Fuel Stratification to Improve the Lean Limit in a Methane-Fueled Heavy-Duty Spark-Ignition Optical Engine

Abstract

<div class="section abstract"><div class="htmlview paragraph">Natural gas is an attractive fuel for heavy-duty internal combustion engines as it has the potential to reduce CO₂, particulate, and NOx emissions. This study reports optical investigations on the effect of methane stratification at lean combustion conditions in a heavy-duty optical diesel engine converted to spark-ignition operation. The combination of the direct injector (DI) and port-fuel injectors (PFI) fueling allows different levels of in-cylinder fuel stratification. The engine was operated in skip-firing mode, and high-speed natural combustion luminosity color images were recorded using a high-speed color camera from the bottom view, along with in-cylinder pressure measurements. The results from methane combustion based on port-fuel injections indicate the lean burn limit at <i>λ</i> = 1.4. To improve the lean limit of methane combustion, fuel stratification is introduced into the mixture using direct injections. Two different volume fractions of direct injections (20% and 40% by volume) were tested in addition to the port-fuel injections. With stratification, the combustion phasing is shifted towards the TDC, and combustion duration decreases, indicating a faster combustion process. The stratification strategy obtained by combining the direct injection of 40 % methane by volume and port-fuel injection of 60 % methane by volume, shows highly stable combustion with COV of IMEPg as 2.85 %, rapid heat release, and shorter burn duration. The high-speed color images provide insights into the spatial and temporal evolution of flame propagation, while a HSV-model (Hue Saturation Value) analysis enables the identification of different combustion regions based on observed color changes during the combustion process.</div></div>