Experimental Investigation on CNG-Diesel Combustion Modes under Highly Diluted Conditions on a Light Duty Diesel Engine with Focus on Injection Strategy

In the last decades, emission legislation on pollutant emissions generated by road transportation sector has become the main driving force for internal combustion engine development. Approximately 20% of worldwide emissions of carbon dioxide from fuel combustion come from the transportation sector, and road vehicles contribute up to 80% of those emissions [1]. Light-duty methane gas engines are usually spark-ignited due to similar combustion characteristics for methane gas and gasoline. Since spark ignition requires a low compression ratio to avoid knock problems, gas engines have lower efficiency than diesel engines. Acombustion concept that has been successfully applied on large stationary engines and to some extent on heavy-duty engines is dual-fuel combustion, where a compression-ignited diesel pilot injection is used to ignite a homogeneous charge of methane gas and air. This dual-fuel combustion concept is well established for large stationary engines and exists as an after-market solution for heavy-duty engines but does not exist at all for light-duty engines. This concept offers a high degree of flexibility for the engine operation because dual fuel combustion does not require heavy modifications of the original diesel engine architecture so diesel operation could remain unaltered.This paper presents an initial study of how combustion characteristics in a multi-cylinder dual fuel methane-diesel light duty engine are altered by the injection control strategy adopted on different high substitution ratio operating points under highly diluted conditions (unthrottled). The measurements have been performed under steady state conditions but the impact of injection strategy on transient operation is discussed and analyzed based on emissions and brake thermal efficiency. Results show that high substitution ratios are difficult to operate at low loads regardless the operation mode selected. RCCI or PPCI combustion could be adopted to promote more stable and robust combustion, although those modes require lower substitution ratios to be achieved. This investigation does not aim for an optimization of the injection parameters in dual fuel combustion mode, but it aims to point out and understand the most relevant characteristics and behaviors of a light duty dual fuel CNG-Diesel engine operating under high substitution ratios.