Mean Piston Speed


Mean piston speed is the average speed of piston in reciprocating engine.

Piston speed usually refers to the average or mean speed of the piston as it moves up and down in the cylinder bore during each crankshaft revolution.

In fact the piston actually comes to a complete stop at the top of the stroke (TDC) and at the bottom of the stroke (BDC), its speed and acceleration at any given point is always changing.

Its important to note that , The formula for mean piston speed yields an average speed.

As we know Speed = Distance /time

so we can write;

MPS = Distance covered by piston in one revolution X rev per sec

lets say L is stroke of the Engine, distance covered will be 2L

therefore MPS =2LN/60 (N =Revolution per Min).

The expression further indicates that besides a power function the MPS permits manipulation of other parameters for certain gain of MPS, shorter the stroke higher is the speed of revolution. A short stroke decreases the height and weight of the engine considerably.

Now let us analyse how it is related to power Power = work done per sec = mean effective pressure * A * L * N/C where C = constant, C=2 for 4 stoke , C=1 for 2 stroke , A= Area, L= Length , N= rev per sec

so, Power = P * L * A * N / C hence Power = constant * (2) * L X N = (constant) X (2LN) so we can say Power = MPS x constant

From above equation we can easily say more the MPS, more the power .

but is it really ?

  1. MPS vs Dyanamic forces , moments , inertia forces Higher the MPS higher the magnitude of dynamic forces , moments & reaction forces (due to inertia) , fluctuating stress (due to varying torque ) .

2. MPS vs Gas Exchange process Higher the piston speed higher the resistance of gases to how in and out of the cylinder, hence The output at the cylinder is lowered   beyond a certain speed as the efficiency of the gas exchange process sharply falls.              

3. MPS Vs Running wear Increased Speed decreases the service life at those components which form rubbing and rotating. The relative surface speed should be such that the rate of wear of piston rings or liners must be within the acceptable limits speed (however is not the only criteria on for wear as it is known that wear on the liner is maximum at top center where the speed is least.)

   4.  MPS vs Fuel consumption

A) If MPS is high, time available for fuel combustion is very less, less time for gas exchange leads to incomplete combustion affecting thermal efficiency of engine which manifests as high fuel consumption

B) At low MPS, Temp attained at the end of the compression is not sufficient for complete combustion as due to low MPS heat is rejected resulting in temperature reduction, incomplete combustion leads to black smoke and higher fuel consumption. 

so to conclude , MPS plays important role in the Engine System, though we can increase the MPS to increase the power it has limitation as discussed above, as well as MPS can not be less than required rpm for efficient combustion .