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Absolute cylinder pressure is used to calculate the dynamic compression ratio, using the following formula: = where is a polytropic value for the ratio of specific heats for the combustion gases at the temperatures present (this compensates for the temperature rise caused by compression, as well as heat lost to the cylinder)
The overall displacement for a typical reciprocating piston engine is calculated by multiplying together three values; the distance travelled by the piston (the stroke length), the circular area of the cylinder, and the number of cylinders in the whole engine. [2] The formula is:
From the geometry shown in the diagram above, the following variables are defined: rod length (distance between piston pin and crank pin) crank radius (distance between crank center and crank pin, i.e. half stroke) crank angle (from cylinder bore centerline at TDC)
The volume ratio / is called the "isentropic expansion ratio". (For the Otto cycle is the same as the compression ratio V 1 / V 2 {\displaystyle V_{1}/V_{2}} ). Mechanically this is the expansion of the hot gaseous mixture in the cylinder known as expansion (power) stroke.
In an engine designed to use the squish effect, at top dead centre the piston crown comes very close (typically less than 1 mm [2]) to the cylinder head. The gases are suddenly "squished" out within the combustion chamber , creating turbulence which promotes thorough air-fuel mixing, a factor beneficial to efficient combustion .
The stroke ratio, determined by dividing the bore by the stroke, traditionally indicated whether an engine was designed for power at high engine speeds or torque at lower engine speeds. [ 2 ] [ 3 ] The term "bore" can also be applied to the bore of a locomotive cylinder or steam engine pistons .
VR5 and VR6 engines are very compact and light, having a narrow V angle which allows a single cylinder block and cylinder head. These engines use a single cylinder head so are technically a straight engine with the name "VR" coming from the combination of German words “Verkürzt” and “Reihenmotor” meaning “shortened inline engine”.
In a multi-cylinder engine, pistons may reach top dead centre simultaneously or at different times depending on the engine configuration. For example: In the V-twin configuration, the two pistons reach TDC at different times, equal to the angular displacement between the cylinders.