A Study of Ignition System Performance and Design, 03-9525Printer Friendly Version
Inclusive Dates: 01/01/05 12/31/05
Background - Because of concerns with rising fuel prices, the ability to increase fuel economy of light-duty gasoline engines has taken on a new priority. Previous internal research projects have identified a new technology bundle to increase fuel economy and decrease emissions in gasoline vehicles. The technique, which consists of running extremely high EGR levels to reduce knock and NOx, allows the user to operate at high compression ratios and at low loads, almost unthrottled. This method cannot be successful, however, without a vigorous ignition system that can ignite a very dilute mixture and promote a rapid burn rate.
Approach - The goal of this project is to investigate the fundamentals of ignition systems to develop an understanding of the important parameters in their design. This understanding will be used to select or design igniters for the SwRI high-efficiency dilute gasoline engine technology and to improve the performance of the standard gasoline engine. Because an ignition system consists of an igniter and ignition circuitry, there are two parts to this project. The first is to investigate igniter hardware and determine the controlling parameters in spark plug performance. To accomplish this, several spark plugs were modified to have different ground strap configurations. The spark event and initial flame kernel formation are observed using a very high-speed camera, and the effect of ground strap configuration on flame growth is noted. The goal here is to examine the relationship between surface area and volume on heat transfer from the ground strap. The second part of this project involves the ignition circuit. Studies have shown that a longer duration spark event is beneficial to ignition in dilute conditions. An extended duration spark circuit was constructed to study the effect of energy level and spark duration on igniter performance. A set of plugs using this circuit was also observed using the high-speed camera, and the effects of energy level and circuit resistance were investigated.
Accomplishments - Ten modified spark plugs were tested in an optically accessible constant volume combustion chamber. The resulting high-speed videos show a marked difference between geometries in that spark plugs with larger surface areas have a slower flame kernel development than those with smaller areas. There also appeared to be an aerodynamic effect based on the cross-sectional shape of the ground strap. This difference also carried over to the engine in the plugs that were tested for their lean limit. The extended duration circuit was shown to increase burn rates in the optical combustion chamber and demonstrated that the ignition event could be stretched from 1 millisecond to 6 milliseconds using proper follow-on circuitry. The extended duration spark circuit has been moved to the engine test cell for dilution limit testing in SwRI’s variable compression ratio engine