Mean Piston Speed

What is Mean Piston Speed ?

The mean piston speed is the average speed of piston in the reciprocating engine.

Or,

Piston speed usually refers to the average speed or mean speed of the piston when it travels up(TDC ) and down (BDC ) in the cylinder bore during each rotations of the crankshaft.

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

The piston either accelerates from zero or slows down to zero rpm.

Formula of Mean Piston Speed

The formula for MPS yields an average speed based on two times the stroke (up and down for one revolution), times the speed of engine (RPM) divided by 60.

MPS = 2 * Stroke * RPM / 60

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

There is a factor of 2 in the equation for a single stroke to occur in 1/2 of the crank revolution (or alternatively: two strokes per one crank revolution) and a ’60’ to convert seconds from minutes to RPM.

For example, a piston in an automobile engine which has a stroke of 90 mm will have a mean piston speed at 3000 rpm of 2 * (90 / 1000) * 3000 / 60 = 9 m/s.

How do you get the expression for “MPS” as 2LN?

The formula for mean piston speed yields an average speed.

As we know that Speed = Distance /time

So, MPS = (Distance covered by piston in one revolution ) X (rev per sec )

let’s say ‘L’ is stroke of the Engine, distance covered will be ‘2L’

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

Term for MPS = 2LN

Explanation for ‘2L’:

in one crack revolution, the piston OF engine travels from TDC to BDC (covering stroke length L) and again from BDC to TDC (covering stroke length L). That is why the word has ‘2L.’

Reason for N (in rpm): As speed = distance / time, the distance traveled above 2L is for a length of 1min (as N is in rotation per minute).

Thus, the speed = 2LN is in meters per minute (in this case).

Relation between MPS and Power

Power :- Work done on the piston during working cycle is P (mean pressure).V (volume displaced by piston)

Power :- P.V. (n/s), n= no. of revolutions per sec. and s = stroke/cycle.

For two stroke engine, s = 1 and for four stroke engine s = 2.

Volume (V) = (area) . (length)
Area = A and Length = L

Now, Power = (constant) . 2 . P . L . A . N

Power = (constant) . P . A . (2LN)

(2LN) = MPS, also power is directly proportional to mps.

Is the mean piston engine speed of 2 strokes and 4-stroke engine the same (i.e. 2LN)?

No,because

MPS= 2S*N/60 for 2 stroke .

and MPS= 4S*N/60 for stroke engine.