Combustion Chamber Development for Flat Firedeck Heavy-Duty Natural Gas Engines

<div class="section abstract"><div class="htmlview paragraph">The widely accepted best practice for spark-ignition combustion is the four-valve pent-roof chamber using a central sparkplug and incorporating tumble flow during the intake event. The bulk tumble flow readily breaks up during the compression stroke to fine-scale turbulent kinetic energy desired for rapid, robust combustion. The natural gas engines used in medium- and heavy-truck applications would benefit from a similar, high-tumble pent-roof combustion chamber. However, these engines are invariably derived from their higher-volume diesel counterparts, and the production volumes are insufficient to justify the amount of modification required to incorporate a pent-roof system. The objective of this multi-dimensional computational study was to develop a combustion chamber addressing the objectives of a pent-roof chamber while maintaining the flat firedeck and vertical valve orientation of the diesel engine. A new combustion chamber was designed based on a commercial 11-liter natural gas engine using stoichiometric combustion, exhaust gas recirculation, and a fixed, early intake valve closing Miller Cycle. Intake port, piston crown, and sparkplug placement modifications were evaluated to provide a combination of tumble and cross-tumble, while reducing but not eliminating the swirl of the base engine. Valve and seat dimensions and orientation of the base engine were maintained, allowing the modifications to be implemented with only a cylinder head casting and piston crown change. For comparative analysis a pent-roof system applied to the same engine was also evaluated. The new design accomplished the objectives of increased burn rate and improved auto-ignition margin. The delay of auto-ignition onset allowed a compression ratio increase from the baseline 13:1 to either 14 or 14.5:1 depending on full-load timing retard decisions. A full-load thermal efficiency improvement of 0.7 to 0.8 points and part-load improvements of 1.5 to 2 points are projected. Performance matched or exceeded that achieved with the pent-roof chamber.</div></div>