Impact of intake valve modulation on engine efficiency and emissions in a stoichiometric spark-ignition natural gas engine at low and mid loads

Abstract

The growing adoption of natural gas as a transportation fuel is driven by its potential to lower greenhouse gas emissions while serving as a viable alternative to conventional diesel engines. However, conventional stoichiometric spark-ignition (SI) natural gas engines suffer from efficiency losses at low to mid loads due to the use of intake air throttling to regulate airflow. While prior studies have explored intake valve closing (IVC) strategies to improve efficiency by reducing throttling losses, few have experimentally demonstrated clear trends for both early and late intake valve closing (EIVC & LIVC) across low and mid loads, particularly in SI natural gas engines. This study addresses that gap through experimental demonstration of early and late intake valve closing strategies across multiple engine speeds at low and mid loads, while analyzing emission trends and impacts of gas exchange, combustion, heat transfer on efficiency. Tests were conducted on a heavy-duty SI natural gas engine under conditions based on the Low Load Cycle (LLC), a representative duty cycle for urban and vocational vehicles where 87% of fuel energy is consumed at low to mid loads. Steady-state tests at 2.8 and 5.6 bar brake mean effective pressure showed brake thermal efficiency improvements of up to 10% and 6%, respectively, primarily due to reduced pumping losses. Additionally, intake valve closing modulation contributed to NOx reductions of up to 40% at both low and mid loads, while CO2 emissions decreased by up to 8%, reflecting reduced fuel consumption. Hydrocarbon (HC) and carbon monoxide (CO) emissions showed no significant changes across most intake valve closing modulations. Combustion analysis revealed that early intake valve closing leads to combustion deterioration, but the associated reduction in heat transfer, as identified through fuel energy distribution analysis, mitigates its impact on brake thermal efficiency.