Test drive BMW and hydrogen: part two

"Water. The only end product of BMW's clean engines is using liquid hydrogen instead of petroleum fuels and enabling everyone to enjoy new technologies with a clear conscience."

BMW way

These words are a quote from an advertising campaign of a German company several years ago. For a long time no one questioned the fact that the Bavarians know very well what they are doing when it comes to motor technology and are one of the undisputed world leaders in this field. Nor would it be thought that a company that has shown solid sales growth in recent years would throw a ton of money on little-known ads for promising technologies with an uncertain future.

At the same time, however, the quoted words are part of a campaign to promote a rather exotic 745-hour hydrogen version of the Bavarian automaker's flagship. Exotic, because according to BMW, the transition to alternatives to hydrocarbon fuels, which the automotive industry has been feeding from the very beginning, will require a change in the entire production infrastructure. The latter is necessary because the Bavarians see a promising development path not in the widely advertised fuel cells, but in the conversion of internal combustion engines to run on hydrogen. BMW believes that the upgrade is a solvable problem and has already made significant progress in solving the main problem of achieving reliable engine performance and eliminating its propensity for uncontrolled combustion processes using pure hydrogen. Success in this direction is due to the competence in the field of electronic control of engine processes and the possibility of using the BMW patented flexible gas distribution systems Valvetronic and Vanos, without which it would be impossible to ensure the normal operation of "hydrogen engines". However, the first steps in this direction date back to 1820, when the designer William Cecil created a hydrogen-fueled engine operating on the so-called "vacuum principle" - a scheme very different from that of the later invented engine with an internal engine. burning. In his first development of internal combustion engines 60 years later, the pioneer Otto used the already mentioned and coal-derived synthetic gas with a hydrogen content of about 50%. However, with the invention of the carburetor, the use of gasoline has become much more practical and safer, and liquid fuel has replaced all other alternatives that have existed until now. The properties of hydrogen as a fuel were rediscovered many years later by the space industry, which quickly discovered that hydrogen had the best energy/mass ratio of any fuel known to mankind.

In July 1998, the European Association of the Automotive Industry (ACEA) committed itself to the European Union to reduce CO2008 emissions from newly registered vehicles in the Union by an average of 2 grams per kilometer by 140. In practice, this meant a 25% reduction in emissions compared to 1995, and the average fuel consumption of the new fleet was about 6,0 l / 100 km. In the near future, additional measures are expected to reduce carbon dioxide emissions by 14% by 2012. This makes the task for car companies extremely difficult and, according to BMW experts, can be solved either by using low-carbon fuels or by completely eliminating carbon from the fuel composition. According to this theory, hydrogen is reappearing in the automotive arena in all its glory.

The Bavarian company became the first car manufacturer to mass-produce hydrogen-powered vehicles. The optimistic and confident claims of Professor Burkhard Geschel, BMW board member responsible for new developments, that "the company will sell hydrogen cars before the expiration of the current 7 Series" have come true. With its latest version, the Hydrogen 7, the seventh series, introduced in 2006, with a 12 hp 260-cylinder engine. this message has already become a reality. The intention seemed quite ambitious, but not without reason. BMW has been experimenting with internal combustion engines running on hydrogen since 1978, and on 11 May 2000 made a unique demonstration of the possibilities of this alternative. An impressive fleet of 15 750 hl vehicles from the previous generation of the week, powered by hydrogen twelve-cylinder engines, completed the 170 km marathon, highlighting the company's success and the promise of new technology. In 000 and 2001, some of these vehicles continued to take part in various demonstrations in support of the hydrogen idea. Then it was time for a new development, based on the next 2002 Series, using a modern 7-liter V-4,4 engine capable of a top speed of 212 km / h, followed by the latest development with a 12-cylinder V-XNUMX. According to the company’s official opinion, the reasons why BMW chose this technology over fuel cells are both commercial and psychological. First, this method will require significantly less investment if the production infrastructure changes. Secondly, because people are used to the good old internal combustion engine, they like it and it will be difficult to part with it. And third, in the meantime, it turned out that this technology is developing faster than fuel cell technology.

In BMW cars, hydrogen is stored in a super-insulated cryogenic vessel, sort of like a high-tech thermos bottle developed by the German refrigeration group Linde. At low storage temperatures, the fuel is in the liquid phase and enters the engine like regular fuel.

At this stage, the designers of the Munich-based company focused on indirect fuel injection, and the quality of the mixture depends on the operating mode of the engine. In part load mode, the engine runs on lean mixtures similar to diesel fuel - the change is made only in the amount of fuel injected. This is the so-called "quality control" of the mixture, in which the engine runs with excess air, but due to the low load, the formation of nitrogen emissions is minimized. When there is a need for significant power, the engine starts to work like a gasoline engine, moving on to the so-called "quantitative control" of the mixture and normal (not lean) mixtures. These changes are possible, on the one hand, due to the speed of electronic control of processes in the engine, and on the other hand, due to the flexible operation of gas distribution control systems - “double” Vanos, working in conjunction with the Valvetronic intake control system without throttle. It should be borne in mind that, according to BMW engineers, the working scheme of this development is only an intermediate stage in the development of technology and that in the future engines will switch to direct hydrogen injection into cylinders and turbocharging. These techniques are expected to result in better vehicle dynamics than a comparable gasoline engine and an increase in overall efficiency of the internal combustion engine by more than 50%. Here we deliberately refrained from touching on the topic of "fuel cells", since this issue has been quite actively used lately. At the same time, however, we must mention them in the context of BMW's hydrogen technology, as the designers in Munich decided to use just such devices to power the on-board electrical network in cars, completely eliminating conventional battery power. This move allows additional fuel savings, as the hydrogen engine does not have to drive the alternator, and the onboard electrical system becomes completely autonomous and independent of the drive path - it can generate electricity even when the engine is not running, as well as produce and consume energy lends itself to full optimization. The fact that only as much electricity as needed can now be produced to power the water pump, oil pumps, brake booster and wired systems also translates into additional savings. However, in parallel with all these innovations, the fuel injection system (gasoline) practically did not undergo expensive design changes. In order to promote hydrogen technologies in June 2002, BMW Group, Aral, BVG, DaimlerChrysler, Ford, GHW, Linde, Opel MAN created the CleanEnergy partnership program, which began with the development of filling stations with liquefied and compressed hydrogen.

BMW is the initiator of a number of other joint projects, including with oil companies, among which the most active participants are Aral, BP, Shell, Total. Interest in this promising area is growing exponentially - in the next ten years, the EU alone will provide direct financial contributions to funds to finance the development and implementation of hydrogen technologies in the amount of 2,8 billion euros. The volume of investments by private companies in the development of "hydrogen" during this period is difficult to predict, but it is clear that it will many times exceed the deductions from non-profit organizations.

Hydrogen in internal combustion engines

It is interesting to note that, due to the physical and chemical properties of hydrogen, it is much more flammable than gasoline. In practice, this means that much less initial energy is required to initiate the combustion process in hydrogen. On the other hand, very lean mixtures can be easily used in hydrogen engines - something that modern gasoline engines achieve through complex and expensive technologies.

The heat between the particles of the hydrogen-air mixture is less dissipated, and at the same time, the auto-ignition temperature and the rate of combustion processes are much higher than that of gasoline. Hydrogen has a low density and a strong diffusivity (the possibility of particles penetrating into another gas - in this case, air).

The low activation energy required for self-ignition is one of the biggest challenges in controlling combustion processes in hydrogen engines because the mixture can easily spontaneously ignite due to contact with hotter areas in the combustion chamber and resistance to following a chain of completely uncontrolled processes. Avoiding this risk is one of the biggest challenges in developing hydrogen engines, but it is not easy to eliminate the consequences of the fact that a highly diffuse burning mixture travels very close to the cylinder walls and can penetrate extremely narrow gaps. such as closed valves, for example... All this must be taken into account when designing these motors.

A high autoignition temperature and a high octane number (about 130) allow an increase in the compression ratio of the engine and, therefore, its efficiency, but again there is a danger of autoignition of hydrogen from contact with the hotter part. in the cylinder. The advantage of the high diffusion capacity of hydrogen is the possibility of easy mixing with air, which in the event of a tank breakdown guarantees quick and safe dispersion of the fuel.

The ideal air-hydrogen mixture for combustion has a ratio of approximately 34:1 (for gasoline this ratio is 14,7:1). This means that when combining the same mass of hydrogen and gasoline in the first case, more than twice as much air is required. At the same time, the hydrogen-air mixture takes up significantly more space, which explains why hydrogen-powered engines have less power. A purely digital illustration of ratios and volumes is quite eloquent - the density of hydrogen ready for combustion is 56 times less than that of gasoline vapor .... However, it should be noted that, in principle, hydrogen engines can also operate with air-hydrogen mixtures up to 180:1 (i.e. very "lean" mixtures), which in turn means that the engine can be operated. without a throttle valve and use the principle of diesel engines. It should also be noted that hydrogen is the undisputed leader in the comparison of hydrogen and gasoline as energy sources in terms of mass - a kilogram of hydrogen is almost three times more energy-intensive than a kilogram of gasoline.

As with gasoline engines, liquefied hydrogen can be injected directly ahead of the valves in the manifolds, but the best solution is injection directly during the compression stroke - in this case, the power can exceed that of a similar gasoline engine by 25%. This is because the fuel (hydrogen) does not displace air as in a gasoline or diesel engine, allowing only air (significantly more than usual) to fill the combustion chamber. Also, unlike gasoline engines, hydrogen engines do not need structural swirling because hydrogen diffuses well enough with air without this measure. Due to the different burning rates in different parts of the cylinder, it is better to place two spark plugs, and in hydrogen engines, the use of platinum electrodes is impractical, since platinum becomes a catalyst leading to fuel oxidation at low temperatures.

H2R

The H2R is a working supersport prototype built by BMW engineers and powered by a twelve-cylinder engine that reaches a maximum output of 285 hp when powered by hydrogen. Thanks to them, the experimental model accelerates from 0 to 100 km / h in six seconds and reaches a top speed of 300 km / h. The H2R engine is based on the standard top-end unit used in the petrol 760i and took only ten months to develop. To prevent spontaneous combustion, the Bavarian specialists have developed a special flow cycle and injection strategy into the combustion chamber, using the possibilities provided by variable valve timing systems of the engine. Before the mixture enters the cylinders, the latter are cooled by air, and ignition is carried out only at the top dead center - due to the high combustion rate with hydrogen fuel, ignition advance is not required.

Conclusions

The financial analysis of the transition to pure hydrogen energy is not yet very optimistic. The production, storage, transportation and supply of light gas are still quite energy-intensive processes, and at the current technological stage of human development such a scheme cannot be effective. However, this does not mean that research and the search for solutions will not continue. Proposals to generate hydrogen from water using electricity from solar panels and store it in large tanks sound optimistic. On the other hand, the process of generating electricity and hydrogen in the gas phase in the Sahara Desert, transporting it to the Mediterranean Sea by pipeline, liquefying and transporting it by cryogenic tankers, unloading it in ports and finally transporting it by truck sounds a little ridiculous at the moment ...

An interesting idea was recently presented by the Norwegian oil company Norsk Hydro, which proposed producing hydrogen from natural gas at production sites in the North Sea, and the residual carbon monoxide was stored in depleted fields under the seabed. The truth lies somewhere in the middle, and only time will tell where the development of the hydrogen industry will go.

Mazda variant

The Japanese company Mazda is also showing its version of the hydrogen engine - in the form of a rotary unit sports car RX-8. This is not surprising, because the design features of the Wankel engine are extremely suitable for using hydrogen as a fuel. The gas is stored under high pressure in a special tank, and the fuel is injected directly into the combustion chambers. Due to the fact that in the case of rotary engines, the areas in which injection and combustion take place are separated, and the temperature in the suction part is lower, the problem of the possibility of uncontrolled ignition is significantly reduced. The Wankel engine also offers enough space for two injectors, which is extremely important for injecting the optimum amount of hydrogen.