The tense confrontation between diesel and gasoline engines reaches its climax. The latest turbo technologies, electronically controlled Common-Rail direct injection systems, high compression ratio - the rivalry brings the two types of engines closer together ... And suddenly, in the midst of an ancient duel, a new player suddenly appeared on the scene. a place under the sun.
After many years of neglect, designers have rediscovered the enormous potential of the diesel engine and accelerated its development through the intensive introduction of new technologies. It got to the point that its dynamic characteristics approached the characteristics of a gasoline competitor and allowed the creation of hitherto unthinkable cars such as the Volkswagen Race Touareg and Audi An R10 TDI with more than serious racing ambitions. The chronology of events of the last fifteen years is well known ... Diesel engines of the XNUMX-ies did not fundamentally differ from their ancestors, created Mercedes-Benz back in 1936. A process of slow evolution followed, which has grown in recent years into a powerful technological explosion. In the late 13s, Mercedes recreated the first automobile turbodiesel, in the late 1s, direct injection made its debut in the Audi model, later diesels received four-valve heads, and in the late XNUMXs, electronically controlled Common Rail injection systems became a reality. . Meanwhile, high pressure direct fuel injection has been introduced into gasoline engines, where the compression ratio today reaches XNUMX: XNUMX in some cases. Recently, turbo technology is also experiencing a renaissance, with the torque values of gasoline engines beginning to approach significantly the torque values of the famous flexible turbo diesel. However, in parallel with the modernization, there is a steady trend towards a serious rise in the price of the gasoline engine ... So, despite the pronounced prejudices and polarization of opinions regarding gasoline and diesel engines in different parts of the world, neither of the two rivals gains tangible dominance.
Despite the coincidence of the qualities of the two types of units, there are still huge differences in the nature, character and behavior of the two heat engines.
In the case of a gasoline unit, a mixture of air and evaporated fuel is formed over a much longer period of time and begins long before the combustion process begins. Whether using a carburetor or modern electronic direct injection systems, the aim of mixing is to obtain a uniform, homogeneous fuel mixture with a well-defined air-fuel ratio. This value is usually close to the so-called “stoichiometric mixture”, in which there are enough oxygen atoms to be able (theoretically) to bond into a stable structure with every hydrogen and carbon atom in the fuel, forming only H20 and CO2. Since the compression ratio is small enough to avoid the premature uncontrolled self-ignition of certain substances in the fuel due to the high compression temperature (the gasoline fraction consists of hydrocarbons with a significantly lower evaporation temperature and a significantly higher combustion temperature). self-ignition from those in the diesel fraction), the ignition of the mixture is initiated by the spark plug and combustion occurs as a front moving at a certain speed limit. Unfortunately, zones with incomplete processes leading to the formation of carbon monoxide and stable hydrocarbons are formed in the combustion chamber, and as the flame front moves, the pressure and temperature at its periphery increase, which leads to the formation of harmful nitrogen oxides (between nitrogen and oxygen from the air). peroxides and hydroperoxides (between oxygen and fuel). The accumulation of the latter to critical values leads to uncontrolled detonation combustion, therefore, modern gasolines use fractions of molecules with a relatively stable chemical "structure" that is difficult to detonate - a number of additional processes are carried out at oil refineries to achieve this resistance. including an increase in the octane number of the fuel. Because of the largely fixed mixture ratio that gasoline engines can handle, the throttle valve plays an important role, which regulates engine load by regulating the amount of fresh air. However, it, in turn, becomes a source of significant losses in the partial load mode, acting as a kind of "throat plug" of the engine.
The idea of the creator of the diesel engine, Rudolf Diesel, is to significantly increase the compression ratio, and hence the thermodynamic efficiency of the machine. Thus, the area of the fuel chamber is reduced, and the combustion energy is not dissipated through the cylinder walls and the cooling system, but is "spent" between the particles themselves, which in this case are much closer to each other. If a previously prepared fuel-air mixture enters the combustion chamber of this type of engine, as in the case of a gasoline engine, then when a certain critical temperature is reached during the compression process (depending on the compression ratio and the type of fuel) long before GMT, the autoignition process will be initiated. uncontrolled volumetric combustion. It is for this reason that fuel is injected into a diesel engine at the last moment, shortly before GMT, at very high pressure, which creates a significant shortage of time for good evaporation, diffusion, mixing, self-ignition and the need to limit the maximum speed, which rarely exceeds the limit. from 4500 rpm This approach establishes the corresponding requirements for the quality of the fuel, which in this case is the fraction of diesel fuel - mainly direct distillates with a significantly lower autoignition temperature, since a more unstable structure and long molecules are a prerequisite for their easier rupture and reaction with oxygen.
A feature of the combustion processes of a diesel engine are, on the one hand, zones with an enriched mixture around the injection holes, where fuel decomposes (cracks) from temperature without oxidation, turning into a source of carbon particles (soot), and on the other. in which there is no fuel at all and, under the influence of high temperature, nitrogen and oxygen of the air enter into a chemical interaction, forming nitrogen oxides. Therefore, diesel engines are always tuned to operate with medium-lean mixtures (that is, with a serious excess of air), and the load is controlled only by the dosage of the amount of injected fuel. This avoids using the throttle, which is a huge advantage over their gasoline counterparts. To compensate for some of the shortcomings of the gasoline engine, the designers have created engines in which the mixture formation process is the so-called "charge stratification".
In partial load mode, the optimal stoichiometric mixture is created only in the area around the spark plug electrodes due to a special injection of an injected fuel jet, a directed air flow, a special piston front profile and other similar methods that ensure ignition reliability. At the same time, the mixture in most of the chamber volume remains lean, and since the load in this mode can be controlled only by the amount of supplied fuel, the throttle valve can remain fully open. This, in turn, leads to a simultaneous decrease in losses and an increase in the thermodynamic efficiency of the engine. In theory, everything looks great, but so far the success of this type of production engine Mitsubishi and VW is not glamorous. In general, so far no one can boast that they have taken full advantage of these technological solutions.
And what if the advantages of two types of engines are combined in a "magical" way? What would be the ideal combination of high compression of a diesel engine, uniform distribution of the mixture throughout the entire volume of the combustion chamber and uniform autoignition in the same volume? Intensive laboratory studies of experimental units of this type in recent years have shown a significant reduction in harmful emissions in exhaust gases (for example, the amount of nitrogen oxides is reduced to 99%!) With an increase in efficiency compared to gasoline engines. It seems that the future does indeed belong to the engines that auto companies and independent development companies recently merged under the heading HCCI - Homogeneous Charge Compression Ignition Engines or Homogeneous Spontaneous Ignition Engines.
Like many other seemingly "revolutionary" developments, the idea of creating such a machine is not new, and while attempts to create a reliable production model are still unsuccessful. At the same time, the growing possibilities of electronic control of the technological process and the great flexibility of gas distribution systems create a very realistic and optimistic prospect for a new type of engine.
In fact, in this case, it is a kind of hybrid of the principles of gasoline and diesel engines. A well-homogenized mixture, like in gasoline engines, enters the combustion chambers of the HCCI, but it self-ignites by the heat from the compression. The new type of engine also does not require a throttle valve as it can run on lean mixtures. However, it should be noted that in this case the meaning of the definition of "lean" differs significantly from the definition of diesel, since HCCI does not have a completely lean and highly enriched mixture, but is a kind of uniformly lean mixture. The principle of operation involves the simultaneous ignition of the mixture in the entire volume of the cylinder without a uniformly moving flame front and at a much lower temperature. This automatically leads to a significant decrease in the amount of nitrogen oxides and soot in the exhaust gases, and, according to a number of authoritative sources, the massive introduction of much more efficient HCCIs into serial automotive production in 2010-2015. Will save humanity about half a million barrels. oil daily.
However, before achieving this, researchers and engineers must overcome the biggest stumbling block at the moment - the lack of a reliable way to control autoignition processes using containing fractions with different chemical composition, properties and behavior of modern fuels. A number of questions are caused by the containment of processes at various loads, speeds and temperature conditions of engine operation. According to some experts, this can be done by returning an accurately metered amount of exhaust gases back to the cylinder, preheating the mixture, or dynamically changing the compression ratio, or directly changing the compression ratio (for example, the SVC prototype Saab) or by changing the valve closing moment using variable valve timing systems.
It is not yet clear how the problem of noise and thermodynamic effects on the engine design will be eliminated due to the spontaneous ignition of a large amount of fresh mixture at full load. The real problem is to start the engine at low cylinder temperatures, since it is quite difficult to initiate self-ignition under such conditions. Many researchers are now working to eliminate these bottlenecks by using sensor prototype observations for continuous electronic monitoring and real-time analysis of cylinder workflows.
According to experts from automotive companies working in this area, including Honda, Nissan, Toyota and GM are likely to create combo cars first that can switch modes of operation, and the spark plug will be used as a kind of helper when the HCCI is struggling. Volkswagen is already implementing a similar scheme in its CCS (Combined Combustion System) engine, which currently only runs on synthetic fuels specially developed for it.
Ignition of the mixture in HCCI engines can be carried out in a wide range of ratios between fuel, air and exhaust gases (it is enough to reach the auto-ignition temperature), and a short combustion duration leads to a significant increase in engine efficiency. Some of the problems of the new type of units can be successfully solved in combination with hybrid systems such as Hybrid Synergy Drive from Toyota - in this case, the combustion engine can only be used in a certain mode, optimal in terms of speed and load. at work, thus bypassing modes in which the engine is experiencing difficulties or becomes ineffective.
Combustion in HCCI engines, achieved through integrated control of temperature, pressure, quantity and quality of the mixture in a position close to GMT, is indeed a big problem against the background of much simpler ignition with a spark plug. On the other hand, HCCI does not need to create turbulent processes, which are important for gasoline and especially diesel engines, due to the simultaneous volumetric nature of autoignition. At the same time, it is for this reason that even small temperature deviations can lead to significant changes in kinetic processes.
In practice, the most important factor for the future of this type of engine is the type of fuel, and the correct design solution can only be found with a detailed knowledge of its behavior in the combustion chamber. Therefore, many auto companies are now working with oil companies (such as Toyota and ExxonMobil), and most of the experiments at this stage are carried out with specially designed synthetic fuels, the composition and behavior of which are calculated in advance. The efficiency of gasoline and diesel in HCCI goes against the logic of classic engines. Due to the high autoignition temperature of gasolines, the compression ratio in them can vary from 12: 1 to 21: 1, and in diesel fuel, igniting at lower temperatures, it should be relatively small - about only 8: 1.
Text: Georgy Kolev