Recently, at the Paris Motor Show, Infiniti (read Renault-Nissan Alliance) introduced a variable compression engine. The proprietary Variable Compression-Turbocharged (VC-T) technology allows you to vary this very degree, literally sucking all the juice out of the engine.
In an "ideal universe" the rule is simple - the higher the compression ratio of the fuel-air mixture, the better. The mixture expands as much as possible, the pistons move as if wound, therefore, the power and efficiency of the motor are maximum. In other words, the fuel is burned extremely efficiently.
Everything would be great if not for the very nature of the fuel. In the course of bullying, his patience once comes to a limit: the more evenly the mixture burns, the better, but at high loads (high compression ratio, high speeds), the mixture begins to explode, and not burn out. This phenomenon is called detonation, and this thing is quite destructive. The walls of the combustion chamber and the piston itself experience serious shock loads and are gradually, but rather quickly destroyed. In addition, the efficiency of the motor drops - the normal operating pressure on the piston drops.
Thus, the most profitable option is when the engine in any mode operates on the verge of detonation, preventing this phenomenon. Infiniti engineers drew up a graph on which they outlined for themselves the effective operating modes of the engine depending on the load, speed and compression ratio of the fuel-air mixture. (Actually, fuel combustion efficiency can be improved in other ways, such as increasing the number of valves per cylinder, adjusting their schedule, even choosing a place above the piston where the injection of a portion of fuel is directed. Of course, we remember this.) The first two parameters, it is clear that they depend both on external factors and on the careful selection of the transmission. And the third - the compression ratio - was also decided to change in the range from 8:1 to 14:1.
Technically, this looks like an introduction to the design of the crank mechanism of an additional element - a rocker arm between the connecting rod and the crankshaft. The rocker is controlled by an electric motor - the lever can be shifted in such a way that the piston stroke range varies within 5 mm. This is enough to significantly change the compression ratio.
There are no advantages without disadvantages. At first glance, they are obvious: an increase in the complexity of the design, some weight gain ... However, it’s a sin to complain about these minuses - the engine turned out to be very balanced, due to which the balancing shafts were removed from the design. It is also likely that the engine is particularly sensitive to the brand and quality of fuel. It seems that this problem - at least to a large extent - is solved by software methods.
The compression ratio is an important characteristic of an internal combustion engine, determined by the ratio of the volume of the cylinder when the piston is at bottom dead center to the volume at top dead center (combustion chamber volume). Increasing the compression ratio creates favorable conditions for the ignition and combustion of the fuel-air mixture and, accordingly, the efficient use of energy. At the same time, the operation of the engine in different modes and different fuels implies a different compression ratio. These properties are fully exploited by the compression ratio change system.
The system provides an increase in engine power and torque, a reduction in fuel consumption and harmful emissions. The main merit of the system for changing the compression ratio is the ability of the engine to operate on different grades of gasoline and even different fuels without deteriorating performance and detonation.
The creation of an engine with a variable compression ratio is a rather complicated technical problem, in the solution of which there are several approaches, which consist in changing the volume of the combustion chamber. Currently, there are prototypes of such power plants.
The pioneer in the creation of an engine with a variable compression ratio is the company SAAB, which introduced in 2000 a five-cylinder internal combustion engine equipped with a Variable Compression. The engine uses a combined cylinder head with cylinder liners. On the one hand, the combined block is fixed on the shaft, on the other hand, it interacts with the crank mechanism. KShM provides a displacement of the combined head from the vertical axis by 4°, which achieves a change in the compression ratio in the range from 8:1 to 14:1.
The required value of the compression ratio is maintained by the engine management system depending on the load (at maximum load - the minimum compression ratio, at the minimum - the maximum compression ratio). Despite the impressive results of the engine in terms of power and torque, the power plant did not go into series, and work on it is currently curtailed.
A more modern development (2010) is a 4-cylinder engine from MCE-5 Development 1.5 l. In addition to the system for changing the compression ratio, the engine is equipped with other progressive systems - direct injection and variable valve timing.
The design of the engine provides for an independent change in the magnitude of the piston stroke in each cylinder. The toothed sector, which acts as a rocker arm, interacts with the working piston on the one hand, and with the control piston on the other. The rocker is connected by a lever to the crankshaft of the engine.
The toothed sector moves under the action of the control piston, which acts as a hydraulic cylinder. The volume above the piston is filled with oil, the volume of which is regulated by a valve. The movement of the sector provides a change in the position of the top dead center of the piston, which results in a change in the volume of the combustion chamber. Accordingly, the compression ratio varies from 7:1 to 20:1.
The MCE-5 engine has every chance of getting into the series in the near future.
He went even further in his research Lotus Cars, introducing a push-pull Omnivore engine(literally - an omnivore). As stated, the engine is able to run on any type of liquid fuel - gasoline, diesel fuel, ethanol, alcohol, etc.
A washer is made in the upper part of the combustion chamber of the engine, which moves by an eccentric mechanism and changes the volume of the combustion chamber. With this design, a record compression ratio of 40:1 is achieved. Poppet valves are not used in the gas distribution mechanism of the Omnivore engine.
Further development of the system is hindered by the low fuel efficiency and environmental friendliness of two-stroke engines, as well as their limited use in cars.
For more than a century of life, the internal combustion engine (ICE) has changed so much that only the principle of operation remains from the ancestor. Almost all stages of modernization were aimed at increasing the coefficient of performance (COP) of the engine. The efficiency indicator can be called universal. Many characteristics are hidden in it - fuel consumption, power, torque, exhaust gas composition, etc. The widespread use of new technical ideas - fuel injection, electronic ignition and engine management systems, 4, 5 and even 6 valves per cylinder - played a positive role in increasing the efficiency of engines.
Nevertheless, as shown by the Geneva Motor Show, the process of modernization of internal combustion engines is still far from being completed. At this popular international auto show, SAAB presented the result of 15 years of work - a prototype of a new engine with a variable compression ratio - SAAB Variable Compression (SVC), which became a sensation in the world of motors.
SVC technology and a number of other advanced and non-traditional technical solutions from the point of view of existing concepts of internal combustion engines made it possible to provide the novelty with fantastic characteristics. So, a five-cylinder engine with a volume of only 1.6 liters, designed for conventional production cars, develops an incredible power of 225 hp. and a torque of 305 Nm. Other, especially important today, characteristics turned out to be excellent - fuel consumption at medium loads was reduced by as much as 30%, CO2 emissions were reduced by the same amount. As for CO, CH and NOx, etc., they, according to the creators, comply with all existing and planned for the near future toxicity standards. In addition to this, the variable compression ratio gives the SVC engine the ability to run on a variety of gasoline grades - from A-76 to AI-98 - with virtually no degradation in performance and excluding the appearance of detonation.
Of course, a significant merit of such characteristics is in the SVC technology, i.e. the ability to change the compression ratio. But before we get acquainted with the device of the mechanism, which made it possible to change this value, let us recall some truths from the theory of the design of the internal combustion engine.
Compression ratio
The compression ratio is the ratio of the sum of the volumes of the cylinder and the combustion chamber to the volume of the combustion chamber. With an increase in the degree of compression, pressure and temperature increase in the combustion chamber, which creates more favorable conditions for the ignition and combustion of the combustible mixture and increases the efficiency of the use of fuel energy, i.e. efficiency. The higher the compression ratio, the higher the efficiency.
There are no problems with the creation of gasoline engines with a high compression ratio and there have never been. And do not do them for the following reason. During the compression stroke of such engines, the pressure in the cylinders rises to very high values. This, of course, causes an increase in the temperature in the combustion chamber and creates favorable conditions for the appearance of detonation. And detonation, as we know (see p. 26), is a dangerous phenomenon. In all engines created up to that time, the compression ratio was constant and was determined depending on the pressure and temperature conditions in the combustion chamber at maximum load, when fuel and air consumption are maximum. The engine does not always work in this mode, one might say, even very rarely. On the highway or in the city, when the speed is almost constant, the motor operates at low or medium loads. In such a situation, for more efficient use of fuel energy, it would be nice to have a higher compression ratio. This problem was solved by SAAB engineers - the creators of SVC technology.
SVC Technology
First of all, it should be noted that in the new engine, instead of the traditional block head and cylinder liners, which were cast directly into the block or pressed, there is one monohead that combines the block head and cylinder liners. To change the degree of compression, or rather, the volume of the combustion chamber, the monohead is made movable. On the one hand, it is mounted on a shaft that performs the function of a support, and on the other, it is supported and driven by a separate crank mechanism. The radius of the crank provides a displacement of the head relative to the vertical axis by 40. This is quite enough to change the volume of the chamber to obtain a compression ratio from 8:1 to 14:1.
The required compression ratio is determined by the SAAB Trionic electronic engine management system, which monitors the load, speed, fuel quality and, based on this, controls the crank hydraulic drive. So, at maximum load, the compression ratio is set to 8:1, and at minimum - 14:1. Combining the cylinder liners with their head, among other things, allowed SAAB engineers to give the channels of the cooling jacket a more perfect shape, which increased the efficiency of the process of removing heat from the walls of the combustion chamber and cylinder liners.
The mobility of the cylinder liners and their heads required changes to the design of the engine block. The joint plane of the block and the head has become lower by 20 cm. As for the tightness of the joint, it is provided by a rubber corrugated gasket, which is protected from damage by a metal casing from above.
Mal, yes daring
For many, it may become incomprehensible how more than two hundred “horses” were “charged” into an engine with such a small volume - after all, such power can adversely affect its resource. When creating the SVC engine, the engineers were guided by completely different tasks. Bringing the motor resource to the required standards is the business of technologists. As for the small volume of the engine, it is done in full accordance with the theory of internal combustion engines. Based on its laws, the most favorable mode of engine operation in terms of increasing efficiency is at high load (at high speeds), when the throttle is fully open. In this case, it maximizes the energy of the fuel. And since engines with a smaller displacement work mainly at maximum loads, their efficiency is also higher.
The secret behind the superiority of small engines in terms of efficiency is due to the absence of so-called pumping losses. They occur at low loads, when the engine is running at low speeds and the throttle is only slightly ajar. In this case, during the intake stroke, a large vacuum is created in the cylinders - a vacuum that resists the downward movement of the piston and, accordingly, reduces efficiency. With a wide open throttle, there are no such losses, since air enters the cylinders almost unhindered.
To avoid pumping losses by 100%, in the new engine, SAAB engineers also used high-pressure air "supercharge" - 2.8 atm., Using a mechanical supercharger - compressor. The compressor was preferred for several reasons: firstly, no turbocharger is capable of generating such boost pressure; secondly, the reaction of the compressor to a change in load is almost instantaneous, i.e. there is no deceleration characteristic of turbocharging. Filling the cylinders with a fresh charge in the SAAB engine was improved both with the help of today's popular modern gas distribution mechanism, in which there are four valves per cylinder, and through the use of an intercooler (Intercooler).
The prototype of the SVC engine, according to the German engine development company FEV Motorentechnie in Aachen, is quite functional. But despite the positive assessment, it will be launched into mass production after some time - after its refinement and fine-tuning to the needs of customers.
As it may seem at first glance, the modern internal combustion engine has reached the highest stage of its evolution. At the moment, various are mass-produced and, appeared, an additional opportunity has been realized.
The list of the most significant developments in recent years includes: the introduction of high-precision injection systems controlled by sophisticated electronics, obtaining high power without increasing the working volume thanks to turbocharging systems, increasing, using, etc.
The result was a noticeable improvement in performance, as well as a decrease in the level of toxicity of exhaust gases. However, that's not all. Designers and engineers around the world continue not only to actively work on improving existing solutions, but also try to create a completely new design.
Suffice it to recall attempts to build, get rid of in the device or dynamically change the compression ratio of the engine. We note right away that although some projects are still under development, others have already become a reality. For example, engines with a variable compression ratio. Let's look at the features, advantages and disadvantages of such internal combustion engines.
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Changing the compression ratio: why is it needed
Many experienced drivers are familiar with such concepts as the octane number for gasoline engines, as well as for diesel engines. For the less knowledgeable reader, we recall that the compression ratio is the ratio of the volume above the piston, which is lowered at BDC (bottom dead center) to the volume when the piston rose to TDC (top dead center).
Gasoline units have, on average, an indicator of 8-14, diesel engines 18-23. The compression ratio is a fixed value and is structurally incorporated during the development of an engine. Also, the requirements for the use of the octane number of gasoline in a particular engine will also depend on the degree of compression. At the same time, either supercharged or supercharged is taken into account.
If we talk about the compression ratio itself, in fact, this is an indicator that determines how much the fuel-air mixture will be compressed in the engine cylinders. Simply put, a well-compressed mixture ignites better and burns more fully. It turns out that increasing the compression ratio allows you to achieve engine growth, get improved engine performance, reduce fuel consumption, etc.
However, there are nuances. First of all, this. Again, if you do not go into details, normally the charge of fuel and air in the cylinders should just burn, and not explode. Moreover, the ignition of the mixture must begin and end at strictly specified moments.
In this case, the fuel has the so-called "knock resistance", that is, the ability to resist detonation. If, however, the compression ratio is greatly increased, then the fuel may begin to detonate in the engine under certain operating conditions of the internal combustion engine.
The result is an uncontrolled explosive combustion process in the cylinders, the rapid destruction of engine parts by a shock wave, a significant increase in temperature in the combustion chamber, etc. As you can see, it is impossible to make a high compression ratio constant precisely for these reasons. In this case, the only way out in this situation is the ability to flexibly change this indicator in relation to different engine operating modes.
Such a “working” motor was recently proposed by engineers of the premium brand Infiniti (an elite division of Nissan). Also, other automakers (SAAB, Peugeot, Volkswagen, etc.) have been and remain involved in similar developments. So let's look at a variable compression engine.
Variable engine compression ratio: how it works
First of all, the available ability to change the compression ratio allows a significant increase in the performance of turbo engines while reducing fuel consumption. In a nutshell, depending on the operating mode and loads on the internal combustion engine, the fuel charge is compressed and burned out in the most optimal conditions.
When the loads on the power unit are minimal, an economical "poor" mixture (a lot of air and little fuel) is supplied to the cylinders. A high compression ratio is well suited for such a mixture. If the load on the engine increases (a “rich” mixture is supplied, in which there is more gasoline), then the risk of detonation naturally increases. Accordingly, to prevent this from happening, the compression ratio is dynamically reduced.
In engines where the compression ratio is constant, a change is a kind of protection against detonation. This angle moves "back". Naturally, such an angle shift leads to the fact that although there is no detonation, power is also lost. As for the engine with a variable compression ratio, there is no need to shift the UOS, that is, there is no power loss.
As for the implementation of the scheme itself, in fact, the task boils down to the fact that there is a physical decrease in the working volume of the engine, however, all characteristics (power, torque, etc.) are preserved.
We note right away that different companies worked on such a solution. As a result, various methods of controlling the compression ratio have appeared, for example, variable combustion chamber volume, connecting rods with the possibility of raising the pistons, etc.
- One of the earliest developments was the introduction of an additional piston into the combustion chamber. The specified piston had the ability to move, while changing the volume. The downside of the whole design was the need to install additional parts in. Also, changes in the shape of the combustion chamber immediately appeared, the fuel burned unevenly and incompletely.
For these reasons, this project was never completed. The same fate befell the development, which had pistons with the ability to change their height. These split-type pistons turned out to be heavy, and difficulties were added regarding the implementation of the control of the lifting height of the piston cover, etc.
- Further developments no longer affected the pistons and the combustion chamber, maximum attention was paid to the issue of lifting the crankshaft. In other words, the task was to implement control of the crankshaft lift height.
The scheme of the device is such that the bearing journals of the shaft are located in special eccentric-type clutches. These clutches are driven by gears that are connected to an electric motor.
Turning the eccentrics allows you to raise or lower, which leads to a change in the height of the pistons in relation to. As a result, the volume of the combustion chamber increases or decreases, while the compression ratio also changes.
Note that several prototypes were built on the basis of a 1.8-liter turbocharged unit from Volkswagen, the compression ratio varied from 8 to 16. The engine was tested for a long time, but the unit never became a serial unit.
- Another attempt to find a solution was an engine in which the compression ratio was changed by lifting the entire cylinder block. The development belongs to the Saab brand, and the unit itself almost did not even get into the series. The engine is known as SVC, 1.6 liter, 5-cylinder, turbocharged unit.
Power was about 220 liters. with., a torque of just over 300 Nm. It is noteworthy that fuel consumption in medium load mode has decreased by almost a third. As for the fuel itself, it became possible to fill in both the AI-76 and the 98th.
Saab engineers divided the cylinder block into two conditional parts. The upper part contained the cylinder heads and liners, while the lower part contained the crankshaft. A kind of connection of these parts of the block, on the one hand, was a movable hinge, and on the other, a special mechanism equipped with an electric drive.
So it was possible to slightly raise the upper part at a certain angle. Such an angle of elevation was only a few degrees, while the compression ratio varied from 8 to 14. At the same time, a rubber casing was supposed to seal the “joint”.
In practice, the parts themselves for lifting the upper part of the block, as well as the protective cover itself, turned out to be very weak elements. Perhaps this is what prevented the motor from getting into the series and the project was closed further.
- Another development was further proposed by engineers from France. The turbo engine with a working volume of 1.5 liters was able to change the compression ratio from 7 to 18 and produced about 225 hp. The torque characteristic is fixed at around 420 Nm.
Structurally, the unit is complex, with a divided. In the area where the connecting rod is attached to the crankshaft, the part was equipped with a special geared rocker arm. At the junction of the connecting rod with the piston, a gear-type bar was also introduced.
On the other side, a piston rail was attached to the rocker, which implemented control. The system was driven from the lubrication system, the working fluid passed through a complex system of channels, valves, and there was also an additional electric drive.
In a nutshell, the movement of the control piston had an effect on the rocker. As a result, the lift height of the main piston in the cylinder also changed. Note that the engine also did not become serial, and the project was frozen.
- The next attempt to create an engine with a variable compression ratio was the solution of Infiniti engineers, namely the VCT (Variable Compression Turbocharged) engine. In this motor, it became possible to change the compression ratio from 8 to 14. A design feature is a unique traverse mechanism.
It is based on the connection of the connecting rod with the lower neck, which is movable. Also used is a system of levers, which are driven by an electric motor.
The controller controls the process by sending signals to the electric motor. The electric motor, after receiving a command from the control unit, shifts the rod, and the lever system implements a change in position, which allows you to change the height of the piston.
As a result, the Infiniti VCT unit with a displacement of 2.0 liters with an output of about 265 hp. allowed to save almost 30% of fuel compared to similar internal combustion engines, which at the same time have a constant compression ratio.
If the manufacturer manages to effectively solve the current problems (design complexity, increased vibrations, reliability, high final cost of unit production, etc.), then the optimistic statements of the company's representatives may well come true, and the engine itself has every chance of becoming serial already in 2018-2019.
Summing up
Based on the information above, it is clear that variable compression engines can provide a significant reduction in fuel consumption in turbocharged gasoline engines.
Against the background of the global fuel crisis, as well as the constant tightening of environmental standards, these engines allow not only to burn fuel efficiently, but also not to limit engine power at the same time.
In other words, such an internal combustion engine is quite capable of offering all the advantages of a powerful gasoline high-speed turbo engine. At the same time, in terms of fuel consumption, such a unit can come close to turbodiesel counterparts, which are popular today, primarily due to their.
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The idea of creating a gasoline engine, where the compression ratio in the cylinders would be a variable value, is not new. So, during acceleration, when the greatest engine output is required, you can sacrifice its economy for a few seconds by reducing the compression ratio - this will prevent detonation, spontaneous combustion of the fuel mixture, which can occur at high loads. With uniform movement, on the contrary, it is desirable to increase the compression ratio in order to achieve more efficient combustion of the fuel mixture and reduce fuel consumption - in this case, the load on the engine is small and the risk of detonation is minimal.
In general, everything is simple in theory, but it turned out to be not so easy to implement this idea in practice. And the Japanese designers were the first who managed to bring the idea to a production model.
The essence of the technology developed by Nissan is to continuously change the maximum piston lift (the so-called top dead center - TDC), depending on the required engine output, which in turn leads to a decrease or increase in the compression ratio in the cylinders. A key detail of this system is the special fastening of the connecting rods, which are connected to the crankshaft through a movable block of rocker arms. The block, in turn, is connected to an eccentric control shaft and an electric motor, which, at the command of the electronics, sets this cunning mechanism in motion, changing the slope of the rocker arms and the TDC position of the pistons in all four cylinders simultaneously.
The difference in compression ratio depending on the TDC position of the piston. On the left picture, the motor is in economy mode, on the right - in maximum efficiency mode. A: When a change in compression ratio is required, the electric motor turns and moves the drive arm. B: The drive lever turns the control shaft. C: when the shaft rotates, it acts on the lever connected to the rocker, changing the angle of the latter. D: Depending on the position of the rocker arm, TDC of the piston rises or falls, thus changing the compression ratio.
As a result, during acceleration, the compression ratio is reduced to 8:1, after which the engine goes into economy mode with a compression ratio of 14:1. Its working volume at the same time varies from 1997 to 1970 cm 3 . The turbo four of the new Infiniti QX50 develops 268 hp. With. and a torque of 380 Nm - significantly more than its predecessor's 2.5-liter V6 (its performance is 222 hp and 252 Nm), while consuming one third less gasoline. In addition, the VC-Turbo is 18 kg lighter than the atmospheric "six", takes up less space under the hood and reaches its maximum torque in the lower speed zone.
By the way, the compression ratio adjustment system not only increases the efficiency of the engine, but also reduces the level of vibration. Thanks to the rocker arms, the connecting rods occupy an almost vertical position during the working stroke of the pistons, while in conventional engines they go from side to side (which is why the connecting rods got their name). As a result, even without balance shafts, this 4-cylinder unit runs as quietly and smoothly as a V6.
But the variable TDC position using a complex system of levers is not the only feature of the new motor. By changing the compression ratio, this unit is also able to switch between two work cycles: the classic Otto, which operates the majority of gasoline engines, and the Atkinson cycle, found mainly in hybrids. In the latter case (with a high compression ratio), due to the larger stroke of the pistons, the working mixture expands more, burning with greater efficiency, as a result, efficiency increases and gasoline consumption decreases.
In addition to two working cycles, this engine also uses two injection systems: classic distributed MPI and direct GDI, which improves fuel efficiency and avoids detonation at high compression ratios. Both systems work alternately, and at high loads - simultaneously. A positive contribution to increasing engine efficiency is also made by a special coating of the cylinder walls, which is applied by plasma spraying, and then hardened and honed. The result is an ultra-smooth “mirror-like” surface that reduces piston ring friction by 44%.
Another unique feature of the VC-Turbo motor is the Active Torque Road active vibration dampening system, which is based on a reciprocating actuator, integrated into its upper mount. This system is controlled by an acceleration sensor that detects engine vibrations and in response generates damping vibrations in antiphase. Active bearings were first used in Infiniti in 1998 on a diesel engine, but that system turned out to be too cumbersome, so it was not widely used. The project was shelved until 2009, when Japanese engineers took up its improvement. It took another 8 years to solve the problem of excess weight and dimensions of the vibration damper. But the result is impressive: thanks to ATR, the 4-cylinder unit of the new Infiniti QX50 is 9 dB quieter than its predecessor's V6!
