VC-T engine. Image: Nissan
Japanese automaker Nissan Motor introduced a new type of gasoline internal combustion engine, which in some respects is superior to advanced modern ones diesel engines.
The new Variable Compression-Turbo (VC-T) engine is capable of change the compression ratio gaseous combustible mixture, that is, to change the stroke pitch of the pistons in the cylinders of the internal combustion engine. This parameter is usually fixed. Apparently, VC-T will be the first in ICE world With variable degree compression of the mixture.
Compression ratio - the ratio of the volume of the above-piston space of the engine cylinder internal combustion with the piston position at the bottom dead center(total volume of the cylinder) to the volume of the above-piston space of the cylinder when the piston is at the top dead center, that is, to the volume of the combustion chamber.
Increasing the compression ratio in general case increases its power and increases Engine efficiency, that is, it helps reduce fuel consumption.
In conventional gasoline engines, the compression ratio is usually from 8:1 to 10:1, and in sports cars And racing cars can reach 12:1 or more. As the compression ratio increases, the engine requires fuel with a higher octane number.
VC-T engine. Image: Nissan
The illustration shows the difference in piston pitch at different compression ratios: 14:1 (left) and 8:1 (right). In particular, the mechanism for changing the compression ratio from 14:1 to 8:1 is demonstrated. It happens this way.
- If it is necessary to change the compression ratio, the module is activated Harmonic Drive and moves the actuator lever.
- The actuator lever turns drive shaft (Control Shaft on the diagram).
- When the drive shaft turns, it changes the angle of inclination multi-link suspension (Multi-link on the diagram)
- The multi-link suspension determines the height to which each piston can rise in its cylinder. Thus, the compression ratio changes. The bottom dead center of the piston appears to remain the same.
Changing the compression ratio in an internal combustion engine can in some ways be compared to changing the angle of attack in controllable pitch propellers, a concept that has been used in aircraft and aircraft for many decades. propellers. The variable propeller pitch makes it possible to maintain the propulsion efficiency close to optimal, regardless of the speed of the carrier in the flow.
Degree change technology internal combustion engine compression makes it possible to maintain engine power while complying with strict engine efficiency standards. This is probably the most real way comply with these standards. “Everyone is working on variable compression ratio and other technologies to significantly improve fuel economy. gasoline engines"says James Chao, managing director for Asia-Pacific and consultant for IHS, "at least for the last twenty years or so." It is worth mentioning that in 2000 Saab company showed a prototype of this Saab engine Variable Compression (SVC) for the Saab 9-5, for which it has won a number of awards at technical exhibitions. Then the Swedish company was bought by the concern General Motors and stopped working on the prototype.
Saab Variable Compression (SVC) engine. Photo: Reedhawk
The VC-T engine is promised to be launched on the market in 2017 with Infiniti QX50 cars. The official presentation is scheduled for September 29 at Paris Motor Show. This two liter four-cylinder engine will have about the same power and torque as the 3.5-liter V6 engine it will replace, but will provide 27% fuel economy compared to it.
Nissan engineers also say the VC-T will be cheaper than today's advanced turbocharged diesel engines and will fully meet current emissions standards for nitrogen oxide and other emissions. exhaust gases- such rules apply in the European Union and some other countries.
After Infiniti new engines are planned to equip others Nissan cars and possibly partner company Renault. 
VC-T engine. Image: Nissan
It can be assumed that the complicated internal combustion engine design at first it is unlikely to be reliable. It makes sense to wait a few years before buying a car with a VC-T engine, unless you want to participate in testing experimental technology.
The idea of creation gasoline engine, where the compression ratio in the cylinders would be a variable value, is not new. So, during acceleration, when the greatest output of the engine is required, you can sacrifice its efficiency for a few seconds by reducing the compression ratio - this will prevent detonation and spontaneous combustion fuel mixture, which can occur when high loads. At uniform motion On the contrary, it is advisable 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 low and the risk of detonation is minimal.
In general, everything is simple in theory, but implementing this idea in practice turned out to be not so easy. And Japanese designers became the first who managed to bring the idea to a production model.
The essence of the developed by Nissan Corporation technology is to, depending on the required engine output, continuously change maximum height lifting the pistons (the so-called top dead center - TDC), which in turn leads to a decrease or increase in the compression ratio in the cylinders. The key detail of this system is the special fastening of the connecting rods, which are connected to crankshaft through moving block 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 inclination 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. In the left picture the motor is in economy mode, in the right - in maximum efficiency mode. A: When a change in compression ratio is required, the electric motor turns and moves the actuator arm. B: The drive lever turns the control shaft. C: When the shaft rotates, it acts on the lever associated with the rocker arm, changing the angle of the latter. D: Depending on the position of the rocker arm, the TDC of the piston rises or falls, thereby changing the compression ratio.
As a result, during acceleration the compression ratio decreases to 8:1, after which the engine switches to an economical operating mode with a compression ratio of 14:1. Its working volume varies from 1997 to 1970 cm 3 . The turbo-four of the new Infiniti QX50 develops a power of 268 hp. With. and a torque of 380 Nm - significantly more than the 2.5-liter V6 of its predecessor (its figures are 222 hp and 252 Nm), while consuming a third less gasoline. In addition, the VC-Turbo is 18 kg lighter than the naturally aspirated six, takes up less space under the hood and achieves maximum torque at lower revs.
By the way, the compression ratio adjustment system not only increases the efficiency of the engine, but also reduces the level of vibrations. 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 move from side to side (which is why the connecting rods got their name). As a result, even without balance shafts, this 4-cylinder unit is as quiet and smooth as a V6.
But the variable TDC position using a complex system of levers is not the only feature of the new engine. By changing the compression ratio, this unit is also capable of switching between two operating cycles: the classic Otto cycle, according to which the majority of gasoline engines operate, and the Atkinson cycle, found mainly in hybrids. In the latter case (at a high compression ratio) due to the larger piston stroke working mixture expands more, burning with greater efficiency, as a result, efficiency increases and gasoline consumption decreases.
In addition to two operating cycles, this engine also uses two injection systems: classic distributed MPI And direct GDI, which increases the efficiency of fuel combustion and avoids detonation at high compression ratios. Both systems operate alternately, and at high loads - simultaneously. A positive contribution to increasing engine efficiency is 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 its integrated upper support Active Torque Road active vibration reduction system, which is based on a reciprocating actuator. This system is controlled by an acceleration sensor that detects engine vibrations and in response generates dampening vibrations in antiphase. Active supports Infiniti was first used in 1998 on diesel engine, but that system turned out to be too cumbersome, so it was not widespread. The project lay shelved until 2009, when Japanese engineers began improving it. It took another 8 years to solve the problem of excess weight and size 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 the V6 of its predecessor!
Closely related to efficiency. In gasoline engines, the compression ratio is limited to detonation combustion. These restrictions are of particular importance for engine operation at full load, while at partial load the high compression ratio does not pose a risk of detonation. To increase engine power and improve efficiency, it is desirable to reduce the compression ratio, but if the compression ratio is low across all engine operating ranges, this will lead to reduced power and increased fuel consumption at part loads. In this case, the compression ratio values, as a rule, are chosen much lower than those values at which the most economical engine performance is achieved. Knowingly worsening the efficiency of engines, this is especially pronounced when operating at partial loads. Meanwhile, a decrease in the filling of the cylinders with the combustible mixture, an increase in the relative amount of residual gases, a decrease in the temperature of parts, etc. create opportunities to increase the compression ratio at partial loads in order to improve engine efficiency and increase its power. To solve this compromise problem, engine options with variable compression ratios are being developed.
Widespread use in engine designs has made this direction of work even more relevant. The fact is that with supercharging, the mechanical and thermal loads on engine parts increase significantly, and therefore they have to be strengthened, increasing the weight of the entire engine as a whole. In this case, as a rule, the service life of parts operating under more loaded conditions is reduced, and engine reliability is reduced. In case of transition to variable degree compression, the working process in the engine during supercharging can be organized in such a way that, due to a corresponding decrease in the compression ratio at any boost pressure, the maximum operating cycle pressures (i.e., operating efficiency) will remain unchanged or will change slightly. At the same time, despite the increase useful work per cycle, and, consequently, engine power, maximum loads its parts may not increase, which makes it possible to boost the engines without introducing changes to their design.
Very essential for the normal course of the combustion process in an engine with a variable compression ratio is right choice shape of the combustion chamber, providing the most shortcut flame spread. The change in the flame propagation front must be very rapid in order to take into account various modes engine operation during vehicle operation. Considering the use of additional parts in crank mechanism, it is also necessary to develop systems with a low coefficient of friction so as not to lose the benefits of using a variable compression ratio.
One of the most common engine options with a variable compression ratio is shown in the figure.
Rice. Engine diagram with variable compression ratio:
1 – connecting rod; 2 – piston; 3 – eccentric shaft; 4 - additional connecting rod; 5 – connecting rod journal crankshaft; 6 – rocker arm
At partial loads, additional 4 occupies the lowest position and raises the piston stroke area. The compression ratio is maximum. At high loads, the eccentric on shaft 3 raises the axis of the upper head of the additional connecting rod 4. At the same time, the over-piston clearance increases and the compression ratio decreases.
In 2000, an experimental SAAB gasoline engine with a variable compression ratio was presented in Geneva. His unique features allow you to reach a power of 225 hp. with a working volume of 1.6 liters. and maintain fuel consumption comparable to an engine half the size. The ability to steplessly change the displacement allows the engine to run on gasoline, diesel fuel or alcohol.
The engine cylinders and the block head are made as a monoblock, that is, as a single block, and not separately as in conventional engines. A separate block also consists of a crankcase and a connecting rod and piston group. The monoblock can move in the block crankcase. The left side of the monoblock rests on the axis 1 located in the block, which serves as a hinge; the right side can be raised or lowered using a connecting rod 3 controlled by an eccentric shaft 4. To seal the monoblock and the block crankcase, a corrugated rubber cover 2.
Rice. SAAB variable compression engine:
1 – axis; 2 – rubber cover; 3 – connecting rod; 4 – eccentric shaft.
The compression ratio changes when the monoblock is tilted relative to the crankcase by means of a hydraulic drive while the piston stroke remains unchanged. Deviation of the monoblock from the vertical leads to an increase in the volume of the combustion chamber, which causes a decrease in the compression ratio.
As the angle of inclination decreases, the compression ratio increases. The maximum deviation of the monoblock from the vertical axis is 4%.
At minimum crankshaft rotation speed and fuel supply reset, as well as at low loads, the monoblock occupies the lowest position, in which the volume of the combustion chamber is minimal (compression ratio - 14). The charging system is switched off and air flows directly into the engine.
Under load, due to rotation of the eccentric shaft, the connecting rod deflects the monoblock to the side, and the volume of the combustion chamber increases (compression ratio - 8). In this case, the clutch connects the supercharger, and air begins to flow into the engine under excess pressure.
Rice. Changing the air supply to the SAAB engine under different modes:
1 – throttle valve; 2 – bypass valve; 3 – clutch; a – at low crankshaft speed; b – at load conditions
The optimal compression ratio is calculated by the control unit electronic system taking into account the crankshaft rotation speed, degree of load, type of fuel and other parameters.
Due to the need to quickly respond to changes in the compression ratio in this engine had to abandon the turbocharger in favor of mechanical supercharging with intermediate air cooling with maximum pressure boost 2.8 kgf/cm2.
Fuel consumption for the developed engine is 30% less than that of conventional engine the same volume, and the exhaust gas toxicity indicators comply with current standards.
The French company MCE-5 Development has developed for the Peugeot-Citroen concern an engine with a variable compression ratio VCR (Variable Compression Ratio). This solution uses the original kinematics of the crank mechanism.
In this design, the transmission of motion from the connecting rod to the pistons is carried out through a double gear sector 5. C right side engine support is located rack 7, on which sector 5 rests. Such engagement ensures strictly reciprocating movement of the cylinder piston, which is connected to the rack 4. Rack 7 is connected to the piston 6 of the control hydraulic cylinder.
Depending on the operating mode of the engine, a signal from the engine control unit changes the position of the piston 6 of the control cylinder connected to the rack 7. Shifting the control rack 7 up or down changes the position of the TDC and BDC of the engine piston, and with them the compression ratio from 7:1 to 20:1 in 0.1 s. If necessary, it is possible to change the compression ratio for each cylinder separately.
Rice. Engine with variable compression ratio VCR:
1 – crankshaft; 2 – connecting rod; 3 – toothed support roller; 4 – piston rack; 5 – gear sector; 6 – piston of the control cylinder; 7 – support control rack.
VC-T engine. Image: Nissan
Japanese automaker Nissan Motor has introduced a new type of gasoline internal combustion engine, which in some respects is superior to advanced modern diesel engines.
The new Variable Compression-Turbo (VC-T) engine is capable of change the compression ratio gaseous combustible mixture, that is, to change the stroke pitch of the pistons in the cylinders of the internal combustion engine. This parameter is usually fixed. Apparently, the VC-T will be the world's first internal combustion engine with a variable compression ratio.
Compression ratio is the ratio of the volume of the piston space above the cylinder of an internal combustion engine with the piston position at bottom dead center (total cylinder volume) to the volume of the cylinder space above the piston with the piston position at top dead center, that is, to the volume of the combustion chamber.
Increasing the compression ratio generally increases its power and increases engine efficiency, that is, helps reduce fuel consumption.
In regular gasoline engines, the compression ratio is typically between 8:1 and 10:1, but in sports cars and race cars it can be as high as 12:1 or more. As the compression ratio increases, the engine requires fuel with a higher octane number.
VC-T engine. Image: Nissan
The illustration shows the difference in piston pitch at different compression ratios: 14:1 (left) and 8:1 (right). In particular, the mechanism for changing the compression ratio from 14:1 to 8:1 is demonstrated. It happens this way.
- If it is necessary to change the compression ratio, the module is activated Harmonic Drive and moves the actuator lever.
- The actuator lever turns the drive shaft ( Control Shaft on the diagram).
- When the drive shaft turns, it changes the angle of the multi-link suspension ( Multi-link on the diagram)
- The multi-link suspension determines the height to which each piston can rise in its cylinder. Thus, the compression ratio changes. The bottom dead center of the piston appears to remain the same.
Changing the compression ratio in an internal combustion engine can in some ways be compared to changing the angle of attack in controllable pitch propellers, a concept that has been used in propellers and propellers for many decades. The variable propeller pitch makes it possible to maintain the propulsion efficiency close to optimal, regardless of the speed of the carrier in the flow.
The technology of changing the compression ratio of the internal combustion engine makes it possible to maintain engine power while complying with strict standards for engine efficiency. This is probably the most realistic way to comply with these standards. “Everyone is now working on variable compression ratio and other technologies to significantly improve the efficiency of gasoline engines,” says James Chao, managing director for Asia Pacific and consultant for IHS, “at least for the last twenty years or so.” . It is worth mentioning that in 2000, Saab showed a prototype of such a Saab Variable Compression (SVC) engine for the Saab 9-5, for which it received a number of awards at technical exhibitions. Then the Swedish company was bought by General Motors and stopped working on the prototype.
Saab Variable Compression (SVC) engine. Photo: Reedhawk
The VC-T engine is promised to be launched on the market in 2017 with Infiniti QX50 cars. The official presentation is scheduled for September 29 at the Paris Motor Show. This 2.0-liter four-cylinder engine will have about the same power and torque as the 3.5-liter V6 it replaces, but will deliver 27 percent fuel economy over it.
Nissan engineers also say the VC-T will be cheaper than today's advanced turbocharged diesel engines, and will fully comply with current nitrogen oxide and other exhaust gas emissions regulations in force in the European Union and some other countries.
After Infiniti, it is planned to equip other cars from Nissan and, possibly, partner company Renault, with new engines.
VC-T engine. Image: Nissan
It can be assumed that the complicated design of the internal combustion engine is unlikely to be reliable at first. It makes sense to wait a few years before buying a car with a VC-T engine, unless you want to participate in testing experimental technology.
