Luigi P. Tozzi

Advanced Combustion System Solutions for Increasing Thermal Efficiency in Natural Gas Engines While Meeting Future Demand for Low NOx Emissions

Key requirements for state of the art industrial gas engines are high engine thermal efficiency, high engine brake mean effective pressure (BMEP), low NOx emissions, and acceptable spark plug life. Fundamentally, as engine thermal efficiency and power density increase, along with the requirement of reduced NOx emissions, the pressure at the time of ignition increases. This results in a higher spark breakdown voltage that negatively affects spark plug life. This problem is resolved with a smaller electrode gap and high spark energy to overcome quenching effects during ignition kernel development. High flow fields in the spark gap region are required to assure the spreading of the discharge, which reduces the rate of electrode erosion. In addition, these high flow fields overcome mixture inhomogeneities by developing large ignition kernels. These large ignition kernels, inside the prechamber spark plug, produce high velocity flame jets into the main chamber enhancing combustion, which results in thermal efficiency gains at lower NOx levels and higher BMEP. The advanced combustion system solution discussed in this paper is the combination of high-energy ignition and a prechamber spark plug with flow fields at the electrode gap. Future developments include improved ion signal quality detonation detection resulting in additional gains in thermal efficiency.Copyright

Acoustic and Ion Sensing of Lean Blowout in an Aircraft Combustor Simulator

** † ‡ § ** This paper describes work to develop practical, fast diagnostic techniques that can be used to monitor the proximity of a combustor to blowout using measurements of the flame’s acoustic and ion signatures. A novel ignition system with ion sensing capability is used for blowout detection in conjunction with acoustic sensing. Data was acquired from a commercial single nozzle combustor fueled with Jet-A. As lean blowout is approached, short duration, localized extinction and re-ignition events were observed. Several signal processing schemes were developed to detect these blowout precursors, including signal thresholding and spectral analysis. The analyses revealed changes in the low frequency acoustic and ion spectra and increased presence of intermittent precursor events in both acoustic and ion data closer to blowout.

Solutions for Improving Spark Plug Life in High Efficiency, High Power Density, Natural Gas Engines

Short spark plug life, resulting in increased engine downtime and operating costs, is the primary factor limiting the power density and thermal efficiency in lean burn natural gas engines. Fundamentally, as engine power density increases, spark plug life decreases. Common approaches to increasing spark plug life include use of high melting temperature electrode materials and increased electrode surface area. However, future targets for engine efficiency and power density require more effective system solutions. In order to achieve these system solutions, work has been focused on developing an empirically derived electrode erosion model. This model quantifies spark plug life as a function of spark discharge characteristics, spark plug electrode design, and flow fields in the vicinity of the spark plug gap for different engine power densities. Furthermore, quenching effects resulting from large surface electrodes and smaller spark gaps have been included to verify ignitability for given in-cylinder charge density and air/fuel ratio conditions. A good agreement between experimental data and model predictions has been demonstrated. Finally, a solution for extending spark plug life in high efficiency, high power density, natural gas engines has been proposed. This solution combines high spark power with a spark plug design consisting of small electrode gap, large electrode surface, and with enhanced flow fields at the electrode gap.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

Industrial Gas Turbine Exciters: Design Considerations for More Effective Products

Ignition systems for industrial gas turbines, in use for decades, continue to evolve with improving technology. A recent development is the use of microprocessors and solid-state semiconductor power switching devices (digital systems). Extensive testing of digital systems has demonstrated excellent ignitability, reliability, and plug life compared to traditional analog systems. Furthermore, digital systems demonstrate a potential for combustion feedback.Copyright