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The compression ratio is a single number that can be used to predict the performance of any engine (such as an Internal-combustion Engine or a Stirling Engine ). It is the Ratio between the volume of the Cylinder , when the Piston is at the bottom of its Stroke , and the volume when the piston is at the top of its stroke. A high compression ratio allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place increased oxygen and fuel molecules into a reduced space; thus, they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree. Higher compression ratios, however, also make Engine Knocking , also known as detonation or Pre-ignition , more likely and this can reduce an engine's efficiency and even physically damage it. FORMULA The ratio is calculated by the following formula: :, where : = Cylinder Bore (diameter) : = Piston stroke length : = 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.
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, the pre-ignition cylinder pressure can be estimated using the following relationship: : where is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, is the compression ratio, and is the ratio of specific heats of the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture. For example, if an engine running on gasoline has a compression ratio is 10:1, the cylinder pressure at top dead center (TDC) is : This figure, however, will also depend on cam (i.e. valve) timing. As a rule of thumb, cylinder pressure for common automotive designs should at least equal 10 Bar , or, roughly estimated in Pounds Per Square Inch (psi) as between 15 and 20 times the compression ratio, or in this case between 150 psi and 200 psi, depending on cam timing. Purpose-built racing engines, stationary engines etc. will return figures outside this range. Factors including late intake valve closure (relatively speaking for camshaft profiles outside of typical production car range, but not necessarily into the realm of competition engines) can produce a misleadingly low figure from this test. Excessive connecting rod clearance, combined with extreme oil pump out (rare but not impossible) can sling enough oil to coat the cylinder walls with enough oil to facilitate reasonable ring seal artificially give a misleadingly high figure, on engines with compromised ring seal. This can actually be used to some slight advantage. If a compression test does give a low figure, and it has been determined it is not due to intake valve closure/camshaft characteristics, then one can differentiate between the cause being valve/seat seal issues and ring seal by squirting engine oil into the spark plug orifice, in a quantity sufficient to disperse across the piston crown and the circumference of the top ring land, and thereby effect the mentioned seal. If a second compression test is performed shortly thereafter, and the new reading is much higher, it would be the ring seal that is problematic, whereas if the compression test pressure observed remains low, it is a valve sealing (or more rarely head gasket, or breakthrough piston or rarer still cylinder wall damage) issue. If there is a significant (> 10%) difference between cylinders, that may be an indication that Valves or Cylinder Head Gasket s are leaking, Piston rings are worn or that the Block is cracked. If a problem is suspected then a more comprehensive test using a Leak-down Tester can locate the leak. 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, until engine wear takes its toll. One exception is the Experiment al Saab Variable Compression Engine (SVC). This engine, designed by Saab Automobile , uses a technique that dynamically alters 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 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 SVC project was shelved by General Motors , when it took over 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 . This work led to him devising the Octane Rating system that is still in use today. The company 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 . The Atkinson Cycle engine was one of the first attempts at variable compression. Since the compression ratio is the ratio between dynamic and static volumes of the combustion chamber the Atkinson cycle's method of increasing the length of the powerstroke compared to the intake stroke ultimately altered the compression ratio at different stages of the cycle. DYNAMIC COMPRESSION RATIO The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke ( Bottom Dead Center - BDC), and that the volume compressed is the actual volume. However: intake valve closure (sealing the cylinder) always takes place after BDC, which causes some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. This "corrected" compression ratio is commonly called the "''dynamic compression ratio''". This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio. The actual position of the piston can be determined by trigonometry, using the stroke length and the . (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.) The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure. SEE ALSO
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