An internal combustion engine fuel , primarily the use of heat emitted by the different fuels in an engine converted into useful work. Liquid fuels such as gasoline , diesel and other petroleum refineries are in accordance with the fuel standard physical , chemical , refining of crude oil , its physical and chemical properties have strict rules , and thus more stable. Because of the different gaseous fuels from oil and gas fields , or from various refinery , during regardless of their composition or nature has a greater difference. Natural gas as a fuel for automobiles , there are some basic requirements , with particular attention to the following aspects of nature .
□ a component
□ the main component of natural gas is an alkane of light components from the field of basic alkoxy CH4 (82% ~ 98%) and no more than ethane C2H6 (6%), propane C3H8 (1.5%) and butane C4H10 ( approximately 1% ) components. Average oil, methane content in the gas field difference is quite large . Our gas field in Sichuan methane content and more than 95 %, while the associated natural gas field in the methane content was 70% to 80%. Some lower methane content , and containing heavier hydrocarbons , such as butane , pentane, hexane and the like .
□ natural gas as a vehicle fuel , liquefied natural gas (LNG) in the best conditions . Because the LNG is a cryogenic gas ( under pressure of -162 ℃) liquefying generated pure methane can be said , the most popular is the vehicle fuel fuel . CNG main component should be extremely small amounts of methane and ethane. If oil associated gas as a vehicle fuel should contain carbon hydrocarbon removal . The main components of propane and butane as LPG , but because of their different sources and differentiated .
□ If it is derived from oil , gas light hydrocarbon recovery in LPG, places propane , butane , and few have the presence of alkenes , suitable for making vehicle fuel . And refinery by-products obtained from LPG, except propane, butane contain many propylene C3H6, C4H8 , etc. butene and butadiene C4H8 olefin component . Olefins density smaller , lower calorific value , poor ignition performance. As the unsaturated hydrocarbon , readily oxidized to produce colloid , the evaporator of the fuel system , the diaphragm cement mixer , corrosion affecting operation of the engine . In addition, incomplete combustion , prone to carbon deposition, accelerated wear and tear parts . Therefore unfit refinery byproduct LPG vehicle fuel . Unless addressed separately to the olefin content is within the specified value.
□ In addition, CNG and LPG also contain some impurities, its content should meet the relevant national standards. Currently CNG vehicle industry standards are being developed. LPG has oil , gas and petroleum products, liquefied petroleum gas LPG fuel (LPG) Two national standard, but none of the provisions of olefin content , and therefore should develop standards for automotive LPG use .
□ Second, the calorific value
□ the fuel calorific value is the value of the heat of combustion of the fuel units calorimeter measured . The fuel in the combustion products H2O condensation process will be included in the latent heat release calorimeter in the measured values ​​, the measured value is called a high calorific value . This part of the latent heat in the engine can not be utilized , so to heat this part of the heat is subtracted from the value . Issued after the effective heat of combustion is called the low calorific value fuel in the cylinder . In calculating the calorific value of natural gas calorific value is calculated according to fuel . Per unit mass of gaseous hydrocarbons measured values ​​do not differ too much heat . C1 slightly more heat value , the calorific value of the carbon number increases slightly reduced. Due to the volume of gas as a fuel metering unit easy , and less carbon number hydrocarbon density, so according to the calorific value per unit volume measured relatively less. Fuel gas is a mixture of light hydrocarbons , the heating value of natural gas can be calculated by the following formula : HU = Σr iHui/100 formula HU - low calorie gas , KJ/Nm3; Hui_ combustible component of the i-th LHV , kJ/Nm3 ri_ - volume I a combustible slow points ( or mole ) percentage ,% there are some changes due to the different composition of the various gaseous fuels origin of natural gas fuel , climate and other conditions , the use of formula can be calculated analysis of the low calorific value gaseous fuel .
□ Third, antiknock
□ gasoline as a motor fuel has a very important quality , which is its antiknock . Now grade gasoline antiknock performance is - octane value gas in a car engine, in particular modified natural gasoline engine , similar to gas , i.e., outside the cylinder a combustible mixture of fuel and air is formed to after the cylinder , then exogenous ( EDM ) point forward , it will encounter a similar level of gasoline antiknock performance problems. The so-called anti-knock refers to the ability to resist detonation combustion of fuel .
□ What is the detonation combustion it? After spark ignition gasoline engine , the flame began to spread in uniform combustible gas mixture in the cylinder . Flame front because unburned mixture has been compressed mixed gas combustion and radiation heat transfer and the temperature and pressure rise , which accelerates a chemical change , i.e. a so-called pre-reaction flame . Farther away from the center of the flame , the deeper the unburned air-fuel mixture before the flame reaction . If the flames spread to the surface in a timely manner , burning it is normal combustion . If not yet reached before the normal flame , the unburned mixture of chemical preparation process has been completed will be a natural formation of new flame center and speeds up to ~ 2000m 1500 / s of flame propagation . This burn with explosive properties was carried out very quickly , so no time to promote the expansion , resulting in shock and a sharp metal percussion . At the same time , the exhaust black smoke, engine power decreased, a phenomenon known as detonation combustion , deflagration or detonation short .
□ knocking many factors , such as compression ratio of the structure, the shape of the combustion chamber , the running speed, load and the like. Good but the engine has been manufactured , the nature of the fuel itself is used as a low ignition point , the speed of the oxidation reaction to produce knock decisive role, which is fuel antiknock . Because of iso-octane antiknock best, so it 's antiknock set at 100 , which is the octane rating of 100. Compared with other octane fuel , gasoline antiknock as No. 93 is 93% iso-octane , the octane number of gasoline on this as 93. For the anti-knock performance over iso-octane methane , ethane more difficult to measure directly . Now published antiknock methane, ethane and other gas fuels , namely octane number are approximate. Austria Liszt engine and test equipment company in the 1960s, a similar body fuel anti-knock performance indicators , called methane value (Methane Number). I.e. methane antiknock capacity of 100 to measure other antiknock fuel gas to the fuel gas at the various retailers are not used for the engine operating conditions . U.S. Cooper also proposed a value of n-butane (NBN) as a measure of gas fuel antiknock parameters. Currently, there is no uniform method of measure and gaseous fuel antiknock world.
□ However, this reference can be seen octane , anti-knock properties of methane, ethane and other natural gas fuel is very good. Thus the extreme limit of the corresponding higher compression Bibi , this is the natural gas engine compression ratio can be higher than the causes of gasoline , and compression ratio, the high thermal efficiency of the engine , will save fuel .


According to the calorific value of natural gas and gasoline calculated under standard conditions , one cubic meter of natural gas is equivalent to 1.1 to 1.3 liters of gasoline .