Under the social background of energy shortage, environmental deterioration and increasing destruction of ecological balance, the study of alternative fuels has become an inevitable choice for the automotive industry to achieve sustainable development. Automotive new energy mainly includes electric energy, hydrogen energy, natural gas, liquefied petroleum gas (LPG), alcohol (methanol, ethanol) fuel, dimethyl ether (DME), solar energy, biomass energy, etc.
The calorific value of dimethyl ether (DME) is higher than that of methanol and ethanol, of which methanol is the lowest. Compared with gasoline, these three alternative fuels have the possibility of combustion. The explosion limit of methanol and ethanol is wider than that of gasoline, which is conducive to the maneuverability of the engine.
Compared with diesel, the cetane number of DME is higher than diesel, the carbon proportion of DME is less, it is easy to burn, and the spontaneous combustion temperature is also lower than diesel, which is conducive to engine ignition and start-up. Compressed natural gas (CNG) is a very good alternative fuel. Its carbon content is lower than diesel, and its hydrogen content is significantly higher than diesel. However, its spontaneous combustion temperature is too high and ignition is a little difficult.
The performance of the current alternative fuel is basically equivalent to that of the existing gasoline and diesel, so it can ensure the thermal efficiency, stability and other important properties of the engine.
- Electric energy
There are many sources of electric energy, which can be transformed from any form of energy such as hydropower, wind energy, nuclear energy and coal. Therefore, electric energy is very rich. In this way, we can vigorously develop electric vehicles called “important means of transportation in the 21st century”. Electric vehicle has been praised by people for its advantages of no pollution, easy start, low noise and easy operation. Therefore, the development of electric vehicles is imperative. Electric vehicle is a kind of vehicle which is driven by electric energy in whole or in part and meets the requirements of road traffic and safety regulations. Electric vehicles include pure electric vehicles, hybrid electric vehicles and fuel cell vehicles. They are the most representative and promising new energy vehicles.
- Hydrogen energy
Compared with natural gas, gasoline and LPG, hydrogen has low unit mass and high calorific value, which is about 2.7 times that of gasoline. Wide flammability limit, easy to realize lean combustion, improve economy, reduce the maximum combustion temperature and greatly reduce NOx emission. At the same time, the spontaneous combustion temperature of hydrogen (585 ℃) is higher than that of natural gas and gasoline, which is conducive to improving the compression ratio and the thermal efficiency of hydrogen energy internal combustion engine. Although the spontaneous combustion temperature of hydrogen energy is higher than that of natural gas, gasoline and other fuels, its ignition energy is very low, which can be as low as 0.02mj at least. In this way, there is almost no fire when hydrogen energy internal combustion engine works, and it has good starting performance. The emission of harmful substances in hydrogen energy is less, and the main product of combustion is water, which does not produce CO and ch. because the quenching distance of hydrogen flame is shorter than that of gasoline, the combustion of combustible mixture near the quench layer of cylinder wall is more complete, and the emission of NOx is greatly reduced.
Due to the problems of hydrogen fuel cell system in energy density, volume, reaction speed and cost, the vehicle powered by fuel cell is still a long way from industrialization. The fuel of internal combustion engine can not only realize the characteristics of clean and renewable hydrogen energy, but also make use of the fully established industrial foundation of internal combustion engine. Moreover, the thermal efficiency of hydrogen engine is high, the comprehensive efficiency is equivalent to that of fuel cell, and the production and use cost is low. It is easy to be developed and applied in terms of service performance and cost. At present, the main factors restricting hydrogen energy internal combustion engine are: low boiling point (- 253 ℃), poor storage and transportation performance and difficult preparation.
- Natural gas
China’s natural gas reserves are about 38 trillion m3, which is also relatively rich compared with oil. The application of natural gas in automobile has the advantages of low pollution, low price and long service life of engine, but it is difficult to use and the cost of storage tank is high. Therefore, we can research and develop new storage technology and strive to develop natural gas automobile. Liquefied petroleum gas is processed from petroleum products, and its resource reserves are limited. However, it can be used as an important supplement to automobile fuel because of its advantages such as low pollution, good economy and convenient storage and transportation.
Natural gas engines are mainly divided into two categories: one is spark plug ignition engine, which generally has the problems of low thermal efficiency and poor lean combustion capacity at low and medium load; The other is the diesel / natural gas dual fuel engine ignited by diesel. This kind of engine needs two sets of fuel supply systems, which has the problem of high hydrocarbon emission at low load. Chinese scholars have found that the combustion mode of Diesel / natural gas dual fuel engine is mainly premixed combustion, so the emission of particulate matter and soot is low.
Due to the obvious advantages of natural gas vehicles in terms of emissions, compared with gasoline vehicles in use, the particulate emissions of natural gas vehicles are almost zero, and the emissions of NOx, CO and HC are also significantly reduced. Therefore, natural gas vehicles are of great significance in improving air quality. At the same time, natural gas vehicle technology has also been unprecedented development, from atmospheric natural gas vehicles to compressed natural gas vehicles (cngvs) and liquefied natural gas vehicles (lngvs).
Nevertheless, there are still some problems in the use of natural gas vehicles, the most prominent of which are the decline of engine power, engine corrosion and early wear. According to data reports, when using natural gas as fuel, the power of cars generally decreases by about 15%, and even more in some cases. As a result of the decrease of power, on the one hand, it leads to the lack of power when the vehicle is overloaded, climbing or accelerating, on the other hand, it leads to the relative increase of fuel consumption and pollutant emissions. In addition, when the automobile uses natural gas as fuel, it is found that the combustion chamber components are obviously corroded, even the crankshaft is corroded, and the valves, piston rings and cylinders are seriously worn. Compared with gasoline, the automobile overhaul period is usually shortened by 1 / 3 ~ 1 / 2.
- Liquefied petroleum gas
Liquefied petroleum gas (LPG) is a by-product in the process of refining gasoline and diesel from crude oil. It has a wide range of sources and can be obtained from oilfield associated gas or natural gas through catalytic cracking unit in refinery. LPG is mainly composed of various hydrocarbons such as propane (C3H8), butane (C4H10) and a small amount of olefins. It is gaseous at room temperature, pressurized to about 1.6Mpa or liquid after cooling. After liquefaction, the volume is reduced to about 1 / 250 of the gaseous state. Therefore, it is easy to store and transport in liquid state. It is an ideal new energy. Its octane number is in the range of 100 ~ 200. Compared with gasoline engine, it has good explosion resistance, can appropriately increase the compression ratio, and its calorific value is close to gasoline value, which can improve power performance and economy. LPG is a clean fuel. Since the flame temperature is lower than that of gasoline and diesel, the amount of NOx is reduced accordingly. LPG is a gas at normal temperature, easy to mix evenly with air, complete reaction, low emission of CO and particles and almost smokeless. The unburned hydrocarbon in the exhaust gas is stable and will not form harmful photochemical smoke in the atmosphere. In addition, LPG and air are mixed into the engine cylinder, which can burn fully and completely and deposit less carbon, which can prolong the service life of engine oil and engine.
At present, the application research of LPG in the engine is mainly LPG / diesel dual fuel engine, which uses the injected diesel to ignite the mixture of LPG and air premixed in the combustion chamber, so as to make the engine work externally. In case of lack of LPG, it can also be used as a diesel engine. This engine also needs to be equipped with a set of air supply system and fuel storage system on the basis of the original engine. When using dual fuel, LPG and diesel burn in the combustion chamber with different mixing ratio under different operating conditions, which not only retains the characteristics of high power and high speed of diesel engine, but also reduces emissions and improves fuel economy. At present, the main problems of this energy are: the density of LPG is higher than that of air, which is easy to deposit, and once leaked, it is easy to form combustible mixture in a small range; LPG has high viscosity in the combustion chamber. When the olefin content is too high, cementation and carbon deposition are easy to occur, causing damage to engine valves, spark plugs and piston rings.
- Alcohol fuel
Among alternative fuels, alcohol fuel is the most promising and easily renewable liquid new energy fuel. At present, the most widely used alcohol alternative fuels are methanol and ethanol. The calorific value of methanol and ethanol is low, but because they are oxygenated fuels, their theoretical air-fuel ratio is lower than that of petroleum fuels, and the greater the percentage of oxygen, the smaller the theoretical air-fuel ratio. In this way, the theoretical calorific value of alcohol fuel mixture is roughly equivalent to that of petroleum fuels. When alcohol substitute fuel is used in internal combustion engine, this characteristic can ensure the same power performance as the original engine.
The octane number of methanol and ethanol is very high, which is very suitable to be used as part of the alternative fuel of gasoline engine. Compared with diesel fuel, alcohol fuel has lower cetane number, viscosity, calorific value and density, and is not easy to dissolve with diesel fuel. However, due to the fact that the thermal efficiency of alcohol fuel when burned on diesel engine is much higher than that when burned on gasoline engine, and its low emission, when used on supercharged diesel engine, it can reduce the intake temperature and improve the working medium density through the intake pipe, which has been systematically studied abroad, The results show that the soot particles (PM) are significantly improved, NOx is reduced, HC is increased, and the change of CO is related to the load. By supplying alcohol through the intake port of diesel engine, soot particles are significantly improved, NOx is reduced, HC and Co are increased.
The application technology of alcohol fuel in diesel engine is applied in the way of mixed combustion, that is, the proportional mixing application of alcohol fuel and fuel oil. At present, it has not been popularized on a large scale.
- Dimethyl ether
Dimethyl ether (DME) is one of the simplest ether compounds. Its molecular formula is CH3OCH3. It is a gas at room temperature and pressure and a liquid at medium pressure. The surface of liquid dimethyl ether looks like water, colorless, almost odorless, non-toxic, non carcinogenic, less corrosive and no pollution to the environment. Its cetane number is high (above 55), which is suitable for compression ignition engine. For diesel engine, the spontaneous combustion temperature and low-temperature fluidity of fuel are the most important. The spontaneous combustion temperature of DME is 15 ℃ lower than that of diesel. DME can quickly mix with air in the cylinder and has a short ignition delay period, which is conducive to the cold start of the engine and can reduce the premixed combustion; The latent heat of vaporization of DME is large, about twice that of diesel. The vaporization of DME can reduce the mixture temperature and further reduce NOx emission. Therefore, DME is suitable for high-speed diesel engines. Although the fuel with high cetane number is easy to crack in the combustion process, resulting in black smoke in the exhaust and increased fuel consumption. However, DME molecules are bound by C-O and C-H bonds, which is conducive to reducing the soot and particles generated.
At present, the main problems are: dimethyl ether has low viscosity, which is easy to cause leakage of high-pressure oil supply system and early wear of coupling parts, which brings difficulty to practical application; There are weak links in the research of dimethyl ether combustion and pollutant emission. Whether methyl formate is the main combustion product in low-temperature combustion, the detailed process of spontaneous combustion and ignition under high temperature and high pressure, its isomerization and peroxidation mechanism and the formation mechanism of trace emissions need to be studied.
- Solar energy
The thermoelectric utilization of solar energy refers to the direct conversion of solar radiation into heat and electricity for people’s use.
Solar energy is mainly used to generate electricity for electric vehicles. At present, it is mainly monocrystalline silicon cells. It is expected that the common application pattern of crystalline silicon cells and thin film cells can be realized in 2020, and then multi-layer composite GaAs solar cells can be further developed. At present, the utilization of solar photovoltaic in China is still in the primary stage, and the battery cost is high. The application of key technologies in automobiles has not yet broken through.
- Biodiesel
Biodiesel refers to the organic fatty acid ester fuel made by transesterification process with oil crops, wild oil plants, engineering microalgae and other aquatic plant oils as well as animal oils and catering waste oils as raw materials. Fame is a kind of biomass energy and a renewable diesel fuel that can replace petrochemical diesel.
Biodiesel vehicles refer to vehicles that use all or part of biodiesel as fuel.
Biodiesel can be used in diesel engines with 100% concentration. China has promulgated the national standard of biodiesel for diesel fuel blending (BD100). However, at present, the main biodiesel in the world still uses biodiesel and mineral oil, which generally accounts for 2% ~ 20% of the total volume of biodiesel oil. At present, the scale application of biodiesel in the industry is generally between B5 (5% biodiesel + 95% standard diesel) ~ B20 (20% biodiesel + 80% standard diesel).
The biggest advantage of biodiesel is its excellent environmental protection characteristics. According to the research results of American scientists, the use of biodiesel can reduce the air toxicity by 90%, and the carbon dioxide emission is 60% lower than that of diesel, which can meet the emission standards of Europe II and II; Low vehicle cost. The vehicles using biodiesel are the same as ordinary diesel vehicles, and the vehicles do not need any modification; Good safety; It has good low-temperature start-up and lubrication performance. Biodiesel has high flash point and low toxicity. It is an environmentally friendly renewable fuel. As a clean and high-quality renewable energy, the development of biodiesel is of great significance today when the world’s oil reserves continue to decrease.
The disadvantage of biodiesel is poor combustion effect. The viscosity of biodiesel is higher than that of diesel, resulting in poor injection effect. Due to the low volatility of biodiesel, the combustion is incomplete, which affects the combustion efficiency of vehicles; The cost of biodiesel production is high; It consumes a lot of cultivated land resources. Biodiesel has unique advantages as an alternative fuel to diesel. However, due to raw materials and processing, compared with petrochemical diesel, biodiesel has poor oxidation stability, which makes it difficult to use and store biodiesel in practice.