Variable Compression Ratio Engines

Introduction:
The name internal combustion engines was given to the engines in which the combustion process takes place inside the engine, unlike engines that use steam as the driving energy, where the combustion process takes place outside the external combustion engines. The combustion process takes place inside the engine in a short time up to 0.003 of a second, and to complete the combustion process during that short period of time, we need to compress the charge to bring the particles of the mixture close until the combustion process is completed and all the fuel in the mixture is burned during that short period. It was found that the higher the charge compression, the higher the combustion efficiency, in addition to reducing the amount of unburned fuel (hydrocarbon) in the exhaust gases. The amount of charge compression is measured by means of the so-called compression ratio, which is the ratio of the volume of the mixture in the chamber before compression (the piston is at its bottom position,
With an increase in the compression ratio, the engine’s performance and efficiency improve, but the compression ratio cannot be increased beyond a certain limit, as the increase in the compression ratio in gasoline engines leads to the occurrence of the slapping phenomenon (combustion of the mixture spontaneously, as a result of intense pressure and high temperature of the mixture), and therefore with the need to increase the ratio Compression is more, it is not possible to increase the compression ratio from 8: 1 to 12: 1 for gasoline engines (spark ignition engines), because of the risk of slapping (self-ignition of the mixture).
The compression ratio depends on the distance of the combustion chamber – the size of the combustion chamber, and on the distance of the cylinder (the distance between the upper dead point and the lower dead point equal to twice the distance of the crankshaft knee (the crankshaft axis and the axis of fixing the big end of the connecting rod) – the volume of the cylinder. The volume occupied by the mixture before compression (combustion chamber volume + cylinder volume) to the volume occupied by the mixture after compression (combustion chamber volume called clearance volume):
compression ratio = (combustion chamber volume + cylinder volume) ÷ ( combustion chamber volume),
as shown in Figure 1.

Figure 1: The ratio of compression is the volume of charge before compression to the volume of charge after compression

 
Conventional gasoline engines (spark ignition) are designed with a Fixed Compression Ratio (FCR); This percentage is the maximum percentage that can be reached before the phenomenon of detonation occurs. Since the possibility of slapping is greater at full load due to the high increase in combustion temperature, the maximum compression ratio is designed to suit full load operating conditions. Researchers in the field of engines have concluded that under partial loads, the compression ratio can be increased than the maximum designed for the engine (at full load) without the risk of slapping phenomenon, with the benefit of increasing the efficiency of combustion, and increasing the power at those loads. Hence the idea of ​​a motor that has a variable compression ratio (increasing the compression ratio at partial loads and decreasing it at high loads). To implement this idea, the size and dimensions associated with the combustion chamber or cylinder must be changed.
Advantages of variable compression ratio engines: The
idea of ​​variable compression ratio engines will improve engine performance, increase its efficiency, and reduce exhaust emissions. With the increase in the compression ratio, the pressure inside the cylinder increases and the temperature increases at the beginning of the combustion period, which leads to a decrease in the proportion of the remaining burning gases in the cylinder and to an increase in the speed of the flame front. Accordingly, the combustion period is less recess, and therefore the combustion time is reduced. This loss in time due to an increase in the compression ratio at partial loads leads to reduced fuel consumption. It also leads to a significant increase in thermal efficiency.
The distinguishing feature of these engines is that they operate at different compression ratios, according to the needs of the engine’s performance. Variable engines with a compression ratio. The compression ratio can be changed continuously by changing the size of the combustion chamber. At low capacities, these engines operate at a high compression ratio to obtain high fuel efficiency, and at high capacities they operate at a low compression ratio to avoid the problem of engine slamming.  
It has been proven that these engines give higher power for the same engine dimensions and have a higher power-to-weight ratio without any damage to the specific fuel efficiency.
The challenge for this VCR technology is to improve thermal efficiency, thus improving fuel consumption while reducing exhaust emissions. One of the most important characteristics affecting thermal efficiency is the compression ratio.
Thermal efficiency of standard air cycles

Where r is the compression ratio, k is the ratio of specific heat at constant volume to specific heat at constant pressure, which is about 1.4.

Figure 2: The effect of increasing the compression ratio on the thermal efficiency of ideal air cycles

 
An increase in the compression ratio will lead to an increase in thermal efficiency and an improvement in fuel consumption. In general, the operating conditions of spark ignition engines are widely variable, such as stopping and moving during traffic within cities, or traveling at a constant speed outside the city and traveling at high speeds on highways. In conventional spark-ignition engines, the maximum compression ratio is determined by cylinder conditions at high load. In the event of an increase in the compression ratio at the designed limit, the mixture self-ignites and leads to the slapping phenomenon, which causes severe combustion problems and may lead to engine damage. Since most spark ignition engines operate under city operating conditions and at low loads, this will result in reduced thermal efficiency and increased fuel consumption. Under-loading the engine at partial loads leads to a temperature drop at the end of the power stroke. Therefore, in this case, the compression ratio can be increased without the risk of slapping.
 
Different designs of variable compression ratio engines: One
of the challenges facing companies operating in the automotive industry is the design, development and production of a variable compression ratio engine. Several patents have been registered in this field. The current ideas for a motor with a different compression ratio center on the design of a motor in which the compression ratio at partial loads is increased than the ratio designed for the engine at full load, and the compression ratio is increased by reducing room space. Combustion (clearance space) in the following suggested ways:
[1] Moving the crankshaft chairs up:
Installing the crankshaft on eccentric bearings that move the crankshaft axis up when giving a rotational movement of the chairs.
In this design, the engine crankshaft is repositioned so that the engine bearings are mounted on a non-axial mount that can rotate to raise or lower the piston’s upper dead-point position within the cylinder. The axle is raised and lowered by rotating the non-axial bearing. This design is simple in terms of not changing the moving parts, but the engine needs to make a connection to the output shaft connected to the flywheel to accommodate this axial difference caused by moving the crankshaft axle. In this way, the compression ratio can be increased from 8 to 14 by rotating the axial stand by 55 degrees.   

Figure 3: Axial crankshaft assembly [1]

 
[2] Changing the connecting rod dimensions: 

Increase the connecting rod length by using additional couplings. /span> The
design proposed by Nissan, is to use multiple joints to achieve a variable compression ratio, adding a control joint between the connecting shaft and the crankshaft to achieve this. For this, a crankshaft is used with a small distance between the crankshaft axis and the axis of the large end of the connecting rod that allows the installation of this connection. Thus, the compression ratio can be increased from 10 to 15 by turning that joint with an actuator by 70 degrees in which the piston position is shifted by 3.1 mm. This design has the advantage that it also increases the travel length of the piston, which leads to reduced piston acceleration (reduced inertial forces).

Figure 3: Multiple couplings for changing the compression ratio (Nissan Company) [2]

 
[3] Giving a vertical movement to the cylinder head:
moving the head of the combustion chamber downwards./span> The
idea of ​​moving the cylinder head (from SAP) is to assemble the cylinder head with the interlocking of the cylinders in one composition, then through connections and actuators the group can be moved in relation to a block the engine. With this system, the amount of compression ratio can be increased from 8 to 14.

Figure 4: Changing the volume of the combustion chamber by moving the cylinder head (SAP company) [3]

[4] Changing the compression ratio by using a secondary piston or valve:
Using a secondary piston or valve inside the combustion chamber when moved changes the chamber volume.
Ford has patented an invention for changing the compression ratio by using a secondary piston or valve. The piston can be in an intermediate position, corresponding to the ideal position of the compression ratio for special conditions. The volume of the combustion chamber is increased to reduce the compression ratio by moving a small secondary piston in contact with the chamber space. This design requires a large diameter cylinder as changing the volume of the combustion chamber via a secondary valve or piston within the chamber reduces the space available for movement of the intake and exhaust valves.
&This design gave acceptable results when testing the engines in the laboratory, but it is difficult to achieve this in the engines produced in the market. The engine cylinder cooling system needs an advanced and highly efficient cooling system, and the spare piston needs a special lubrication system to obtain high efficiency for this type of design.    

Figure 5: Using a valve to change the compression ratio of the engine (Ford Company) [4]

 
[5] Changing the height of the piston head: the
action of a two-part piston, allows the piston head to move upwards, to increase the piston height/span>.
Daimler-Benz has designed a variable-height piston, and this design is based on a template idea for application in mass production of variable compression ratio engines, as it leads to minor changes in the design of conventional engines compared to other designs. Unfortunately, this idea will lead to an increase in the weight of the moving parts, and control that change with the higher speed of the group. The change in elevation is carried out by the engine oil, and this is one of the challenges in implementing the design. This design reduces the maximum fire loads which makes the design more efficient. But the design will reduce the size available for expansion.

Figure 6: The piston of Daimler-Benz variable-compression ratio engines [5]

 
  [6] Pressure-reacting piston The
action of a two-part piston allows the piston head to move downward at high loads against the force of disc springs between the two parts of the piston. /span>
The University of Michigan has developed a pressure reaction piston for spark ignition engines. This piston consists of a separate piston head and piston wall with a set of disc springs in between. The piston in this way allows the piston head to move downward as the pressure in the combustion chamber increases. The compression of the piston leads to an increase in the clearance volume, a decrease in the compression ratio and a decrease in the maximum pressure value of the cylinder. This design effectively reduces the maximum pressure value at high loads without additional control devices, while allowing the engine to operate at a high compression ratio under low load conditions (piston head up). This design can easily be easily applied to conventional engines with only a simple change to the piston and connecting rod design. This results in an improvement in the specific brake fuel consumption of this engine compared to a conventional engine at the higher load range from 8 to 18 percent,

Figure 7: A piston reacting to pressure [6]

 
[7]
Movement of the large end of the crankshaft bearings : Installing the large end of the connecting rod on eccentric bearings that moves the axis of fixing the large end of the connecting rod upward when giving a rotational movement of the chairs./span>
One company moved the crankshaft large end bearings by a non-pivoting method to affect the end of the ride in the upper dead-point where the non-pivoting chairs could be repositioned by means of a large gear. Thus, the upper dead-point can be shifted by 10mm with only 40° rotation of the non-pivot chairs. Increasing the piston height at the upper dead point will result in a decrease in the height of the other piston on the same knee as the crankshaft. To reduce the size of the engine, this design was applied to a two-cylinder engine, this engine is considered to be of less weight and small in size, and the cost of adding to this design is reasonable compared to adding it to a four-cylinder engine. Applying this to a four-cylinder engine requires the addition of several drive gears.

Figure 8: Changing the connecting rod big end anchor point [7]

A summary of the proposals for variable compression ratio engines is shown in Table 1.MSuggested designCharacteristicsThe obstacle in the implementation of mass production1crankshaft axis movementThe crankshaft chairs are loaded on a non-axial bearing and the compression ratio is changed by making a circular motion of the bearing.The engine needs a fixed output shaft and therefore needs a connection between the crankshaft and the output shaft.2Connecting arm shape adjustmentThe length of the connecting rod is changed by means of a multi-link system to change the compression ratio of the engine.Connecting rod joints increase vibrations.3Moving the cylinder headThe compression ratio can be changed by moving the cylinder head by tilting the head relative to the mass of the cylinders.Needs full engine build modification.4Changing the compression ratio by using a secondary piston or valveThe compression ratio can be changed by moving a secondary piston or valve inside the combustion chamberAdding another part inside the combustion chamber space may affect the valve space and travel. The presence of additional cavities entering the chamber may impede the complete combustion of the charge.5,6Piston head height adjustmentAdjusting the piston height by means of a hydraulic system or a spring system that changes the compression ratioA significant increase in the weight of the reciprocating parts inside the engine, in addition to a high heat loss to increase the heat transfer rate due to the increased surface to volume ratio.7Moving the seat of the big end of the connecting rodChanging the position of the upper dead point by loading the large end bearings of the connecting rod onto non-axial bearings and driving them by means of large bearing swivel gears.The system needs many drive gears

 
General benefit of variable compression ratio engines:
These engines are a prime solution for weak mixture combustion. It allows for an increase in compression pressure and temperature to maintain favorable conditions for combustion coefficients (no ignition loss with rapid flame spread) even under weak mixture operating conditions. This provides this engine with better control of pollutants than a similar engine without pollutant control systems or with the pollutant control system in a conventional engine. It also increases the shelf life of the triple catalyst. Since the proportion of the geometric size is controlled for this type of motor, these motors increase the slapping limit, whatever the load. In this type of engine there must be a correlation between the compression ratio and the spark supply to obtain the best thermal efficiency results. These engines reduce the temperature of the exhaust gases, which in turn reduces engine thermal stresses and reduces the possibility of fuel overflow at high power. The advantages of variable compression ratio motors can be summarized as follows:
(a) To make the efficiency equal and optimal over the operating speed of the engine.
b- Reducing the rate of fuel consumption and reducing harmful exhaust emissions.
C – The possibility of using different types of fuel in the engine with optimum efficiency.
D – Smooth rotation for empty load speed and high acceleration at full load.
E- It gives higher graphical thermal efficiency than conventional fixed compression ratio engines.
f- It allows the empty load speed to be reduced as it reduces ignition losses and irregularities in the thermal cycle leading to reduced vibrations.
G- Reduce the low frequency noise due to the stability of the maximum pressure value.
H- Smooth combustion as the resulting heat rate is equal to the high and low compression ratios.
i- Emissions from the car at cold start can be greatly reduced due to rapid heating of the catalyst.
K – Improve the low torque of the gasoline engine without fear of slamming.
L- A futuristic technology for low-mix combustion engines.
M- Reduce CO2 emissions by reducing the size of the engine for the same output power.
N- Good performance at empty load at low ambient temperature.
u – constant friction loss due to the approximately constant value of the maximum pressure.
The future of variable compression ratio engines: Variable compression ratio
engines have not yet reached the market, despite patents and decades of experience. And of course there are some prototypes of that engine that have been tested. In many cases, switching from the design and installation of the conventional engine to the proposed engine creates an obstacle to the implementation of the new proposal. The following are some of the manufacturing and commercial obstacles that hinder the application of this technology:
1- The proposed methods require fundamental changes in the design and installation of these engines, which constitutes a major obstacle to the process of implementing and manufacturing the engine commercially in the markets.
2- Increasing the number of elements in a combustion chamber of limited size will affect the shape and size of the charge inlet and outlet openings and valve movements.
3- With the presence of additional cavities inside the combustion chamber that may lead to the incomplete combustion of the charge, which increases the emissions of hydrocarbons in the exhaust.
4- There is a noticeable increase in the moving mass inside the engine, especially in the design of the variable height piston.
5- Some of the proposals lead to increased vibrations because the additional parts connected to the connecting arm.
6- In some cases, the design needs a complete engine change.
 
Challenges Facing Variable Compression Ratio Engines
High loads for a long operating time will affect the operational life of the engine, with the increase in average loads, the wear in the engine will increase, which will increase the leakage of gases from the combustion chambers, noise and increase the rates of oil consumption. Although variable compression ratio engines are characterized by less vibrations caused by low frequency due to the change in crankshaft torque, as they are regular cycles. But there is a need to reduce the noise caused by the complexity in the design and the increase in the parts, and for this the engine must be of a strong and consistent body (of high rigidity).
 
Financial outlook:
Choosing the right technology for variable compression ratio engines will be an important point in determining the engine price for the appropriate application for future cars. The different technologies currently offered must be compared in terms of advantages and disadvantages for the engine parts, methods of operation and maintenance, and the amount of its operational life, as well as the possibility of making adjustments easily to the engines currently in the market.
The real benefit of these engines is the possibility to use the forced charging system (dual technology), without fear of slamming, this is used to either increase the engine capacity of the same dimensions, or reduce the engine size while retaining the same power. This direction will reduce heat and friction losses, reduce airflow losses, and have a lighter engine while increasing engine efficiency.
Variable compression ratio motor technology will allow the application of the Atkinson thermal cycle and the ability to use a weak mixture.
This technology also allows the use of many types of fuels with these engines, as the compression ratio can be changed and adjusted to suit the properties of the fuel used.