§ How it works

In one crank revolution the piston travels the stroke twice: once down, once up. Mean piston speed is that distance divided by the time one revolution takes.

\bar{v}_p = \frac{2\, S\, n}{60}

With stroke $S$ in metres and $n$ in rpm the result is in m/s. It is the standard yardstick for how hard an engine's bottom end is working: most road engines stay below about 20 m/s at redline, and racing engines reach 25 m/s or more.

Instantaneous speed

The piston stops at each end of the stroke and is fastest somewhere before mid-stroke. For crank radius $r = S/2$ , rod length $l$ and crank angle $\theta$ :

v(\theta) = r\,\omega \left( \sin\theta + \frac{r \sin\theta \cos\theta}{\sqrt{l^{2} - r^{2}\sin^{2}\theta}} \right)

A shorter rod pushes the velocity peak closer to TDC and makes it taller, which is why the curve on the chart leans to the left.

Acceleration at TDC

a_{\text{TDC}} = r\,\omega^{2} \left( 1 + \frac{r}{l} \right)

This is where the rod has to yank the piston to a stop and throw it back down. The load grows with the square of rpm, so a 10 % higher redline means about 21 % more stress on rods, pins and bolts.

§ How it works

In one crank revolution the piston travels the stroke twice: once down, once up. Mean piston speed is that distance divided by the time one revolution takes.

\bar{v}_p = \frac{2\, S\, n}{60}

Instantaneous speed

The piston stops at each end of the stroke and is fastest somewhere before mid-stroke. For crank radius $r = S/2$ , rod length $l$ and crank angle $\theta$ :

v(\theta) = r\,\omega \left( \sin\theta + \frac{r \sin\theta \cos\theta}{\sqrt{l^{2} - r^{2}\sin^{2}\theta}} \right)

A shorter rod pushes the velocity peak closer to TDC and makes it taller, which is why the curve on the chart leans to the left.

Acceleration at TDC

a_{\text{TDC}} = r\,\omega^{2} \left( 1 + \frac{r}{l} \right)

This is where the rod has to yank the piston to a stop and throw it back down. The load grows with the square of rpm, so a 10 % higher redline means about 21 % more stress on rods, pins and bolts.

Piston Speed

Inputsdrag · tap · type

Fig. 04Piston velocity over one revolution

§ How it works

Instantaneous speed

Acceleration at TDC

Piston Speed

Inputsdrag · tap · type

Fig. 04Piston velocity over one revolution

§ How it works

Instantaneous speed

Acceleration at TDC