Library / Automotive and vehicle dynamics / VEH-07
Automotive and vehicle dynamics
Mean piston speed
Average speed of the piston over a cycle, which is twice the stroke per revolution.
- Ref
- VEH-07
- Rev
- v1
- Field
- Automotive and vehicle dynamics
- Result
- Ūp [m/s]
The piston travels a distance 2S (up and down) in each crankshaft revolution, and the crankshaft turns N times per minute, so the average speed is 2 S N / 60 with S converted to metres. It is a cycle average of the speed magnitude. The instantaneous speed peaks at roughly 1.6 times the mean and falls to zero at each end of the stroke.
Average speed of the piston over a cycle, which is twice the stroke per revolution. It sets the mechanical stress and friction level of an engine and limits its safe rpm.
Calculator
- Mean piston speed in km/h61.92 km/h
- Crank angular speed628.3 rad/s
- Time for one piston stroke5 ms
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- At the defaults the piston averages 17.2 m/s, a typical value for a petrol engine at 6000 rpm. Production petrol engines are usually kept below about 20 to 25 m/s, and race engines run higher with special materials.
- A shorter stroke allows more revs for the same piston speed: with a 70 mm stroke the engine could run at 8000 rpm at 18.67 m/s, only a little above the 17.2 m/s here.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (m/s) | Local step | Change in result |
|---|---|---|---|---|---|---|
| S Piston stroke | 40 to 200 mm | 8 | 40 | 32 | +10% | +10 % |
| N Engine speed | 500 to 9,000 rpm | 1.433 | 25.8 | 24.37 | +10% | +10 % |
Sweep of Engine speed
| Engine speed (rpm) | Mean piston speed (m/s) |
|---|---|
| 500 | 1.433 |
| 1,563 | 4.479 |
| 2,625 | 7.525 |
| 3,688 | 10.57 |
| 4,750 | 13.62 |
| 5,813 | 16.66 |
| 6,875 | 19.71 |
| 7,938 | 22.75 |
| 9,000 | 25.8 |
Sweep one variable
Variables and how each one contributes
Mean piston speed
What the equation gives you. Everything else feeds it.
Piston stroke
The piston stroke, in mm. Directly proportional: a longer stroke means the piston must move faster at the same rpm. Raising it by 10 % (to 94.6) moves the result by +10 %, from 17.2 to 18.92 m/s.
Engine speed
The engine speed, in rpm. Directly proportional: the piston speed follows the revs exactly. Raising it by 10 % (to 6,600) moves the result by +10 %, from 17.2 to 18.92 m/s.
Worked example with the values above
- Distance travelled per revolution: 2 S = 2 × 86 mm = 0.172 m.
- Revolutions per second: N / 60 = 6,000 / 60 = 100 rev/s.
- Ūp = 0.172 × 100 = 17.2 m/s.
Assumptions and limits
- Mean speed over a full revolution of a simple crank-slider mechanism, with no allowance for the connecting rod ratio.
- Steady engine speed.
Common mistakes
- Entering the stroke in metres while the formula expects mm.
- Treating the mean speed as the peak speed. The peak instantaneous speed is higher, about 1.6 times the mean.
Statistics and uncertainty
The mean piston speed follows directly from two measured quantities. The stroke is known to better than 0.1 %, and a tachometer to about 1 %, so the result is within about 1 %. The limit it is compared with is much less certain: the acceptable piston speed depends on the engine design, lubrication, materials and duty cycle, and there is no sharp limit.
Where it is used
- Checking that a high-revving engine stays inside a safe piston speed.
- Comparing the durability and friction level of different engines.
Maths you need
- Speed = distance / time: average speed over a cycle
- Units: mm to m, rpm to rev/s