Geometrical Properties of Reciprocating Engines

The following parameters define the basic geometry of a reciprocating engine :

where

Vc : is Clearance volume.

Vd   : is Displacment or swept volume.

VL   or VS : is Total or stroke volume .

TC   : Top Center.

BC : Bottom   Center.

Ratio of cylinder bore to piston stroke :

                          Rbs =    B / L

Ratio of connecting rod lenght to crank radius :


                          R = l / a

Also the stroke and crank reduis are related by

                          L = 2 a

Typical values of these parameters are :

The cylinder volume V at any crank postion angle is


An important characteristic speed is the Mean piston speed Smps :

                                                           Smps = 2 L N

Where N is the rotational speed of the crankshaft .

The Instantanous piston velocity sp is obtained from

                                                       sp = ds / dt

Where S is can br obtained from the following equation " obtained from the upper figure " .

Compression ratio	It is 8-12 for SI engine	It is 8-12 for CI engine
B / L	0.8 - 1.2 for small and medium size engines	Decreasing to about 0.5 for large slow - speed CI engine
R	3 - 4 for small and meduim size engines	Increasing 5-9 for large slow speed CI engines