Frictional Analysis of a Small Two-Stroke Utility Engine via Tear-Down Testing 2007-32-0076

Two stroke engines are popular power sources for ambulatory applications because of their superior power to weight ratio. Due to fuel short-circuiting however, they suffer from poor fuel economy and high levels of emissions. Small utility two-stroke engines are typically tuned rich to improve ignition stability and cooling. Additionally the presence of fuel in the crankcase requires high rates of two-stroke oil usage to insure proper lubrication. Given rising petroleum prices and concern for the environment, reducing the fuel consumption and emissions of two-stroke engines is of increasing importance.

With the extreme price sensitivity of small two-stroke engines more sophisticated options such as direct fuel injection, are not possible. Instead, the best starting point for fuel consumption improvement is to insure proper carburetor tuning and lubrication of the engine and optimization of the engine's intake and exhaust tuning.

For this study a typical small utility engine was mounted to a motoring dynamometer. The engine was operated at a standard speed for a given length of time with various oil/fuel ratios. Once the engine ran dry the ignition was switched off and the frictional torque was measured as the engine cooled. Frictional measurements were analyzed as a function of lubricant quantity and engine temperature. Finally friction was measured as the engine was progressively dismantled, removing the exhaust system, carburetor, spark plug, piston and connecting rod. Frictional contributions from each source were then tabulated.