Assessment of Engine Thermal Conditions on Combustion and Emissions: A Comprehensive Design-Oriented Experimental Investigation of Coolant and Lubricating Oil Temperature Variations in an Automotive Diesel Engine

Abstract

Engine thermal conditions play a decisive role in governing combustion behavior, efficiency, and pollutant formation in compression ignition engines, particularly under varying operating and warm-up conditions. While coolant temperature has traditionally been considered the primary thermal control parameter, the independent and combined influence of lubricating oil temperature on in-cylinder combustion and emissions remains insufficiently understood. This study presents a comprehensive, design-oriented experimental investigation into the effects of controlled coolant and lubricating oil temperature variations on combustion characteristics and exhaust emissions in an automotive diesel engine. Experiments were conducted on a four-stroke, direct-injection diesel engine equipped with independent thermal management systems for coolant and lubricating oil circuits, enabling precise regulation of their respective temperatures over a wide operational range. The engine was tested at selected steady-state operating conditions representative of typical automotive usage. High-resolution in-cylinder pressure measurements were employed to derive combustion metrics, including ignition delay, heat release rate, and combustion phasing parameters (CA10, CA50, and CA90). Concurrently, regulated gaseous emissions namely nitrogen oxides (NOₓ), carbon monoxide (CO), unburned hydrocarbons (HC), and smoke opacity were measured using calibrated exhaust analyzers. The results demonstrate that both coolant and lubricating oil temperatures exert a significant and distinct influence on combustion dynamics and emission formation. Elevated coolant temperatures were found to increase in-cylinder wall temperatures, leading to reduced ignition delay, advanced combustion phasing, and enhanced thermal efficiency, albeit accompanied by increased NOₓ emissions. In contrast, higher lubricating oil temperatures primarily affected mechanical and thermal boundary conditions by reducing lubricant viscosity and friction losses, which in turn improved combustion stability and reduced CO and HC emissions, particularly at lower loads. The coupled variation of coolant and oil temperatures revealed nonlinear interactions, highlighting the importance of coordinated thermal management strategies. The findings underline the critical role of integrated engine thermal control in balancing efficiency and emissions. From a design and control perspective, the study provides valuable insights into optimal thermal operating windows and supports the development of advanced coolant and oil temperature management strategies aimed at improving combustion performance and meeting increasingly stringent emission regulations.