner burning engines.
Hydrogen fuel
Gaseous Hydrogen
Hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
is being researched as a fuel for vehicles because it produces no
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
emissions. If hydrogen fuel becomes more popular, it has the potential to be less expensive than other fuels, if low-cost production via electrolysis can be implemented. 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
Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.
To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully l ...
fuels. In gaseous form, hydrogen is difficult to store and has a low volumetric energy density. It can be produced in many different ways, but many of the methods produce carbon dioxide. The most promising method is
electrolysis. Safety of the hydrogen storage tanks in the event of an accident has been investigated, and various tests show that they do not present any problems. When used as a fuel, hydrogen has a wide range of flammability and low ignition energy. These properties allow hydrogen to be burned using a wide range of air-fuel mixtures, but problems arise with premature ignition.
Crankcase ventilation is very important when burning hydrogen, because of the low ignition energy. Proper ventilation is needed to prevent ignition in the crankcase and the formation of water in the engine oil.
Hydrogen and
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
are very similar as fuels, so the differences between the components needed to burn them are trivial, and interoperable systems are easily made.
Liquid Hydrogen
The volumetric energy density can be increased if hydrogen is stored as a cryogenic liquid. Liquid hydrogen provides almost a third as much energy per unit volume as gasoline.
Liquid hydrogen fuel has some disadvantages. The technology is very new and the infrastructure for liquid hydrogen filling stations is currently very limited. Many of the processes commonly used in creating the fuel give off
greenhouse gases
A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
, and the hydrogen produced is usually derived from finite resources. The storage of the liquid hydrogen is a major problem. 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 it warms, it
evaporates
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
. The pressure in the fuel tank then increases, and some gas must be released. Release valves are installed in these vehicles so that the pressure in the tank does not get too high, but a small amount of fuel is lost.
See also
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Flexible-fuel vehicle
A flexible-fuel vehicle (FFV) or dual-fuel vehicle (colloquially called a flex-fuel vehicle) is an alternative fuel vehicle with an internal combustion engine designed to run on more than one fuel, usually gasoline blended with either ethanol fu ...
*
Hybrid vehicle
*
CleanEnergy
*
Natural gas vehicle
A natural gas vehicle (NGV) is an alternative fuel vehicle that uses compressed natural gas (CNG) or liquefied natural gas (LNG). Natural gas vehicles should not be confused with autogas vehicles powered by liquefied petroleum gas (LPG), ma ...
References
External links
IAV Alternative DrivesAlternative Fuels
{{DEFAULTSORT:Bivalent (Engine)
Engines