Efficiency Effects of Supplemental Oil Cooling in Small Air-Cooled Engines

Abstract

Small air-cooled gasoline engines are typically designed to operate fuel rich as a way to avoid engine overheating. Though fuel-rich operation provides benefits in thermal management, there are drawbacks to fuel-rich operation as well, most importantly fuel efficiency and exhaust emissions. This article investigates the efficiency and emission improvements enabled by a supplemental oil cooling loop installed on a small air-cooled Honda GXR120 engine. The engine was retrofitted with electronic fuel injection to allow for direct control of air-fuel equivalence ratios, i.e., lambda (λ), while an auxiliary oil circuit pumped oil from the crankcase sump, routed it through an oil-to-water energy (heat) exchanger, and returned it to the engine through a port in the valve cover to cool the cylinder head. A set of experiments was conducted to evaluate the engine performance across low, medium, and high-load conditions and variable λ values. Experimental results demonstrate that a simple targeted oil cooling system reduces engine block temperature below the temperature achieved by enriched fuel operation. This thermal mitigation allows the engine to operate stably at a high load with a stoichiometric mixture, yielding an increase in efficiency with 22% reduction in electrical specific fuel consumption (ESFC) alongside an 87% reduction in carbon monoxide (CO) emissions. Supplemental oil cooling provides a viable, retrofittable pathway toward improved efficiency and reduced emissions in small air-cooled engines.