enginespeed

Enginespeed, commonly written as engine speed, denotes the rate at which an engine's crankshaft completes revolutions. It is normally expressed in revolutions per minute (RPM) and, in physics contexts, in radians per second (rad/s). In internal combustion engines, enginespeed is governed by throttle position and the engine control unit (ECU), which regulate fuel delivery and ignition to maintain a target speed at idle and under load.

A tachometer shows enginespeed to the operator. Modern ECUs coordinate gear shifts to keep RPM within safe or efficient ranges. Power depends on both torque and speed; horsepower increases with RPM up to a peak and then becomes limited by the engine’s design and efficiency. A redline marks the maximum safe RPM for a given engine.

Ranges vary by engine type. Gasoline engines commonly idle around 600–1000 RPM, cruise around 1,500–3,000 RPM, and have redlines around 6,000–7,500 RPM; high-performance units can exceed that. Diesel engines typically operate at lower RPM but deliver high torque. Turbocharging, gearing, and cam timing influence where peak power occurs, and thus the practical operating speed band.

Enginespeed affects wear, fuel economy, and emissions. Higher speeds increase frictional losses and exhaust temperatures, while operating in efficient RPM ranges improves combustion and durability. In hybrids and electric vehicles, enginespeed is managed alongside electric motor speed to optimize overall performance and efficiency.