Research on Extended Expansion General-Purpose Engine - Theoretical Analysis of Multiple Linkage System and Improvement of Thermal Efficiency

摘要

Research has been conducted on an extended expansion engine, using a multiple linkage system to increase the thermal efficiency of general-purpose engines. A four-jointed linkage was used between the connecting rod and the crank pin of a standard piston-crank system. The end of the linkage rotates at half the speed of the crankshaft, resulting in piston strokes unequal length in each revolution. The length of the expansion stroke is greater than that of the compression stroke, thereby providing an extended expansion cycle. This thermal cycle is just Atkinson cycle, and precisely different from Miller cycle, which often required supercharging or variable valve timing control system. Theoretical thermal efficiency and numerical simulations were used to determine the increase in thermal efficiency at a compression ratio of 8.5 and an expansion ratio of 12.3. The results show that this extended expansion increases indicated thermal efficiency 4.0% against a conventional engine, from 27.3 to 31.3%. In addition, the new design displayed increases of 2.0 and 2.7% in indicated thermal efficiency against naturally aspirated Miller cycle engines with early valve closure and late valve closure, respectively. An air-cooled, single-cylinder engine equipped with this mechanism was constructed and its thermal efficiency was compared in tests against a conventional engine in the same power class. The results of these tests matched the simulation results almost exactly, with the new engine displaying 4.1% higher indicated thermal efficiency and 5.0% higher break thermal efficiency than in the conventional engine. Using this system, it can be achieved to improve thermal efficiency without increasing the compression ratio which leads to knocking. This system is well suited to air-cooled, general-purpose engines operated under the continuous high loads and may often be operated by low-octane fuels in a developing country.