Experimental test and thermodynamic analysis on scaling-down limitations of a reciprocating internal combustion engine

Due to the enormous energy densities of liquid hydrocarbon fuels for future utilization on micro scale, there is a concern about the feasibility of scaling down reciprocating internal combustion engines from small scale to meso scale. By building a specialized test bench, the performance and combustion characteristics of a miniature internal combustion engine with a displacement of 0.99 cc were tested, and the thermodynamic simulation was carried out to achieve a more complete understanding of in-cylinder mass and energy change of the miniature internal combustion engine. The miniature internal combustion engine had higher brake-specific fuel consumption, lower thermal efficiency, lower brake mean effective pressure, and serious cyclic variation; however, friction mean effective pressure seems to be less sensitive to engine speed. Simulation results showed that the miniature internal combustion engine had a poor volumetric efficiency, which was not more than 50%. The step-by-step processes of scaling down the miniature internal combustion engine were also simulated; it was found that the maximum indicated mean effective pressure loss was due to the imperfection of gas exchange processes, and the next was the imperfection of combustion. It is considered that for the scaled-down miniature internal combustion engines, more attention should be pay on improving the processes of gas exchange and combustion, and achieving meso-scale internal combustion engines with cylinder bore less than 1 mm is thermodynamically possible in future if these imperfections, especially that of the gas exchange process, can be effectively perfected.