Michael B. Riley

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

An Advanced Turbocharging System for Improved Fuel Efficiency

Turbochargers provide an efficient method of utilizing exhaust energy to boost intake air pressure for improved engine performance and efficiency. However transient operation requires increased air delivery (via quicker compressor response) to allow more rapid fueling for acceleration in both diesel and natural gas engines. In diesel engines rapid boosting will avoid increased particulates caused by excessive fueling during acceleration. Further, in applications that use either a wastegate, inlet bypass or variable vanes in the turbine to limit boost pressures, the excess energy in the exhaust is thrown away. The SuperTurbo™ (or superturbocharger) can recover much of the wasted energy and return it to the crankshaft, increasing overall efficiency. Woodward has developed a mechanical geartrain connected to the turbine shaft that transmits power through a variable speed hydraulic transmission to the crankshaft of an engine. This combination, a superturbocharger, provides the needed characteristics of (a) recovery of energy at high speed/load operating points (turbocompounding), (b) very rapid acceleration of the turbine shaft during transients (supercharging), (c) elimination of boost limitation devices, and (d) a variable speed hydraulic transmission that will be lower cost than a high-speed, high-power electrical system. While the air requirements are different for diesel and natural gas engines, both have sufficient exhaust energy to drive a turbine beyond the needs of the compressor for much of the performance map. Part load operation may be different as natural gas engines are usually throttled. The choice of a diesel or natural gas application was influenced by the availability of a suitable engine. The first prototype superturbocharger was built and tested on a Mack E7G natural gas engine, replacing the wastegated turbocharger of the stock engine. Preliminary results show fuel economy improvements of almost 6% at high speeds and high loads. In addition the load response of the engine was greatly increased due to the ability to accelerate turbine shaft speed, increasing boost. However there was a peak power output drop due to limitations in boost and imperfect sizing.Copyright