Performance analysis of a two-stage expansion air engine

This study proposes an air engine consisting of one small and one large cylinder to conduct two-stage expansion in series, in which high-pressure air first expands in the small cylinder and then residual-pressure air is transferred to the large cylinder for another expansion, by fully using the high-pressure air and increasing the power output and efficiency of the engine. First, mathematical models of a single-cylinder engine and a two-stage expansion engine were constructed. Second, the relations between the rotational speed and the output power, torque, efficiency, and cylinder pressure were established using MATLAB simulation software for analyzing the air engine in comparison with experimental approaches. The experimental results indicated that the two-stage expansion engine generated up to 1.7 kW of power and 12.42 Nm of torque at air pressure of 12 bar, which was superior to the performance of a single-cylinder engine. By varying the intake and exhaust timing sequences, the relations among the rotational speed, output power, torque, efficiency, and cylinder pressure were investigated. The results showed that early intake in the first cylinder improved power output by 5.3% as the speed increased, whereas early intake and exhaust in the second cylinder increased power output by 7%.