Experimental comparison of open chamber and passive prechamber spark ignition on the combustion performance and emissions for a heavy-duty Natural gas engine

Abstract

In the natural gas engine industry, due to concerns about global climate change and greenhouse gas emissions, methane (CH₄) emissions are becoming a strong focus. Operating in the fuel-lean regime yields high thermal efficiency and low engine-out NOx emissions, but it can result in elevated methane emissions. Passive prechamber igniters are designed to increase in-cylinder turbulence and accelerate the combustion process, enabling faster and more stable combustion compared to traditional open chamber spark plugs. This improves stability and has the potential to reduce unburned hydrocarbon emissions. Additionally, faster burn rates help prevent end gas autoignition, the primary cause of engine knock, enabling the use of higher compression ratios and more advanced spark timing to increase thermal efficiency. The focus of this work aims to quantify the impact of passive prechamber and traditional open chamber ignition systems on the emissions and performance of a high compression ratio, fumigated natural gas engine over a wide range of excess air ratios from stoichiometric to ultra-lean. Data collected from these experiments was then used to evaluate the advantages and drawbacks of passive prechamber igniters when compared to a traditional SI open chamber. Under near stoichiometric conditions up to modestly lean operation (λ∼ 1.4), it was found that the passive prechamber igniter delivers a rapid combustion process that is ∼2× faster than the open chamber system. This yielded improved combustion stability and lower propensity for end gas autoignition. However, the combustion is so rapid, that “knock” or pressure waves are induced, which yield significantly elevated combustion noise. These differences diminished as the excess air ratio was increased to λ > 1.5, and the ignition systems yielded a similar combustion process. Ultimately, the open chamber igniter delivered lower NOx, lower methane emissions, higher thermal efficiency, and a slightly higher lean limit compared to prechamber igniter.