Over the past two years, we have conducted two experimental test series aimed at examining typical performance of gasoline V-twin engines in the 25 hp class, and the suitability of assumed mechanical efficiency in correcting observed measurements. We used engines manufactured by Honda, Kawasaki, Kohler, and Subaru (Robin). The tests were conducted at the Engines Laboratory of the California Polytechnic State University, San Luis Obispo (Cal Poly). The Kohler engines are fuel injected while the others three are carbureted. We tested twenty-eight engines in total. The first series of tests included four horizontal shaft engines from each of the manufacturers (sixteen in total), and followed the general guidelines of SAE standard J1349-199506.

This paper reports primarily on the subsequent series of twelve engine tests, which included vertical shaft engines of an equivalent family (and displacement class), from three of the original manufacturers: Honda, Kawasaki and Kohler. All three engines have roughly the same engine speed range (2000–4000), and all three reportedly reach peak power at 3600rpm. This is typical of small engines, which may be used to drive small generators in addition to being installed on other equipment.

Vertical shaft engines are typically tested on a vertical shaft dynamometer, or one that converts from a horizontal to vertical position. However, these dynamometers are typically either of the water brake or eddy current type. They cannot motor the engine, and thus cannot measure friction mean effective pressure (FMEP) directly, which is the preferred method to quantify friction and mechanical efficiency for engine testing. However, testing vertical shaft engines on a horizontal shaft motoring dynamometer requires an angled gear drive to mate the engine to the dynamometer, and thus adds a loss that complicates the accurate measurement of FMEP and brake output. We present here results using a simple method with which our measurements can be corrected for this loss, in tests of this sort.

The study thus expands on our previous results, and shows the extent by which engine to engine variations are affected by shaft configurations, within a given model family, and within similar offerings by different manufacturers. We also analyzed our results to contrast the methodology of SAE J1349-199506 with that of the updated J1349-201109, specifically with respect to using an assumed value of mechanical efficiency to characterize FMEP and correct dynamometer data on small, general utility engines.

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