The state of perfection at present reached in the muffling of autoengines is well known. As far as this problem is concerned, it may be considered solved as regards suppression of noise. Not a greatdeal of scientific data are available. Some tests were carried on at the University of Michigan, an abstract of the, report being published in the Horseless Age for May, 1915. Five types of mufflers were tested and investigated as to back pressure, horsepower loss, and muffling ability. Of the five, the one given the highest rank on all three counts has the construction shown in figure 1. The engine used was a nominal 25 horsepower automobile engine (Hudson 0-54, 4 1/8 by 1 inches), the test speed ranging from 750 to 1,800 revolutions per minute. At the latter speed the brake horsepower reached 40. The muffler weighted 14.5 pounds, which is equivalent to 0.36 pound per horsepower, based on the maximum power, and. had a volume capacity of 847 cubic inches, which is approximately nine times the cylinder displacement.
All of the other mufflers weighed more, so that 0.36 pound per horsepower may perhaps be considered the present minimum in automobile practice. This is a feature, however of not as great importance as it would be in airplane practice. This muffler showed a back pressure of only slightly over 1 pound at the maximum speed, the loss of horsepower being only 1.4 per cent at the maximum. For automobile practice this must be considered an excellent showing.
We were fortunate enough to obtain the loan of two mufflers especially designed for an 8-cylinder V-type engine aid commercially manufactured. The smaller one of these mufflers is intended for a single cylinder and is 5 inches in diameter by 12 inches long. It has the construction shown in figure 2. The larger one, intended for four cylinders, is 5 inches in diameter and. 28 inches long. The internal construction is probably made up of a multiplication of the elements of the smaller muffler. Both of these mufflers were tested by connecting them directly to the ends of the side manifolds by means of slip joints. Speed changes were noted by forcing the mufflers on and palling them off by hand. This method of testing puts a heavier load on the smaller muffler than it is designed for, but since we did not have eight of them this was the only method available. As a matter of fact, the horsepower loss at rated output for the larger muffler is prohibitive. On the other hand, several observers judged that the larger muffler more effectively quieted the exhaust noise. The two, however, are so close together regarding this point that it became difficult to judge of the difference in connection with the other noises. Our conclusion is that both mufflers are good with respect to quieting and that the greater efficiency of quieting in the lager muffler is bought at too great an increase m the back pressure. We have no hesitation in saying that the smaller muffler is as good. a. solution of the problem as we have yet seen.
