Variable compression ratio

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Variable compression ratio is a technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. 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.


Gasoline 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 burned and therefore more air is needed. To achieve this, turbochargers or superchargers 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 a 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 (VCR) is becoming increasingly desirable as oil prices increase and car buyers have an increased interest in fuel economy. In addition, Global Climate Warming requires measures from the international community. To the Automobile industry, it means stricter limits to car emissions, especially CO2. Variable compression ratio is one cost effective way of achieving these targets. In addition, VCR allows free use of different fuels besides petrol e.g. LPG or ethanol.

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.


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). Earlier 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. However, new solutions like Waulis approach does not use a second piston and is implemented to existing 4-cylinder engine with minor modifications. This is a promising sign towards full commercial production readiness and cost-efficient innovative solution which will change the future of VCR engines.

Two-stroke engines[edit]

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 Yamaha,[1] 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.[2][3]

Engine designs[edit]

The first VCR engine built and tested was by Harry Ricardo in the 1920s. This work led to him devising the octane rating system that is still in use today. Many companies have been undertaking their own research into VCR Engines, including Nissan, Volvo, PSA/Peugeot-Citroën and Renault but so far with no publicly demonstrated results.[4]

Waulis Motors Ltd[edit]

Waulis Motors Ltd was founded in 2011 by experienced engine technology experts in Espoo, Finland. The patented technology itself is based on the decades research and planning made by Aulis Pohjalainen. As an outcome of that process the first working prototype saw the daylight in Summer 2011. Company’s invention aims to remove the pressure related disadvantages of the existing conventional motors and provide a motor which has substantially lower loss of thermal energy, increased output per litre, and which decreases CO2- emissions about 30-40 % compared to conventional motors.

Waulis develops a new innovative patented technology solution, that resolves the existing changes in cylinder pressure adjustment in a cost-effective way. Compared to Waulis, existing VCR- technology solutions are much more complex to produce, challenging to control, and substantially much more expensive to implement. In other words, Waulis develops a solution, which manufacturing costs in contrast to achieved benefits are remarkable competitive compared to its rivals.

Waulis design describes a crank device and an adjusting device of a combustion engine. The system adjusts the cylinder pressure of the motor in accordance with the required power. Adjustment of the cylinder pressure is achieved by changing the compression ratio by means of the adjusting device. The adjusting device moves an eccentric wheel to a set position using an adjusting wheel; this results in the connecting rod moving the piston to a desired distance from the cylinder head. The adjusting device measures the volume of the air entering the cylinder and adjusts the compression ratio appropriately. The adjusting device also takes into account the speed of rotation to adjust the compression pressure accordingly.

Peugeot MCE-5[edit]

Principle of the MCE-5 ("Multi Cycle Engine - 5 parameters"), the variable compression ratio engine made by Peugeot.

The Peugeot design works by varying the effective length of the con-rods connecting the piston to the crank. When the con-rod is shorter, the compression ratio is lower and vice versa. On the left hand-side of the diagram is the conventional piston of an internal combustion engine. On the right is an hydraulic cylinder with double-acting piston. This acts through a rod-crank system with a gear wheel, whose movement adjusts the effective con-rod length and thus the compression ratio in the left cylinder.[5]

Saab SVC[edit]

SAAB Automobile rekindled interest in variable compression when they introduced their SVC engine to the world at the Geneva motor show in 2000. SAAB had 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. The SAAB SVC was an advanced and workable addition to the world of VCR engines, but it never reached production due to the company's bankruptcy.

The design, an implementation of the Larsen VCR engine,[6] consisted of a monobloc head, which contained all of the valve gear, and the crankshaft/crankcase assembly. These parts were connected 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. Unlike the Peugeot design, the effective con-rod length is fixed. A supercharger was chosen in preference to a turbocharger to achieve the necessary response time and high boost pressure.

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 hinged 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.

Gomecsys VCR Technology (state of 2012)[edit]

Gomecsys Gen4 VCR crankshaft for a 4-inline engine

Gomecsys is a Dutch Engineering company that has developed its own variable compression ratio technology. Strong improvements have been made over the last 5 years and currently the company has its 4th generation VCR engines running on the dyno. One of the big advantages of the system is the simplicity. The complete VCR system is integrated on the crankshaft and every 4 stroke engine can be upgraded by replacing a normal crankshaft with a Gomecsys VCR crankshaft. Additional fuel saving technologies incorporated in the system boost the overall CO2 reduction to 18%, and that's without downsizing.[citation needed]

See also[edit]


  1. ^ Yamaha Environment-Friendly "SD (Super Diesel) Engine"
  2. ^ Green Car Congress: Lotus, QUB and Jaguar to Develop Variable Compression Ratio, 2-Stroke OMNIVORE Research Engine [1]
  3. ^ Lotus Engineering Omnivore Variable Compression Ratio Engine to Debut in Geneva [2]
  4. ^ Variable compression ratio engines
  5. ^, February 25, 2009 [3].
  6. ^ US patent 5025757, Gregory J. Larsen, "Reciprocating piston engine with a varying compression ratio", issued 1991-06-25