Experimental Investigation of Engine Performance and Emission Characteristics of a Compression Ignition Engine Fueled with Exhaust-Gas-Assisted Preheated Ethanol–Diesel Blends

Improving the utilization of low-cetane renewable oxygenated fuels without compromising compression ignition (CI) engine efficiency remains a critical challenge. This study addresses this limitation with a self-sustaining thermal management strategy that couples ethanol–diesel blending (E10–E40) with a waste-heat-driven helical coil heat exchanger. By recovering exhaust thermal energy that would otherwise be wasted, the system raises blend temperature to an operational optimum of 40 °C without external power input, directly counteracting ethanol’s high latent heat of vaporization and low cetane number. Experiments on a single-cylinder CI engine across multiple loads show that this thermal conditioning significantly reduces blend viscosity and density, facilitating superior atomization. While ethanol’s lower energy density increased brake-specific fuel consumption (BSFC) by up to 41.94%, the preheating loop stabilized combustion and mitigated the brake thermal efficiency (BTE) deficit. Environmental benefits were most pronounced at high load, with NOx and CO2 emissions decreasing by up to 11.43% and 20%, respectively. The E20–D80 blend preheated to 40 °C provided the most robust balance between mechanical performance and emission reduction, with one-way ANOVA and uncertainty analysis confirming the statistical reliability of the measured trends. These findings establish waste-heat recovery as a practical, hardware-ready pathway for expanding the operational limits of alcohol-based fuels in conventional CI engines.