Piston speed (mean)

The mean piston-speed is the distance that the piston travels, if during the complete stroke, it's speed remains constant. In fact however, the piston speed varies (in light red), from zero at the top- and bottom dead centers, to the maximum speed just above the middle of the stroke, where it is around 1,6 times higher than the mean piston speed (in dark red).

To determine the mean piston speed, one can compare it with everyday speed. One doesn't simply measure, at random, any amount of speeds during a longer drive and add them together to get average speed, but the driven distance is divided through the time that it took. Thus, the mean piston speed is calculated. Of course, the piston speed can be calculated at any given point, but this is more complicated.

As one can see by the light red curve, which is similar to a sinus curve, the course of the piston speed remains about the same with each crankshaft rotation. If one calculates the average and compares this with the maximum value of the piston speed, one arrives at the factor of approx 1,6, it makes no difference whether the engine has a long- or a short stroke, or a large- or a small cubic capacity. This arises from from the motion sequence of a crank mechanism.

Checking the calculation!     Do you need help? Please enter numbers only in two of the spaces, then click on the free space!
s	Stroke	mm
n	Engine RPM	RPM
vm	Mean piston speed	m/s

Explanation of the constants

The conversion of millimeters (stroke) to the meter per sec. (piston speed) provides the factor of 1000. The conversion of the minutes at 1 RPM to the seconds of the meter per seconds (piston speed) multiply the factor by 60. The resulting figure of 60.000 must still be divided by 2, because one full crankshaft rotation (RPM) means that two strokes take place.

Whereas previously a piston speed of 16 meters per sec. was considered the limit for normal engines, nowadays, it's been moved upwards. This does not mean that most engines can now cope with more than 16 m/s, but that the range of up to 25 m/s (Formula 1) e.g., through using certain materials, and having lighter moving masses is more controllable. In this case, high demands are made on the material combinations between piston, cylinder sleeve and motor oil. The tension- and pressure strain on the connecting rod also increases greatly. How extreme piston speeds, despite high RPM can be avoided, can be read up here. 06/11

Example

Given that:

s = 0,085 m; n = 5500 1/min

Assignment:

v_m in m/s

Solution:

           s · n
v_m = =
            30

0,085 m · 5500 1/min
= 15,58 m/s
               30