Cubic capacity

The rolled is the Volume swept by the displacement of a moving part in a room hermetically closed for a unit movement. This concept is used for the Pompe S and all the Moteur S using a Fluide.

The unit movement is a return ticket in the case of a linear device like a piston or a turn in the case of a rotary device.

The cubic capacity is homogeneous with a volume per number of movement, it will thus be expressed in m ³ /tour or m ³ /aller-return in the system IF. By abuse language, one expresses the cubic capacity as being homogeneous with a volume, one makes implicit the fact then that it is brought back to a unit movement. Thus the cubic capacity is more usually expressed in liter for the car S and in cm ³ for the motor bikes.

In the particular case of the internal combustion engine, the cubic capacity is the volume swept by a piston between the Point dead high (PMH) and the Dead bottom center (PMB), therefore for a return ticket.

Example: An engine of car of a cubic capacity of 2 Liters (actually 2L/tr) aspires and drives back 2 liters of fluid for'''''' a turn of the crankshaft. When the crankshaft makes a turn, all the pistons made a return ticket.

Calculation of a cubic capacity for a device with Piston S

The surface of a piston corresponds to the diameter opened by the Alésage of the cylinders and is calculated according to the formula of the surface of a disc: : S = \ pi \ times R^2 = \ pi \ times \ frac {D^2} {4} \,

With R the ray of the section of the piston and D its diameter.

By multiplying the surface of the piston by the race and the number of pistons, one obtains the total cubic capacity. The race C is the distance parcourrue between the two extreme positions of the piston. It is equal to twice the eccentricity of the crankshaft.

V_ {rolled} = NR \ times C \ times S = NR \ times C \ times \ pi \ times R^2

avec
NR , the number of cylinders,
C , the travel of the piston,
S , the surface of the section of the piston. One will pay obviously attention to the units as for any calculation.

Case of the internal combustion engines

In a qualitative way, plus an engine a " has; grosse" cubic capacity more it develops an important couple, n the other hand such an engine will not be able to turn very quickly because of the size and thus of the mass of the pistons and other moving parts.

Concretely, an engine of motor cycle of a cubic capacity of 50cm ³ (0,05 liter) can turn to more 12000tr/min whereas an engine of truck, of a cubic capacity of 16 liters will exceed only seldom the 2000tr/min.

The couple developed depends on the cubic capacity and the effective average pressure (SME).

SME derives from the concept of SME or average pressure indicated, which is average intensity of pressure on the surface of piston during a race doubles compression-expansion.

SME is not entirely transformed into couple, because a fraction (15 to 25%) is absorbed by the Friction S of the parts between them, for the ejection of exhaust fumes and the aspiration of fresh gases (losses by pumping), like for the drive of the distribution, Pompe S with oil, with water, of food and possibly of injection and or compressor, and or ventilator.

The Moteur S with low number of cylinders are less powerful (with equal cubic capacity), because they turn less quickly; on the other hand they have franker recoveries. The engines with great number of cylinders have a more elastic behavior, the made-to-order of electrical motors less, and vibrate because driving times are more frequent thus balance the unit best.

See too

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