Studies in laboratory engines equipped with radioactive piston rings show that wear is highest during a cold startup. Corrosion by condensed combustion products is responsible.
Engine operating variables and additives in fuels and motor oils influence corrosion and, therefore, startup wear. Long shutdown periods, low engine temperature, and high intake-air humidity increase wear. In fuels, antirusts offer some control; for example, an amine dialkyl phosphate eliminates 40% of the wear. In motor oils, detergents are the most helpful ingredients; barium salts of organo-phosphorus compounds or sulfonate-phenates lower wear 30%. But, taken together, antirust in the fuel and detergent in the motor oil do not reinforce each other.
Ample room remains for further improvement. Strong polar compounds that chemisorb and form tenacious protective films on metal surfaces do the best job. Particularly valuable would be fuel additives and motor-oil additives that work better together.