Mean Piston Speed Calculator

How it works

mean piston speed = 2 × stroke × rpm ÷ 60 · instantaneous v(θ) = r·ω·[sin θ + r·sin 2θ ÷ (2√(l² − r² sin² θ))], r = stroke ÷ 2, ω = 2π·rpm ÷ 60 · ft/min = m/s × 196.85 · max rpm = limit × 60 ÷ (2 × stroke)

Each crankshaft revolution moves the piston down and back up one stroke, so its average speed is twice the stroke times revolutions per second. Mean piston speed is the classic yardstick of mechanical stress and wear: inertia loads rise with its square, and ring and bearing life fall as it climbs. Most road engines redline around 15–20 m/s (3,000–4,000 ft/min), sporty engines reach 20–25 m/s and racing engines, including modern Formula 1 power units, run around 25 m/s. The peak instantaneous speed, a little after 70° past top dead centre, is about 60% higher and depends on the rod-to-stroke ratio.

Worked example

An engine with an 86 mm stroke at 7,000 rpm: 2 × 0.086 × 7,000 ÷ 60 = 20.07 m/s (3,950 ft/min) — the high-performance range. With a 142 mm rod the peak speed is about 32.9 m/s, and a 20 m/s limit would be reached at 6,977 rpm.

Sources: Heywood, J. B. — Internal Combustion Engine Fundamentals (McGraw-Hill)

Frequently asked questions