Variable compression ratio
Encyclopedia
Variable compression ratio is the technology to adjust internal combustion engine
cylinder compression ratio
s on the fly. This is done to increase fuel efficiency
while under varying loads. Higher loads require lower ratios to be more efficient and vice versa. Variable compression engines allow for the volume above the piston at 'Top dead centre
' to be changed. For automotive use this needs to be done dynamically in response to the load and driving demands.
s or supercharger
s are used to increase the inlet pressure. This would result in detonation of the fuel/air mixture unless the compression ratio was decreased, i.e. the volume above the piston made greater. This can be done to greater or lesser extent with massive increases in power being possible. The down side of this is that under light loading, the engine can lack power and torque. The solution is to be able to vary the inlet pressure and adjust the compression ratio to suit. This gives the best of both worlds, a small efficient engine that behaves exactly like a modern family car engine but turns into a highly tuned one on demand.
Variable compression ratio is becoming increasingly desirable as oil prices increase
and car buyers have an increased interest in fuel economy.
The cylinder head can be altered by using a hydraulic system which is connected to the crank shaft and responds according to the load and acceleration required.
introduced a variable compression to the world at the Geneva motor show in 2000, in the form of their SVC
engine, but has yet to reach production . The design consisted of a monobloc head, which contained all of the valve gear, and the crankshaft/crankcase assembly. These parts were connected together by a pivot which allowed 4 degrees of movement controlled by a hydraulic actuator. This mechanism allows the distance between the crankshaft centre line and the cylinder crown to be varied.
A supercharger was chosen in preference to a turbocharger to achieve the necessary response time and high boost pressure.
v ,v,
, which dynamically vary the size of the combustion chamber
. As of late (in the 2000s) this technology has seen some renewed interest, due it being able to burn a wide range of fuels (e.g. including alcohols) such as the Lotus
Omnivore.
is an advanced and workable addition to the world of VCR engines, the first being built and tested by Harry Ricardo
in the 1920s. This work led to him devising the octane rating
system that is still in use today. SAAB has recently been involved in working with the 'Office of Advanced Automotive Technologies', to produce a modern petrol VCR engine that showed an efficiency comparable with that of a Diesel
. Many companies have been carrying out their own research in to VCR Engines, including Nissan
, Volvo
, PSA/Peugeot
-Citroën
and Renault
but so far with no publicly demonstrated results.
To alter Vc, the SVC 'lowers' the cylinder head
closer to the crankshaft
. It does this by replacing the typical one-part engine block with a two-part unit, with the crankshaft in the lower block and the cylinders in the upper portion. The two blocks are hinge
d together at one side (imagine a book, lying flat on a table, with the front cover held an inch or so above the title page). By pivoting the upper block around the hinge point, the Vc (imagine the air between the front cover of the book and the title page) can be modified. In practice, the SVC adjusts the upper block through a small range of motion, using a hydraulic actuator.
The Cortina engine is the only variable compression concept that slides the entire reciprocating assembly toward and away from the cylinder head. The advantage of this approach is that there is no complication to the workings of the reciprocating assembly itself, nor is there any complication outside of the engine.
The way this is accomplished is by sliding the reciprocating assembly on a splined output shaft that is positioned perpendicular to a typical engine’s output orientation.
The novel use of a barrel cam in place of a tradition crankshaft uses fewer parts than competing designs and adds the option of a true Atkinson cycle. It also has a more favorable leverage curve than a traditional crankshaft. Since the connecting rods stay straight, with a barrel cam cycling the pistons, it also is possible to box in the lower cylinder for use as an internal supercharger.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
cylinder compression ratio
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
s on the fly. This is done to increase fuel efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
while under varying loads. Higher loads require lower ratios to be more efficient and vice versa. Variable compression engines allow for the volume above the piston at 'Top dead centre
Dead centre
In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead centre while the latter is known as bottom dead centre ....
' to be changed. For automotive use this needs to be done dynamically in response to the load and driving demands.
Advantages
Petrol engines have a limit on the maximum pressure during the compression stroke, after which the fuel/air mixture detonates rather than burns. To achieve higher power outputs at the same speed, more fuel must be burnt and therefore more air is needed. To achieve this, turbochargerTurbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s or supercharger
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
s are used to increase the inlet pressure. This would result in detonation of the fuel/air mixture unless the compression ratio was decreased, i.e. the volume above the piston made greater. This can be done to greater or lesser extent with massive increases in power being possible. The down side of this is that under light loading, the engine can lack power and torque. The solution is to be able to vary the inlet pressure and adjust the compression ratio to suit. This gives the best of both worlds, a small efficient engine that behaves exactly like a modern family car engine but turns into a highly tuned one on demand.
Variable compression ratio is becoming increasingly desirable as oil prices increase
Energy crisis
An energy crisis is any great bottleneck in the supply of energy resources to an economy. In popular literature though, it often refers to one of the energy sources used at a certain time and place, particularly those that supply national electricity grids or serve as fuel for vehicles...
and car buyers have an increased interest in fuel economy.
The cylinder head can be altered by using a hydraulic system which is connected to the crank shaft and responds according to the load and acceleration required.
Production
Variable compression engines have existed for decades but only in laboratories for the purposes of studying combustion processes. These designs usually have a second adjustable piston set in the head opposing the working piston. (very much like model aircraft 'Diesel' engines) Variable compression engines have been highly desirable but technically unobtainable for production vehicles due to the mechanical complexity and difficulty of controlling all of the parameters. The advances in low cost microcontrollers and a wealth of experience in their application to engine management now makes the control possible. SAAB AutomobileSaab Automobile
Saab Automobile AB, better known as Saab , is a Swedish car manufacturer owned by Dutch automobile manufacturer Swedish Automobile NV, formerly Spyker Cars NV. It is the exclusive automobile Royal Warrant holder as appointed by the King of Sweden...
introduced a variable compression to the world at the Geneva motor show in 2000, in the form of their SVC
Saab Variable Compression engine
The Saab Variable Compression engine was a development project of Saab Automobile, for which it won an award both in 2000 and 2001.In a normal internal combustion engine, because cylinder bore diameter, piston stroke length and combustion chamber volume are almost always constant, the compression...
engine, but has yet to reach production . The design consisted of a monobloc head, which contained all of the valve gear, and the crankshaft/crankcase assembly. These parts were connected together by a pivot which allowed 4 degrees of movement controlled by a hydraulic actuator. This mechanism allows the distance between the crankshaft centre line and the cylinder crown to be varied.
A supercharger was chosen in preference to a turbocharger to achieve the necessary response time and high boost pressure.
v ,v,
Two-stroke engines
Due the comparative simplicity of cylinderhead design (lacking intake valves) it is somewhat easier to implement in two-stroke engines. From the late 90s on up models which expand on this idea have been available, such as from YamahaYamaha
Yamaha may refer to:* Yamaha Corporation, a Japanese company with a wide range of products and services** Yamaha Motor Company, a Japanese motorized vehicle-producing company...
, which dynamically vary the size of the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
. As of late (in the 2000s) this technology has seen some renewed interest, due it being able to burn a wide range of fuels (e.g. including alcohols) such as the Lotus
Lotus Cars
Lotus Cars is a British manufacturer of sports and racing cars based at the former site of RAF Hethel, a World War II airfield in Norfolk. The company designs and builds race and production automobiles of light weight and fine handling characteristics...
Omnivore.
Saab variable-compression engine
The SAAB SVCSaab Variable Compression engine
The Saab Variable Compression engine was a development project of Saab Automobile, for which it won an award both in 2000 and 2001.In a normal internal combustion engine, because cylinder bore diameter, piston stroke length and combustion chamber volume are almost always constant, the compression...
is an advanced and workable addition to the world of VCR engines, the first being built and tested by Harry Ricardo
Harry Ricardo
Sir Harry Ricardo was one of the foremost engine designers and researchers in the early years of the development of the internal combustion engine....
in the 1920s. This work led to him devising the octane rating
Octane rating
Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...
system that is still in use today. SAAB has recently been involved in working with the 'Office of Advanced Automotive Technologies', to produce a modern petrol VCR engine that showed an efficiency comparable with that of a Diesel
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
. Many companies have been carrying out their own research in to VCR Engines, including Nissan
Nissan Motors
, usually shortened to Nissan , is a multinational automaker headquartered in Japan. It was a core member of the Nissan Group, but has become more independent after its restructuring under Carlos Ghosn ....
, Volvo
Volvo
AB Volvo is a Swedish builder of commercial vehicles, including trucks, buses and construction equipment. Volvo also supplies marine and industrial drive systems, aerospace components and financial services...
, PSA/Peugeot
Peugeot
Peugeot is a major French car brand, part of PSA Peugeot Citroën, the second largest carmaker based in Europe.The family business that precedes the current Peugeot company was founded in 1810, and manufactured coffee mills and bicycles. On 20 November 1858, Emile Peugeot applied for the lion...
-Citroën
Citroën
Citroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...
and Renault
Renault
Renault S.A. is a French automaker producing cars, vans, and in the past, autorail vehicles, trucks, tractors, vans and also buses/coaches. Its alliance with Nissan makes it the world's third largest automaker...
but so far with no publicly demonstrated results.
To alter Vc, the SVC 'lowers' the cylinder head
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
closer to the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
. It does this by replacing the typical one-part engine block with a two-part unit, with the crankshaft in the lower block and the cylinders in the upper portion. The two blocks are hinge
Hinge
A hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...
d together at one side (imagine a book, lying flat on a table, with the front cover held an inch or so above the title page). By pivoting the upper block around the hinge point, the Vc (imagine the air between the front cover of the book and the title page) can be modified. In practice, the SVC adjusts the upper block through a small range of motion, using a hydraulic actuator.
Cortina Variable Compression engine
American inventor Paul Cortina is the latest entry into variable compression engine development.The Cortina engine is the only variable compression concept that slides the entire reciprocating assembly toward and away from the cylinder head. The advantage of this approach is that there is no complication to the workings of the reciprocating assembly itself, nor is there any complication outside of the engine.
The way this is accomplished is by sliding the reciprocating assembly on a splined output shaft that is positioned perpendicular to a typical engine’s output orientation.
The novel use of a barrel cam in place of a tradition crankshaft uses fewer parts than competing designs and adds the option of a true Atkinson cycle. It also has a more favorable leverage curve than a traditional crankshaft. Since the connecting rods stay straight, with a barrel cam cycling the pistons, it also is possible to box in the lower cylinder for use as an internal supercharger.