Improved ignition in high-BMEP, lean-burn gas engines
by Alan Oetken
As high-BMEP gas engines move toward leaner air-fuel-ratio operations to meet increasingly more stringent emission requirements, the air-fuel mixtures become more difficult to ignite. Lean mixtures have slower laminar flame speeds that can result in engine misfire. Also, in large-bore engines the flame front has to travel further, so high fuel-charge turbulence is used often to increase the flame front speed, but this increased turbulence makes the flame initiation more susceptible to ignition variability.
In high-performance gas engines, shot-to-shot combustion variation typical of some spark plug designs can can lead to poor ignition quality, excessive knock margins, poor engine efficiency, and higher exhaust emissions.
Traditional J-gap spark plugs try to address performance issues by increasing the spark energy, which shortens plug life. To counter decreasing spark plug life, J-gap manufacturers often increase the electrode area, which has a “quenching” effect on the ignition, or they use precious metals that increase manufacturing complexity and cost. In addition, the J-gap spark plugs’ spherical flame front has slower combustion speed as the flame front moves across the combustion chamber. This type of flame front is often enhanced by high in-chamber turbulence, which increases ignition variability.
The relatively recent introduction of pre-chamber spark plugs have addressed some of these issues. But conventional, pre-chamber spark plugs tend to extend into the combustion chamber, such that heat is absorbed in the end cap, which increases the risk of pre-ignition and thermal run-away. In addition, conventional pre-chamber spark plugs have difficulty managing residual exhaust gases during the exhaust stroke, causing inconsistent exchange of burned gas and fresh charge in the pre-chamber, which also leads to misfire and knock.
Woodward’s new FTI (fast turbulent ignition) igniters are a leap forward in spark igniters for high-BMEP, lean-burn gas engines fueled by natural gas or biogases. When customized for specific engine requirements, FTI igniters can extend operating limits to lower NOx emissions and increase engine efficiency by enabling engine timing to operate closer to top-dead-center (TDC). These new igniters use a unique pre-chamber design with patented features that dramatically reduces the combustion coefficient of variation (COV) while extending the igniter’s operating life without precious metal electrodes.
The FTI igniter’s proprietary design provides high-velocity ignition jets for consistent “shot-to-shot” ignition. The multi-jet flame front coming out of the FTI igniters generates its own in-chamber turbulence. This self-generating turbulence adds to the existing in-chamber turbulence promoting rapid flame growth throughout the fuel mixture. FTI pre-chamber design increases the jet velocity by as much as two times that of conventional pre-chamber plugs.
Since the FTI doesn’t rely on the “hot surface” method to obtain fast combustion like conventional pre-chamber plugs, it reduces the possibility of pre-ignition (knock) and thermal run-away often associated with pre-chamber spark plugs, especially as BMEP is increased. Secondly, the FTI igniter properly manages residual exhaust exchange in the pre-chamber, keeping residual gases effects to a minimum after the exhaust stroke and properly recharging the chamber during each shot.
Woodward’s FTI igniters provide patented innovations that deliver longer life with more consistent ignition of lean-fuel mixtures in high-BMEP gas engines. These improvements enable better engine performance, higher combustion efficiencies and reduced emissions, while reducing both misfire and knock in challenging applications.
For the product specification, click here.
Alan Oetken is project manager of new product launches at Woodward, Inc., Loveland, Colo., USA. The product announcement originally appeared in Woodward’s Energy Control Newsletter, July 2015, and this article, with revisions, later appeared in Diesel & Gas Turbine Worldwide, July-August 2015, page 30.
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