Construction features of engines with variable compression ratio

Variable and high compression gasoline engines.

The company Nissan announced a “revolutionary” VC-T engine with a variable compression ratio from 8 to 14, it is achieved by a more complex design of the final drive and allows for up to 25% of fuel savings, i.e. the average consumption for the Infiniti QX-50 is expected 7l/100km. Here’s a comparison, even a normal reliable turbodiesel in a hybrid version with this “miracle” of engineering. “Woe from Wit” AS Griboedov has long written, the pressure to change the boost is much easier. I will try to explain my engine (1.6 HPR 150 hp) compression ratio 10.5 and boost pressure 0.8 bar, ie in the cylinders gets more air charge in 1.8 times greater than the work in the atmospheric version. At change of a compression ratio from 14 to 8 this parity makes 1.75 times, to reduce air charge it is enough to reduce pressure supercharging without changing working volume of the cylinder, it is not necessary to complicate KCM, a crankshaft is that place where all should be superreliable. And another thing, reducing the compression ratio to 8 also reduces the stroke of the piston, ie to get the same torque would still have to burn more fuel compared to the engine at 14, ie the effect is achieved only by running the engine for part of the time at a high compression ratio.So much effort to achieve intermediate positive results, very questionable, given the increased complexity at high enough specific load.I suppose its mileage to exhaustion will be beautiful, but not long.

2.In the line of atmospheric engines Mazda for CX-30 appeared 2-liter engine capacity of 180 hp and the degree of compression of 16.3 ! Peak torque (224 Nm) at 3000 rpm, moved down by 1000 rpm compared to the 2-liter 150 hp engine with a compression ratio of 13. Consumption of 95-gasoline-6.6/5.1/5.6 l/100 km. 2019 Mazda CX-30 2.0 Skyactiv-X (180 hp) | Specifications, fuel consumption. Urban fuel consumption 5.2 l/100 km Highway fuel consumption 4.3 l/100 km Mixed cycle fuel consumption 4.6 l/100 km CO2 emissions 105 g/km Fuel Gasoline Acceleration time 0 – 100 km/h 8.5 sec Top speed 204 km/h Environmental standard Euro 6d Power/weight ratio 7.6 kg/hp, 131.6 hp/tonne Engine Power 180 hp @ 6000 rpm Power per liter displacement 90.1 hp/litre Torque 224 Nm @ 3000 rpm Front layout, longitudinal Engine displacement 1998 cm Number of cylinders 4 Cylinder arrangement Cylinder diameter 83.5 stroke 91.2 Compression Ratio 16.3 Number of valves per cylinder 4 Power System Direct injection Type supercharger Uncharged 2 liter engine 150 hp with a compression ratio of 13 for the CX-30 has a torque of 213 Nm @ 4000 rpm, It’s worth noting that with a compression ratio of 16.3, the motor’s torque peak has shifted to 1,000 rpm lower. This engine has a rather complex piston shape and an unusual ignition system-eliminating the occurrence of detonation at such a compression ratio is not an easy task.

High geometric compression ratio and simultaneously variable compression ratio petrol engines.


The geometric compression ratio (the degree of expansion of the gases at the RX stroke) for such an engine is 17-18. This allows for a more efficient use of the combusted fuel energy. The actual compression ratio is usually determined by the intake valve opening-closing angles and the amount of air entering the cylinder. The actual compression ratio for this motor is between 10-12 and is somewhat dependent on the crankshaft (camshafts) RPM. Real SS in this motor is regulated by the exhaust valves, the intake valves work as usual.At the beginning of the compression stroke the exhaust valves(valve) are in ajar position and release some air when the piston starts moving up. To do this, it is sufficient to additionally grind a more gentle cam on the camshaft, which additionally opens the exhaust valves (valve) for a short time at the beginning of the compression stroke. Immediately all the problems with fuel ignition disappear – normal spark ignition is used. Supercharging may not be used, filling the cylinder with air as full as possible – the excess air at the beginning of the compression stroke is vented out of the cylinder.

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This process is carried out without changing the design of the valve train, unlike the BC-T engine of the Infiniti. The essence is that the design of the exhaust camshaft is changed. Schematically its section is shown in the sketch:

1.Tubular exhaust camshaft. (eccentric) opening the exhaust valve on the exhaust stroke. 3.Cam (eccentric) opening the exhaust valve at the beginning of the compression stroke to partially vent the cylinder. 4.Inner eccentric on the camshaft-adjusts the amount of venting at the start of the compression stroke by changing the position of eccentric 3. The sketch shows the position with maximum air release (minimum GVW).

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Modern technology allows you to significantly increase the life of engines, especially if you apply some new technical solutions.First of all, the engine should warm up to its operating temperature for a minimum time and never overheat.Solutions when the engine warms up and start driving on lean mixture allows the engine to warm up faster and protect it from unnecessary load.This is also facilitated by the radiator divided into two parts in the proportion of 1/3 to 2/3, in addition to this application of the radiator shutters, a different scheme of arrangement Parts wear is often caused by particles of dust and sand getting into the engine together with air, fuel or oil (more often with air), air pre-cleaning by cyclone (drive from electric onboard network), it creates supercharging pressure, then – a usual air filter (zero resistance filter). For an engine with such a high CS the boost pressure will not exceed 0.4 bar. Particular attention should also be paid to cleaning the fuel from foreign particles, improve the degree of cleaning if possible. The quality of the fuel that enters the engine is one of the most important factors. The peculiarity is that the boost pressure does not depend on the engine speed, in this arrangement it is determined only by the cyclone operating mode, i.e. it is possible to change it depending on the selected engine operating mode (warm-up, eco, normal, sport). In the same engine, it is possible to run it in the variant of atmospheric (P=0,05-0,1 bar), “atmospheric with greater air supply (P=0,2 bar)”, “semi-turbocharged (P=0,3 bar)” and turbocharged (P=0,4 bar), where P-pressure for this engine at various modes of its operation. Thus, several different engines “living” in one case and having different habits from “silent” to “beast” are received. Full boost pressure for an engine with this compression ratio is likely to be somewhere around 0.4 bar, you need to find the “golden mean” when the maximum boost can get high engine output without risk of detonation and without significantly limiting its speed while maintaining engine life.Less strenuous modes to adjust will be easier. In addition the mass of the charge of the air entering the cylinder depends on its temperature, in our country is a very large dispersion of winter and summer temperatures and it is quite logical to change the boost pressure depending on this or a natural change in atmospheric pressure (eg when driving in the mountains), it is possible.This already nuances, but important enough-As selected engine operating mode with a constant charge of air entering the cylinders is possible to transfer engine operation to leaner fuel-air 2 consecutive cyclones with low boost pressure (no more than 0.4 bar in total) solve several problems at once: 1.Pre-cleaning of the air from dust with an efficiency of at least 95%. The first cyclone performs coarse dust (coarse particles) purification of the air, the second cyclone performs fine dust purification. Then the air filter of zero resistance. 2. The mass of the air charge entering the cylinders remains constant at any engine RPM and is manually selected when you select the driving mode. 3. Cyclones are more simple and reliable in comparison with a turbine, driven by the onboard electric network. 4.

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The boost pressure of each of the 2 cyclones varies in steps of 0.05 bar between 0-0.2 bar. The total range is 0-0.4 bar in increments of 0.05 bar. Depending on the selected driving mode, temperature and air pressure overboard.

Such engine combines the advantages of high and variable compression ratio.

P.S. In my opinion it is more effective to change the boost pressure, in the range of 0.05-0.4 bar. This cleans the air and the exhaust camshaft is structurally simpler. But as if there are two different ways to change the compression ratio in such an engine.

What features do variable compression ratio engines have

Many consumers are attracted to internal combustion engines, capable of producing high RPM, excellent fuel consumption and great power at the same time. So far it was possible to realize it on diesel turbo-motors. If the developers do not stop and find a solution to the existing problems, the diesel will have a serious competitor in the form of an economical, powerful and high-speed turbocharged gasoline engine.

At times it seems as if the current combustion engines have already reached their maximum in efficiency, performance and economy. This is the end of evolution, after which a round of new engine development should begin.

But it only seems that way. In reality, a number of upgraded and modernized ICEs, capable of running on gasoline, diesel fuel, gas and combined fuel, which is realized in hybrid cars, are now being mass-produced.

In recent years the engineers managed to implement the high-precision injection system, to increase the engine power without increasing its displacement, to apply the turbocharging system, to increase the number of valves, to use the variable valve timing and much more.

As a result of the carried out modifications it was possible to significantly improve the parameters of the engine, reduce their toxicity, raise productivity. In parallel with the modification of existing engines, designers are trying to create something new. Some projects exist only on paper, while others have been successfully implemented. Engines with variable or variable compression ratio (MCR) belong to the last category.

Why Variable or Variable Rate Compression (VCC) engines are used

Experienced motorists are probably familiar with the concept of compression ratio in an engine. But it is worth clarifying that this is the ratio of the volume above the engine’s working piston, which drops to its lowest dead center, to the volume when that piston reaches the top dead center.

For gasoline-powered engines, the standard compression ratio is between 8 and 14, but for diesel engines, it is increased to 18-23. For each engine, the compression ratio value acts as a fixed value, which are laid down still at the stage of creation and development of the engine. Depending on what degree of compression is characteristic for a particular power unit, the engine will have the appropriate requirements for the octane or cetane number of fuel used for gasoline and diesel engines, respectively. Additionally, the developers take into account the presence or absence of turbocharging system in the engine. That is, a turbocharged engine or just an atmospheric one.

In simple terms, the compression ratio determines the force of compression of the mixture of fuel and air in the cylinders of the power unit. And here it is important to understand that with strong compression, the fuel-air mixture is able to ignite better and burn out completely. By increasing this parameter, the efficiency of the internal combustion engine will increase, the engine output will improve, and fuel consumption will decrease.

But this effect has a downside. It is associated with a possible detonation effect. Under normal conditions, the fuel-air mixture, compressed in the cylinders, at ignition should not explode, but just burn. In parallel, the ignition process must start and end strictly at certain points in time.

Fuel has a special characteristic called detonation resistance. That is, it is the ability of the fuel to resist the effect of detonation. If the compression ratio is too high, gasoline or diesel can detonate, i.e. explode, which occurs under certain operating conditions of the internal combustion engine.

What are the peculiarities of the engines

Detonation results in uncontrolled processes in which the fuel in the cylinders burns off by explosions. This leads to accelerated wear of engine components, create shock waves, a significant increase in the internal combustion engine temperature with all the ensuing consequences. In this regard, it is impossible to create conditions for the engine in which the compression ratio will always be high.

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The only objectively effective solution to this situation is a flexible change in parameters depending on the specific operating mode of the power plant. That is, changing the compression ratio in certain conditions. This gives a real opportunity to increase engine efficiency, improve the quality of combustion of the air-fuel mixture, improve fuel economy indicators and achieve better efficiency. And since the increase in compression parameters occurs briefly and only in specified engine operating conditions, no destructive effects are observed.

The advantages of variable compression ratio engines look obvious. But in practice, it was extremely difficult to create such an engine. Some auto companies eventually succeeded. Among them, it is worth noting such manufacturers as Infiniti, Peugeot, Saab, Volkswagen, etc.

Operating principle

The main feature and advantage of engines with a variable compression ratio can be considered the ability to significantly improve performance indicators, while reducing fuel consumption.

To put it as simply as possible, the specific current engine operation mode and engine load are taken into account which allows the fuel-air mixture to be compressed and ignited in the most optimal and suitable conditions.

At a time when the power plant is under minimum load, a so-called lean mixture is fed inside the working cylinders. This means that a larger volume of air and less fuel is added. This lean mixture works best when the compression ratio is high enough. Strong compression promotes efficient combustion of even small amounts of fuel mixed with air.

If, however, the engine loads are increased, the cylinders are fed with a rich mixture which is dominated by gasoline. But if the same compression ratio is maintained, but with an enriched mixture, the risk of a detonation effect during engine operation increases considerably. This is the situation that requires the compression ratio to be lowered dynamically.

Currently, power units with constant compression ratios are being actively used. In order to prevent and protect against the risk of detonation, the system for changing the ignition timing advance angle is being employed. This allows the ignition angle to be moved backwards, as it were. The disadvantage of this system is that detonation is prevented, but in parallel the engine loses power and the consumption increases.

In the case of engines with ACC, there is no need to use the advance angle. Hence, there is no loss of power during operation.

If we talk about the implementation of the variable compression ratio scheme, the task of engineers and designers is to physically reduce the size or working volume of the power unit (more precisely, its combustion chamber, where compression of the air-fuel mixture occurs), while maintaining all its characteristics in the form of power, torque, etc.

Quite a number of car companies were working to implement something similar. And many managed to achieve serious success.

At the same time, automakers used different ways to implement the idea. That is, they controlled the degree of compression with the help of changing the volume of the combustion chamber, connecting rods, crankshafts, etc.

The earliest development is considered to be the one, in which an additional piston integrated into the combustion chamber was provided for the engine. In this case the variable compression ratio was achieved by moving this piston, which in the process of its movement could change the volume.

But the solution had too many disadvantages. The main of them is the need to install additional components in the cylinder block. This immediately complicated the process of engine production and increased its cost. At the same time the developers were faced with the problem of a modified combustion chamber shape. As a result, the fuel entering it could not burn evenly and fully.

Considering objective reasons, the project was soon shut down. The work on it was not completed, because there were too many obvious drawbacks. Similarly unsuccessful was another project, where engineers tried to use pistons capable of changing their height. The designers used split pistons, which in practice turned out to be too heavy. In addition, there were difficulties with attempts to implement a control system for lifting the piston cover.

What does atmospheric engine mean

Much more interesting are the projects of several automakers. Not all of them were successful and have a chance to be implemented in mass production, but they managed to make big steps forward.


Why an SAR is used

German automaker, based on the mistakes of its predecessors, did not touch the pistons and combustion chamber. But the company engineers paid attention to the crankshaft lift. That is, they wanted to create a system that would allow controlling the height of the crankshaft lift.

The result was a scheme where the crankshaft journals were placed inside special couplings. And it was eccentric-type couplings were used. They were driven by gears which were directly connected with electric motor.

Because the eccentrics made turns, the crankshaft was lowered and raised. This changed the elevation of the working pistons relative to the block head. This made it possible to decrease and increase the current volume of the engine combustion chamber, changing in parallel the compression ratio itself.

German engineers managed to create several prototypes at once. As a base engine for their developments in Volkswagen used turbo engine with a volume of 1.8 liters. In their developments, the compression ratio was able to change in the range of 8-16 units. Long and numerous tests were conducted. But for some reason, the engine never went into production.

Another fairly good attempt was made by Saab engineers. Their development was different in that they wanted to change the compression ratio by raising the entire block.

At one point, it seemed that the engine would soon go into mass production. The engine received the quite famous in its time marking SVC. It was an engine with a displacement of 1.6 liters with 5 cylinders and turbocharging system.

The output power of the power plant reached 220 horsepower with a torque of over 300 Nm. Due to its innovations the level of fuel consumption was reduced by almost 35%. At the same time the engine was omnivorous, because could work well on the high-octane 98 gasoline, and conventional cheap AI 76.

The solution of Saab’s engineers was as follows. They divided the engine cylinder block, which allowed to get conventionally two parts of the engine. At the top was the cylinder head itself and the cylinder liners, and a place at the bottom was reserved for the crankshaft. The connecting element between the two parts of the engine was a movable joint and a special mechanism operating at the expense of the electric drive.

Such a solution allowed to lift the upper part of the cylinder block at an angle. The angle was literally just a few degrees, allowing the compression ratio to change in a fairly wide range from 8 to 14. To seal the joint, a special casing made of high-strength but elastic rubber was used.

In theory, everything looked very promising and promising. But when faced with the practical realization of their idea, it turned out that the movable top unit and the casing were not reliable, strong, or durable enough. They promised to be the engine’s weak link. It is possible that this was the reason why the engine was rejected for series production, where it was possible to realize variable compression ratio systems.


The French engineers of the Peugeot automobile company were the next to try their forces. The Peugeot developers took a turbocharged engine with a working volume of 1.5 liters as a basis and provided it with an opportunity to change the current compression ratio. The variation range of the parameter was 7-18 units. Such engine gave 225 horsepower at the output, and its torque reached 420 Nm.

The design of the engine was very complex, using a split connecting rod. In the area of its attachment to the crankshaft, the connecting rod additionally received a special toothed rocker arm. In the area where the piston and connecting rod are connected, the engineers used a rack and pinion.

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On the other side, a piston rack was fixed to the installed rocker arm. This piston was responsible for steering. The movement was created by the lubrication system. The oil went through a path consisting of many channels and valves. In addition, an electric drive was used here.

In short, the control piston acted on the rocker arm by moving it. This made it possible to change the lift height of the main cylinder piston.

In the future, the project was frozen, no hints at the thawing and serial production of such a motor has not yet appeared.


While the most promising and promising project is the development of the company Infiniti, which have created their own engine, which has a variable compression ratio.

This engine is labeled VCT. Here was implemented the ability to change the parameters of the compression ratio in the range from 8 to 14 units. The project is based on the use of a special traverse mechanism.

Engineers created a movable connection between the bottom journals and the connecting rod. Plus, a lever system is used here. These levers are driven by an electric motor.

The controller is responsible for the control process. It sends the appropriate signals to the actuator, that is, the electric motor. When the electric motor receives a command from the control unit, the rod begins to move and the levers change their position. This makes it possible to change the current height of the working piston.

Basic principle of operation

At the output of the Japanese engineers from Infiniti turned out a powerful turbocharged gasoline engine with a working volume of 2.0 liters and 265 horsepower. The main achievement of the motor was the preservation of high efficiency with excellent fuel economy indicators. Compared to similar power plants where the compression ratio is constant, after implementing the system of changing the compression ratio the economy was almost 30%.

  • Yes, while the developers themselves admit that the engine has a number of drawbacks. They manifest themselves in the following:
  • The design is quite complicated;
  • There are increased vibrations during operation;
  • Reliability is still lame and inferior to conventional ICE with a constant compression ratio;

Increased cost of the motor, etc.

But at present, engineers are actively working to eliminate existing weaknesses. If they manage to solve at least a few problems, which is realistic and possible, then the prototype has every chance to become a production engine for cars of the company. Based on the optimistic forecasts of official representatives, the project requires another 1-2 years. That is, there are chances that in 2019 we will see the Japanese turbo engine with variable compression ratio, installed as standard in some models of Infiniti cars.

Considering everything said before, it’s safe to say that engines with variable compression ratio have excellent prospects. They have the potential to improve current performance characteristics such as economy and efficiency, without sacrificing power. There are still some problems and design limitations that prevent the motors from being put into series production.

This is one of the most realistic ways to reduce the fuel consumption of gasoline turbocharged engines, thereby creating stiff competition for turbocharged diesel engines.

Since there is currently a fuel crisis in the world, and in parallel the requirements for environmental standards are increasing, the emergence of an engine with improved fuel combustion efficiency without the need for power limitations will be a big step forward in the development of internal combustion engines.

In theory, such powertrains with variable compression ratios can provide significant advantages to the modern turbocharged gasoline engine. And in terms of consumption, gasoline engines will be almost as good as advanced turbo-diesel engines. The popularity of the latter is becoming more and more evident and noticeable.

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