Thermodynamic Limits and Chemical Energy Conversion Efficiency in Internal Combustion Engines

Abstract

Internal combustion engines remain critical for global energy conversion in transportation and heavy industry despite the rising share of renewable and electric power sources. However, their thermal efficiency is fundamentally limited by thermodynamic laws, and practical engines achieve only 25-40% efficiency due to irreversible processes such as friction, heat losses, incomplete combustion, and pumping losses. Additionally, these engines contribute significantly to pollutant emissions including CO₂, NOₓ, and particulate matter, driving stringent regulatory pressures worldwide. This paper reviews the thermodynamic limits imposed by the Carnot cycle and the gap between ideal and real engine efficiencies, focusing on entropy generation caused by irreversibilities within the engine cycle. Key engineering challenges such as material thermal limitations, mechanical losses, and operational constraints are examined, highlighting the role of thermal barrier coatings, advanced cooling systems, and structural design improvements in mitigating these issues. Emerging technologies in fuel chemistry, including the use of Dimethyl Ether and high-octane synthetic fuels, alongside advanced combustion strategies like Homogeneous Charge Compression Ignition (HCCI) and Reactivity Controlled Compression Ignition (RCCI), demonstrate promising routes to enhance efficiency and reduce emissions. Furthermore, integrative system-level approaches combining ultra-high-pressure injection, low-temperature combustion, and thermoelectric generation offer future potential to break through current efficiency ceilings. This study underscores the necessity of collaborative advancements across structural design, fuel innovation, combustion control, and materials science to push the thermal efficiency of internal combustion engines closer to their theoretical limits, while simultaneously addressing environmental challenges.