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When combustion products, commonly called “blowby,” are prevented from reaching an engine's crankcase, sludge formation is inhibited and lubricant life extended. This paper discloses the concept, developed at the Army Fuels and Lubricants Research Laboratory, which prevents leakage of blowby gases into the crankcases of piston engines. “Blowby diversion” is accomplished, by one or a combination of four basic methods, at the piston ring zone where the blowby gases are intercepted after leaking by the bottom compression ring, but before reaching the crankcase. Single-cylinder engine studies have demonstrated that the principle of blowby diversion is feasible and can prevent better than 90% of the blowby from entering the crankcase. The studies further indicated a significant reduction in sludge formation rates (increase in engine cleanliness), with decreases in ring wear and air pollution. Blowby diversion appears to be reliable and easily adapted to conventional engines at low cost.