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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:
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)
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 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 .
During use, the cylinder liner is subject to wear from the rubbing action of the piston rings and piston skirt. This wear is minimized by the thin oil film which coats the cylinder walls and also by a layer of glaze which naturally forms as the engine is run-in. On some engines, the cylinder liner is replaceable, in case it becomes worn or damaged.
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.
The Trojan Car used an undersquare, split piston, two stroke, two-cylinder inline engine; this was partly for this tax advantage and partly because its proportions allowed flexing V-shaped connecting rods for the two pistons of each U-shaped cylinder, which was cheaper and simpler than two connecting rods joined with an additional bearing.
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.