Experimental Study for Energy Balance and Exergy Analysis of a Single-Cylinder, Spark-Ignition, Air-Cooled Engine

<div>In a time when small and micro energy sources are becoming increasingly important due to current environmental challenges, the efficient recovery of low-grade waste heat has emerged as a key strategy to enhance overall energy sustainability. Although extensive research has been conducted on energy and exergy distributions in large-scale internal combustion engines, experimental studies focusing on small, air-cooled gasoline engines remain limited, particularly regarding the quantification of their recoverable energy potential. Addressing this gap, this work analyzes and quantifies the global energy distribution and exergy availability in a single-cylinder, spark-ignition, air-cooled Robin EY15 engine operating at rotational speeds between 1500 and 4600 min<sup>−1</sup>, and throttle valve openings from one-quarter to full. The defined control volume includes the engine and the load system. The mass flows analyzed are fuel flow (standard gasoline), intake air, exhaust gas (assumed as air) and cooling air, while the energy flows are net power and miscellaneous heat losses. It is found that the maximum net and exergy efficiencies of the engine are 14.1% and 13%, respectively, at 2500 min<sup>−1</sup> and full open throttle. The major energy dissipation ways are the cooling air 24.3%–73.6% and miscellaneous losses 9%–61% (percentage related to total energy flow provided by the fuel). Based on exergy analysis, between 6%–9.7% and 22%–29.7%, respectively, of that energy flows are transformable into mechanical work; however, the exhaust gases has the higher potential, between 22.7% and 34.7%. The rate of exergy destroyed ranges between 69.6% and 89.7%, meaning that the maximum achievable efficiency would range from 10.3% up to 30.5% throughout the tested engine speed–load conditions. These findings provide useful insights into the low-grade heat recovery potential of small-scale combustion engines and contribute new experimental data to the field of micro energy systems.</div>