In closed crankcase ventilation systems, the oil-loaded blow-by is fed back into the intake air. Due to the incomplete combustion of the oil, it produces higher particle and hydrocarbon emissions, a reduced catalytic converter lifetime, and deposits in the intake manifold.A survey of state-of-the-art ventilation systems is given. The procedures for measuring the ventilation aerosol on the engine and the requisite measuring instruments, especially for determining the particle size distribution, are described. The results of the measurements prove the usually unsatisfactory separation efficiency of the production parts.A significant increase in separation efficiencies can be obtained by using nonwovens. To optimize the construction concerned, several measurements with varied separation parameters of the nonwovens were conducted on an oil mist test bench.