Compression ratio
From Free net encyclopedia
The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.
The ratio is calculated by the following formula:
- <math>\mbox{CR} = \frac { ( \pi b^2 s) / 4 + V_c } {V_c} </math>, where
- <math>b</math> = cylinder bore (diameter)
- <math>s</math> = piston stroke length
- <math>V_c</math> = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.
Due to pinging (detonation), the CR in a gasoline/petrol or LPG or CNG-powered engine will usually not be much higher than 10:1.
In engines with a ping sensor and an electronic control unit, the CR can be as high as 12.5:1 (2005 Audi A6 3.2)
In a turbo charged or super charged engine the CR will be around 8.5:1
In a diesel engine the CR will be 20:1 and higher.
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Fault finding and diagnosis
Measuring the compression pressure of an engine with a pressure gauge connected to the spark plug opening gives an indication of the engine's state and quality.
If the nominal compression ratio of an engine is given, e.g. as 1:10, then the measured pressure in each cylinder should be equal to or higher than 10 bar (see Boyle's law and combined gas law). If there is a significant (> 10%) difference between cylinders, that may be an indication that valves or cylinder head gaskets are leaking, piston rings are broken, cylinders are worn or that the engine block is cracked.
Saab Variable Compression engine
Because cylinder bore diameter, piston stroke length, and combustion chamber volume are almost always constant, the compression ratio for a given engine is almost always constant.
One exception is the experimental Saab Variable Compression (SVC) engine. This engine uses a technique to dynamically alter the volume of the combustion chamber (Vc), which, via the above equation, changes the compression ratio (CR).
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 block, 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 pivotting 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.
However, the SVC project was shelved by General Motors (the owner of Saab Automobile) due to cost.
Variable Compression Ratio (VCR) Engines
The SAAB SVC is a very late addition to the world of VCR engines, the first being built and tested by Harry Ricardo in the 1920s, it lead to him devising the octane rating system that is still in use today. The company has recently been involved with 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.