A good air-cleaner is an essential part of automotive engine equipment. Many types of cleaner are on the market and the user must choose on the basis of the three essential requirements of maximum cleaning efficiency, minimum attention from the operator and minimum power-loss.
With respect to these three essentials, the development of a laboratory method of testing air-cleaners starts with the premise that the test for efficiency should consist of feeding in a weighed quantity of dust, and an account be made for that which is not separated by the cleaner. The first method was to insert a white outing-flannel cloth in the airstream from the cleaner. The varied degrees of soiling of the cloths from different cleaners were a relative measure of their efficiency. This method was found unsatisfactory for several reasons.
An attempt was made to use a dry centrifugal cleaner of predetermined efficiency, in series with the cleaner under test, to catch a portion of the dust escaping. It was found, however, that the efficiency of the centrifugal cleaner varied with the fineness of the dust, and that the dust escaping another cleaner was so fine that almost none of it was caught by the centrifugal cleaner.
It was discovered that soft felt ⅜ in., or more, thick was 100-per cent efficient when used as a filter. A filter was arranged using soft felt ⅝ in. thick interposed in the air line from the cleaner under test to the carbureter. The filter was arranged so that the felt could be removed and weighed. In making a test, the apparatus was set-up, using an air jet for suction instead of an engine. A venturi meter was inserted in the line to assure correct air velocity. The apparatus was operated without feeding dust until the weight of the felt became constant. Then, 50 grams of dust was fed into the cleaner and, from the increase in weight of the felt filter, the efficiency of the cleaner was computed. The best cleaners, as determined by this method, were then checked by the use of an engine under load on a Sprague cradle-dynamometer. Determinations were made also of the cleaner's effect on the horsepower of the engine, as well as of the water consumption of washers or any other factors that might affect the operation of the cleaner or engine.
While it is possible and may appear feasible to some to embody more refinement in the method outlined, the author believes, due to his experience and observation, that this is not essential to the determination of a satisfactory air-cleaner. In making the final choice, the most efficient cleaner may not be selected. Operators do not, as a rule, give the necessary attention to such auxiliary apparatus. If a cleaner requires little or no attention from the operator, it might be the wiser choice even though it may be slightly less efficient. In cases where tractors are called upon to operate under extremely dusty conditions, it has been found advisable to combine a centrifugal cleaner, which requires no attention, with an oily fiber filter. This combination gives an over-all efficiency of 99.9 per cent and the filter need only be cleaned and re-oiled once a week.
The method of attachment to the engine is very important, in that tight connections must be provided so that unclarified air shall not leak into the carbureter inlet. The author recommends standard flange connections, or their equivalent, throughout.