The effects of dual-coil ignition and axial swirl on spark-assisted controlled autoignition

Abstract

The work involved an experimental study of a dual-coil ignition system for improved combustion stability and control during four-stroke gasoline controlled autoignition combustion. The ignition system was based upon the ‘redundant-coil’ set-up previously used in motorsport applications, where two conventional induction coils are connected to a common spark plug via an individually assigned diode. Such systems have recently been used for improved tolerance to exhaust gas recirculation in automotive spark ignition engines, but their effects have apparently not been previously reported for spark-assisted controlled autoignition. The basis for the work was therefore to validate such operation at low loads, where spark assist has been most widely suggested as a means of stabilising controlled autoignition combustion. The experiments were undertaken using a thermodynamic single-cylinder research engine equipped with an electrohydraulic, fully variable valvetrain on the inlet and the exhaust. This valvetrain was primarily used to invoke negative valve overlap and later, additionally, to allow study of the asymmetric intake valve operation and the influence of the increased axial swirl ratio within the bulk gas flow. The key observation from the work was that a staged single strike per coil strategy was found to improve the combustion stability significantly when operating at the low-load controlled autoignition boundary, especially when combined with increased axial swirl to promote ignitability between successive strikes. Overall, it was concluded that the system provides a new method for ensuring robust controlled autoignition operation at the lowest loads, with additional ability to help to control the combustion noise.