Development of Positive Crankcase Ventilation System of Four-stroke Engine for Small Motorcycles

In most motorcycle engines, blow-by gas is introduced into the air intake system, including the air cleaner, for the purpose of reducing the blow-by gas emissions in the crankcase. In such a system, the water and gasoline components of the blow-by gas get mixed into the engine oil under some driving conditions. This oil contamination shortens the oil replacement intervals. In order to reduce this problem, we devised a system to ventilate the crankcase positively and installed it on the engine of a small motorcycle. The system had a construction to draw fresh air with a one way valve that is actuated by the change in pressure inside the crankcase. The inside of the crankcase is ventilated by the fresh air. In the case of small motorcycle engines, if the gas positively ventilated from the inside of the crankcase were guided to the downstream of the throttle valve, the engine performance under low -loads, including idling, is significantly changed by the inflow of the gas. To avoid such changes, a construction was adopted to guide the blow-by gas to the upstream of the throttle valve. Furthermore, a system was adopted to control the quantity of fresh air sucked into the crankcase with a solenoid valve controlled by engine speed and throttle angle. The influence on engine performance by blow-by gas was thereby minimized. The positive crankcase ventilation system (PCV system) reduced the amount of water contained in the engine oil by 73% and that of gasoline by 42%, as compared with the conventional system.