Domenico Chiera

Mechanism for High Velocity Turbulent Jet Combustion From Passive Prechamber Spark Plug

Next generation passive prechamber spark plugs for high BMEP natural gas engines require long ignition delay for durability, fast combustion for efficiency, and low COV for lean engine operation. Additionally, a successful plug should have long life, low cost, and have a robust knock margin, with best-in-class NOx vs. fuel consumption.This paper discusses the underlying physics of the novel passive prechamber spark plug, the Woodward–Lean Quality Plug (WW-LQP.) The WW-LQP has demonstrated good ignition delay, fast combustion, and low COV at λ > 1.8+, while improving fuel consumption by more than 1% on a lean natural gas engine.The key operating principles are developed for achieving complete combustion of the prechamber “charge”, leading to high prechamber pressure rise and resulting in high velocity turbulent flame jets, which in-turn provides for fast combustion in the main chamber. The design physics are verified by CFD simulations and on-engine experiments, including pressure measurements in both the prechamber and main combustion chamber.Copyright

Lean Limit Extension for High BMEP Gas Engines via Novel Electronic Ignition and Prechamber Plug: Higher Efficiency and Lower NOx in Open Chamber Engines

Large bore natural gas engines have the perennial challenge to achieve ever higher efficiency with ever lower NOx emissions, while maintaining stable combustion, avoiding misfire and engine knock. A primary strategτy to achieve these goals is to run leaner and leaner. However, leaner mixtures lead to reduced combustion stability and the operating space between misfire and engine knock shrinks. Leaner operation requires a high performance ignition system. This report will highlight the fundamental challenges related to lean operation and the progress Woodward has made to create a novel high performance prechamber spark plug to achieve good combustion stability in a passive prechamber spark plug under lean conditions. The spark plug in combination with the appropriate ignition system enables faster and more stable combustion under increasingly lean conditions, improving fuel efficiency and emissions. Engine simulation modeling is used to demonstrate the benefits of lean gas mixtures and reduced combustion duration to enhance the NOx versus fuel consumption trade-off for a range of air fuel ratios. With this database available, a design requirements flow-down is performed such that combustion performance requirements can be specified a priori, which if met would ensure the high level engine emissions and performance targets would be met. With combustion requirements in hand, CFD simulations are used to identify the mechanisms by which flame propagation is improved with prechamber spark plugs in general, and by the Lean Quality Plug (WW-LQP) prechamber spark plug under development at Woodward. Experimental validation was carried out to confirm the benefits of lean operation and improvement of combustion stability (COV) on the NOx-efficiency trade-off. Operation with Woodward’s WW-LQP spark plug and IC1100 AC ignition system showed improved fuel efficiency at constant NOx on a high BMEP engine. Additionally, the enhanced stability and low COV of the WW-LQP enables extension of the natural gas lean limit closer to λ = 2.00 for an open chamber engine.Copyright

Proposed Combustion Strategy for Increasing Thermal Efficiency in Open Chamber Stationary Gas Engines

The quest for high engine brake thermal efficiency (BTE) in medium size (140mm – 190mm bore), lean-burn gas applications becomes increasingly difficult as lower emission levels (250mg/Nm3 NOx) are targeted. A traditional approach to offsetting this negative trend has been to design the piston and the intake ports to create high turbulence and homogeneous mixtures leading to faster combustion burn rates with leaner mixtures. This paper proposes a new combustion strategy aimed at optimizing fuel-air mixture stratification in the main combustion chamber. This would result in maximum fuel concentration within a passive prechamber plug leading to high turbulence flame jet (HTFJ) penetration in the main combustion chamber and, therefore, faster combustion burn rates. Experimental correlation of a combustion model is provided for flame jet ignition in a quiescent, mildly stratified combustion chamber through three different cases. The first case uses a traditional J-gap spark plug; the second, a prechamber plug that is not optimized for the fuel distribution present in this combustion chamber. Finally, the third case makes use of a prechamber plug that has been configured to have properly oriented HTFJ. These three cases constitute the basis of the proposed combustion strategy leading to significant increase in engine brake thermal efficiency (BTE).Copyright