This paper presents an analytical approach of estimating the minimum torque to start an engine at low temperature down to -30 °C. Obviously, the output torque from a starter motor has to overcome the conventional mechanical friction torque from piston, bearings, valvetrain and auxiliaries, and gas torque from the cylinder pressure/vacuum rise due to piston motion. Using the mechanical friction models developed previously by the author, the paper shows that although piston, bearings and valvetrain are still significant mechanical friction sources for a cold engine, the gas torque is actually the biggest contributor to the minimum required turning torque. The analysis also showed that the gas resistance torque was sensitive to the crankshaft position at the start. For example, for a V8 gasoline engine, the gas resistance torque is the highest if the engine starts at the intake valve close (IVC) position.
