Rapid wearing out of the engines of farm tractors, trucks and automobiles led the University of California to undertake a study of the dust problem and the efficiency of air-cleaners in removing field and road dust from the air before it passes into the engine. Work was begun in 1922 and several reports have been made on the methods devised and the progress made during the last 2 years. Results to June, 1924, were given in the paper published in August, 1924. The present paper gives results of the studies to the end of 1924 and includes data from tests of 12 new makes or models of air-cleaner not previously tested or not fully tested.
Of outstanding importance is the discovery that the quantity of dust inspired by any cleaner or carbureter is greatly reduced if the intake is placed high and faces away from the direction of motion of the vehicle. The cleaners on two 3½-ton Liberty trucks used in road construction work in California, with intakes located 46 and 48 in. above ground and facing forward, encountered approximately 0.07 and 0.08 grams of dust per mile respectively, whereas two other trucks of the same make and model and used in the same work, which had no air-cleaners but whose intakes were directed backward, evidently encountered much less dust as the average engine wear in these two trucks was almost exactly the same as in the truck fitted with a cleaner that caught and held slightly more than 0.08 grams per mile but which probably passed as much dust into the engine as it caught and held. A 1-ton truck which had its intake located 47 in. from the ground and facing sidewise under the hood encountered slightly more than 0.001 grams of dust. This truck, however, was used mainly on paved roads.
The makes and models of cleaner tested since the last progress report was given at the Spring Lake Meeting of the Society last June are illustrated and described and the author presents graphs showing the efficiency of all the devices tested and the amount of restriction they imposed on the flow of air to the carbureter when clean and when they had been fed from 25 to 100 grams of No. 1 standard dust, which is entirely free from moisture, has a specific gravity of 0.680, and 98.60 per cent of which passes through a screen of 200 meshes per in.
The accompanying tables give the type, weight, material and dimensions of the 12 additional cleaners tested since June, 1924, the results obtained in service tests to determine the quantity of dust entering the carbureter intake, a summary of results of tests of the 12 cleaners showing their efficiency in service runs and the amount of restriction caused by each cleaner as measured in inches of water due to vacuum in a U-tube manometer, and results of a fill-up test of these cleaners to give a general indication of how a cleaner might act if subjected to gross neglect and abuse by determining how much or how little increase of restriction occurs when from 1 to 10 lb. of No. 2 standard dust has been fed to it.
Other tests of great importance which might be made would have as their objects the determination of length of time the cleaner will last in normal service, how long it will function without attention, amount of work or time required to keep it clean and in proper working condition, weak places in its construction and variation in efficiency among cleaners of the same make and model.
The University of California is preparing a program of laboratory tests of bearing wear, which is closely allied with the dust problem. The author says that the function of the University is not to do routine testing but to devise methods and prove them by their practical application. It is ready to undertake the task in cases where the regular test methods are not applicable, as in testing radiator-fan-type cleaners.-


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