If you have the opportunity to fly over Western Siberia at night, through the window you will see a grotesque sight, reminiscent of the Kuwaiti desert after the withdrawal of Saddam's troops during the first war in Iraq. The landscape is littered with huge burning "torches", which is clear evidence that many Russian oil producers still consider natural gas a byproduct and unnecessary product in the process of finding oil fields ...
Experts believe that this waste will be stopped in the near future. For many years, natural gas was considered a surplus product and was burned or simply released into the atmosphere. It is estimated that so far Saudi Arabia alone has dumped or burned more than 450 million cubic meters of natural gas during oil production ...
At the same time, the process is reversed - the majority of modern oil companies have been using natural gas for a long time, realizing the value of this product and its importance, which may only increase in the future. This view of things is especially characteristic of the United States, where, in contrast to the already depleting oil reserves, there are still large gas fields. The latter circumstance is automatically reflected in the industrial infrastructure of a huge country, the work of which is inconceivable without cars, and even more so without large trucks and buses. There are more and more transport companies abroad that are modernizing the diesel engines of their truck fleets to work both with combined gas-diesel systems and only with blue fuel. More and more ships are switching to natural gas.
Against the backdrop of liquid fuel prices, the price of methane sounds fantastic, and many are beginning to doubt that there is a catch - and not without reason. Considering that the energy content per kilogram of methane is higher than per kilogram of gasoline, and one liter (i.e. one cubic decimeter) of gasoline weighs less than a kilogram, anyone can conclude that a kilogram of methane contains much more energy than a liter of gasoline. Understandably, even without this obvious jumble of numbers and undefined inequalities, operating a car that runs on natural gas or methane will cost you much less money than running a car running on gasoline.
But here's the classic big "BUT" ... Why, since the "scam" is so great, almost no one in our country uses natural gas as a vehicle fuel, and cars adapted for its use in Bulgaria are less common. a phenomenon from kangaroo to the Rhodope pine mountain? The answer to this perfectly normal question is not given by the fact that the gas industry around the world is developing at a frantic pace and is currently considered the safest alternative to liquid fuel oil. Hydrogen engine technology still has unclear predictions for the future, control of processes in the cylinders of "hydrogen" engines is extremely difficult, and the question of which economical method of extracting pure hydrogen is not yet clear. Against this background, the future of methane, to put it mildly, is brilliant - especially since there are huge deposits of natural gas in politically safe countries, that new technologies (mentioned in the previous issue of cryogenic liquefaction and chemical transformation of natural gas into liquid) are getting cheaper, while the price of classic hydrocarbon products are growing. Not to mention the fact that methane has every chance of becoming the main source of hydrogen for the fuel cells of the future.
The real reason for the abandonment of hydrocarbon gases as automotive fuels is still low oil prices over the decades, which have pushed the development of automotive technology and related road transport infrastructure towards the provision of energy for gasoline and diesel engines. Against the background of this general trend, attempts to use gas fuel are rather sporadic and insignificant.
Even after the end of World War II, the shortage of liquid fuels in Germany led to the emergence of cars equipped with the simplest systems for using natural gas, which, although much more primitive, differ little from the systems used by Bulgarian taxis today. from gas cylinders and reducers. Gas fuels gained more importance during the two oil crises in 1973 and 1979-80, but even then we can only talk about short flashes that went almost unnoticed and did not lead to significant development in this area. For more than two decades since this latest acute crisis, liquid fuel prices have remained consistently low, reaching absurdly low prices in 1986 and 1998 at $ 10 a barrel. It is clear that such a situation cannot have a stimulating effect on alternative types of gas fuel ...
At the beginning of the 11th century, the market situation is gradually but surely moving in a different direction. Since the 2001 September XNUMX terrorist attacks, there has been a gradual but steady upward trend in oil prices, which has continued to rise as a result of increased consumption by China and India and difficulties in finding new deposits. However, car companies are much more awkward in the direction of mass production of cars adapted to run on gaseous fuels. The reasons for this cumbersomeness can be found both in the inertia of thinking of the majority of consumers, accustomed to traditional liquid fuels (for Europeans, for example, diesel fuel remains the most realistic alternative to gasoline), and in the need for huge investments in pipeline infrastructure. and compressor stations. When this is added to the complex and expensive storage systems for fuel (especially compressed natural gas) in the cars themselves, the big picture starts to clear up.
On the other hand, gaseous fuel propulsion systems are becoming more diversified and follow the technology of their gasoline counterparts. Gas feeders already use the same sophisticated electronic components to inject fuel into the liquid (still rare) or gas phase. Also, more and more models of production vehicles appear with a factory setting for monovalent gas supply or with the possibility of dual gas / petrol supply. Increasingly, another advantage of gaseous fuels is being realized - due to its chemical structure, gases are more fully oxidized, and the level of harmful emissions in the exhaust gases of cars using them is much lower.
A new beginning
However, to break into the market, it will be necessary to apply targeted and direct financial incentives to end users of natural gas as a vehicle fuel. To attract customers, methane sellers in Germany are already providing gas-powered car buyers with special bonuses, the nature of which sometimes seems simply incredible - for example, the Hamburg gas distribution company reimburses individuals for gas purchases. cars from designated dealers for one year. The only condition for the user is to stick the sponsor's advertising sticker on his car ...
The reason why natural gas in Germany and Bulgaria (in both countries the vast majority of blue fuel is piped from Russia) is much cheaper than other fuels is to be found in a number of legal prerequisites. The market price for gas is logically related to the price of oil: as the price of oil rises, so does the price of blue fuel, but the difference in prices for gasoline and gas for the end consumer is mainly related to lower taxation of natural gas. In Germany, for example, the price of gas is legally fixed until 2020, and the scheme of this “fixation” is as follows - during the specified period, the price of natural gas may rise along with the price of oil, but its proportional advantage over other energy sources must be maintained at constant level. It is clear that with such a regulated legal framework, low prices and the absence of any problems in the construction of "gas engines", the only problem for the growth of this market is the undeveloped network of gas stations - in huge Germany, for example, there are only 300 such points, and in Bulgaria there are many smaller.
The prospects for eliminating this infrastructural deficit at the moment look great - in Germany, the association Erdgasmobil and the French oil giant TotalFinaElf intends to invest huge amounts of money in the construction of several thousand new gas stations, and in Bulgaria several companies have taken on a similar task. It is possible that soon the whole of Europe will use the same developed network of filling stations for natural and liquefied petroleum gas, as consumers in Italy and the Netherlands - the countries whose development in this area we told you in the previous issue.
Honda Civic GX
At the 1997 Frankfurt Motor Show, Honda unveiled the Civic GX, claiming it is the world's most environmentally friendly car. It turned out that the Japanese ambitious statement is not just another marketing gimmick, but the truth, which remains relevant to this day, and can be seen in practice in the latest edition of the Civic GX. The vehicle is designed to run on natural gas only, and the engine is designed to take full advantage of the high octane rating of the gaseous fuel. Unsurprisingly, vehicles of this type today can offer exhaust emission levels lower than those required in the future European Euro 5 economy, or 90% lower than American ULEVs (ultra low emission vehicles). . The Honda engine runs extremely smoothly and the high compression ratio of 12,5: 1 makes up for the lower volumetric energy value of natural gas compared to gasoline. The 120-liter tank is made of composite material, and the equivalent gas mileage is 6,9 liters. The famous Honda VTEC variable valve timing system works perfectly with the special properties of the fuel and further improves the engine charge. Due to the lower burning rate of natural gas and the fact that the fuel is "dry" and does not have lubricating properties, the valve seats are made of special heat-resistant alloys. Pistons are also made of stronger materials, as gas cannot cool the cylinders when it evaporates like gasoline.
The hoses of the Honda GX are injected with natural gas in the gas phase, the volume of which is 770 times that of the equivalent amount of gasoline. The biggest technological challenge for Honda engineers was creating the right injectors to work in these conditions and prerequisites - in order to achieve optimal power, the injectors must cope with the difficult task of simultaneously supplying the required amount of gas, for which, in principle, liquid petrol is injected. This is a problem for all engines of this type, since the gas occupies a much larger volume, displaces part of the air and requires injection directly into the combustion chambers.
In the same 1997 Fiat A similar model Honda GX was also demonstrated. The “bivalent” version of the Marea can use two types of fuel - gasoline and natural gas, and the gas is pumped by a second, completely independent fuel system. The engine always starts on liquid fuel and then automatically switches to gas. The 1,6-liter engine has 93 hp. with gas fuel and 103 hp. from. when using gasoline. Basically, the engine runs mainly on gas, unless the latter runs out or the driver has a clear desire to use gasoline. Unfortunately, the “dual nature” of divalent energy does not allow us to take full advantage of the benefits of high-octane natural gas. Fiat is currently producing a Mulipla version with this type of PSU.
Over time, similar models appeared in the range Opel (Astra and Zafira Bi Fuel for LPG and CNG versions), PSA (Peugeot 406 LPG and Citroen Xantia LPG) and VW (Golf Bifuel). Classics in this area are considered Volvowhich produce variants S60, V70 and S80 capable of operating on both natural gas and biogas and LPG. All of these vehicles are equipped with gas injection systems using special injectors, electronically controlled technological processes and fuel-compatible mechanical components such as valves and pistons. CNG fuel tanks can withstand a pressure of 700 bar, although the gas itself is stored in them at a pressure of no more than 200 bar.
BMW is a renowned proponent of sustainable fuels and has been developing various powertrains for vehicles with alternative sources for many years. Back in the early 90s, the Bavarian company created models of the 316g and 518g series, which use natural gas as fuel. In its latest developments, the company decided to experiment with fundamentally new technologies and, together with the German refrigeration group Linde, the oil company Aral and the energy company E.ON Energy, developed a project for the use of liquefied gases. The project is developing in two directions: the first is development for the supply of liquefied hydrogen, and the second is the use of liquefied natural gas. The use of liquefied hydrogen is still considered a promising technology, which we will discuss later, but the system for storing and using liquefied natural gas is quite real and can be applied in practice in the automotive industry in the next few years.
At the same time, natural gas is cooled to a temperature of -161 degrees and condenses at a pressure of 6-10 bar, while passing into the liquid phase. The tank is much more compact and lighter in comparison with compressed gas cylinders and is practically a cryogenic thermos made of super-insulating materials. Thanks to modern Linde technology, despite the very thin and light tank walls, liquid methane can be stored in this state for two weeks without problems, even in hot weather and without the need for refrigeration. The first LNG filling station, in the construction of which € 400 was invested, is already operating in Munich.
Combustion processes in gaseous fuel engines
As already mentioned, natural gas contains mainly methane, while LPG contains propane and butane in proportions depending on the season. As the molecular weight increases, the detonation resistance of paraffinic (unbranched) hydrocarbon compounds such as methane, ethane, and propane decreases, molecules break down more easily and more peroxides accumulate. Thus, diesel engines use diesel fuel rather than gasoline, since in the former case, the autoignition temperature is lower.
Methane has the highest hydrogen / carbon ratio of all hydrocarbons, which in practice means that for the same weight, methane has the highest energy value among hydrocarbons. The explanation of this fact is complex and requires a certain knowledge of the chemistry and energy of relationships, so we will not deal with this. Suffice it to say that the stable methane molecule provides an octane number of about 130.
For this reason, the combustion rate of methane is much lower than that of gasoline, small molecules allow methane to burn more completely, and its gaseous state results in less oil leaching from the cylinder walls in cold engines compared to gasoline mixtures. . Propane, in turn, has an octane rating of 112, which is still higher than most gasolines. Poor propane-air mixtures burn at a lower temperature than gasoline, but rich ones can lead to thermal overload of the engine, since propane does not have the cooling properties of gasoline due to its entry into the cylinders in gaseous form.
This problem has already been solved with the use of liquid propane direct injection systems. Since propane is easily liquefied, it is not difficult to build a system for storing it in a car, and heating the intake manifolds is simply not necessary, since propane does not condense like gasoline. This in turn improves the thermodynamic efficiency of the engine, where it is safe to use thermostats that maintain a lower coolant temperature. The only significant drawback of gaseous fuels is the fact that neither methane nor propane lubricates the exhaust valves, so experts say it is a "dry fuel" that favorably affects piston rings but harms valves. You cannot rely on gases to transfer most of the additives to engine cylinders, but engines that run on these fuels do not need as many additives as gasoline engines. Mixture quality control is a very important factor in gas engines, as rich mixtures lead to higher exhaust temperatures and overloading of valves, while poor mixtures create a problem due to a decrease in the already low combustion rate, which is again a prerequisite for thermal overloading of the valve. ... The compression ratio in propane engines can easily be increased by two or three units, and in methane engines even more. The resulting increase in nitrogen oxides is offset by lower overall emissions. The optimal propane mix is a little "leaner" - 15,5: 1 (air to fuel) versus 14,7: 1 for gasoline, and this is taken into account when designing evaporators, metering devices or injection systems. Since both propane and methane are gases, engines do not need to enrich during cold starts or acceleration.
The overtaking angle is calculated according to a different curve than for gasoline engines - at low rpm, overtaking on ignition should be higher due to slower combustion of methane and propane, but at high rpm, gasoline engines need more increase. mixture (the combustion rate of gasoline decreases due to the short time of pre-flame reactions - i.e. the formation of peroxides). That is why there is a completely different algorithm in electronic ignition control systems for gas engines.
Methane and propane also increase the high voltage requirements of the spark plug electrodes — a “drier” mixture is “harder” to break through than a spark because it is a less conductive electrolyte. Therefore, the distance between the electrodes of spark plugs suitable for such engines is usually different, the voltage is higher, and in general the issue of spark plugs is more complex and subtle than for gasoline engines. Most modern gas engines use a lambda probe for optimal metering of the mixture in terms of quality. Having ignition systems on two separate curves is especially important for vehicles equipped with dual systems (for natural gas and petrol), since the sparse network of natural gas filling points often requires the forced use of petrol.
The optimum compression ratio for natural gas is about 16: 1, and the ideal air-fuel mixture is 16,5: 1. If the gas engine is not specifically adapted to achieve such a high compression ratio, then the high octane number of natural gas cannot be utilized to the maximum extent and the engine will lose about 15% of its potential power. When using natural gas, the amount of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas is reduced by 90%, and nitrogen oxides (NOx) - by about 70%, compared with the emissions of conventional gasoline engines. Oil change intervals for gas engines are typically doubled.
In the past few years, dual-fuel fuel delivery systems have become increasingly popular. I hasten to note that we are not talking about "bivalent" engines running alternately on gas or petrol and having spark plugs, but about special diesel-gas systems in which part of the diesel fuel is replaced by natural gas supplied by a separate power system. This technology is based on standard diesel engines.
The principle of operation is based on the fact that methane has a self-ignition temperature above 600 degrees - i.e. above a temperature of about 400-500 degrees at the end of the compression cycle of a diesel engine. This, in turn, means that the methane-air mixture does not ignite by itself when compressed in the cylinders, and the injected diesel fuel, which ignites at about 350 degrees, is used as a kind of spark plug. The system could run entirely on methane, but in this case it would be necessary to install an electrical system and a spark plug. Typically, the percentage of methane increases with load, the vehicle runs on diesel at idle, and the methane / diesel ratio reaches 9/1 under high load. These proportions can also be changed according to the preliminary program.
Some companies produce diesel engines with so-called. Micropilot power supply systems, in which the role of the diesel system is limited to the injection of a small amount of fuel needed only to ignite methane. Therefore, these engines cannot operate autonomously on a diesel engine and are usually used in industrial machines, cars, buses and ships, where costly re-equipment is economically justified - after its wear, this leads to significant savings, engine life. increases significantly, and emissions of harmful gases are significantly reduced. Mikropilot machines can operate on both liquefied and compressed natural gas.
Types of systems used for additional installation
The variety of gas supply systems for gaseous fuels is constantly growing. In principle, species can be divided into several types. When propane and methane are used, these are blended atmospheric pressure systems, gas phase injection systems, and liquid phase injection systems. From a technical point of view, propane-butane injection systems can be divided into several generations:
The first generation are systems without electronic control in which the gas is mixed in a simple mixer. This is usually the case with old carburetor engines.
Second generation - injection with one injector, analogue lambda probe and three-way catalyst.
Third generation - injection with one or more injectors (one per cylinder), microprocessor-controlled and with both a self-study program and a self-diagnosis code table.
Fourth generation - sequential (cylindrical) injection depending on the position of the piston, with the number of injectors equal to the number of cylinders, and with feedback through a lambda probe.
Fifth generation - Multi-point sequential injection with feedback and communication with a microprocessor to control petrol injection.
In the most modern systems, the gas computer fully uses the data of the main microprocessor to control the parameters of the gasoline engine, including the injection time. Data transmission and control are also fully linked to the main petrol program, which avoids the need to create entire XNUMXD gas injection maps for each car model - the smart device simply reads programs from the petrol processor. and adapts them to gas injection.