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Spark plug design features are established by both the engine designer and the spark plug engineer to obtain the optimum engine and spark plug performance for the specific application. The paper describes the elements which influence spark plug design, discusses factors affecting the spark plug voltage requirement, operating temperature and heat range, and presents test data showing the effect of various firing end configurations on engine performance. The engine tests were performed in a medium displacement V-8 engine at a simulated 30 mph road load operating condition. The results suggest that the ignition of lean fuel/air ratios can be extended by projection of the spark plug gap deeper into the combustion chamber and that there exists an optimum relationship between electrode size and gap spacing. Further tests, with ignition systems having widely varying spark discharge characteristics, indicate the very short spark durations to be detrimental in igniting lean fuel/air ratios.

Some of the ignition systems currently being developed for gasoline engines have widely different electrical discharge characteristics. By selecting certain of these systems and by modifying conventional systems, the authors undertook an evaluation of the effect of the electrical discharge characteristics of the ignition system on fuel economy in current passenger car engines. It was found that changes in the electrical discharge characteristics from the present conventional ignition system could not improve fuel economy. However, a change in spark plug gap projection was found to be beneficial in the several makes of engines tested. Spark plug temperature recordings of typical vehicles in actual service indicated substantial low power operation. Tests based on low power accelerations revealed considerable losses in fuel economy and performance due to malfunctioning of the spark plugs which was undetected by the driver.