The Effects of Crankcase Oil Viscosity on Engine Friction at Low Temperatures 2000-01-2052
With Euro III emissions standards requiring a 50% reduction in current diesel car emissions from January 1st 2000, there is a need to improve fuel consumption and thus reduce exhaust emissions. One of the ways this is being addressed is by the introduction of lower viscosity crankcase oils and by the use of friction modifiers within these oils. The resulting reduced engine friction contributes to improved fuel economy and should also aid engine cold start ability.
To investigate the cold start abilities of different crankcase oils, a matrix consisting of six different oil formulations was tested at -20°C, - 25°C and -30°C in a two litre four cylinder diesel engine. The tests were conducted using a dynamometer, with the engine being driven by the dynamometer at nominal cranking speed for 30 seconds and then increased to cold idle speed for a further 30 seconds. The main parameters of interest were the torque-to-turn (torque required to change engine speed from static to cranking speed or from cranking to cold idle speed), motoring torque, oil pressure and main bearing temperature.
Changing from the most to the least viscous oils, a reduction of almost 36% was found in the torque-to-turn required during the engine initial cranking phase. The low viscosity oil also exhibited a faster increase in gallery oil pressure during the speed changes. However, the lower oil viscosities appeared to worsen the engine friction during the ramp up to cold idle speed. This is thought to be due to back-pressure on the oil pump, and may lead to difficulties in engines reaching idle speed after the engine has fired.
During testing of the most viscous oil, a 5W/30 with a Kinematic Viscosity @ 100°C of 12.5 mm2/s, more than 23 seconds elapsed before normal oil pressure was reached. This could lead to premature engine component wear under cold climate conditions.
Main bearing temperatures were as expected, with more viscous oils giving faster bearing temperature increases.